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REVIEW ARTICLE
published: 25 August 2014
doi: 10.3389/fpls.2014.00412
Current perspectives on the hormonal control of seed
development in Arabidopsis and maize: a focus on auxin
Antonella Locascio1, 2*†, Irma Roig-Villanova3†, Jamila Bernardi4† and Serena Varotto1
1Department of Agronomy Food Natural Resources Animals Environment - University of Padova, Padova, Italy
2IBMCP-CSIC, Universidad Politécnica de Valencia, Valencia, Spain
3Dipartimento di Bioscienze, Università degli Studi di Milano, Milan, Italy
4
Edited by:
Lucia Colombo, University of Milan,
Italy
Reviewed by:
Marcelo Carnier Dornelas,
Universidade Estadual de Campinas,
Brazil
Mario Enrico Pè, Institute of Life
Sciences ScuolaSuperiore
Sant’Anna, Italy
*Correspondence:
Antonella Locascio, IBMCP-CSIC,
Universidad Politécnica de Valencia,
Avda de los Naranjos s/n, ed.8E,
46020 Valencia, Spain
e-mail: anlo3@upvnet.upv.es
†These authors have contributed
equally to this work.
The seed represents the unit of reproduction of flowering plants, capable of developing
into another plant, and to ensure the survival of the species under unfavorable
environmental conditions. It is composed of three compartments: seed coat, endosperm
and embryo. Proper seed development depends on the coordination of the processes that
lead to seed compartments differentiation, development and maturation. The coordination
of these processes is based on the constant transmission/perception of signals by
the three compartments. Phytohormones constitute one of these signals; gradients of
hormones are generated in the different seed compartments, and their ratios comprise
the signals that induce/inhibit particular processes in seed development. Among the
hormones, auxin seems to exert a central role, as it is the only one in maintaining
high levels of accumulation from fertilization to seed maturation. The gradient of auxin
generated by its PIN carriers affects several processes of seed development, including
pattern formation, cell division and expansion. Despite the high degree of conservation
in the regulatory mechanisms that lead to seed development within the Spermatophytes,
remarkable differences exist during seed maturation between Monocots and Eudicots
species. For instance, in Monocots the endosperm persists until maturation, and
constitutes an important compartment for nutrients storage, while in Eudicots it is reduced
to a single cell layer, as the expanding embryo gradually replaces it during the maturation.
This review provides an overview of the current knowledge on hormonal control of seed
development, by considering the data available in two model plants: Arabidopsis thaliana,
for Eudicots and Zea mays L., for Monocots. We will emphasize the control exerted by
auxin on the correct progress of seed development comparing, when possible, the two
species.
Keywords: seed development, maize, Arabidopsis, endosperm, embryo, phytohormones, auxin
INTRODUCTION
In order to ensure their continuation, Spermatophytes
(Gymnosperm and Angiosperm plants) adapted seed devel-
opment, a product of their sexual reproduction, which permits
the maintenance of their lineages, allows them to be spread in
the environment, and when needed, provides resistance during
unfavorable environmental conditions (through the state of
dormancy).
The seed comprises three compartments: embryo, endosperm
and seed coat. The embryo represents the structure of the future
adult plant. It encloses all the elements and fundamental pat-
terns necessary for the new plant to develop after germination.
The endosperm constitutes the reservoir for all the nutrients that
the embryo will use during development and until the new plant
becomes autotrophic. The seed coat derives from the integu-
ments of the ovule and protects the vital part of the seed from
mechanical injury, predators and drying out.
The seed originates from a double fertilization event, in which
one sperm cell fertilizes the egg cell of the megagametophyte gen-
erating the diploid embryo, and a second sperm cell fertilizes the
diploid central cell, from which derives the triploid endosperm
(Reiser and Fischer, 1993; West and Harada, 1993; Goldberg et al.,
1994).
Briefly, the seed development process can be divided into
two main phases: (a) morphogenesis, or cellular phase, and (b)
maturation (Figure 1). Morphogenesis covers all the processes
including formation and structural development of the different
compartments of the mature seed. In this stage the resources that
provide the accessible food reserve for the embryo are also dis-
tributed and allocated. The mechanisms that lead to the definition
of the structures composing the seed are highly coordinated and
extremely complex. They involve a tight hormonal control and a
continuous interchange of signals from and to the maternal tis-
sues, and between the two major seed compartments, embryo and
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Istituto di Agronomia Genetica e Coltivazioni Erbacee, Università Cattolica del Sacro Cuore, Piacenza, Italy
Locascio et al. Phytohormones and seed development coordination
FIGURE 1 | Seed development in Arabidopsis and maize. (A) Schematic
representation of seed development in Arabidopsis. Embryo development
stages are indicated. The evolution of the endosperm is shown from the
formation of the coenocyte, where the multiple anticlinal cell divisions
generate nuclei placed all around the peripheral cytoplasm, followed by the
formation of the peripheral endosperm layer. This layer evolves into the
cellular endosperm after periclinal divisions and cell wall formation events.
Later in the developmental program, the volume of the central vacuole
progressively decreases to finally disappear, and the endosperm is absorbed
almost completely and replaced by the growing embryo in the mature seed.
At the end of maturation only three types of endosperm remain: the
single-cell layered endosperm, the micropylar endosperm surrounding the
embryo radicle, and the chalazal endosperm, adjacent to the chalazal cyst. (B)
Schematic representation of seed development in maize. Stages indicate
days after pollination (DAP). In parallel with Arabidopsis, the progression of
seed development is showed from the definition of the coenocyte, to the
cellularization of the endosperm and progressive disappearance of the central
vacuole. The process of maturation, besides others modifications, ends with
the expansion of the endosperm that finally occupies the largest part of the
seed and the accumulation of starch in its cells that progressively undergo
programmed cell death. (C) Schematic trend of hormone accumulation during
seed development. The high level of auxin (AUX) present during all the seed
development phases suggests that this hormone has a key role throughout
the entire program of seed formation. The pattern of Cytokinins (CK)
accumulation is the opposite with respect to auxin. CKs have a prominent
role during the phase that involves cell divisions, decreasing progressively
during the maturation phase, when cell expansion prevails. The
brassinosteroids (BR) follow the same pattern of CKs. The highest
concentration of BRs is shown at the beginning of seed development, and is
detected in the maternally derived tissues (i.e., integuments). Their levels
decrease at the end of maturation. The pattern of accumulation of
Gibberellins (GA) is characteristic, showing two peaks corresponding to
specific phases of seed development: the stage of embryo differentiation,
when the GAs promote cell growth and expansion, and the end of the
maturation phase, when they activate proteolytic enzymes that mobilize
resources from the endosperm necessary for germination. Abscissic acid
(ABA) shows an accumulation pattern complementary to the GAs, being the
main hormone that inhibits all the processes induced by GAs.
endosperm. The incessant communication among the three parts
composing the seed will ensure its coordinated development.
Maturation is the physiological process that ends with the
onset of the state of seed dormancy. In this stage the seed loses up
to 95% of its water content (desiccation), nutrients are stored in
the endosperm (Monocots) or in the cotyledons (Eudicots), cell
cycle activities are stopped, RNA and protein synthesis decrease
(Sheridan and Clark, 1987; Goldberg et al., 1989, 1994; Raz et al.,
2001). Embryo growth during maturation is exclusively charac-
terized by events of cellular expansion without cell divisions, and
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Locascio et al. Phytohormones and seed development coordination
subsequently cell differentiation. During late maturation the seed
is metabolically quiescent and highly tolerant to hydric stress
(state of dormancy).
The study of plant embryogenesis and seed development
has been facilitated by the characterization of mutants (Parcy
and Giraudat, 1997; Gazzarrini et al., 2004; Yang et al., 2008;
Pignocchi et al., 2009; Xing et al., 2013). Thanks to functional
analysis of mutants and misexpression experiments some of the
genes and “signals” affecting seed development have been dis-
covered (Garcia et al., 2003; Luo et al., 2005; Ohto et al., 2005,
2009; Chourey et al., 2006; Wang et al., 2010a). In addition, QTL
mappings allowed the identification of several loci with signif-
icant effect on seed weight and size (Orsi and Tanksley, 2009).
Nevertheless, the molecular mechanisms that control the transi-
tion into the maturation phase and those that precede cell growth
and division are not yet fully elucidated.
Many of the studies on seed development use Arabidopsis
thaliana as model plant for the Eudicots and Zea mays L. for the
Monocots. Despite the fact that Monocots and Eudicots share the
same seed structures, the processes that lead to seed development
and maturation reveal remarkable differences between the two
groups.
In this review, we discuss the relevance of the communica-
tions between the three compartments of the seed during its
development, by a comparative analysis of the latest findings in
Arabidopsis and maize. We will summarize the elements that par-
ticipate in the “flow” of signals that control seed development,
and describe in more detail the regulation of this process exerted
by the phytohormones, particularly by auxin.
THE PROCESS OF SEED DEVELOPMENT IN ARABIDOPSIS
ESTABLISHMENT OF THE SEED COMPARTMENTS
The process of seed formation, development and maturation
of Arabidopsis plants has been well described in several recent
reviews (Becraft and Asuncion-Crabb, 2000; Berger, 2003; Olsen,
2004; Santos-Mendoza et al., 2008; Sun et al., 2010). Soon after
fertilization, the endosperm nuclei undergo successive mitotic
divisions without cell wall formation, generating the multinu-
cleate endosperm, or coenocyte (pre-globular stage) (Figure 1A).
This phase is followed by cellularization of the endosperm, and
the definition of three regions: the micropylar, the peripheral
and chalazal endosperm (Sorensen et al., 2002). As the embryo
sac expands, the central vacuole enlarges displacing the cyto-
plasm of the endosperm to a peripheral position. The cellularized
endosperm acts as nourishing tissue that is consumed by the
embryo during maturation. The main storage products (lipids
and proteins) accumulate in the profusely grown cotyledons. In
mature seeds, the embryo fills the seed volume, while a single
peripheral endospermic cell layer persists. It contains only a few
storage products and the function of these cells is important
during seed dormancy, germination and seedling nourishment
(Bethke et al., 2007; Holdsworth et al., 2008).
IMPORTANCE OF COMMUNICATION BETWEEN THE SEED
COMPARTMENTS
A strong interdependent relationship has been described among
seed compartments (Nowack et al., 2010). A failure in the
development of one of these compartments, or in the “communi-
cations” through their structures, will cause defects in the mature
seed, and in some cases even abortion and embryo death. The
characterization of mutants presenting phenotypes affecting seed
development has helped to unravel some of the communication
pathways between seed structures (Chaudhury and Berger, 2001;
Berger, 2003; Berger et al., 2006; Nowack et al., 2010).
Mutations affecting endosperm formation at different stages
have been described. For instance, knockout mutants such as short
hypocotyl under blue1 (shb1), miniseeds3 (mini3), and haiku1 and
2(iku1 and 2) display a reduced seed size due to alterations in the
cellularization of the endosperm. SHB1 binds to the promoters
of MINI3 and IKU2, which act in the same pathway to induce
their expression and trigger endosperm proliferation and seed
growth (Kang and Ni, 2006). These mutants also display a lim-
itation of cell elongation in the surrounding integuments. Thus,
the effect of the endosperm on the development of the seed coat
points out the relevance of the communication between these
two compartments. The reduced seed size in mini3,iku1,and
iku2 knockout mutants has also recently been associated with a
reduced triacylglycerol content in the embryo (Fatihi et al., 2013).
Genetic evidences revealed the influence of the mater-
nal tissues on endosperm development, exerting their control
through chromatin remodeling mechanisms (Drews et al., 1998;
Grossniklaus et al., 1998; Chaudhury and Berger, 2001).
Furthermore, it has been shown that endosperm develop-
ment is required for proper embryo development (Berger, 2003;
Berger et al., 2006). Mutations affecting endosperm develop-
ment (Portereiko et al., 2006; Bemer et al., 2008), cellularization
(Pignocchi et al., 2009) or breakdown (Waters et al., 2013a)affect,
interrupt or even prevent embryo development and growth. A
failure in endosperm cellularization or development will also
affect the amount of nutrients stored in the seed, which are essen-
tial for embryo maturation. One of the nutrients that largely
affects the progression of embryo development is sucrose (Ruan
et al., 2010). APETALA2 (AP2), which is a floral patterning reg-
ulator, together with AtSUC5 (Baud et al., 2005)isinvolved
in the control of sucrose ratio and seed mass. ap2 mutant dis-
plays alteration in seed size leading to a bigger seed compared
tothewildtype(Jofuku et al., 2005). In addition to sugars, the
growth-promoting phytohormones cytokinins, brassinosteroids,
and auxins are considered important signaling molecules in seed
development (Sun et al., 2010). A major role for brassinosteroids
and auxins in the control of seed size has been elucidated (Schruff
et al., 2006; Martinez-Andujar et al., 2012; Jiang et al., 2013; Jiang
and Lin, 2013). An overview of the specific role of each of these
hormones in seed development will be given later in this review.
Finally, LEAFY COTYLEDON1 (LEC1), LEC2,andFUSCA3
(FUS3) are genes principally expressed in the embryo, and
required to maintain cell fate. Nevertheless, it has recently been
shown that, toward the phase of maturation, the expression of
these genes is also detectable in the endosperm. Thus, their func-
tion is determinant for both embryo development and for initi-
ation and maintenance of the maturation phase. Unsurprisingly,
alterations in their expression cause dramatic effects on seed phe-
notype (Bäumlein et al., 1994; Meinke et al., 1994; Lotan et al.,
1998; Gazzarrini et al., 2004).
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Locascio et al. Phytohormones and seed development coordination
THE PROCESS OF SEED DEVELOPMENT IN MAIZE
Maize is widely considered the model plant for studies regarding
seed development in Monocots. The program of seed develop-
ment in Monocots is basically the same as that in the Eudicots;
from the double fertilization event to the endosperm cellulariza-
tion the processes are highly conserved (Brown et al., 1996a,b)
(Figure 1). The main difference between the two models is in the
later stage of endosperm development. While in Arabidopsis the
endosperm is absorbed at the end of the maturation phase to
provide space for the embryo to grow, in maize the endosperm
persists and covers other important roles on embryo development
and seed organization.
After cellularization of the coenocyte four main cell types
differentiate and characterize the fully developed endosperm:
the basal endosperm transfer layer (BETL) or transfer cells, the
aleurone layer, the starchy endosperm and the cells of the embryo-
surrounding region (ESR) (Figure 1B). The transfer cell domain
localizes in the basal part of the endosperm. It derives from three
initial cells in the chalazal region of the endosperm coenocyte
that after division assume transfer cell identity (Figure 1B). The
function of the BETL is to load and distribute the nutrients com-
ing from the maternal tissues to the endosperm. Genes expressed
at the early stages of the transfer cell differentiation are of spe-
cial interest to understand how these cells are specified. Many of
them are involved in signal transduction and/or transcriptional
regulation between maternal tissue and developing seed recently
reviewed in Lopato et al. (2014). The aleurone layer delimitates
the transfer cell region and constitutes the outer layer of cells of
the endosperm. It represents the region of separation between the
seed coat and the starchy endosperm. Aleurone cell differentia-
tion occurs exclusively in response to surface position and does
not involve specific maternal signals input (Gruis et al., 2006;
Reyes et al., 2010). However, recent studies showed that some
phytohormones have a prominent role in the determination of
aleurone cell fate (Geisler-Lee and Gallie, 2005; Bethke et al., 2006;
Forestan et al., 2010). The specification of the aleurone cells also
depends on the expression of specific genes such as Crinkly4 (Cr4)
(Becraft et al., 1996)andDefective in kernel1 (Dek1)(Becraft
and Asuncion-Crabb, 2000; Lid et al., 2002, 2005). Interestingly,
dek1 mutant specifically lacks the aleurone cell layer, but still
maintains the transfer cells, supporting the idea that aleurone
and transfer cells originated from different specification pro-
cesses (Lid et al., 2002). A large number of mutants with defects
in endosperm and embryo have been described (Neuffer and
Sheridan, 1980; Sheridan and Neuffer, 1980). In the defective ker-
nel (dek) mutants, both embryo and endosperm development are
generally altered, while defective endosperm (de) mutants show
alterations in endosperm development (described in Manzocchi
et al., 1980; Pasini et al., 2008). A subclass of dek mutants is rep-
resented by the empty pericarp (emp) mutants (Scanlon et al.,
1994), which display reduced endosperm and pericarp loss. In
these mutants the defects in embryo organization seem to be
the origin of the compromised endosperm, pointing out, as in
Arabidopsis, the importance of communication between the seed
compartments. After seed maturation, the aleurone cell layer par-
ticipates in the process of seed germination by synthesizing the
enzymes that hydrolyze the resources stored in the endosperm
and by constituting, together with the embryo, a source of oil
storage (Saoussem et al., 2009).
The starchy endosperm is the largest part of the seed in which
starch and proteins accumulate serving the embryo germination.
It originates from the inner cell generated in the first periclinal cell
division of the endosperm, in which the external cell will generate
the aleurone. dek1 and cr4 mutants curiously maintain the starchy
endosperm and replace the aleurone layer with starchy cells.
The ESR is the cavity of the endosperm where the embryo
develops. It supplies the nutrients and constitutes the route
of communication between the embryo and the surrounding
endosperm.
The characterization of seed structures formation in maize,
and the genetic dissection of the process of seed develop-
ment have been mainly performed by a mutational approach.
Complementary to the study of mutants, advanced techniques
of molecular biology offer nowadays the possibility to investi-
gate all those cases in which the analysis of certain mutations
would not be possible. For instance, the cases in which the
defects in embryo/seed structures are as severe as to cause lethal-
ity. The identification of these mutations by techniques of deep
sequencing provides information about gene identity and its
implication in the process that has been interrupted or disturbed
by the mutation. Recently, Lu et al. (2013) and Sekhon et al.
(2014) made important contributions to the study of maize seed
development. In their works they analyzed gene transcription
by RNA-sequencing and obtained detailed information about
differential gene expression between embryo and endosperm.
Transcripts were classified, organized in subgroups and several
interesting regulatory networks between the two compartments
were proposed.
HORMONAL COORDINATION OF SEED DEVELOPMENT
Developing seeds consist of multiple tissues and cells with specific
patterns of proliferation and differentiation. In order to integrate
and organize cell distribution within the tissue/organ, determine
cell fate, and control the progression through development, a pre-
cise spatial and temporal coordination is required. Cells are able
to control all these activities through the production and per-
ception of “signals.” The transmission and perception of these
signals is important not only among seed structures, but also
within the same compartment to control the progression of the
developmental process. Hormones constitute part of these signals
(Figure 1C).
In the next sections, we summarize the current knowledge
about the mechanisms of communication between the three
structures of the seed, especially through the hormonal route.
We cover the process of seed development from fertilization
of the ovule to maturity. While other scientists have reviewed
several mechanisms governing the signaling between the three
seed compartments (Nowack et al., 2010), in this review we will
focus on the hormonal control of seed development, especially
emphasizing the role of auxin.
