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Open access, freely available online
September 2004 | Volume 2 | Issue 9 | e311
Phytohormones: What Are they?
Plant growth and development
involves the integration of many
environmental and endogenous signals
that, together with the intrinsic genetic
program, determine plant form.
Fundamental to this process are several
growth regulators collectively called the
plant hormones or phytohormones.
This group includes auxin, cytokinin,
the gibberellins (GAs), abscisic acid
(ABA), ethylene, the brassinosteroids
(BRs), and jasmonic acid (JA), each
of which acts at low concentrations to
regulate many aspects of plant growth
and development.
With the notable exception of the
steroidal hormones of the BR group,
plant hormones bear little resemblance
to their animal counterparts (Figure 1).
Rather, they are relatively simple, small
molecules such as ethylene gas and
indole-3-acetic acid (IAA), the primary
auxin in the majority of plant species.
The concept of plant hormones
originates from a classical experiment
on phototropism, the bending of plants
toward light, carried out by Charles
Darwin and his son Francis in 1880.
The Darwins were able to demonstrate
that when oat seedlings were exposed
to a lateral light source, a transported
signal originating from the plant apex
promoted differential cell elongation
in the lower parts of the seedling that
resulted in it bending toward the light
source. This signal was subsequently
shown to be IAA, the fi rst known plant
hormone.
What Do They Do?
Virtually every aspect of plant
growth and development is under
hormonal control to some degree.
A single hormone can regulate an
amazingly diverse array of cellular
and developmental processes, while
at the same time multiple hormones
often infl uence
a single process.
Well-studied
examples include
the promotion
of fruit ripening
by ethylene,
regulation of the
cell cycle by auxin
and cytokinin,
induction of seed
germination and
stem elongation
by GA, and the
maintenance of
seed dormancy by
ABA. Historically,
the effects of each
hormone have
been defi ned
largely by the
application
of exogenous
hormone.
More recently,
the isolation
of hormone
biosynthetic
and response mutants has provided
powerful new tools for painting a
clearer picture of the roles of the
various phytohormones in plant growth
and development.
How Do They Work?
Plant biologists have been
fascinated by the regulatory capacity
of phytohormones since the time of
their discovery, and the notion that
hormone levels or responses could be
manipulated to improve desired plant
traits has long been an area of intense
interest. Perhaps the best-known
example of this is the isolation of dwarf
varieties of wheat and rice that led to
the “green revolution” in the second
half of the 20th century, which is
credited with saving millions of people
around the globe from starvation.
These dwarf varieties have shorter
stems than wild-type, making these
plants less susceptible to damage by
wind and rain. The molecular isolation
of these “dwarfi ng genes” has revealed
that they encode components of the
GA biosynthesis and response pathways
(Peng et al. 1999; Sasaki et al. 2002).
To elucidate the molecular
mechanisms underlying phytohormone
action, several researchers have
utilized the genetically facile model
plant Arabidopsis thaliana to isolate
mutations that confer altered response
to applied hormone. Molecular and
biochemical analysis of the gene
Primer
Hormonal Regulation of Plant Growth
and Development
William M. Gray
Citation: Gray WM (2004) Hormonal regulation of
plant growth and development. PLoS Biol 2(9): e311.
Copyright: © 2004 William M. Gray. This is an open-ac-
cess article distributed under the terms of the Cre-
ative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly
cited.
Abbreviations: ABA, abscisic acid; ARF, auxin response
factor; BR, brassinosteroid; GA, gibberellin; HK,
histidine kinase; IAA, indole-3-acetic acid; JA, jasmonic
acid; SCF, SKP1/Cullin/F-box protein
William M. Gray is in Plant Biology at the University
of Minnesota, St. Paul, Minnesota, United States of
America. E-mail: grayx051@tc.umn.edu
DOI: 10.1371/journal.pbio.0020311
DOI: 10.1371/journal.pbio.0020311.g001
Figure 1. Chemical Structures of the Plant Hormones
A partial list of the responses elicited by each hormone
is provided below. Ethylene gas promotes fruit ripening,
senescence, and responses to pathogens and abiotic stresses.