THE ROLE OF AUXIN IN ARABIDOPSIS AND MAIZE DEVELOPMENT
Auxin is a key hormone for plant growth and development,
accomplishing important roles during the entire lifespan of the
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Locascio et al. Phytohormones and seed development coordination
plant (Tanaka et al., 2006; Teale et al., 2006; Vanneste and Friml,
2009). It has been shown to be fundamental in the first steps of
seed development, as well as for the determination of embryo
structure and size (Hamann et al., 2002; Friml et al., 2003; Jenik
and Barton, 2005; Cheng et al., 2007; Wabnik et al., 2013). In
Arabidopsis it has been demonstrated that auxin plays a role in
seed dormancy and germination through its crosstalk with other
hormones such as Abscissic acid (ABA) (Liu et al., 2007a, 2013).
In Arabidopsis and maize correct seed development requires
a coordinated crosstalk between the seed tissues (mainly embryo
and endosperm). One of the most important signaling molecules
involved in this communication is auxin. Auxin accumulation
and distribution varies during seed development. In fact, the use
of auxin marker tools revealed that, at the beginning of develop-
ment, in immature Arabidopsis seeds, auxin accumulates in the
embryos, concretely at the root apex, ends of cotyledon primordia
and at the hypophysis (Ni et al., 2001). In maize, auxin concen-
tration increases at the onset of endoreduplication and remains
high throughout the development (Lur and Setter, 1993b). More
specifically, it has been shown that auxin is low during the initial
phase of endosperm development and increases from 9 to 11 days
after pollination (DAP) remaining high until maturation (Lur
and Setter, 1993a,b).InarecentworkbyBernardi et al. (2012)
it was shown that free indole-3-acetic acid (IAA) levels increase
between 8 and 28 DAP with a drop at 20 DAP. These results sug-
gest a role for auxin in all the stages of maize seed development.
Auxin is involved in positional signaling during aleurone devel-
opment and specification (Forestan et al., 2010). Complete loss
of endogenous auxin in the embryo might be lethal, confirm-
ing a key role of this phytohormone in embryo development and
germination.
The mechanism of action of auxin involves three checkpoints:
biosynthesis, polar transport, and perception/transduction of the
signal. These three main processes, which will be extensively dis-
cussed in the next sections, are involved in both maize (Forestan
and Varotto, 2012) and Arabidopsis seed development, influenc-
ing the final size.
IAA biosynthesis
The first natural auxin identified was the IAA, which orig-
inates from its precursor, the amino acid tryptophan (Trp).
Five IAA biosynthetic pathways have been proposed: four inter-
connected Trp-dependent IAA biosynthetic pathways, and a
Trp-independent pathway (Tivendale et al., 2014). Recently,
it has been discovered that YUC (flavin-monoxygenases) and
TAA/TAR (tryptophan amino-transferases), two of the most rel-
evant enzymes involved in the biosynthesis of auxin, act in the
same pathway, the so-called indolic 3-pyruvic acid (IPA) path-
way (Mashiguchi et al., 2011; Stepanova et al., 2011; Won et al.,
2011). Based on these studies, it was proposed that YUCCA
(YUC) proteins catalyze the rate-limiting step of the IPA path-
way. Counts of 11 YUC genes have been made in the Arabidopsis
genome (Zhao et al., 2001; Cheng et al., 2006); four of them,
YUC1,YUC4,YUC10,andYUC11,beingexpressedinanover-
lapping way in the embryo (Cheng et al., 2007). While the double
mutant yuc1 yuc4 does not show any obvious phenotype dur-
ing embryogenesis, the quadruple mutant yuc1 yuc4 yuc10 yuc11
displays several morphological defects, already at the embryonic
globular stage, failing also in developing hypocotyls and primary
roots. This strong phenotype is similar to that displayed by the
mutants in auxin signaling (monopteros,mp;bodenloss,bdl), per-
ception (transport inhibitor response1 (tir1)auxin signaling F box
protein 1 (afb1)afb2 afb3 quadruple mutant) and transport (pin-
formed 1 (pin1) pin3 pin4 pin7 quadruple mutant) that will be
further described in this review, which indicates that the auxin
synthetized by YUC is critical for embryogenesis (Cheng et al.,
2007).
It has been found that the modulation of the auxin biosyn-
thetic genes (i.e., YUC1, 2,4,and10) is controlled by transcrip-
tion factors such as LEC2 during somatic embryogenesis and
particularly, it was described that YUC4 is a direct target of LEC2
(Stone et al., 2008; Wojcikowska et al., 2013). This indicates that
regulation of the expression of key auxin biosynthetic genes by
transcription factors is one of the mechanisms that modulate the
hormone levels in Arabidopsis.
In maize, the seed is the organ that accumulates the greater
content of IAA and most of the free auxin is synthesized in situ
by the Trp-dependent auxin pathway. Compared to the 11 YUC
genes identified in Arabidopsis, only four YUC-like genes have so
farbeenidentifiedinmaize(Bernardi et al., 2012), (Tab le 1 ). The
first evidence of the relevance of TAR and YUC function in maize
emerged from two parallel works based on the study of the expres-
sion pattern of these genes in mn1 mutant. This mutant displays
a reduced IAA content in the seed, and despite a higher expres-
sion of ZmTAR than ZmYUC1 (in both mutant and wild type),
only the ZmYUC1 showed a reduced level of expression in the
mutant, suggesting a key role of this gene in auxin biosynthesis
(Chourey et al., 2010; Le Clere et al., 2010). In maize, three of the
TAR orthologs are highly expressed in the endosperm (Bernardi
et al., 2012), while of the four YUC orthologs, only ZmYUC1 is
mainly expressed in this tissue (Chourey et al., 2010; Le Clere
et al., 2010), and it was reported recently that its expression cor-
relates with IAA accumulation (Bernardi et al., 2012). Moreover,
it was observed that in order to obtain an altered seed phenotype
in Arabidopsis it is necessary to produce a quadruple yuc mutant
(as described in the first part of this section); however, in maize
the single mutation in ZmYUC1 is sufficient to cause alteration
in seed phenotype (Bernardi et al., 2012). This observation per-
mits to speculate that, with respect to Arabidopsis, the YUC genes
of maize show a higher tissue-specificity, while they are largely
redundant in Arabidopsis.
Several mutants defective in auxin biosynthetic genes were
identified in maize, i.e., orange pericarp (Wright et al., 1992);
vanishing tassel2 (Phillips et al., 2011) that is homologous to
TAA1;sparse inflorescence1 (Gallavotti et al., 2008)anddefective
endosperm18 (Torti et al., 1986; Bernardi et al., 2012) both homol-
ogous to AtYUC. Of these, the only mutant showing defective
phenotype in seed is de18. The knockout mutation of ZmYUC1 in
the mutant de18 still retains a high level of TAR despite a decrease
in IAA during early endosperm development (1-7% respect to
wild type) suggesting that TAR and YUC may act on the same
pathway also in maize seed (Bernardi et al., 2012).
The orange pericarp (orp) mutant, which is defective in
Trp-synthesis originating from indole, produces plants that are
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Locascio et al. Phytohormones and seed development coordination
Table 1 | Genes involved in the hormonal control of seed development in Arabidopsis and maize.
Gene name Acronym Biological function of the encoded protein Seed compartment expression Species*
YUCCA 1
YUCCA 4
YUCCA 10
YUCCA 11
YUC1
YUC4
YUC10
YUC11
Key enzymes of tryptophan-dependent auxin
biosynthesis. Function in seed development
and morphogenesis
Embryo, Endosperm,
Seed coat
A
PINFORMED 1
PINFORMED 3
PINFORMED 4
PINFORMED 7
PIN1
PIN3
PIN4
PIN7
Auxin efflux carriers involved in early embryo
development. Establish the apical-basal auxin
gradient
Embryo A
Transport Inhibitor
Response 1
TIR1 Part of the SCF E3-ubiquitin ligase complex.
Functions as auxin receptor and is
responsible for auxin signal transduction
Embryo, Endosperm A
AUXIN SIGNALING F-BOX 1
AUXIN SIGNALING F-BOX 2
AUXIN SIGNALING F-BOX 3
AFB1
AFB2
AFB3
F-box proteins that form a complex with TIR1.
Involved in regulation of auxin response
Seed coat (AFB2 and AFB3)and
Embryo
A
MONOPTEROS/
AUXIN RESPONSE
FAC TO R 5
MP/ARF5 Transcriptional activator. Regulates embryo
development
Embryo A
BODENLOSS/
Auxin/INDOLACETICACID 12
BDL/
Aux/1AA12
Transcriptional repressor. Interacts with MP
preventing it from activating its targets
Embryo A
ETTIN/AUXIN RESPONSE
FAC TO R 3
ETT/ARF3 Controls the integument development Seed coat A
ABERRANT TESTA SHAPE ATS Forms a complex with ETT. Involvement in
integument formation
Seed coat A
MEGAINTEGUMENTA/AUXIN
RESPONSE FACTOR 2
MNT/ARF2 Regulates seed size. Interacts with BIN2.
Growth repressor
Seed coat, Embryo A
ZmYUCCA 1 ZmYUC1 Involved in auxin biosynthesis in maize
endosperm. Controls seed size
Endosperm M
Sparse inflorescence1 Spi1 A YUCCA ortholog in maize. Role in maize
inflorescence development
Embryo M
Vanishing tassel 2 Vt2 A TAA ortholog in maize. Role in vegetative
and reproductive development
Embryo M
Orange pericarp 1
Orange pericarp 2
orp1
orp2
Both orp1 and orp2 encode the beta subunit
of tryptophan synthase. Required for
seedling development
Embryo
Endosperm
M
Miniature 1 Mn1 Cell wall invertase. Role in nutrient allocation
and crosstalk with auxin
Endosperm M
ZmPINFORMED 1
ZmPINFORMED 2
ZmPINFORMED 5
ZmPINFORMED 8
ZmPINFORMED 10
ZmPIN1
ZmPIN2
ZmPIN5
ZmPIN8
ZmPIN10
Auxin efflux carriers involved in polar
transport during embryogenesis and
endosperm formation
Embryo
Endosperm
M
SEMAPHORE1 SEM1 Regulator of knox gene expression. Required
for proper kernel development
Embryo
Endosperm
M
ABERRANT PHYLLOTAXY 1 ABPH1 Cytokinin-inducible type A response
regulator. Negative regulator of SAM size and
positive regulator of PIN1 expression
Embryo M
(Continued)
Frontiers in Plant Science | Plant Evolution and Development August 2014 | Volume 5 | Article 412 |6
Locascio et al. Phytohormones and seed development coordination
Table 1 | Continued
Gene name Acronym Biological function of the encoded protein Seed compartment expression Species*
Histidine phosphotransfer
proteins
AHPs Cytokinin signal transducers. Regulate seed
size
Endosperm, seed coat A
Histidine Kinase AHK Cytokinin receptor. Regulates seed size Seed coat A
Response Regulators ARRs Targets of the AHPs. Together with cytokinin
response proteins regulate endosperm
development
Endosperm A
SHRINK/CYP72C1 SHK1 Decreases brassinosteroids levels. Regulates
cell division and seed size
Embryo Endosperm, Seed coat A
CITOKININ OXYDASE 1
CITOKININ OXYDASE 2
CITOKININ OXYDASE 3
CKX1 CKX2
CKX3
Regulate seed size and weight Endosperm A
DWARF 5 DWF5 Endoplasmic reticulum transmembrane
protein involved in brassinosteroids signaling
Embryo, endosperm, seed coat A
ZmHistidine kinase ZmHK1
ZmHK1a2
ZmHK2
ZmHK3b
ZmHK2a2
Cytokinin receptor-like genes. Control seed
size
Embryo M
DE-ETIOLATED 2 DET2 Gene of brassinosteroids biosynthesis.
Controls embryo development, seed size and
embryo cell number
Embryo, Endosperm A
BRASSINOSTEROIDS
INSENSITIVE 1
BRASSINOSTEROIDS
INSENSITIVE 2
BRI1
BIN2
Protein kinase, regulate brassinosteroids and
phosphorilatesARF2. Growth Repressor
Seed coat, Endosperm A
BRASSINAZOLE-
RESISTANT 1
BZR1 Positive brassinosteroid-signaling protein.
Phosphorylated by BIN2
Endosperm A
ZmStarch synthase I ZmSSI Starch synthase induced by ABA Endosperm M
Only the genes mentioned in the text are showed. *A, Arabidopsis; M, Maize.
seedling lethal that can be partially rescued by supplying Trp, and
accumulates indole and anthranilate (Wright et al., 1992). The
fact that this mutant still produces auxin provided the first evi-
dence that also a Trp-independent biosynthesis occurs in maize.
Auxin polar transport
Theroleexertedbyauxinintheregulationofplantgrowth
strongly depends on its characteristic polar transport. Plants have
evolved a unique mechanism of directional cell-to-cell transport
of this growth regulator, determinant for the generation of a
polarized embryonic axis. The relevance of the apical-basal axis
establishment is that it will determine the body plan of the adult
organism. Auxin transport is realized by the PIN efflux trans-
porters, the auxin influx carriers (AUX/LAX1 family) and the
PGP proteins belonging to the ABCB transporter superfamily
(Bennett et al., 1996; Petrasek et al., 2006). The polar subcellular
localization of the carriers (influx/efflux) establishes the direc-
tional flow of auxin. In Arabidopsis there are eight genes encoding
PIN proteins (PIN1-8), of which only PIN1,PIN3,PIN4,and
PIN7 are expressed in the embryo. Relocation of PIN1 and PIN7
has been shown to be crucial for embryo polarity establishment
(Friml et al., 2003). Although the upstream mechanisms directing
the polarization of auxin during embryogenesis are still practi-
cally unknown, it was shown that 24-h after fertilization auxin
peaks in the funiculus, the chalaza, and the micropyle of the ovule
(but not in the valve), which indicates that the increase in auxin
levels in the young seeds is probably due to a maternal origin
(Dorcey et al., 2009). Friml et al. (2003) described through the
analysis of the DR5rev::GFP marker line (a synthetic promoter
that responds to auxin response factors) an accumulation of auxin
in the apical cell of the embryo, just after the first divisions of
the zygote, while a weak signal was detected in the suspensor.
Later on during development, auxin signaling is localized in the
upmost suspensor cells. At later stages of embryogenesis, auxin
signal is detected at the tips of the developing cotyledons and
provascular veins. Thus, auxin seems to determine the division
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Locascio et al. Phytohormones and seed development coordination
patterns and specification of the cells derived from the zygote,
and this pattern is determined by the activity of the PINs. PIN1
and PIN7 are required in the process of early embryo develop-
ment. In fact, their asymmetric subcellular localization has been
considered responsible for controlling polar auxin flow in post-
embryonic development (Friml et al., 2003), (Figure 2A). PIN7
is localized in the apical membranes of suspensor cells, while
PIN1 localizes in pro-embryo cells, first in a non-polarized man-
ner, while later, at the globular stage, it is recruited to the basal
membrane. PIN7 polarity is then reversed, facing the basal mem-
brane of suspensor cells. The polarization of PIN1 and reversion
of polarity of PIN7 correlate with an apical-basal reversion of
the auxin gradient (Friml et al., 2003). The biological function
of the PINs in Arabidopsis seed development has been clarified
by analyzing the phenotype of their mutants. A high percent-
age of embryos in pin7 mutant do not correctly establish the
apical-basal auxin gradient. Morphologically, the mutant embryo
shows defects at the proembryo stage resembling those of mp
and bdl mutations (see details below), bearing filamentous struc-
tures at late stages of embryo development. However, in many
cases pin7 mutants recover at globular stage, corresponding to
the moment when PIN1 protein starts to be localized at the basal
part of the cellular membrane and PIN4 expression increases. pin1
mutants also display defects at the basal embryo pole. Higher
order mutants were generated by combining PINs single mutants,
specifically expressed in the embryo (pin1,pin3,pin4,andpin7).
The severity of the phenotypes increased additively as higher
order mutants were generated. The quadruple mutant pin1 pin3
pin4 pin7 is the combination that displayed the most severe phe-
notype. These mutants, depending on the genetic background of
the ecotype, can manifest embryo lethality or, when the embryo
survives, will generate a plant with severe apical defects and non-
functional or absent roots. The severity of the phenotype showed
by these multiple mutants indicates the existence of some func-
tional redundancy among the different PINs in the embryo (Friml
et al., 2003).
In the seed of maize, a gradient of auxin might be responsi-
ble for a correct differentiation of both embryo and endosperm
(Forestan et al., 2010). The switch from the apical to basal mem-
brane localization of ZmPIN1 proteins characterizes the coleop-
tilar stage and the following establishment of an auxin flux from
both the differentiated scutellum and the shoot apical meristem
(SAM) that is responsible for the differentiation of embryonic
roots. The final stage, in which auxin polar transport is involved
during embryogenesis, is the formation of leaf primordia, sug-
gested by ZmPIN1 localization in the subapical region of the
meristem (Forestan et al., 2010)(Figure 2B).
Among the auxin transport carrier genes known in
Arabidopsis, the PIN family of efflux carriers has been extensively
studiedinmaize(reviewedinForestan and Varotto, 2012).
In particular, 12 PIN genes, 2 PIN-like and 1 ABCB were
identified (Forestan et al., 2012). ZmPIN1,ZmPIN2,ZmPIN5,
and ZmPIN10 had overlapping expression during early seed
development and their transcripts were localized in different
subcellular domains. ZmPIN8, which was up-regulated in all
the seed stages analyzed, was expressed in BETL cells, maternal
chalazal tissues and aleurone layer (Forestan et al., 2012). In the
FIGURE 2 | Auxin transport during the embryogenesis development of
Arabidopsis and maize. (A) Schematic representation of auxin transport
during embryo development in Arabidopsis. In early embryo (one-cell stage
to 16-cell stage in the figure), PIN7 (blue) is expressed in the suspensor
cells localizing to the apical membranes, mediating auxin transport toward
the proembryo. During the octant (not shown) and 16-cell stage, all
proembryo cells express PIN1 (purple), which is evenly distributed along
the inner cell membranes and not polarly localized. Later, during the
transition to the globular stage, the subcellular localization of PIN1
becomes polar, facing the basal membranes. Simultaneously and similarly,
the polarity of PIN7 localization is reversed, now localized at the basal
membrane of suspensor cells. The localization of PIN1 and PIN7 in the
basal membranes establishes an apical-basal flux of auxins that will be
maintained throughout the life cycle of the plant (adapted from Friml et al.,
2003; Weijers and Jurgens, 2005; Nawy et al., 2008). (B) Model for the
ZmPIN1-mediated auxin transport during early stages of maize
embryogenesis. Medial longitudinal sections of maize embryos at
proembryo, transition and coleoptilar stages are shown. The ZmPIN1
protein (red) localizes in embryo plasma membranes. After the first division
of the zygote, several cell divisions in different planes lead to the formation
of the small embryo and the larger suspensor (proembryo stage). At this
stage, auxins accumulate in the endosperm above the embryo but not in
the embryo itself (not shown), and ZmPIN1 localizes at the cell boundaries
of the undifferentiated proembryo core, without any polarity. Later,
adaxial/abaxial polarity is established by the outgrowth of the scutellum at
the abaxial side of the embryo (early transition stage). ZmPIN1 is polarly
localized in the apical anticlinal membranes, marking the provascular cells
of the differentiating scutellum, indicating an auxin flux toward the tip of the
single maize cotyledon (late transition stage). At the coleoptilar stage there
is the switch from apical to basal gradient of ZmPIN1, followed by a change
of the auxin flux (adapted from Forestan et al., 2010; Chen et al., 2014). col,
coleoptile; SAM, shoot apical meristem; scu, scutellum; su, suspensor. In
both A and B the red arrows indicate the auxin efflux mediated by PINs.
same work it was proposed that some ZmPIN genes could be
subjected to sub-functionalization. Some conserved mechanisms
in Arabidopsis and maize are found during embryogenesis
where ZmPIN1a-c appeared to have a redundant function as
Frontiers in Plant Science | Plant Evolution and Development August 2014 | Volume 5 | Article 412 |8
Locascio et al. Phytohormones and seed development coordination
previously reported for Arabidopsis PIN genes (Vieten et al.,
2005)(Figure 2).
In a recent work by Chen et al. (2014) it was shown that the
auxin signaling from the endosperm was important to pattern
embryo development during early embryogenesis in maize. More
specifically, the auxin signal detected at the surface of the adax-
ial embryo was correlated to the specification of embryo proper,
SAM and scutellum. As previously mentioned, the ZmPIN1 sig-
nals were polarized first in the apical and in the basal membrane
of epidermal L1 cell layer suggesting an auxin flux from these cells
to the inner layers (also detailed in Figure 2). Finally, the same
authors suggested a new function for the ESR during the early
stages of development, being responsible for preventing the auxin
flux from the endosperm to the embryo.
Concerning the endosperm specification, a gradient drop off
specifies the aleurone fate maintaining the outer layer above a
specific IAA threshold. Indeed, in normal maize endosperm, the
auxin concentration is high at the endosperm margin and lower
in the center. In the presence of NPA, an auxin transport inhibitor,
auxin accumulates above this threshold in many cell layers, result-
ing in a multilayered aleurone (Becraft and Asuncion-Crabb,
2000; Becraft and Yi, 2011).Afurthermutantthatshowsareduc-
tion of auxin transport is semaphore1 (sem1). sem1 shows a dwarf
phenotype with defects also in endosperm and embryo patterning
(Scanlon et al., 2002) but the gene mutation responsible for the
alteration in the polar auxin transport has not yet been identified.
Signal transduction
At the molecular level, auxin response is mediated by the
action of AUXIN RESPONSE FACTORS (ARFs). The ARFs are
transcription factors that recognize specific sequences termed
Auxin-Response Elements (AuxREs) present in the promoter of
auxin-responsive genes, activating or repressing their transcrip-
tion (Abel and Theologis, 1996; Ulmasov et al., 1999). However,
the ARFs do not seem to be able to regulate gene expression in
response to auxin by themselves, but require interaction with the
AUXIN/INDOLE-3-ACETIC ACID (AUX/IAA) proteins, which
constitute the repressors of auxin signaling (Kim et al., 1997).
Aux/IAAs function as transcriptional repressors by binding and
sequestering the ARFs. The degradation of the Aux/IAA proteins
is induced by auxin (Guilfoyle and Hagen, 2007). Briefly, the F-
box protein Transport Inhibitor Response 1 (TIR1), which has
been identified as an auxin receptor (Dharmasiri et al., 2005;
Kepinski and Leyser, 2005), interacts with the E3-Ubiquitin lig-
ase Skp1/Cullin/F-box (SCF) complex, generating the SCFTIR1
complex, and determining the substrate specificity toward the
Aux/IAA proteins. These proteins are thus ubiquitinated, and
marked as substrates for proteasomal degradation (Tan et al.,
2007). Dharmasiri et al. (2005) described three additional genes,
AFB1,2,and3, which encode F-box proteins also interacting
with the SCF-complexes. In Arabidopsis, TIR1 and AFB1 are
moderately expressed during embryogenesis, while AFB2 and 3
expression is high. The generation of high order mutants for the
four genes resulted in a progressive decrease in auxin response,
as well as an increase in the severity of the defects in develop-
ment. The severe effect of the quadruple mutation is manifested
in the lack of root, the disappearance of hypocotyl, and often
by the formation of a single cotyledon resembling bdl or mp
mutants.
Genome-wide analyses on the maize reference genome were
performed to identify the Aux/IAA (Wang et al., 2010b)andARF
gene families in maize (Liu et al., 2011; Xing et al., 2011; Wang
et al., 2012). The 31 identified ZmAux/IAA genes showed a puta-
tive expression (based on EST mining) in different tissues and
organs that suggests a temporal and spatial pattern of regulation.
The 31–36 ZmARF are also known to have a tissue-specificity that
changes during plant development (Wang et al., 2010b, 2012; Liu
et al., 2011; Xing et al., 2011). Each ZmARF possesses a sister
pair due to chromosomal duplication of the maize genome, and
they are distributed in all the chromosomes except chromosome 7
(Xing et al., 2011; Wang et al., 2012). Xing et al. (2011) found that
about half of the ZmARF genes identified have an auxin respon-
sive element in their promoter regions and 18 were predicted to
be targets of small RNAs. Furthermore, in the same work, seven
ZmARF genes were constitutively expressed in developing embryo
suggesting the importance of auxin signaling during embryo for-
mation in maize. Despite the genomic information, there is still a
lack of knowledge about the functional role of these auxin-related
factors in maize.
InArabidopsis23ARFsand29Aux/IAAwereidentified
(Okushima et al., 2005). Of these, only three ARFs and one IAA
have been characterized as having a role in seed and embryo
development. ARF5/MONOPTEROS (MP) is an activator of
auxin-responsive genes and constitutes a key element in the devel-
opment process of the embryo. While mp partial loss-of-function
mutants have nearly normal embryo development, their repro-
ductive program is compromised. The phenotype of mp strong
mutant alleles results in alterations of the basal body region of the
embryo, such as malformation of the hypophysis and subsequent
absence of the radicle and root meristem, already detectable from
the octant stage of embryo development, frequently ending in an
embryo lethal phenotype (Berleth and Jurgens, 1993; Hardtke and
Berleth, 1998). The gene BODENLOSS (BDL) encodes IAA12. It
was demonstrated that bdl mutant, holding a gain-of-function
mutation that stabilizes the BDL protein, has a phenotype sim-
ilar to mp. The fact that the double mutant bdl mp shows a
similar phenotype places BDL and MP in the same pathway
(Hamann et al., 2002). In fact, BDL is co-expressed with MP
during early embryogenesis, and it was shown that this protein
physically interacts with MP. These results indicate that BDL and
MP are IAA-ARF interacting proteins, where in this interaction
BDL prevents MP from activating its auxin responsive gene tar-
gets. After BDL degradation in response to auxin, the release of
MP triggers the correct initiation of the basal body region in early
embryogenesis (Hamann et al., 2002).
On the other hand, ARF3/ETTIN (ETT) controls the correct
development of the integuments and thus, the seed coat. The
ett mutant ovules present the same alterations as the aberrant
testa shape (ats) ones, where inner and outer integuments grow
together fused in a single wide structure, resulting in rounded,
aberrant-morphology seeds, variable in size (Kelley et al., 2012).
The authors show that ETT and ATS are able to interact, and
propose a model in which, upon integument initiation, ATS-ETT
complex accumulates in the ovule, in the abaxial layer of the inner
www.frontiersin.org August 2014 | Volume 5 | Article 412 |9
Locascio et al. Phytohormones and seed development coordination
integument, where they negatively regulate PIN1, proposing that
these two proteins participate together in auxin signaling during
seed development (Kelley et al., 2012).
Finally, megaintegumenta (mnt) mutants, defective in ARF2,
present larger seeds than wild-type plants due to the formation
of a bigger integument (Schruff et al., 2006). Despite the fact that
the phenotype of the mnt/arf2 mutant has been already charac-
terized, the ARF2 targets that control seed development are still
unknown. In Arabidopsis, the mechanism by which ARF2 is acti-
vated has been elucidated and involves a phosphorylation medi-
ated by the protein kinase BRASSINOSTEROID-INSENSITIVE 2
(BIN2), which in turn is regulated by brassinosteroids (Vert et al.,
2008). In the mnt/arf2 mutant, seed size and weight are dramat-
ically increased. The enlarged seed coat is due to the presence
of extra cells in the integuments before fertilization, generated
by extra anticlinal cell divisions. The embryo is also bigger than
the wild type, but not the endosperm. Surprisingly for integu-
ment mutants, mnt/arf2 does not show female infertility. Given
that the mnt/arf2 lesion causes other pleiotropic effects on vege-
tative and floral development, Schruff et al. (2006) conclude that
MNT/ARF2 is a repressor of cell division and organ growth.
Nutrient allocation and auxin in maize endosperm
The role of sugars in controlling auxin biosynthesis and
metabolism in both maize and Arabidopsis is well known (Le
Clere et al., 2010; Sairanen et al., 2012;Figure 3). In maize, auxin-
sugar crosstalk involves other aspects peculiar to this Monocot:
nutrient accumulation and endoreduplication in the endosperm.
The transfer cells or BETL, as the major site of cross-talk between
maternal (chalazal) and filial tissue (endosperm and embryo), is
essential for nutrient intake and correct endosperm development.
Indeed, the altered morphology of BETL is a characteristic of
mutants with reduced seed mass such as de18 (Torti et al., 1984),
mn1 (Kang et al., 2009), and reduced grain filling1 (Maitz et al.,
2000). It has been observed that in the situation of abnormal
development of BETL the transport of auxin, and presumably its
accumulation, is impaired. Indeed, de18 mutant exhibits low lev-
els of ZmPIN1 in the transfer cells with respect to the wild type
(Forestan and Varotto, 2010).
Zeins are the main storage proteins of maize kernels, constitut-
ing about 70% of total protein content in endosperm. An increase
in zein synthesis was observed at both transcript and protein level
after exogenous auxin treatment (Lur and Setter, 1993a). In the
defective kernel18 mutant (dek18),thatisdefectiveinauxinaccu-
mulation, 12 and 14 KDa zeins were found to correlate with auxin
content. Indeed, at 20 DAP, these proteins were present in the
wild type and in the mutant treated with exogenous auxin and
not in the dek18 mutant (Lur and Setter, 1993b). Whereas the
interaction between zeins and auxin has not been proved, the
sugar-hormone crosstalk in maize is well known (Chourey et al.,
2010; Le Clere et al., 2010). IAA synthesis seems to be required
for grain filling in maize and this process is related to the cell
wall invertase activity. In mn1, which is a small seeded mutant
defective in invertase, the low sugar content correlates with the
low transcript level of ZmYUC1 during endosperm development
(Le Clere et al., 2010). Indeed, the presence of glucose increased
the expression of ZmYUC1 in cultured kernels (Le Clere et al.,
FIGURE 3 | Schematic representation of the factors affecting seed
development in Arabidopsis and maize. Communication among seed
compartments can involve one or more of the factors displayed on the right.
The figure shows only the elements cited in this review. Single-headed
arrows indicate regulation, double-headed arrows indicate reciprocal
influence or regulation, and dashed arrows indicate an effect demonstrated
only in one of the two species. Full-headed arrows indicate the
communication among seed compartments.
2010). Forestan et al. (2010) showed that IAA accumulation in
the three endosperm compartments (BETL, aleurone and ESR)
occurs shortly before the starch accumulation phase. The inver-
tase inhibitor ZM-INVINH1 has been reported to bind cell wall
invertase during kernel development in maize and is localized to
the ESR (Bate et al., 2004). This evidence suggests that invertase
activity, together with auxin transport, has a key role in the reg-
ulation of events that control carbon partitioning during early
kernel development. The evidence that ZmYUC1 is regulated by
hexose sugars, in particular glucose is in agreement with the data
reported in Arabidopsis (Hartig and Beck, 2006).
During the early phases of endosperm development,
endoreduplication and the synthesis of storage compounds
are tightly interconnected (Sabelli and Larkins, 2009). There
is limited information on how phytohormones control the
endoreduplication in maize. It was shown that the increase of
IAA in the late stage of seed development is coincident with the
onset of endoreduplication (Lur and Setter, 1993a). Furthermore,
analysis of auxin levels in several dek mutants indicated a positive
correlation between IAA and nuclear size (Lur and Setter, 1993b).
Thesameevidencewasfoundstudyingde18 mutant (Bernardi
et al., 2012). The low auxin accumulation in the early stages of
development of this mutant causes a delay in endoreduplication
and a reduced ploidy level with respect to the wild type.
In two recent works about the dissection of the maize seed
transcriptome many genes related to hormone metabolism were
Frontiers in Plant Science | Plant Evolution and Development August 2014 | Volume 5 | Article 412 |10
Locascio et al. Phytohormones and seed development coordination
found enriched, particularly in embryo (Lu et al., 2013; Teoh
et al., 2013). Transcription factor analysis on maize seed evi-
denced that most of the highly expressed genes in the embryo
are involved in epigenetic regulation (i.e., methyltransferases and
acetyltransferases) and hormone signaling pathways (i.e., ARF
and Aux/IAA) (Lu et al., 2013). The role of epigenetics in the con-
trol of seed development is briefly discussed in Section Epigenetic
Control of Endosperm Development by Genomic Imprinting.
ROLE OF THE OTHER PHYTOHORMONES ON SEED DEVELOPMENT
Although auxin plays a principal role in the regulation of embryo
patterning and endosperm development, other hormones have
been found to participate in the control of seed development.
Their functions in this developmental process and the links
between them are briefly discussed in the following sections.
The role of cytokinins
The activity of Cytokinins (CKs), together with auxin, is especially
linked to growth promotion by cell division, development and
differentiation (Bishopp et al., 2011; Vanstraelen and Benkova,
2012). Although the biosynthetic pathway and transmission of
the signal are quite well described (Hwang et al., 2012), the
function of CKs in the seed has still not been exhaustively char-
acterized. In Arabidopsis, the limited existing knowledge comes
from a few reports (Werner et al., 2003; Garcia et al., 2005; Day
et al., 2008). Day et al. (2008) identified the Histidine-containing
phosphotransfer proteins genes (AHPs) as the genes preferentially
activated by CKs in Arabidopsis. After being phosphorylated by
the CKs-receptors (AHKs), the AHP proteins transduce the CK-
signal by entering the nucleus and transferring the phosphate
group to the Arabidopsis response regulators (ARRs). The ARR
proteins constitute a class of regulators in the cytokinin signal-
ing. They comprise the type-A proteins that are normally negative
regulators of CKs signaling, and the type-B that positively reg-
ulate gene expression. They can directly bind DNA through the
MYB-like domain, and contribute to the outputs of the cytokinin
signaling through protein-protein interactions by their glutamine
(Q)-rich domain. The CYTOKININ RESPONSE FACTOR (CRF)
proteins rapidly accumulate in response to CKs. Their function
largely overlap with the type-B ARRs, indicating that CRF and
ARR belong to a two-component system for the transmission of
CKs signaling in the regulation of the development of embryo,
cotyledons and leaves (Rashotte et al., 2006). The B-type ARR
genes (ARR21, ARR19, ARR18, ARR8) together with the CRF2
and CRF3 genes are preferentially expressed in the endosperm
(Day et al., 2008).
Studies of the genes related to CKs production have shown
that during the first stages of seed development their expression is
principally associated to an effect of the hormone on the develop-
ment of endosperm and seed coat. These results suggest that the
control of seed size would involve a crosstalk occurring between
maternal and zygotic tissues (Garcia et al., 2005). Genetic anal-
yses of CK synthetic genes (cytokinin oxidases, CKXs)-AtCKX1
and AtCKX3- have indicated that in the corresponding mutants
the size of the seed, as well as the embryo, was increased (Wer ner
et al., 2003). Similar phenotypes were found observing the triple
mutant of the cytokinin receptors ahk2 ahk3 ahk4 (Riefler et al.,
2006).Theeffectofanincreaseinbothembryoandseedsizewas
suggested to be under control of maternal and/or endospermal
genotypes (Riefler et al., 2006).
Recently, CKs were elucidated to have an important role in
the integration of epigenetic and genetic control of seed devel-
opment. Li et al. (2013) characterized CKX2 as target of IKU
that, as mentioned before, has a relevant role in the promo-
tion of endosperm cellularization, controlling seed size. Several
works have been conducted in maize because of the high level
of CKs detectable in the seed and the relative ease of study due
to the seed size. In most of these studies the relative abundance
of the hormone at the early stages of endosperm development
and embryo differentiation was reported (Jones, 1990; Lur and
Setter, 1993a,b; Dietrich et al., 1995; Brugiere et al., 2003; Veach
et al., 2003; Rijavec et al., 2009). Studies on gene transcription
in different stages of caryopsis development showed that the
genes encoding for enzymes involved in the synthesis of CKs are
expressed mainly in endosperm, pedicel and embryo soon after
pollination. In the pedicel/placental chalazal/basal endosperm
region, CKs levels were 2–3 times higher than in the rest of the
seed (Brugiere et al., 2003). At a later stage of seed develop-
ment, CKs become detectable also in the BETL (Brugiere et al.,
2008; Smehilova et al., 2009; Vyroubalova et al., 2009; Rijavec
et al., 2011). Immunolocalizations and in situ hybridizations
showed that CKs are synthesized in certain regions of the seed,
but they are also transported from the maternal tissue through
the pedicel to the endosperm (Zhang et al., 2009b; Rijavec and
Dermastia, 2010). The authors also found that CKs accumu-
late in the placenta-chalazal cell layer and are able to promote
Programmed Cell Death (PCD).
Similarly to Arabidopsis, the main role of CKs detected in
the caryopsis of maize is establishing seed size, by promoting
endosperm cell divisions. A relationship was elucidated between
CKs accumulation and/or activation, and cyclin activity. In
respecttoArabidopsis,inwhichCKshaveaneffectonlyonCycD3
activity (Riou-Khamlichi et al., 1999), in maize the CKs affect
CycD3 and CycD2 (Gutierrez et al., 2005).
A study on the localization of auxin and cytokinins dur-
ing early seed development elucidated the role of these two
antagonistic hormones. At 6–8 DAP the CKs were detected in
both the BETL region and the ESR, while the signal was very
low in the embryo (Chen et al., 2014). This evidence corre-
lates with the fast cell division and growth at both apical and
basal part of the endosperm. Later in development (9–10 DAP)
CKs signal is mainly localized in the epidermis of the scutel-
lumandintheSAM.Thecontemporarylocalizationofauxin
and CKs during early embryo development is different at the
scutellum tip, where IAA signal is stronger than that of CKs.
In Arabidopsis, transient and antagonistic interaction between
auxin and cytokinins is critical for specifying the root-stem
cell-pool that will determine the definition of the root-stem
axis. Auxin, through a feedback mechanism, is responsible for
the repression of CKs signaling in this region (Muller and
Sheen, 2008). It might be possible that a similar crosstalk occurs
in the tip region of the scutellum of maize: the high auxin
response at the abaxial tip region may repress cytokinin activity
(Chen et al., 2014).
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Locascio et al. Phytohormones and seed development coordination
In a more recent study the functional characterization of seven
ZmHistidine kinase (ZmHK) genes, encoding the cytokinin recep-
tors, was performed (Wang et al., 2014). Arabidopsis transgenic
lines expressing each of the ZmHK genes showed a reduction
in seed size with respect to the normal. This result suggests that
ZmHKs function as repressors of seed development.
The role of brassinosteroids
Brassinosteroids (BRs) are plant steroid hormones involved in
several developmental programs, including seed development.
They function in the pathway that regulates ovule number and
seed size and shape, in some cases complementing CKs and aux-
ins. They also participate in the regulation of seed germination,
by antagonizing the inhibitor effect of ABA (Zhang et al., 2009b),
and being synergic to gibberellins (Leubner-Metzger, 2001).
The mechanisms by which BRs control seed development are
still elusive. Many studies on BRs signaling have been performed
on BR-deficient mutants in rice (Hong et al., 2005; Tanabe et al.,
2005; Morinaka et al., 2006; Jiang and Lin, 2013). Different works
demonstrated the relevance of BRs on seed size determination
also in Arabidopsis, using BR-deficient or insensitive mutants
(Choe et al., 2000; Li et al., 2001; Jiang et al., 2013; Jiang and Lin,
2013). Arabidopsis mutants deficient in BRs (i.e., dwarf5,dwf5;
dwf11;shrink1-D,shk1-D;detiolated2,det2;BR-Insensitive1,bri1)
produce dwarf phenotypes and smaller and fewer seeds (Chory
et al., 1991; Choe et al., 2000; Takahashi et al., 2005; Jiang and
Lin, 2013).
The control of seed development by BRs is mainly
exerted through the function of the protein BRASSINAZOLE
RESISTANT1 (BZR1). The activity of BZR1 varies according to
its state of phosphorylation, dependent on the presence of BRs
and mediated by the BIN2 kinase, which acts as a negative regula-
tor (Li and Nam, 2002; Wang et al., 2002; Yin et al., 2002). BZR1
acts as a master regulator, from which a network of links extends
to proteins acting in the different pathways of seed development
and determination: ARF2, TRANSPARENT TESTA GLABRA2
(TTG2) and TTG16 regulating integument development; SHB1,
IKU1, and 2, MINI3 together with AP2 and the MADS-box
AGL61, 62, and 80 regulating endosperm development; and
epigenetic regulators of endosperm development and paternal
imprinting such as FIS2, MEA, FIE, MET1, SWN, and MSI1
(Sun et al., 2010; Jiang et al., 2013; Jiang and Lin, 2013). Indeed,
the model proposed by Jiang and Lin (2013) indicates that
BRs control the development of embryo and endosperm (and
subsequently seed development) by regulating the expression of
these genes through the direct or indirect action of BZR1.
The altered seed phenotype that dwf5 and shk1-D mutants
display indicates that BRs are also involved in seed shape deter-
mination (Choe et al., 2000; Takahashi et al., 2005). Thus, it
has been observed that seed elongation requires BRs production
and signaling in the maternal tissues (integuments) after fertiliza-
tion. This aspect was especially evident in the det2 mutant, where
the cell length of the integuments was significantly reduced and
partially rescued by exogenous application of BRs (Jiang et al.,
2013).
Summarizing, in Arabidopsis it has been shown that the pro-
duction of BRs in the embryo/endosperm is sufficient to increase
seed volume, while they regulate seed size by an independent
mechanism that involves BR production and signaling in the seed
coat. How these hormones trigger different, localized behaviors in
the different seed compartments still remains unknown (reviewed
in Jiang and Lin, 2013).
Concerning maize, the information about BRs is extremely
limited, and nothing is known about the conservation of the
mechanism of action with respect to Arabidopsis and rice (Salas
Fernandez et al., 2009). Hartwig et al. published one of the pio-
neering works in 2011. They characterized the mutant nana plant
1(na1),whichisamutantonZmDET2, the Arabidopsis ortholog
of DET2 (Hartig and Beck, 2006; Hartwig et al., 2011). The
phenotype of this mutant possesses all the characteristics of a BR-
deficiency mutant, and shows the typical dwarfism. This work is
complemented by a few other studies in which three additional
genes involved in BRs biosynthesis and signaling were isolated
(Tao et al., 2004; Liu et al., 2007b; Makarevitch et al., 2012).
However, no data are available about the specific effect of BRs
on maize seed development, so this is an attractive field to be
explored.
The role of abscissic acid and gibberellins
The action of Abscissic acid and Gibberellins (GAs) on seed
development is strictly correlated and antagonistic. As already
mentioned, during the phase of seed maturation it is possible to
define two important processes: the accumulation of nutrients in
the endosperm (that will be used by the embryo during develop-
ment and early phase of germination) and desiccation (that allow
the embryo to tolerate hydric stress and terminate in the state of
seed dormancy). Both processes are predominantly regulated by
ABA. The concentration of this hormone increases during the late
phase of seed maturation and is maintained until germination. In
order to germinate, however, the seed must recover the water lost
during maturation, since it is necessary to mobilize the resources
for the embryo and activate enzymes and pathways for breaking
of dormancy. This process, named imbibition, is regulated by the
gibberellic acid. Hence, the ratio of ABA and GAs is determinant
in the progression of seed maturation (Weber et al., 2005; Bethke
et al., 2006; Seo et al., 2006; Liu et al., 2010).
The mechanism by which ABA controls the accumulation of
food resources in the aleurone cell layer is based on the regula-
tion of β–ZIP and DOF transcription factors (Vicente-Carbajosa
and Carbonero, 2005; Monke et al., 2012). A study conducted by
Karssen et al. (1983) and subsequently confirmed by Kanno et al.
(2010) revealed that the biosynthesis of ABA occurs both in the
maternal tissue and in the zygote. The synthesis of the hormone
in the first compartment determines a primary peak of ABA accu-
mulation, which is necessary to complete the process of embryo
development. The final stages of embryo and endosperm forma-
tion correlate with a second peak of ABA, necessary to begin the
process of seed desiccation and food storage in the aleurone layer
(Kanno et al., 2010). Regarding other aspects of seed develop-
ment, it is still unknown if ABA has additional functions in the
zygotic compartments. In Arabidopsis, similarly to maize, it has
been shown that the synthesis of starch and thus its accumula-
tion begins at the early stage of seed development (Chen et al.,
2011). During this process the synthesis of starch is regulated by
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Locascio et al. Phytohormones and seed development coordination
phytohormones, such as ABA and IAA, with a major role played
by ABA. It has been reported that the genes for starch metabolism
(i.e., AGPase,SS,DBE,andSBE) are regulated by sugars (Rook
et al., 2001; Bossi et al., 2009; Seiler et al., 2011). Moreover, inter-
esting results by Hu et al. (2012) showed a correlation between
ZmSSI (a sugar-regulated gene for starch-synthesis) and ABA
during the endosperm filling stage. Identification of a sequence
in the promoter of ZmSSI that is recognized by ABSCISIC ACID
INSENSITIVE 4 (ABI4) allowed the characterization of the mech-
anism by which ABA regulates the expression of the gene. ABI4
would act as a transcriptional activator of ZmSSI in response to
ABA treatment.
GAs, conversely to ABA, promote germination by mobiliz-
ing the resources necessary for embryo development (Koornneef
et al., 1982). This is supported by the fact that after the cotyledon
stage, when the events of PCD in the endosperm begin to leave
space to the expanding embryo, some of the genes related with
GA-biosynthesis are activated, followed by the activation of prote-
olytic enzymes and α-amylases (Sreenivasulu and Wobus, 2013).
The study of the synthesis of bioactive GAs during seed develop-
ment revealed that the peak of GAs occurs just before that of ABA
(Singh et al., 2010; Nadeau et al., 2011). High auxin concentra-
tion also triggers the production of bioactive gibberellin (Dorcey
et al., 2009), (Figure 1).
EPIGENETIC CONTROL OF ENDOSPERM DEVELOPMENT BY
GENOMIC IMPRINTING
Although the epigenetic mechanism of regulation is beyond the
scope of this review, it is worth mentioning it, since it is still
linked to hormones accumulation and coordination. In this con-
text, a prominent role is exerted by the epigenetic mechanisms
that determine the parent-of-origin specific gene expression,
described as genomic imprinting. The regulation of gene expres-
sion by imprinting has been extensively studied in maize and rice,
as Monocots, and in Arabidopsis, representing Eudicots (Hsieh
et al., 2009; Luo et al., 2011; Waters et al., 2011).
The process by which a gene is imprinted involves the place-
ment of epigenetic marks on its genomic sequence (i.e., DNA
methylation) or in the nucleosomes (i.e., histone modifications).
Different classes of methyltransferases can deposit the DNA-
methylation marks, however, the methyltransferase operating in
the endosperm is mainly the cytosine-DNA-methyltransferase
MET1 (Law and Jacobsen, 2010). The action of DEMETER
(DME) is antagonistic to MET1 as it removes the methylation
marks from specific imprinted genes in maternal tissues allow-
ing their expression (Choi et al., 2002; Gehring et al., 2006).
Histone H3K27 tri-methylation is the epigenetic mark for silenc-
ing. The proteins that catalyze this modification belong to the
Polycomb group of proteins (PcG). Genes like MEA/FIS1,FIS2,
and FIE/FIS3 belong to the PcG, and exert a pivotal role on
maintaining the pattern of gene expression in the endosperm.
These proteins participate in the formation of the Polycomb
Repressive Complex 2 (PRC2), contributing to the establishment
of an imprinted state, thus controlling gene expression in the
developing seed (Hennig and Derkacheva, 2009).
The process of imprinting takes place already during gameto-
phyte formation (Reik and Walter, 2001; Feil and Berger, 2007;
Waters et al., 2013b). Epigenetic mechanisms involving small
interfering RNAs (siRNAs), which are responsible for silencing of
the complementary gene targets, have been suggested as having a
prominent role in genome-wide hypomethylation of Arabidopsis
endosperm (Hsieh et al., 2009, 2011; Bauer and Fischer, 2011).
In particular, it has been shown that genomic demethylation by
DME induces the production of siRNAs, in both Arabidopsis
(Ibarra et al., 2012)andrice(Rodrigues et al., 2013). It has
been observed that an extensive process of demethylation on
Transposon Elements (TE) or repeats participates in the process
of imprinting by affecting the neighboring genes (Choi et al.,
2002; Gehring et al., 2006). Indeed, in Arabidopsis, one third of
the imprinted genes are flanked by TEs (Bauer and Fischer, 2011;
Gehring et al., 2011; Wolff et al., 2011). The global hypomethy-
lation in the endosperm is likely originated from the central cell
nucleus of the female gametophyte prior to fertilization (Ibarra
et al., 2012). The major role of demethylation in female game-
tophyte (central cell) and male gametophyte (vegetative nucleus)
is to produce siRNAs that are transported to the egg cell (in the
female gametophyte) and the sperm cells (in the pollen) to rein-
force the silencing of TEs (Gutierrez-Marcos et al., 2006; Bauer
and Fischer, 2011; Ibarra et al., 2012). Given that, prior to fertil-
ization the parental alleles are differentially methylated in each
gametophyte, the triploid endosperm generated in the second
event of fertilization will cope with a different state of methylation
between the parental alleles. The different state of methylation
between the alleles is believed to trigger the establishment of
genomic imprinting in specific parental genes (Raissig et al.,
2011). Interestingly, the control of gene expression by imprint-
ing is important in the context of the determination of seed size
and viability of the embryo (Xiao et al., 2003).
Despite that few imprinted genes are common between maize,
rice and Arabidopsis, the mechanism that regulates parent-of-
origin expression seems to be conserved between Monocots and
Eudicots (Gutierrez-Marcos et al., 2006; Rodrigues et al., 2013). It
is well established that the major site where the imprinted genes
accumulate is the endosperm (Luo et al., 2011). Nevertheless,
in both Arabidopsis and maize imprinted genes have also been
identified in the embryo (i.e., maternally expressed in embryo1,
mee1 Jahnke and Scholten, 2009). The identity of the imprinted
genes in both Arabidopsis and maize has been determined, in
many cases, through advanced techniques of transcriptome deep
sequencing and genome-wide search (Gehring et al., 2011; Waters
et al., 2011; Zhang et al., 2011, 2014; Xin et al., 2013). These
genes encoded proteins with a wide range of molecular functions,
ranging from the regulation of pigmentation, starch metabolic
pathways, protein storage, hormone responses, cell wall for-
mation, transcriptional regulation, chromatin modification, and
cytoskeletal function to mRNA regulation.
The distinct pattern of Maternally Expressed Genes (MEGs)
and Paternally Expressed Genes (PEGs) ensures the proper evo-
lution of seed development and is associated with specific stages
of endosperm development (Raissig et al., 2013). In maize, the
imprinted gene Meg1 is involved in nutrient allocation and has a
role in controlling seed size (Costa et al., 2012). Meg1 positively
regulates transfer cell specification and development, therefore
increasing nutrient uptake. This effect is especially pronounced
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Locascio et al. Phytohormones and seed development coordination
at 10 DAP, when a peak of auxin in BETL, ESR and aleurone
layer coincides with the onset of nutrient accumulation for starch
and zein storage protein synthesis (Lur and Setter, 1993a; Sabelli
and Larkins, 2009; Forestan et al., 2010). The involvement of
imprinted genes in hormone signaling pathways and transcrip-
tional regulation of endosperm development is evidence of the
importance of auxin in correct seed development (Xin et al.,
2013). The role of imprinted genes (in particular MEGs) in
driving the correct nutrient synthesis is coupled with the impor-
tance of the transport mediated by auxin. It is not surprising
that PIN1 was identified as specific MEG (Xin et al., 2013).
Interestingly, YUC10 orthologs in rice, maize and Arabidopsis
were found to be exclusively paternally expressed, meaning that
the function of YUC10 could be crucial for endosperm develop-
ment (Waters et al., 2013b). The relevance of MEGs and PEGs
in this stage of development is to facilitate the communica-
tion and coordination between endosperm, embryo and maternal
tissues.
miRNAs CONTROL OF SEED DEVELOPMENT
Another relevant mechanism controlling gene expression during
seed development is exerted by microRNAs (miRNAs). miRNAs
are single-stranded RNA molecules of 21–22 nucleotides in length
(with some exceptions) processed from a precursor molecule
defined as pre-miRNA (Bartel, 2004). The RNA duplex is subse-
quently transported out of the nucleus, where the complementary
sequence (called star) is removed to allow mature miRNA to be
ready for action. The mature miRNA binds with perfect or imper-
fect complementarity to sites in the 5or 3untranslated regions
(UTR) or coding sequences (CDS) of genes, causing its cleavage
or translational repression (Grennan, 2008).
Plant microRNAs play important regulatory roles in many
biological and metabolic processes, including development, hor-
mone signaling, and responses to environmental stress (Reinhart
et al., 2002). miRNAs are frequently grouped in families that
have a specific set of transcript targets and appear to be evolu-
tionarily conserved between plant species (Wu et al., 2009; Wang
et al., 2011). It has been observed that miRNAs are expressed from
early to later stages during seed development (Kang et al., 2009;
Nodine and Bartel, 2010; Rubio-Somoza and Weigel, 2011). More
specifically, they seem to be implicated in the control of embryo-
genesis and embryo patterning, also affecting the germination
process (Seefried et al., 2014). Mutants lacking essential compo-
nents of miRNA biogenesis and/or processing, manifest a severely
compromised seed development or even lethality (Nodine and
Bartel, 2010). The relevance of miRNAs function in the control
of seed development is especially evident by observing dicer-like1
mutants, in which alterations are mainly at the level of embryo
apical-basal-radial symmetry (Lang et al., 1994; Ray et al., 1996;
Schauer et al., 2002).
During the process of seed maturity miRNAs also affect seed
size. During gametophyte and early seed development, for exam-
ple, miR172 targets several APETALA2-like transcription factors,
thus controlling seed size and yield; ap2 loss-of-function causes
an increase of seed weight (Jofuku et al., 2005; Tang et al., 2012).
The miR172-AP2 interaction is conserved between Arabidopsis
and maize (Wang et al., 2005).
Others miRNAs affecting seed size belong to two families,
miR159 and miR319 (Palatnik et al., 2007; Li et al., 2011).
miR159ab double mutant manifests a reduced seed size and seed
shape alterations (Allen et al., 2007). miR319 has a central role
in coordinating multiple miRNAs and is strictly connected to
phytohormone regulation (Luo et al., 2011). This last seems to
be an upstream regulator that targets several TCP transcription
factors, which in turn activate the hormonal machinery through
the recruitment of other miRNAs (Palatnik et al., 2003; Luo
et al., 2011). The function of miR319 seems to be conserved also
in maize where, in addition, this specific miRNA together with
miR171 target genes that participate in secondary pathways of
auxin and GA signaling transduction, thus affecting embryo dif-
ferentiation (Zhang et al., 2009a; Kang et al., 2012; Shen et al.,
2013).
Regarding the control exerted by miRNAs on phytohormones,
it has been shown in Arabidopsis that auxin metabolism is con-
trolled by at least four conserved miRNA families (miR160,
miR167, miR390, and miR393), which mainly exert control
by regulating ARF proteins (i.e., ARF6, ARF8, ARF10, ARF16,
and 17) (Rhoades et al., 2002; Mallory et al., 2005; Marin et al.,
2010; Windels and Vazquez, 2011; Kinoshita et al., 2012).
It has been observed that miRNAs in some cases also
participate in the hormones cross-talk. For instance, it was
reported that auxin and ABA signal transduction pathways are
targets of differentially expressed miRNAs (Reyes and Chua,
2007). It was shown that miR159 and 160 affect the pro-
cess of germination by regulating ABA sensitivity. Interestingly,
miR160 is implicated in the regulation of auxin metabolism
(Rhoades et al., 2002; Mallory et al., 2005); the mutants
expressing a miR160-resistant form of ARF10 are hyper-
sensitive to ABA (Liu et al., 2007a), therefore suggesting
a point of cross-talk between ABA and auxin in imbibed
seeds.
A deep sequencing approach has been used to identify seed
specific miRNAs in maize (Zhang et al., 2006, 2009a; Wang et al.,
2011; Kang et al., 2012). Most miR167 and miR319 families
were found enriched in seeds rather than leaves (Kang et al.,
2012). Target prediction of maize miRNAs found that miR167,
as in Arabidopsis, targets ARFs (Zhang et al., 2009a). The con-
servation of both targets and miRNA in Arabidopsis and maize
suggests conserved mechanisms of regulation in Monocots and
Eudicots.
Another important example of conservation was recently
reported in a study of miRNA regulation during the early devel-
opment of barley grains. In this case, it was suggested that
the miRNAs contribute to the control of the development of
cereal grain particularly regulating phytohormone response path-
ways (Curaba et al., 2012). Specifically, the regulation of TIR1
and potentially three ARFs by the miR393 and miR167 families
resulted conserved.
The more recent knowledge on miRNAs and their molecu-
lar connections and involvement in multiple hormonal responses
and crosstalk, with patterning genes in specific developmental
processes and also in seed development is discussed in detail
in other excellent reviews (Nonogaki, 2010; Rubio-Somoza and
Weigel, 2011; Curaba et al., 2014).
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Locascio et al. Phytohormones and seed development coordination
CONCLUDING REMARKS
Spermatophytes have evolved seeds to ensure spreading and sur-
vival. Seed development and the inherent establishment of the
final seed size is a multi-step process controlled by a complex
network. Besides being a valuable model for basic research, it
is also an important and interesting trait for farmers and the
related industries, as seeds represent the basis for the food, feed
and bio-based economy. However, despite the enormous advance
on the study of the mechanisms controlling seed size, there are
still open questions to answer. The general interest of the indus-
try nowadays, apart from increasing production, is to modify
thecompoundsaccumulatedintheseedsinordertoimprove
their quality. With this perspective in mind, in these last years,
many researchers have focused their attention on comparative
studies between species, and the translation of information from
model plants to crops. In this review we have summarized the
known genes and elements involved in the process of seed devel-
opment, particularly focusing on the auxin hormonal pathway,
and underlining the concept that the proper distribution, trans-
duction and coordination of all the hormonal signals is essential
for seed development.
The early stages of the seed developmental program are very
similar in Eudicots and Monocots. Nevertheless, remarkable dif-
ferenceshavebeendescribedinthestageofseedmaturation:
while in Monocots the endosperm persists until maturation,
being the compartment of nutrient storage for the embryo, in
Eudicots it is gradually replaced by the embryo until being
reduced to a single cell layer (Figure 1).
The initial genetic studies on seed biology aimed the identi-
fication and characterization of genes involved in fundamental
processes associated with embryogenesis and seed development,
based mainly on the characterization of mutants, misexpression
experiments and the use of marker lines. Most of the genes iden-
tified showed relationship with phytohormones (Tab l e 1 ). The
knowledge of the pathways and events regulated by each hormone
leads to the interesting possibility of specific hormones/hormone
inhibitors being applied to the crops at specific moments in their
development, in order to obtain the desired results in agricultural
production.
Phytohormones are among the most relevant signals involved
in the communication between seed structures. The communi-
cation among the three compartments of the seed (seed coat,
endosperm and embryo) has been revealed to have a key role
for the correct seed formation. In fact, failure in this commu-
nication can cause alteration in seed size, defective embryos and
future seedlings and, in extreme cases, seed abortion. The charac-
terization of mutants affecting hormonal signaling constitutes an
interesting opportunity to describe relationships, relevance and
mechanisms by which the signals are exchanged between seed
compartments.
The difficulty in studying the role of genes whose muta-
tions cause lethal phenotype in embryo/seed has recently been
overcome with modern molecular biology techniques and by
the use of -omics approaches. As mentioned in this review Lu
et al. (2013), through the massive study of gene expression by
RNA-sequencing, contributed to the identification of genes dif-
ferentially expressed between embryo and endosperm, proposing
interesting regulatory networks between the two compartments,
in maize.
Several comparative studies between species are allowing the
identification of the conserved and divergent elements acting
in the regulatory mechanisms governing seed development. As
shown in this review most of the crosstalk involving the factors
that affect seed development (hormones, epigenetics and sugars)
are conserved between maize and Arabidopsis (Figure 3). In addi-
tion, the identification and characterization of orthologous genes
will permit the description of unknown genes function, and the
definition of patterns of regulation that would otherwise be dif-
ficult to describe. For instance, the recent study by Chen et al.
(2014) highlighted the different patterns of auxin and cytokinin
accumulation during embryogenesis in the two model species
Arabidopsis and maize. While in Arabidopsis the involvement
of IAA in embryo development immediately after fertilization is
clear, in maize this hormone is not detected in the embryo, but
in the endosperm, during early embryogenesis. This result indi-
cates the presence of a phase shift in the level of IAA accumulation
during the early stages of embryo development when comparing
the two species. Nevertheless, later in the development, the same
pattern of hormone accumulation is observed at both scutellum
(maize) and at the emerging cotyledon tips (Arabidopsis). The
polar transport and the consequent IAA flux, which is essential in
establishing the apical-basis pattern of the embryo, are conserved
in both species as well. However, in maize after the differentiation
of embryo meristems, auxin polar transport is also essential for
the regular differentiation of both the leaf primordia at the SAM
and the seminal root at the root apical meristem.
Similarly to auxin, cytokinin response is delayed in maize
with respect to Arabidopsis. However, the antagonistic effect of
auxin and cytokinins during embryogenesis is again conserved in
these two species. The existence of a functional convergence in
seed development between Arabidopsis and maize was moreover
deduced by observing the phenotype of the Arabidopsis triple
receptor kinase mutant ahk2/ahk3/ahk4 and the effect of ZmHKs
overexpression in Arabidopsis transgenic lines. The seeds showed
an increased size with respect to the wild type in the triple mutant
while a reduction in size in the overexpressing lines.
At the later stages of seed development, ABA and IAA seem
to co-regulate the expression of many genes involved in starch
biosynthesis in maize endosperm and the accumulation of nour-
ishing proteins in the arabidopsis cotyledons. This evidence
underlines the importance of auxin during all the stages of seed
development, from embryogenesis to maturation (Figure 1).
Although some of the factors controlling the seed development
process have been identified, many are still unknown. For exam-
ple, to date, there is no information available about the role of
brassinosteroids in maize seed development.
The arduousness of these studies in maize is not only linked to
methodology-limitation, but also mainly attributable to genomic
complexities. In cereal crops, the high level of gene dupli-
cation with respect to Arabidopsis hinders the identification
of orthologs, or the simple comparison of gene function. In
Monocots several cases of sub-functionalization were reported. In
maize, for example, it was shown that the PIN gene has been sub-
jected to events of duplication that generated the four ZmPIN1a-d
www.frontiersin.org August 2014 | Volume 5 | Article 412 |15
Locascio et al. Phytohormones and seed development coordination
genes with different levels of tissue-specific expression. It has been
suggested that these four genes could have gained new functions
assembling the specific roles of AtPIN3,AtPIN4,andAtPIN7,
which have still not been identified in the maize genome. Despite
the problems concerning the study of crops with respect to the
Eudicot model plants, specific areas of research are improving in
Monocots. One of these is the characterization of the process of
genome imprinting. This study is more advanced in maize than
in Arabidopsis, due to the bigger seed size, the persistence of
the endosperm and the straightforward physical separation of the
seed structures.
Imprinting is fundamental for both embryo and endosperm
differentiation during seed development, which interestingly is
the sole plant developmental phase characterized by maternal
dependency. The study of the dynamic of the imprinted genes
during early and late stages of development, and their cross-talk
with hormones will shed light on both the biological signifi-
cance of this mechanism of gene expression regulation, and the
connection between epigenetic mechanisms and hormonal con-
trol during seed development and maturation. The identification
of miRNAs differentially expressed in the seed and their corre-
sponding targets has established a new complicated link between
miRNAs dynamics and the traditional role of hormones in seed
development. However, the study of conservation of both miR-
NAs and target gene expression between different species during
seed development still needs further investigations.
The use of the present and new marker lines and mutants,
as well as the technical advances on transcriptomics, proteomics,
genomics and metabolomics, will greatly contribute to the under-
standing of the environmental, hormonal, genetic, and epigenetic
mechanisms and cross-talks that control seed production and
development, and to establish parallelisms between Monocots
and Eudicots in the near future.
ACKNOWLEDGMENTS
TheauthorsaregratefultoDr.S.Masiero,M.Abbas,andA.
Garside for revising the manuscript and for critical discussion.
We would like to apologize with all our colleagues because for
space constraints their work could not be discussed in this review.
Antonella Locascio, Irma Roig-Villanova, and Jamila Bernardi
were funded by the Ministry for Education and University, Italy
(FIRB grant no. RBFR08UG7).
REFERENCES
Abel, S., and Theologis, A. (1996). Early genes and auxin action. Plant Physiol. 111,
9–17. doi: 10.1104/pp.111.1.9
Allen, R. S., Li, J., Stahle, M. I., Dubroue, A., Gubler, F., and Millar, A. A. (2007).
Genetic analysis reveals functional redundancy and the major target genes of the
Arabidopsis miR159 family. Proc. Natl. Acad. Sci. U.S.A. 104, 16371–16376. doi:
10.1073/pnas.0707653104
Bartel, D. P. (2004). MicroRNAs: genomics, biogenesis, mechanism, and function.
Cell 116, 281–297. doi: 10.1016/S0092-8674(04)00045-5
Bate, N. J., Niu, X., Wang, Y., Reimann, K. S., and Helentjaris, T. G. (2004).
An invertase inhibitor from maize localizes to the embryo surrounding
region during early kernel development. Plant Physiol. 134, 246–254. doi:
10.1104/pp.103.027466
Baud, S., Wuilleme, S., Lemoine, R., Kronenberger, J., Caboche, M., Lepiniec, L.,
et al. (2005). The AtSUC5 sucrose transporter specifically expressed in the
endosperm is involved in early seed development in Arabidopsis. Plant J. 43,
824–836. doi: 10.1111/j.1365-313X.2005.02496.x
Bauer, M. J., and Fischer, R. L. (2011). Genome demethylation and
imprinting in the endosperm. Curr. Opin. Plant Biol. 14, 162–167. doi:
10.1016/j.pbi.2011.02.006
Bäumlein, H., Miséra, S., Luerßen, H., Kölle, K., Horstmann, C., Wobus, U.,
et al. (1994). The FUS3 gene of Arabidopsis thaliana is a regulator of gene
expression during late embryogenesis. Plant J. 6, 379–387. doi: 10.1046/j.1365-
313X.1994.06030379.x
Becraft, P. W., and Asuncion-Crabb, Y. (2000). Positional cues specify and main-
tain aleurone cell fate in maize endosperm development. Development 127,
4039–4048.
Becraft, P. W., Stinard, P. S., and McCarty, D. R. (1996). CRINKLY4: a TNFR-
like receptor kinase involved in maize epidermal differentiation. Science 273,
1406–1409. doi: 10.1126/science.273.5280.1406
Becraft, P. W., and Yi, G. (2011). Regulation of aleurone development in cereal
grains. J. Exp. Bot. 62, 1669–1675. doi: 10.1093/jxb/erq372
Bemer, M., Wolters-Arts, M., Grossniklaus, U., and Angenent, G. C. (2008). The
MADS domain protein DIANA acts together with AGAMOUS-LIKE80 to
specify the central cell in Arabidopsis ovules. Plant Cell 20, 2088–2101. doi:
10.1105/tpc.108.058958
Bennett, M. J., Marchant, A., Green, H. G., May, S. T., Ward, S. P., Millner,
P. A., et al. (1996). Arabidopsis AUX1 gene: a permease-like regula-
torofrootgravitropism.Science 273, 948–950. doi: 10.1126/science.273.52
77.948
Berger, F. (2003). Endosperm: the crossroad of seed development. Curr. Opin. Plant
Biol. 6, 42–50. doi: 10.1016/S1369526602000043
Berger, F., Grini, P. E., and Schnittger, A. (2006). Endosperm: an integrator
of seed growth and development. Curr. Opin. Plant Biol. 9, 664–670. doi:
10.1016/j.pbi.2006.09.015
Berleth, T., and Jurgens, G. (1993). The role of the monopteros gene in organising
the basal body region of the Arabidopsis embryo. Development 118, 575–587.
doi: 10.1016/0168-9525(93)90246-E
Bernardi, J., Lanubile, A., Li, Q. B., Kumar, D., Kladnik, A., Cook, S. D.,
et al. (2012). Impaired auxin biosynthesis in the defective endosperm18
mutant is due to mutational loss of expression in the ZmYuc1 gene encoding
endosperm-specific YUCCA1 protein in maize. Plant Physiol. 160, 1318–1328.
doi: 10.1104/pp.112.204743
Bethke, P. C., Hwang, Y. S., Zhu, T., and Jones, R. L. (2006). Global patterns of gene
expression in the aleurone of wild-type and dwarf1 mutant rice. Plant Physiol.
140, 484–498. doi: 10.1104/pp.105.074435
Bethke, P. C., Libourel, I. G., Aoyama, N., Chung, Y. Y., Still, D. W., and Jones, R. L.
(2007). The Arabidopsis aleurone layer responds to nitric oxide, gibberellin, and
abscisic acid and is sufficient and necessary for seed dormancy. Plant Physiol.
143, 1173–1188. doi: 10.1104/pp.106.093435
Bishopp, A., Benkova, E., and Helariutta, Y. (2011). Sending mixed messages:
auxin-cytokinin crosstalk in roots. Curr. Opin. Plant Biol. 14, 10–16. doi:
10.1016/j.pbi.2010.08.014
Bossi, F., Cordoba, E., Dupre, P., Mendoza, M. S., Roman, C. S., and Leon, P.
(2009). The Arabidopsis ABA-INSENSITIVE (ABI) 4 factor acts as a central
transcription activator of the expression of its own gene, and for the induc-
tion of ABI5 and SBE2.2 genes during sugar signaling. Plant J. 59, 359–374. doi:
10.1111/j.1365-313X.2009.03877.x
Brown, R., Lemmon, B., and Olsen, O.-A. (1996a). Development of the endosperm
in rice (Oryza sativa L.): cellularization. J. Plant Res. 109, 301–313. doi:
10.1007/BF02344477
Brown, R. C., Lemmon, B. E., and Olsen, O. A. (1996b). Polarization predicts the
pattern of cellularization in cereal endosperm. Protoplasma 192, 168–177. doi:
10.1007/BF01273889
Brugiere, N., Humbert, S., Rizzo, N., Bohn, J., and Habben, J. E. (2008). A mem-
ber of the maize isopentenyl transferase gene family, Zea mays isopentenyl
transferase 2 (ZmIPT2), encodes a cytokinin biosynthetic enzyme expressed
during kernel development. Cytokinin biosynthesis in maize. Plant Mol. Biol.
67, 215–229. doi: 10.1007/s11103-008-9312-x
Brugiere, N., Jiao, S., Hantke, S., Zinselmeier, C., Roessler, J. A., Niu, X., et al.
(2003). Cytokinin oxidase gene expression in maize is localized to the vascula-
ture, and is induced by cytokinins, abscisic acid, and abiotic stress. Plant Physiol.
132, 1228–1240. doi: 10.1104/pp.102.017707
Chaudhury, A. M., and Berger, F. (2001). Maternal control of seed
development. Semin. Cell. Dev. Biol. 12, 381–386. doi: 10.1006/scdb.
2001.0267
Frontiers in Plant Science | Plant Evolution and Development August 2014 | Volume 5 | Article 412 |16
Locascio et al. Phytohormones and seed development coordination
Chen, J., Huang, B., Li, Y., Du, H., Gu, Y., Liu, H., et al. (2011). Synergistic influence
of sucrose and abscisic acid on the genes involved in starch synthesis in maize
endosperm. Carbohydr. Res. 346, 1684–1691. doi: 10.1016/j.carres.2011.05.003
Chen, J., Lausser, A., and Dresselhaus, T. (2014). Hormonal responses dur-
ing early embryogenesis in maize. Biochem. Soc. Trans. 42, 325–331. doi:
10.1042/BST20130260
Cheng, Y., Dai, X., and Zhao, Y. (2006). Auxin biosynthesis by the YUCCA flavin
monooxygenases controls the formation of floral organs and vascular tissues in
Arabidopsis. Genes Dev. 20, 1790–1799. doi: 10.1101/gad.1415106
Cheng, Y., Dai, X., and Zhao, Y. (2007). Auxin synthesized by the YUCCA
flavin monooxygenases is essential for embryogenesis and leaf formation in
Arabidopsis. Plant Cell 19, 2430–2439. doi: 10.1105/tpc.107.053009
Choe, S., Tanaka, A., Noguchi, T., Fujioka, S., Takatsuto, S., Ross, A. S., et al. (2000).
Lesions in the sterol delta reductase gene of Arabidopsis cause dwarfism due to a
block in brassinosteroid biosynthesis. Plant J. 21, 431–443. doi: 10.1046/j.1365-
313x.2000.00693.x
Choi, Y., Gehring, M., Johnson, L., Hannon, M., Harada, J. J., Goldberg, R. B.,
et al. (2002). DEMETER, a DNA glycosylase domain protein, is required for
endosperm gene imprinting and seed viability in arabidopsis. Cell 110, 33–42.
doi: 10.1016/S0092-8674(02)00807-3
Chory, J., Nagpal, P., and Peto, C. A. (1991). Phenotypic and genetic analysis
of det2, a new mutant that affects light-regulated seedling development in
Arabidopsis. Plant Cell 3, 445–459. doi: 10.1105/tpc.3.5.445
Chourey, P. S., Jain, M., Li, Q. B., and Carlson, S. J. (2006). Genetic control of cell
wall invertases in developing endosperm of maize. Planta 223, 159–167. doi:
10.1007/s00425-005-0039-5
Chourey, P. S., Li, Q. B., and Kumar, D. (2010). Sugar-hormone cross-talk in seed
development: two redundant pathways of IAA biosynthesis are regulated differ-
entially in the invertase-deficient miniature1 (mn1) seed mutant in maize. Mol.
Plant 3, 1026–1036. doi: 10.1093/mp/ssq057
Costa, L. M., Yuan, J., Rouster, J., Paul, W., Dickinson, H., and Gutierrez-Marcos,
J. F. (2012). Maternal control of nutrient allocation in plant seeds by genomic
imprinting. Curr. Biol. 22, 160–165. doi: 10.1016/j.cub.2011.11.059
Curaba, J., Singh, M. B., and Bhalla, P. L. (2014). miRNAs in the crosstalk
between phytohormone signalling pathways. J. Exp. Bot. 65, 1425–1438. doi:
10.1093/jxb/eru002
Curaba, J., Spriggs, A., Taylor, J., Li, Z., and Helliwell, C. (2012). miRNA regu-
lation in the early development of barley seed. BMC Plant Biol. 12:120. doi:
10.1186/1471-2229-12-120
Day, R. C., Herridge, R. P., Ambrose, B. A., and Macknight, R. C. (2008).
Transcriptome analysis of proliferating Arabidopsis endosperm reveals bio-
logical implications for the control of syncytial division, cytokinin signal-
ing, and gene expression regulation. Plant Physiol. 148, 1964–1984. doi:
10.1104/pp.108.128108
Dharmasiri, N., Dharmasiri, S., Weijers, D., Lechner, E., Yamada, M., Hobbie, L.,
et al. (2005). Plant development is regulated by a family of auxin receptor F box
proteins. Dev. Cell 9, 109–119. doi: 10.1016/j.devcel.2005.05.014
Dietrich, K. M. Jr., Blevins, D. G., Reinbott, T. M., and Morris, R. O. (1995).
Changes in CKs and CK oxidase activity in developing maize kernels and the
effects of exogenous CK on kernel development. Plant Physiol. Biochem. 33,
327–336. doi: 10.1016/j.plaphy.2012.12.010
Dorcey, E., Urbez, C., Blazquez, M. A., Carbonell, J., and Perez-Amador, M. A.
(2009). Fertilization-dependent auxin response in ovules triggers fruit develop-
ment through the modulation of gibberellin metabolism in Arabidopsis. Plant
J. 58, 318–332. doi: 10.1111/j.1365-313X.2008.03781.x
Drews, G. N., Lee, D., and Christensen, C. A. (1998). Genetic analysis of
female gametophyte development and function. Plant Cell 10, 5–17. doi:
10.1105/tpc.10.1.5
Fatihi, A., Zbierzak, A. M., and Dormann, P. (2013). Alterations in seed devel-
opment gene expression affect size and oil content of Arabidopsis seeds. Plant
Physiol. 163, 973–985. doi: 10.1104/pp.113.226761
Feil, R., and Berger, F. (2007). Convergent evolution of genomic imprinting in
plants and mammals. Tre n d s Gen e t . 23, 192–199. doi: 10.1016/j.tig.2007.02.004
Forestan, C., Farinati, S., and Varotto, S. (2012). The maize PIN gene family of
auxin transporters. Front. Plant Sci. 3:16. doi: 10.3389/fpls.2012.00016
Forestan, C., Meda, S., and Varotto, S. (2010). ZmPIN1-mediated auxin transport
is related to cellular differentiation during maize embryogenesis and endosperm
development. Plant Physiol. 152, 1373–1390. doi: 10.1104/pp.109.150193
Forestan, C., and Varotto, S. (2010). PIN1 auxin efflux carriers localization studies
in Zea mays. Plant Signal Behav. 5, 436–439. doi: 10.4161/psb.5.4.11339
Forestan, C., and Varotto, S. (2012). The role of PIN auxin efflux carriers in
polar auxin transport and accumulation and their effect on shaping maize
development. Mol. Plant 5, 787–798. doi: 10.1093/mp/ssr103
Friml, J., Vieten, A., Sauer, M., Weijers, D., Schwarz, H., Hamann, T., et al. (2003).
Efflux-dependent auxin gradients establish the apical-basal axis of Arabidopsis.
Nature 426, 147–153. doi: 10.1038/nature02085
Gallavotti, A., Barazesh, S., Malcomber, S., Hall, D., Jackson, D., Schmidt, R. J.,
et al. (2008). Sparse inflorescence1 encodes a monocot-specific YUCCA-like
gene required for vegetative and reproductive development in maize. Proc. Natl.
Acad. Sci. U.S.A. 105, 15196–15201. doi: 10.1073/pnas.0805596105
Garcia, D., Fitz Gerald, J. N., and Berger, F. (2005). Maternal control of integu-
ment cell elongation and zygotic control of endosperm growth are coor-
dinated to determine seed size in Arabidopsis. Plant Cell 17, 52–60. doi:
10.1105/tpc.104.027136
Garcia, D., Saingery, V., Chambrier, P., Mayer, U., Jurgens, G., and Berger, F. (2003).
Arabidopsis haiku mutants reveal new controls of seed size by endosperm. Plant
Physiol. 131, 1661–1670. doi: 10.1104/pp.102.018762
Gazzarrini, S., Tsuchiya, Y., Lumba, S., Okamoto, M., and McCourt, P. (2004).
The transcription factor FUSCA3 controls developmental timing in Arabidopsis
through the hormones gibberellin and abscisic acid. Dev. Cell 7, 373–385. doi:
10.1016/j.devcel.2004.06.017
Gehring, M., Huh, J. H., Hsieh, T. F., Penterman, J., Choi, Y., Harada, J. J.,
et al. (2006). DEMETER DNA glycosylase establishes MEDEA polycomb
gene self-imprinting by allele-specific demethylation. Cell 124, 495–506. doi:
10.1016/j.cell.2005.12.034
Gehring, M., Missirian, V., and Henikoff, S. (2011). Genomic analysis of parent-of-
origin allelic expression in Arabidopsis thaliana seeds. PLoS ONE 6:e23687. doi:
10.1371/journal.pone.0023687
Geisler-Lee, J., and Gallie, D. R. (2005). Aleurone cell identity is suppressed
following connation in maize kernels. Plant Physiol. 139, 204–212. doi:
10.1104/pp.105.064295
Goldberg, R. B., Barker, S. J., and Perez-Grau, L. (1989). Regulation of gene
expression during plant embryogenesis. Cell 56, 149–160. doi: 10.1016/0092-
8674(89)90888-X
Goldberg, R. B., De Paiva, G., and Yadegari, R. (1994). Plant embryogenesis: zygote
to seed. Science 266, 605–614. doi: 10.1126/science.266.5185.605
Grennan, A. K. (2008). Arabidopsis MicroRNAs. Plant Physiol. 146, 3–4. doi:
10.1104/pp.104.900244
Grossniklaus, U., Vielle-Calzada,J. P., Hoeppner, M. A., and Gagliano, W. B. (1998).
Maternal control of embryogenesis by MEDEA, a polycomb group gene in
Arabidopsis. Science 280, 446–450. doi: 10.1126/science.280.5362.446
Gruis, D. F., Guo, H., Selinger, D., Tian, Q., and Olsen, O. A. (2006). Surface posi-
tion, not signaling from surrounding maternal tissues, specifies aleurone epider-
malcellfateinmaize.Plant Physiol. 141, 898–909. doi: 10.1104/pp.106.080945
Guilfoyle, T. J., and Hagen, G. (2007). Auxin response factors. Curr. Opin. Plant
Biol. 10, 453–460. doi: 10.1016/j.pbi.2007.08.014
Gutierrez, R., Quiroz-Figueroa, F., and Vazquez-Ramos, J. M. (2005). Maize cyclin
D2 expression, associated kinase activity and effect of phytohormones during
germination. Plant Cell Physiol. 46, 166–173. doi: 10.1093/pcp/pci007
Gutierrez-Marcos, J. F., Costa, L. M., and Evans, M. M. (2006). Maternal game-
tophytic baseless1 is required for development of the central cell and early
endosperm patterning in maize (Zea mays). Genetics 174, 317–329. doi:
10.1534/genetics.106.059709
Hamann, T., Benkova, E., Baurle, I., Kientz, M., and Jurgens, G. (2002). The
Arabidopsis BODENLOS gene encodes an auxin response protein inhibiting
MONOPTEROS-mediated embryo patterning. Genes Dev. 16, 1610–1615. doi:
10.1101/gad.229402
Hardtke, C. S., and Berleth, T. (1998). The Arabidopsis gene MONOPTEROS
encodes a transcription factor mediating embryo axis formation and vascular
development. EMBO J. 17, 1405–1411. doi: 10.1093/emboj/17.5.1405
Hartig, K., and Beck, E. (2006). Crosstalk between auxin, cytokinins, and sugars
in the plant cell cycle. Plant Biol. (Stuttg.) 8, 389–396. doi: 10.1055/s-2006-
923797
Hartwig, T., Chuck, G. S., Fujioka, S., Klempien, A., Weizbauer, R., Potluri, D. P.,
et al. (2011). Brassinosteroid control of sex determination in maize. Proc. Natl.
Acad. Sci. U.S.A. 108, 19814–19819. doi: 10.1073/pnas.1108359108
www.frontiersin.org August 2014 | Volume 5 | Article 412 |17
Locascio et al. Phytohormones and seed development coordination
Hennig, L., and Derkacheva, M. (2009). Diversity of Polycomb group com-
plexes in plants: same rules, different players? Tren d s G e net . 25, 414–423. doi:
10.1016/j.tig.2009.07.002
Holdsworth, M. J., Bentsink, L., and Soppe, W. J. (2008). Molecular networks regu-
lating Arabidopsis seed maturation, after-ripening, dormancy and germination.
New Phy tol . 179, 33–54. doi: 10.1111/j.1469-8137.2008.02437.x
Hong, Z., Ueguchi-Tanaka, M., Fujioka, S., Takatsuto, S., Yoshida, S., Hasegawa, Y.,
et al. (2005). The Rice brassinosteroid-deficient dwarf2 mutant, defective in the
rice homolog of Arabidopsis DIMINUTO/DWARF1, is rescued by the endoge-
nously accumulated alternative bioactive brassinosteroid, dolichosterone. Plant
Cell 17, 2243–2254. doi: 10.1105/tpc.105.030973
Hsieh, T. F., Ibarra, C. A., Silva, P., Zemach, A., Eshed-Williams, L., Fischer, R. L.,
et al. (2009). Genome-wide demethylation of Arabidopsis endosperm. Science
324, 1451–1454. doi: 10.1126/science.1172417
Hsieh, T. F., Shin, J., Uzawa, R., Silva, P., Cohen, S., Bauer, M. J., et al. (2011).
Regulation of imprinted gene expression in Arabidopsis endosperm. Proc. Natl.
Acad. Sci. U.S.A. 108, 1755–1762. doi: 10.1073/pnas.1019273108
Hu, Y. F., Li, Y. P., Zhang, J., Liu, H., Tian, M., and Huang, Y. (2012). Binding of
ABI4 to a CACCG motif mediates the ABA-induced expression of the ZmSSI
gene in maize (Zea mays L.) endosperm. J. Exp. Bot. 63, 5979–5989. doi:
10.1093/jxb/ers246
Hwang, I., Sheen, J., and Muller, B. (2012). Cytokinin signaling networks. Annu.
Rev. Plant Biol. 63, 353–380. doi: 10.1146/annurev-arplant-042811-105503
Ibarra, C. A., Feng, X., Schoft, V. K., Hsieh, T. F., Uzawa, R., Rodrigues, J. A.,
et al. (2012). Active DNA demethylation in plant companion cells reinforces
transposon methylation in gametes. Science 337, 1360–1364. doi: 10.1126/sci-
ence.1224839
Jahnke, S., and Scholten, S. (2009). Epigenetic resetting of a gene imprinted in plant
embryos. Curr. Biol. 19, 1677–1681. doi: 10.1016/j.cub.2009.08.053
Jenik, P. D., and Barton, M. K. (2005). Surge and destroy: the role of auxin in plant
embryogenesis. Development 132, 3577–3585. doi: 10.1242/dev.01952
Jiang, W.B., Huang, H. Y., Hu, Y. W., Zhu, S. W., Wang, Z. Y., and Lin, W. H. (2013).
Brassinosteroid regulates seed size and shape in Arabidopsis. Plant Physiol. 162,
1965–1977. doi: 10.1104/pp.113.217703
Jiang, W. B., and Lin, W. H. (2013). Brassinosteroid functions in Arabidopsis seed
development. Plant Signal. Behav. 8:e25928. doi: 10.4161/psb.25928
Jofuku, K. D., Omidyar, P. K., Gee, Z., and Okamuro, J. K. (2005). Control of seed
mass and seed yield by the floral homeotic gene APETALA2. Proc. Natl. Acad.
Sci. U.S.A. 102, 3117–3122. doi: 10.1073/pnas.0409893102
Jones, A. M. (1990). Location of transported auxin in etiolated maize shoots using
5-azidoindole-3-acetic Acid. Plant Physiol. 93, 1154–1161. doi: 10.1104/pp.
93.3.1154
Kang, B. H., Xiong, Y., Williams, D. S., Pozueta-Romero, D., and Chourey, P. S.
(2009). Miniature1-encoded cell wall invertase is essential for assembly and
function of wall-in-growth in the maize endosperm transfer cell. Plant Physiol.
151, 1366–1376. doi: 10.1104/pp.109.142331
Kang, M., Zhao, Q., Zhu, D., and Yu, J. (2012). Characterization of microR-
NAs expression during maize seed development. BMC Genomics 13:360. doi:
10.1186/1471-2164-13-360
Kang, X., and Ni, M. (2006). Arabidopsis SHORT HYPOCOTYL UNDER BLUE1
contains SPX and EXS domains and acts in cryptochrome signaling. Plant Cell
18, 921–934. doi: 10.1105/tpc.105.037879
Kanno, Y., Jikumaru, Y., Hanada, A., Nambara, E., Abrams, S. R., Kamiya, Y., et al.
(2010). Comprehensive hormone profiling in developing Arabidopsis seeds:
examination of the site of ABA biosynthesis, ABA transport and hormone
interactions. Plant Cell Physiol. 51, 1988–2001. doi: 10.1093/pcp/pcq158
Karssen, C. M., Brinkhorst-Van Der Swan, D. L., Breekland, A. E., and Koornneef,
M. (1983). Induction of dormancy during seed development by endogenous
abscisic acid: studies on abscisic acid deficient genotypes of Arabidopsis thaliana
(L.) Heynh. Planta 157, 158–165. doi: 10.1007/BF00393650
Kelley, D. R., Arreola, A., Gallagher, T. L., and Gasser, C. S. (2012). ETTIN (ARF3)
physically interacts with KANADI proteins to form a functional complex essen-
tial for integument development and polarity determination in Arabidopsis.
Development 139, 1105–1109. doi: 10.1242/dev.067918
Kepinski, S., and Leyser, O. (2005). The Arabidopsis F-box protein TIR1 is an auxin
receptor. Nature 435, 446–451. doi: 10.1038/nature03542
Kim, J., Harter, K., and Theologis, A. (1997). Protein-protein interactions among
the Aux/IAA proteins. Proc. Natl. Acad. Sci. U.S.A. 94, 11786–11791. doi:
10.1073/pnas.94.22.11786
Kinoshita, N., Wang, H., Kasahara, H., Liu, J., Macpherson, C., Machida, Y.,
et al. (2012). IAA-Ala Resistant3, an evolutionarily conserved target of miR167,
mediates Arabidopsis root architecture changes during high osmotic stress.
Plant Cell 24, 3590–3602. doi: 10.1105/tpc.112.097006
Koornneef, M., Jorna, M. L., Brinkhorst-Van Der Swan, D. L., and Karssen, C. M.
(1982). The isolation of abscisic acid (ABA) deficient mutants by selection of
induced revertants in non-germinating gibberellin sensitive lines of Arabidopsis
thaliana (L.) heynh. Theor. Appl. Genet. 61, 385–393. doi: 10.1007/BF00272861
Lang, J. D., Ray, S., and Ray, A. (1994). sin 1, a mutation affecting female fertil-
ity in Arabidopsis, interacts with mod 1, its recessive modifier. Genetics 137,
1101–1110.
Law, J. A., and Jacobsen, S. E. (2010). Establishing, maintaining and modifying
DNA methylation patterns in plants and animals. Nat. Rev. Genet. 11, 204–220.
doi: 10.1038/nrg2719
Le Clere, S., Schmelz, E. A., and Chourey, P. S. (2010). Sugar levels regulate
tryptophan-dependent auxin biosynthesis in developing maize kernels. Plant
Physiol. 153, 306–318. doi: 10.1104/pp.110.155226
Leubner-Metzger, G. (2001). Brassinosteroids and gibberellins promote
tobacco seed germination by distinct pathways. Planta 213, 758–763. doi:
10.1007/s004250100542
Li, J., and Nam, K. H. (2002). Regulation of brassinosteroid signaling by
a GSK3/SHAGGY-like kinase. Science 295, 1299–1301. doi: 10.1126/sci-
ence.1065769
Li, J., Nam, K. H., Vafeados, D., and Chory, J. (2001). BIN2, a new
brassinosteroid-insensitive locus in Arabidopsis. Plant Physiol. 127, 14–22. doi:
10.1104/pp.127.1.14
Li, J., Nie, X., Tan, J. L., and Berger, F. (2013). Integration of epigenetic and genetic
controls of seed size by cytokinin in Arabidopsis. Proc. Natl. Acad. Sci. U.S.A.
110, 15479–15484. doi: 10.1073/pnas.1305175110
Li, Y., Li, C., Ding, G., and Jin, Y. (2011). Evolution of MIR159/319 microRNA
genes and their post-transcriptional regulatory link to siRNA pathways. BMC
Evol. Biol. 11:122. doi: 10.1186/1471-2148-11-122
Lid, S. E., Gruis, D., Jung, R., Lorentzen, J. A., Ananiev, E., Chamberlin, M., et al.
(2002). The defective kernel 1 (dek1) gene required for aleurone cell devel-
opment in the endosperm of maize grains encodes a membrane protein of
the calpain gene superfamily. Proc. Natl. Acad. Sci. U.S.A. 99, 5460–5465. doi:
10.1073/pnas.042098799
Lid, S. E., Olsen, L., Nestestog, R., Aukerman, M., Brown, R. C., Lemmon, B.,
et al. (2005). Mutation in the Arabidopisis thaliana DEK1 calpain gene per-
turbs endosperm and embryo development while over-expression affects organ
development globally. Planta 221, 339–351. doi: 10.1007/s00425-004-1448-6
Liu, P. P., Montgomery, T. A., Fahlgren, N., Kasschau, K. D., Nonogaki, H., and
Carrington, J. C. (2007a). Repression of AUXIN RESPONSE FACTOR10 by
microRNA160 is critical for seed germination and post-germination stages.
Plant J. 52, 133–146. doi: 10.1111/j.1365-313X.2007.03218.x
Liu, T., Zhang, J., Wang, M., Wang, Z., Li, G., Qu, L., et al. (2007b). Expression and
functional analysis of ZmDWF4, an ortholog of Arabidopsis DWF4 from maize
(Zea mays L.). Plant Cell Rep. 26, 2091–2099. doi: 10.1007/s00299-007-0418-4
Liu, X., Zhang, H., Zhao, Y., Feng, Z., Li, Q., Yang, H. Q., et al. (2013). Auxin con-
trols seed dormancy through stimulation of abscisic acid signaling by inducing
ARF-mediated ABI3 activation in Arabidopsis. Proc. Natl. Acad. Sci. U.S.A. 110,
15485–15490. doi: 10.1073/pnas.1304651110
Liu, Y., Jiang, H., Chen, W., Qian, Y., Ma, Q., Cheng, B., et al. (2011). Genome-w ide
analysis of the auxin response factor (ARF) gene family in maize (Zea mays).
Plant Growth Regul. 63, 225–234. doi: 10.1007/s10725-010-9519-0
Liu, Y., Ye, N., Liu, R., Chen, M., and Zhang, J. (2010). H2O2 mediates the regula-
tion of ABA catabolism and GA biosynthesis in Arabidopsis seed dormancy and
germination. J. Exp. Bot. 61, 2979–2990. doi: 10.1093/jxb/erq125
Lopato, S., Borisjuk, N., Langridge, P., and Hrmova, M. (2014). Endosperm trans-
fer cell-specific genes and proteins: structure, function and applications in
biotechnology. Front. Plant Sci. 5:64. doi: 10.3389/fpls.2014.00064
Lotan, T., Ohto, M., Yee, K. M., West, M. A., Lo, R., Kwong, R. W., et al. (1998).
Arabidopsis LEAFY COTYLEDON1 is sufficient to induce embryo develop-
ment in vegetative cells. Cell 93, 1195–1205. doi: 10.1016/S0092-8674(00)
81463-4
Lu, X., Chen, D., Shu, D., Zhang, Z., Wang, W., Klukas, C., et al. (2013). The
differential transcription network between embryo and endosperm in the
early developing maize seed. Plant Physiol. 162, 440–455. doi: 10.1104/pp.113.
214874
Frontiers in Plant Science | Plant Evolution and Development August 2014 | Volume 5 | Article 412 |18
Locascio et al. Phytohormones and seed development coordination
Luo, M., Dennis, E. S., Berger, F., Peacock, W. J., and Chaudhury, A. (2005).
MINISEED3 (MINI3), a WRKY family gene, and HAIKU2 (IKU2), a leucine-
rich repeat (LRR) KINASE gene, are regulators of seed size in Arabidopsis. Proc.
Natl. Acad. Sci. U.S.A. 102, 17531–17536. doi: 10.1073/pnas.0508418102.
Luo, M., Taylor, J. M., Spriggs, A., Zhang, H., Wu, X., Russell, S., et al. (2011).
A genome-wide survey of imprinted genes in rice seeds reveals imprinting
primarily occurs in the endosperm. PLoS Genet. 7:e1002125. doi: 10.1371/jour-
nal.pgen.1002125
Lur, H.-S., and Setter, T. L. (1993a). Endorsperm development of maize defec-
tive kernel (dek) mutants. Auxin and cytokinin levels. Ann. Bot. 72, 1–6. doi:
10.1006/anbo.1993.1074
Lur, H. S., and Setter, T. L. (1993b). Role of auxin in maize endosperm development
(Timing of nuclear DNA endoreduplication, zein expression, and cytokinin).
Plant Physiol. 103, 273–280. doi: 10.1104/pp.103.1.273
Maitz, M., Santandrea, G., Zhang, Z., Lal, S., Hannah, L. C., Salamini, F., et al.
(2000). rgf1, a mutation reducing grain filling in maize through effects on basal
endosperm and pedicel development. Plant J. 23, 29–42. doi: 10.1046/j.1365-
313x.2000.00747.x
Makarevitch, I., Thompson, A., Muehlbauer, G. J., and Springer, N. M. (2012). Brd1
gene in maize encodes a brassinosteroid C-6 oxidase. PLoS ONE 7:e30798. doi:
10.1371/journal.pone.0030798
Mallory, A. C., Bartel, D. P., and Bartel, B. (2005). MicroRNA-directed regulation
of Arabidopsis AUXIN RESPONSE FACTOR17 is essential for proper develop-
ment and modulates expression of early auxin response genes. Plant Cell 17,
1360–1375. doi: 10.1105/tpc.105.031716
Manzocchi, L. A., Daminati, M. G., Gentinetta, E., and Salamini, F. (1980). Viable
defective endosperm mutants in maize. I. Kernel weight, protein fractions and
zein subunits in mature endosperms. Maydica 25, 105–116.
Marin, E., Jouannet, V., Herz, A., Lokerse, A. S., Weijers, D., Vaucheret, H., et al.
(2010). miR390, Arabidopsis TAS3 tasiRNAs, and their AUXIN RESPONSE
FACTOR targets define an autoregulatory network quantitatively regulating
lateral root growth. Plant Cell 22, 1104–1117. doi: 10.1105/tpc.109.072553
Martinez-Andujar, C., Pluskota, W. E., Bassel, G. W., Asahina, M., Pupel, P.,
Nguyen, T. T., et al. (2012). Mechanisms of hormonal regulation of endosperm
cap-specific gene expression in tomato seeds. Plant J. 71, 575–586. doi:
10.1111/j.1365-313X.2012.05010.x
Mashiguchi, K., Tanaka, K., Sakai, T., Sugawara, S., Kawaide, H., Natsume, M., et al.
(2011). The main auxin biosynthesis pathway in Arabidopsis. Proc. Natl. Acad.
Sci. U.S.A. 108, 18512–18517. doi: 10.1073/pnas.1108434108
Meinke, D. W., Franzmann, L. H., Nickle, T. C., and Yeung, E. C. (1994).
Leafy cotyledon mutants of Arabidopsis. Plant Cell 6, 1049–1064. doi:
10.1105/tpc.6.8.1049
Monke, G., Seifert, M., Keilwagen, J., Mohr, M., Grosse, I., Hahnel, U., et al.
(2012). Toward the identification and regulation of the Arabidopsis thaliana
ABI3 regulon. Nucleic Acids Res. 40, 8240–8254. doi: 10.1093/nar/gks594
Morinaka, Y., Sakamoto, T., Inukai, Y., Agetsuma, M., Kitano, H., Ashikari, M.,
et al. (2006). Morphological alteration caused by brassinosteroid insensitivity
increasesthebiomassandgrainproductionofrice.Plant Physiol. 141, 924–931.
doi: 10.1104/pp.106.077081
Muller, B., and Sheen, J. (2008). Cytokinin and auxin interaction in root stem-
cell specification during early embryogenesis. Nature 453, 1094–1097. doi:
10.1038/nature06943
Nadeau, C. D., Ozga, J. A., Kurepin, L. V., Jin, A., Pharis, R. P., and Reinecke, D. M.
(2011). Tissue-specific regulation of gibberellin biosynthesis in developing pea
seeds. Plant Physiol. 156, 897–912. doi: 10.1104/pp.111.172577
Nawy, T., Lukowitz, W., and Bayer, M. (2008). Talk global, act local-
patterning the Arabidopsis embryo. Curr. Opin. Plant Biol. 11, 28–33. doi:
10.1016/j.pbi.2007.10.007
Neuffer, M. G., and Sheridan, W. F. (1980). Defective kernel mutants of maize. I.
Genetic and lethality studies. Genetics 95, 929–944.
Ni, D. A., Wang, L. J., Ding, C. H., and Xu, Z. H. (2001). Auxin distribution and
transport during embryogenesis and seed germination of Arabidopsis. Cell Res.
11, 273–278. doi: 10.1038/sj.cr.7290096
Nodine, M. D., and Bartel, D. P. (2010). MicroRNAs prevent precocious gene
expression and enable pattern formation during plant embryogenesis. Genes
Dev. 24, 2678–2692. doi: 10.1101/gad.1986710
Nonogaki, H. (2010). MicroRNA gene regulation cascades during early stages of
plant development. Plant Cell Physiol. 51, 1840–1846. doi: 10.1093/pcp/pcq154
Nowack, M. K., Ungru, A., Bjerkan, K. N., Grini, P. E., and Schnittger, A. (2010).
Reproductive cross-talk: seed development in flowering plants. Biochem. Soc.
Trans. 38, 604–612. doi: 10.1042/BST0380604
Ohto, M. A., Fischer, R. L., Goldberg, R. B., Nakamura, K., and Harada, J. J.
(2005). Control of seed mass by APETALA2. Proc. Natl. Acad. Sci. U.S.A. 102,
3123–3128. doi: 10.1073/pnas.0409858102
Ohto, M. A., Floyd, S. K., Fischer, R. L., Goldberg, R. B., and Harada, J. J.
(2009). Effects of APETALA2 on embryo, endosperm, and seed coat develop-
ment determine seed size in Arabidopsis. Sex. Plant Reprod. 22, 277–289. doi:
10.1007/s00497-009-0116-1
Okushima, Y., Overvoorde, P. J., Arima, K., Alonso, J. M., Chan, A., Chang, C., et al.
(2005). Functional genomic analysis of the AUXIN RESPONSE FACTOR gene
family members in Arabidopsis thaliana: unique and overlapping functions of
ARF7 and ARF19. Plant Cell 17, 444–463. doi: 10.1105/tpc.104.028316
Olsen, O. A. (2004). Nuclear endosperm development in cereals and Arabidopsis
thaliana.Plant Cell 16(Suppl.), S214–S227. doi: 10.1105/tpc.017111
Orsi, C. H., and Tanksley, S. D. (2009). Natural variation in an ABC trans-
porter gene associated with seed size evolution in tomato species. PLoS Genet.
5:e1000347. doi: 10.1371/journal.pgen.1000347
Palatnik, J. F., Allen, E., Wu, X., Schommer, C., Schwab, R., Carrington, J. C., et al.
(2003). Control of leaf morphogenesis by microRNAs. Nature 425, 257–263.
doi: 10.1038/nature01958
Palatnik, J. F., Wollmann, H., Schommer, C., Schwab, R., Boisbouvier, J., Rodriguez,
R., et al. (2007). Sequence and expression differences underlie functional spe-
cialization of Arabidopsis microRNAs miR159 and miR319. Dev. Cell 13,
115–125. doi: 10.1016/j.devcel.2007.04.012
Parcy, F., and Giraudat, J. (1997). Interactions between the ABI1 and the ectopically
expressed ABI3 genes in controlling abscisic acid responses in Arabidopsis veg-
etative tissues. Plant J. 11, 693–702. doi: 10.1046/j.1365-313X1997.11040693.x
Pasini, L., Stile, M., Puja, E., Valsecchi, R., Francia, P., Carletti, G., et al. (2008).
The integration of mutant loci affecting maize endosperm development in a
dense genetic map using an AFLP-based procedure. Mol. Breed. 22, 527–541.
doi: 10.1007/s11032-008-9196-0
Petrasek, J., Mravec, J., Bouchard, R., Blakeslee, J. J., Abas, M., Seifertova, D., et al.
(2006). PIN proteins perform a rate-limiting function in cellular auxin efflux.
Science 312, 914–918. doi: 10.1126/science.1123542
Phillips, K. A., Skirpan, A. L., Liu, X., Christensen, A., Slewinski, T. L., Hudson, C.,
et al. (2011). Vanishingtassel2 encodes a grass-specific tryptophan aminotrans-
ferase required for vegetative and reproductive development in maize. Plant Cell
23, 550–566. doi: 10.1105/tpc.110.075267
Pignocchi, C., Minns, G. E., Nesi, N., Koumproglou, R., Kitsios, G., Benning, C.,
et al. (2009). ENDOSPERM DEFECTIVE1 is a novel microtubule-associated
protein essential for seed development in Arabidopsis. Plant Cell 21, 90–105.
doi: 10.1105/tpc.108.061812
Portereiko, M. F., Lloyd, A., Steffen, J. G., Punwani, J. A., Otsuga, D., and Drews,
G. N. (2006). AGL80 is required for central cell and endosperm development in
Arabidopsis. Plant Cell 18, 1862–1872. doi: 10.1105/tpc.106.040824
Raissig, M. T., Baroux, C., and Grossniklaus, U. (2011). Regulation and flexibil-
ity of genomic imprinting during seed development. Plant Cell 23, 16–26. doi:
10.1105/tpc.110.081018
Raissig, M. T., Bemer, M., Baroux, C., and Grossniklaus, U. (2013). Genomic
imprinting in the Arabidopsis embryo is partly regulated by PRC2. PLoS Genet.
9:e1003862. doi: 10.1371/journal.pgen.1003862
Rashotte, A. M., Mason, M. G., Hutchison, C. E., Ferreira, F. J., Schaller, G. E., and
Kieber, J. J. (2006). A subset of Arabidopsis AP2 transcription factors mediates
cytokinin responses in concert with a two-component pathway. Proc. Natl. Acad.
Sci. U.S.A. 103, 11081–11085. doi: 10.1073/pnas.0602038103
Ray, A., Lang, J.D., Golden, T., and Ray, S. (1996). SHORT INTEGUMENT (SIN1),
a gene required for ovule development in Arabidopsis, also controls flowering
time. Development 122, 2631–2638.
Raz, V., Bergervoet, J. H., and Koornneef, M. (2001). Sequential steps for develop-
mental arrest in Arabidopsis seeds. Development 128, 243–252.
Reik, W., and Walter, J. (2001). Genomic imprinting: parental influence on the
genome. Nat. Rev. Genet. 2, 21–32. doi: 10.1038/35047554
Reinhart, B. J., Weinstein, E. G., Rhoades, M. W., Bartel, B., and Bartel, D. P. (2002).
MicroRNAs in plants. Genes Dev. 16, 1616–1626. doi: 10.1101/gad.1004402
Reiser, L., and Fischer, R. L. (1993). The ovule and the embryo sac. Plant Cell 5,
1291–1301. doi: 10.1105/tpc.5.10.1291
www.frontiersin.org August 2014 | Volume 5 | Article 412 |19
Locascio et al. Phytohormones and seed development coordination
Reyes, F. C., Sun, B., Guo, H., Gruis, D. F., and Otegui, M. S. (2010).
Agrobacterium tumefaciens-mediated transformation of maize endosperm as
a tool to study endosperm cell biology. Plant Physiol. 153, 624–631. doi:
10.1104/pp.110.154930
Reyes, J. L., and Chua, N. H. (2007). ABA induction of miR159 controls transcript
levels of two MYB factors during Arabidopsis seed germination. Plant J. 49,
592–606. doi: 10.1111/j.1365-313X.2006.02980.x
Rhoades, M. W., Reinhart, B. J., Lim, L. P., Burge, C. B., Bartel, B., and Bartel,
D. P. (2002). Prediction of plant microRNA targets. Cell 110, 513–520. doi:
10.1016/S0092-8674(02)00863-2
Riefler, M., Novak, O., Strnad, M., and Schmulling, T. (2006). Arabidopsis
cytokinin receptor mutants reveal functions in shoot growth, leaf senescence,
seed size, germination, root development, and cytokinin metabolism. Plant Cell
18, 40–54. doi: 10.1105/tpc.105.037796
Rijavec, T., and Dermastia, M. (2010). Cytokinins and their function in developing
seeds. Acta Chim. Slov. 57, 617–629.
Rijavec, T., Jain, M., Dermastia, M., and Chourey, P. S. (2011). Spatial and temporal
profiles of cytokinin biosynthesis and accumulation in developing caryopses of
maize. Ann. Bot. 107, 1235–1245. doi: 10.1093/aob/mcq247
Rijavec, T., Kovac, M., Kladnik, A., Chourey, P. S., and Dermastia, M. (2009).
A comparative study on the role of cytokinins in caryopsis development in
the maize miniature1 seed mutant and its wild type. J. Integr. Plant Biol. 51,
840–849. doi: 10.1111/j.1744-7909.2009.00863.x
Riou-Khamlichi, C., Huntley, R.,Jacqmard, A., and Murray, J. A. (1999). Cytokinin
activation of Arabidopsis cell division through a D-type cyclin. Science 283,
1541–1544. doi: 10.1126/science.283.5407.1541
Rodrigues, J. A., Ruan, R., Nishimura, T., Sharma, M. K., Sharma, R., Ronald, P. C.,
et al. (2013). Imprinted expression of genes and small RNA is associated with
localized hypomethylation of the maternal genome in rice endosperm. Proc.
Natl. Acad. Sci. U.S.A. 110, 7934–7939. doi: 10.1073/pnas.1306164110
Rook, F., Corke, F., Card, R., Munz, G., Smith, C., and Bevan, M. W. (2001).
Impaired sucrose-induction mutants reveal the modulation of sugar-induced
starch biosynthetic gene expression by abscisic acid signalling. Plant J. 26,
421–433. doi: 10.1046/j.1365-313X.2001.2641043.x
Ruan, Y. L., Jin, Y., Yang, Y. J., Li, G. J., and Boyer, J. S. (2010). Sugar input,
metabolism, and signaling mediated by invertase: roles in development, yield
potential, and response to drought and heat. Mol. Plant 3, 942–955. doi:
10.1093/mp/ssq044
Rubio-Somoza, I., and Weigel, D. (2011). MicroRNA networks and devel-
opmental plasticity in plants. Trends Plant Sci. 16, 258–264. doi:
10.1016/j.tplants.2011.03.001
Sabelli, P. A., and Larkins, B. A. (2009). The development of endosperm in grasses.
Plant Physiol. 149, 14–26. doi: 10.1104/pp.108.129437
Sairanen, I., Novak, O., Pencik, A., Ikeda, Y., Jones, B., Sandberg, G., et al.
(2012). Soluble carbohydrates regulate auxin biosynthesis via PIF proteins in
Arabidopsis. Plant Cell 24, 4907–4916. doi: 10.1105/tpc.112.104794
Salas Fernandez, M. G., Becraft, P. W., Yin, Y., and Lubberstedt, T. (2009). From
dwarves to giants? Plant height manipulation for biomass yield. Trends Plant
Sci. 14, 454–461. doi: 10.1016/j.tplants.2009.06.005
Santos-Mendoza, M., Dubreucq, B., Baud, S., Parcy, F., Caboche, M., and Lepiniec,
L. (2008). Deciphering gene regulatory networks that control seed development
and maturation in Arabidopsis. Plant J. 54, 608–620. doi: 10.1111/j.1365-
313X.2008.03461.x
Saoussem, H., Sadok, B., Habib, K., and Mayer, P. M. (2009). Fatty acid accumu-
lation in the different fractions of the developing corn kernel. Food Chem. 117,
432–437. doi: 10.1016/j.foodchem.2009.04.038
Scanlon, M. J., Henderson, D. C., and Bernstein, B. (2002). SEMAPHORE1 func-
tions during the regulation of ancestrally duplicated knox genes and polar auxin
transport in maize. Development 129, 2663–2673.
Scanlon, M. J., Stinard, P. S., James, M. G., Myers, A. M., and Robertson, D. S.
(1994). Genetic analysis of 63 mutations affecting maize kernel development
isolated from Mutator stocks. Genetics 136, 281–294.
Schauer, S. E., Jacobsen, S. E., Meinke, D. W., and Ray, A. (2002). DICER-LIKE1:
blind men and elephants in Arabidopsis development. Trends Plant Sci. 7,
487–491. doi: 10.1016/S1360-1385(02)02355-5
Schruff, M. C., Spielman, M., Tiwari, S., Adams, S., Fenby, N., and Scott, R. J.
(2006). The AUXIN RESPONSE FACTOR 2 gene of Arabidopsis links auxin sig-
nalling, cell division, and the size of seeds and other organs. Development 133,
251–261. doi: 10.1242/dev.02194
Seefried, W. F., Willmann, M. R., Clausen, R. L., and Jenik, P. D. (2014). Global reg-
ulation of embryonic patterning in Arabidopsis by microRNAs. Plant Physiol.
165, 670–687. doi: 10.1104/pp.114.240846
Seiler, C., Harshavardhan, V. T., Rajesh, K., Reddy, P. S., Strickert, M., Rolletschek,
H., et al. (2011). ABA biosynthesis and degradation contributing to ABA
homeostasis during barley seed development under control and terminal
drought-stress conditions. J. Exp. Bot. 62, 2615–2632. doi: 10.1093/jxb/
erq446
Sekhon,R.S.,Hirsch,C.N.,Childs,K.L.,Breitzman,M.W.,Kell,P.,Duvick,
S., et al. (2014). Phenotypic and transcriptional analysis of divergently selected
maize populations reveals the role of developmental timing in seed size deter-
mination. Plant Physiol. 165, 658–669. doi: 10.1104/pp.114.235424
Seo, M., Hanada, A., Kuwahara, A., Endo, A., Okamoto, M., Yamauchi, Y., et al.
(2006). Regulation of hormone metabolism in Arabidopsis seeds: phytochrome
regulation of abscisic acid metabolism and abscisic acid regulation of gibberellin
metabolism. Plant J. 48, 354–366. doi: 10.1111/j.1365-313X.2006.02881.x
Shen, Y., Jiang, Z., Lu, S., Lin, H., Gao, S., Peng, H., et al. (2013). Combined small
RNA and degradome sequencing reveals microRNA regulation during imma-
ture maize embryo dedifferentiation. Biochem. Biophys. Res. Commun. 441,
425–430. doi: 10.1016/j.bbrc.2013.10.113
Sheridan, W. F., and Clark, J. K. (1987). Maize embryogeny: a promising experi-
mental system. Tre n d s Ge n e t . 3, 3–6. doi: 10.1016/0168-9525(87)90153-3
Sheridan, W. F., and Neuffer, M. G. (1980). Defective kernel mutants of maize II.
Morphological and embryo culture studies. Genetics 95, 945–960.
Singh, D. P., Filardo, F. F., Storey, R., Jermakow, A. M., Yamaguchi, S., and Swain, S.
M. (2010). Overexpression of a gibberellin inactivation gene alters seed develop-
ment, KNOX gene expression, and plant development in Arabidopsis. Physiol.
Plant. 138, 74–90. doi: 10.1111/j.1399-3054.2009.01289.x
Smehilova, M., Galuszka, P., Bilyeu, K. D., Jaworek, P., Kowalska, M., Sebela, M.,
et al. (2009). Subcellular localization and biochemical comparison of cytoso-
lic and secreted cytokinin dehydrogenase enzymes from maize. J. Exp. Bot. 60,
2701–2712. doi: 10.1093/jxb/erp126
Sorensen, M. B., Mayer, U., Lukowitz, W., Robert, H., Chambrier, P., Jurgens, G.,
et al. (2002). Cellularisation in the endosperm of Arabidopsis thaliana is coupled
to mitosis and shares multiple components with cytokinesis. Development 129,
5567–5576. doi: 10.1242/dev.00152
Sreenivasulu, N., and Wobus, U. (2013). Seed-development programs: a systems
biology-based comparison between dicots and monocots. Annu. Rev. Plant Biol.
64, 189–217. doi: 10.1146/annurev-arplant-050312-120215
Stepanova, A. N., Yun, J., Robles, L. M., Novak, O., He, W., Guo, H., et al. (2011).
The Arabidopsis YUCCA1 flavin monooxygenase functions in the indole-3-
pyruvic acid branch of auxin biosynthesis. Plant Cell 23, 3961–3973. doi:
10.1105/tpc.111.088047
Stone, S. L., Braybrook, S. A., Paula, S. L., Kwong, L. W., Meuser, J., Pelletier, J.,
et al. (2008). Arabidopsis LEAFY COTYLEDON2 induces maturation traits and
auxin activity: implications for somatic embryogenesis. Proc. Natl. Acad. Sci.
U.S.A. 105, 3151–3156. doi: 10.1073/pnas.0712364105
Sun, X., Shantharaj, D., Kang, X., and Ni, M. (2010). Transcr iptional and hormonal
signaling control of Arabidopsis seed development. Curr. Opin. Plant Biol. 13,
611–620. doi: 10.1016/j.pbi.2010.08.009
Takahashi, N., Nakazawa, M., Shibata, K., Yokota, T., Ishikawa, A., Suzuki, K.,
et al. (2005). shk1-D, a dwarf Arabidopsis mutant caused by activation of
the CYP72C1 gene, has altered brassinosteroid levels. Plant J. 42, 13–22. doi:
10.1111/j.1365-313X.2005.02357.x
Tan, X., Calderon-Villalobos, L. I., Sharon, M., Zheng, C., Robinson, C. V., Estelle,
M., et al. (2007). Mechanism of auxin perception by the TIR1 ubiquitin ligase.
Nature 446, 640–645. doi: 10.1038/nature05731
Tanabe, S., Ashikari, M., Fujioka, S., Takatsuto, S., Yoshida, S., Yano, M., et al.
(2005). A novel cytochrome P450 is implicated in brassinosteroid biosynthe-
sis via the characterization of a rice dwarf mutant, dwarf11, with reduced seed
length. Plant Cell 17, 776–790. doi: 10.1105/tpc.104.024950
Tanaka, H., Dhonukshe, P., Brewer, P. B., and Friml, J. (2006). Spatiotemporal
asymmetric auxin distribution: a means to coordinate plant development. Cell
Mol. Life Sci. 63, 2738–2754. doi: 10.1007/s00018-006-6116-5
Tang, X., Bian, S., Tang, M., Lu,Q., Li, S., Liu, X., et al. (2012). MicroRNA-mediated
repression of the seed maturation program during vegetative development in
Arabidopsis. PLoS Genet. 8:e1003091. doi: 10.1371/journal.pgen.1003091
Tao, Y., Zheng, J., Xu, Z., Zhang, X., Zhang, K., and Wang, G. (2004).
Functional analysis of ZmDWF1, a maize homolog of the Arabidopsis
Frontiers in Plant Science | Plant Evolution and Development August 2014 | Volume 5 | Article 412 |20
Locascio et al. Phytohormones and seed development coordination
brassinosteroids biosynthetic DWF1/DIM gene. Plant Sci. 167, 743–751. doi:
10.1016/j.plantsci.2004.05.012
Teale, W.D., Paponov, I. A., and Palme,K. (2006). Auxin in action: signalling, trans-
port and the control of plant growth and development. Nat. Rev. Mol. Cell Biol.
7, 847–859. doi: 10.1038/nrm2020
Teoh, K. T., Requesens, D. V., Devaiah, S. P., Johnson, D., Huang, X., Howard, J.
A., et al. (2013). Transcriptome analysis of embryo maturation in maize. BMC
Plant Biol. 13:19. doi: 10.1186/1471-2229-13-19
Tivendale, N. D., Ross, J. J., and Cohen, J. D. (2014). The shifting
paradigms of auxin biosynthesis. Trends Plant Sci. 19, 44–51. doi:
10.1016/j.tplants.2013.09.012
Torti, G., Lombardi, L., Manzocchi, L., and Salamini, F. (1984). Indole-3-acetic acid
content in viable defective endosperm mutants of maize. Maydica 29, 335–343.
Torti, G., Manzocchi, L., and Salamini, F. (1986). Free and bound indole-acetic acid
is low in the endosperm of the maize mutant defective endosperm-B18. Theor.
Appl. Genet. 72, 602–605. doi: 10.1007/BF00288997
Ulmasov, T., Hagen, G., and Guilfoyle, T. J. (1999). Activation and repression
of transcription by auxin-response factors. Proc. Natl. Acad. Sci. U.S.A. 96,
5844–5849. doi: 10.1073/pnas.96.10.5844
Vanneste, S., and Friml, J. (2009). Auxin: a trigger for change in plant development.
Cell 136, 1005–1016. doi: 10.1016/j.cell.2009.03.001
Vanstraelen, M., and Benkova, E. (2012). Hormonal interactions in the regu-
lation of plant development. Annu.Rev.CellDev.Biol.28, 463–487. doi:
10.1146/annurev-cellbio-101011-155741
Veach, Y. K., Martin, R. C., Mok, D. W., Malbeck, J., Vankova, R., and Mok,
M. C. (2003). O-glucosylation of cis-zeatin in maize. Characterization of
genes, enzymes, and endogenous cytokinins. Plant Physiol. 131, 1374–1380. doi:
10.1104/pp.017210
Vert, G., Walcher, C. L., Chory, J., and Nemhauser, J. L. (2008). Integration of auxin
and brassinosteroid pathways by Auxin Response Factor 2. Proc. Natl. Acad. Sci.
U.S.A. 105, 9829–9834. doi: 10.1073/pnas.0803996105
Vicente-Carbajosa, J., and Carbonero, P. (2005). Seed maturation: developing an
intrusive phase to accomplish a quiescent state. Int. J. Dev. Biol. 49, 645–651.
doi: 10.1387/ijdb.052046jc
Vieten, A., Vanneste, S., Wisniewska, J., Benkova, E., Benjamins, R., Beeckman, T.,
et al. (2005). Functional redundancy of PIN proteins is accompanied by auxin-
dependent cross-regulation of PIN expression. Development 132, 4521–4531.
doi: 10.1242/dev.02027
Vyroubalova, S., Vaclavikova, K., Tureckova, V., Novak, O., Smehilova, M., Hluska,
T., et al. (2009). Characterization of new maize genes putatively involved in
cytokinin metabolism and their expression during osmotic stress in relation to
cytokinin levels. Plant Physiol. 151, 433–447. doi: 10.1104/pp.109.142489
Wabnik, K., Robert, H. S., Smith, R. S., and Friml, J. (2013). Modeling framework
for the establishment of the apical-basal embryonic axis in plants. Curr. Biol. 23,
2513–2518. doi: 10.1016/j.cub.2013.10.038
Wang, A., Garcia, D., Zhang, H., Feng, K., Chaudhury, A., Berger, F., et al. (2010a).
The VQ motif protein IKU1 regulates endosperm growth and seed size in
Arabidopsis. Plant J. 63, 670–679. doi: 10.1111/j.1365-313X.2010.04271.x
Wang, B., Chen, Y., Guo, B., Kabir, M. R., Yao, Y., Peng, H., et al. (2014).
Expression and functional analysis of genes encoding cytokinin receptor-like
histidine kinase in maize (Zea mays L.). Mol. Genet. Genomics 289, 501–512.
doi: 10.1007/s00438-014-0821-9
Wang, H., Nussbaum-Wagler, T., Li, B., Zhao, Q., Vigouroux, Y., Faller, M., et al.
(2005). The origin of the naked grains of maize. Nature 436, 714–719. doi:
10.1038/nature03863
Wang, L., Liu, H., Li, D., and Chen, H. (2011). Identification and characterization
of maize microRNAs involved in the very early stage of seed germination. BMC
Genomics 12:154. doi: 10.1186/1471-2164-12-154
Wang, Y., Deng, D., Bian, Y., Lv, Y., and Xie, Q. (2010b). Genome-wide analysis of
primary auxin-responsive Aux/IAA gene family in maize (Zea mays. L.). Mol.
Biol. Rep. 37, 3991–4001. doi: 10.1007/s11033-010-0058-6
Wang, Y., Deng, D., Shi, Y., Miao, N., Bian, Y., and Yin, Z. (2012). Diversification,
phylogeny and evolution of auxin response factor (ARF) family: insights
gained from analyzing maize ARF genes. Mol. Biol. Rep. 39, 2401–2415. doi:
10.1007/s11033-011-0991-z
Wang, Z. Y., Nakano, T., Gendron, J., He, J., Chen, M., Vafeados, D., et al. (2002).
Nuclear-localized BZR1 mediates brassinosteroid-induced growth and feed-
back suppression of brassinosteroid biosynthesis. Dev. Cell 2, 505–513. doi:
10.1016/S1534-5807(02)00153-3
Waters, A., Creff, A., Goodrich, J., and Ingram, G. (2013a). “What we’ve got here
is failure to communicate”: zou mutants and endosperm cell death in seed
development. Plant Signal. Behav. 8:e24368. doi: 10.4161/psb.24368
Waters, A. J., Bilinski, P., Eichten, S. R., Vaughn, M. W.,Ross-Ibarr a, J., Gehring, M.,
et al. (2013b). Comprehensive analysis of imprinted genes in maize reveals allelic
variation for imprinting and limited conservation with other species. Proc. Natl.
Acad. Sci. U.S.A. 110, 19639–19644. doi: 10.1073/pnas.1309182110
Waters, A. J., Makarevitch, I., Eichten, S. R., Swanson-Wagner, R. A., Yeh, C.
T., Xu, W., et al. (2011). Parent-of-origin effects on gene expression and
DNA methylation in the maize endosperm. Plant Cell 23, 4221–4233. doi:
10.1105/tpc.111.092668
Weber, H., Borisjuk, L., and Wobus, U. (2005). Molecular physiology of
legume seed development. Annu. Rev. Plant Biol. 56, 253–279. doi:
10.1146/annurev.arplant.56.032604.144201
Weijers, D., and Jurgens, G. (2005). Auxin and embryo axis formation: the ends in
sight? Curr. Opin. Plant Biol. 8, 32–37. doi: 10.1016/j.pbi.2004.11.001
Werner, T., Motyka, V., Laucou, V., Smets, R., Van Onckelen, H., and Schmulling,
T. (2003). Cytokinin-deficient transgenic Arabidopsis plants show multiple
developmental alterations indicating opposite functions of cytokinins in the
regulation of shoot and root meristem activity. Plant Cell 15, 2532–2550. doi:
10.1105/tpc.014928
West, M., and Harada, J. J. (1993). Embryogenesis in higher plants: an overview.
Plant Cell 5, 1361–1369. doi: 10.1105/tpc.5.10.1361
Windels, D., and Vazquez, F. (2011). miR393: integrator of environmental cues
in auxin signaling? Plant Signal. Behav. 6, 1672–1675. doi: 10.4161/psb.6.11.
17900
Wojcikowska, B., Jaskola, K., Gasiorek, P., Meus, M., Nowak, K., and Gaj, M. D.
(2013). LEAFY COTYLEDON2 (LEC2) promotes embryogenic induction in
somatic tissues of Arabidopsis, via YUCCA-mediated auxin biosynthesis. Planta
238, 425–440. doi: 10.1007/s00425-013-1892-2
Wolff, P., Weinhofer, I., Seguin, J., Roszak, P., Beisel, C., Donoghue, M. T.,
et al. (2011). High-resolution analysis of parent-of-origin allelic expression
in the Arabidopsis Endosperm. PLoS Genet. 7:e1002126. doi: 10.1371/jour-
nal.pgen.1002126
Won, C., Shen, X., Mashiguchi, K., Zheng, Z., Dai, X., Cheng, Y., et al.
(2011). Conversion of tryptophan to indole-3-acetic acid by TRYPTOPHAN
AMINOTRANSFERASES OF ARABIDOPSIS and YUCCAs in Arabidopsis.
Proc. Natl. Acad. Sci. U.S.A. 108, 18518–18523. doi: 10.1073/pnas.11084
36108
Wright, A. D., Moehlenkamp, C. A., Perrot, G. H., Neuffer, M. G., and Cone,
K. C. (1992). The maize auxotrophic mutant orange pericarp is defective
in duplicate genes for tryptophan synthase beta. Plant Cell 4, 711–719. doi:
10.1105/tpc.4.6.711
Wu, G., Park, M. Y., Conway, S. R., Wang, J. W., Weigel, D., and Poethig, R. S.
(2009). The sequential action of miR156 and miR172 regulates developmental
timing in Arabidopsis. Cell 138, 750–759. doi: 10.1016/j.cell.2009.06.031
Xiao, W., Gehring, M., Choi, Y., Margossian, L., Pu, H., Harada, J. J., et al.
(2003). Imprinting of the MEA Polycomb gene is controlled by antagonism
between MET1 methyltransferase and DME glycosylase. Dev. Cell 5, 891–901.
doi: 10.1016/S1534-5807(03)00361-7
Xin, M., Yang, R., Li, G., Chen, H., Laurie, J., Ma, C., et al. (2013). Dynamic
expression of imprinted genes associates with maternally controlled nutrient
allocation during maize endosperm development. Plant Cell 25, 3212–3227. doi:
10.1105/tpc.113.115592
Xing, H., Pudake, R. N., Guo, G., Xing, G., Hu, Z., Zhang, Y., et al. (2011).
Genome-wide identification and expression profiling of auxin response factor
(ARF) gene family in maize. BMC Genomics 12:178. doi: 10.1186/1471-2164-
12-178
Xing, Q., Creff, A., Waters, A., Tanaka, H., Goodrich, J., and Ingram, G. C. (2013).
ZHOUPI controls embryonic cuticle formation via a signalling pathway involv-
ing the subtilisin protease ABNORMAL LEAF-SHAPE1 and the receptor kinases
GASSHO1 and GASSHO2. Development 140, 770–779. doi: 10.1242/dev.
088898
Yang, S., Johnston, N., Talideh, E., Mitchell, S., Jeffree, C., Goodrich, J., et al.
(2008). The endosperm-specific ZHOUPI gene of Arabidopsis thaliana regulates
endosperm breakdown and embryonic epidermal development. Development
135, 3501–3509. doi: 10.1242/dev.026708
Yin, Y., Wang, Z. Y., Mora-Garcia, S., Li, J., Yoshida, S., Asami, T., et al.
(2002). BES1 accumulates in the nucleus in response to brassinosteroids to
www.frontiersin.org August 2014 | Volume 5 | Article 412 |21
Locascio et al. Phytohormones and seed development coordination
regulate gene expression and promote stem elongation. Cell 109, 181–191. doi:
10.1016/S0092-8674(02)00721-3
Zhang, B., Pan, X., and Anderson, T. A. (2006). Identification of 188 con-
served maize microRNAs and their targets. FEBS Lett. 580, 3753–3762. doi:
10.1016/j.febslet.2006.05.063
Zhang, L., Chia, J. M., Kumari, S., Stein, J. C., Liu, Z., Narechania, A., et al. (2009a).
A genome-wide characterization of microRNA genes in maize. PLoS Genet.
5:e1000716. doi: 10.1371/journal.pgen.1000716
Zhang, L. Y., Bai, M. Y., Wu, J., Zhu, J. Y., Wang, H., Zhang, Z., et al. (2009b).
Antagonistic HLH/bHLH transcription factors mediate brassinosteroid regula-
tion of cell elongation and plant development in rice and Arabidopsis. Plant Cell
21, 3767–3780. doi: 10.1105/tpc.109.070441
Zhang, M., Xie, S., Dong, X., Zhao, X., Zeng, B., Chen, J., et al. (2014). Genome-
wide high resolution parental-specific DNA and histone methylation maps
uncover patterns of imprinting regulation in maize. Genome Res. 24, 167–176.
doi: 10.1101/gr.155879.113
Zhang, M., Zhao, H., Xie, S., Chen, J., Xu, Y., Wang, K., et al. (2011). Extensive,
clustered parental imprinting of protein-coding and noncoding RNAs in devel-
oping maize endosperm. Proc. Natl. Acad. Sci. U.S.A. 108, 20042–20047. doi:
10.1073/pnas.1112186108
Zhao, Y., Christensen, S. K., Fankhauser, C., Cashman, J. R., Cohen, J. D., Weigel,
D., et al. (2001). A role for flavin monooxygenase-like enzymes in auxin
biosynthesis. Science 291, 306–309. doi: 10.1126/science.291.5502.306
Conflict of Interest Statement: The authors declare that the research was con-
ducted in the absence of any commercial or financial relationships that could be
construed as a potential conflict of interest.
Received: 27 April 2014; accepted: 03 August 2014; published online: 25 August 2014.
Citation: Locascio A, Roig-Villanova I, Bernardi J and Varotto S (2014) Current per-
spectives on the hormonal control of seed development in Arabidopsis and maize: a
focus on auxin. Front. Plant Sci. 5:412. doi: 10.3389/fpls.2014.00412
This article was submitted to Plant Evolution and Development, a section of the
journal Frontiers in Plant Science.
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