IAA (an auxin) regulates cell division and expansion, vascular
differentiation, lateral root development, and apical dominance.
Cytokinins are adenine derivatives fi rst identifi ed by their ability
to promote cytokinesis. JA is a volatile signal that modulates
pollen development and responses to pathogen infection. The
BRs regulate cell expansion and photomorphogenesis (light-
regulated development). GAs are diterpenoid compounds that
promote germination, stem elongation, and the induction of
fl owering. ABA promotes seed dormancy and is involved in
several stress signaling pathways.
PLoS Biology | www.plosbiology.org 1271
products defi ned by these mutations,
coupled with expression studies aimed
at identifying the downstream target
genes that mediate hormonal changes
in growth and development, has
begun to unlock some of the mysteries
behind phytohormone action. While
no hormone transduction pathway is
completely understood, we now have
a rudimentary understanding of many
of the molecular events underlying
hormone action. Several reviews
covering the individual hormone
pathways in greater detail have recently
been published (Turner et al. 2002;
Gomi and Matsuoka 2003; Himmelbach
et al. 2003; Kakimoto 2003; Dharmasiri
and Estelle 2004; Guo and Ecker 2004;
Wang and He 2004).
Common Themes
Regulation by proteolysis has
emerged as a resounding theme
in plant hormone signaling. The
ubiquitin-mediated degradation of
key regulatory proteins has been
demonstrated, or is at least likely, for
all of the phytohormone response
pathways (Smalle and Vierstra
2004). In the case of auxin, the
response pathway is normally subject
to repression by a large family of
transcriptional regulators called the
Aux/IAA proteins (Figure 2). These
proteins dimerize with members of
the auxin response factor (ARF)
family of transcription factors, thus
preventing ARFs from activating
auxin-responsive genes (Tiwari et al.
2004). Upon an auxin stimulus, an
SCF (SKP1/Cullin/F-box protein)
ubiquitin ligase (Deshaies 1999)
containing the TIR1 F-box protein
ubiquitinates the Aux/IAA proteins,
marking them for degradation by the
26S proteasome thereby de-repressing
the response pathway (Gray et al.
2001). The hormone promotes the
Aux/IAA–TIR1 interaction; however,
the molecular mechanisms behind
this regulation are unclear. Most yeast
and animal SCF substrates must be
post-translationally modifi ed, usually
by phosphorylation, before they are
recognized by their cognate F-box
protein. Despite numerous efforts to
identify auxin-induced modifi cation of
Aux/IAA proteins, no such signal has
been discovered, raising the distinct
possibility that auxin uses a novel
mechanism to regulate SCF–substrate
interactions.
Ethylene and cytokinin are both
perceived by receptors sharing
similarity to bacterial two-component
regulators. Common in prokaryotes,
but apparently restricted to plants and
fungi in eukaryotes, these modular
signaling systems involve a membrane-
bound receptor containing an
intracellular histidine kinase (HK)
domain (Wolanin et al. 2002). Ligand
binding activates the kinase, resulting
in autophosphorylation and initiation
of a series of phosphotransfer reactions
that culminates with the activation
of a response regulator protein that
functions as the effector component
of the pathway. Cytokinin signaling
appears to largely follow this paradigm
(Kakimoto 2003). Ethylene response,
however, appears more complex (Guo
and Ecker 2004).
Ethylene is perceived by a family
of fi ve receptors. ETR1 and ERS1
contain a consensus HK domain,
however, the HK domains of ETR2,
ERS2, and EIN4 are degenerate and
lack elements necessary for catalytic
activity. This fact, together with studies
of “kinase-dead” mutants of ETR1,
suggests that HK activity is not required
for ethylene response. Mutations that
abolish ethylene binding in any of the
fi ve receptor genes are dominant and
confer ethylene insensitivity, indicating
that the receptors function as negative
regulators of the ethylene pathway.
Genetic and molecular studies have
positioned these receptors upstream
of the Raf-like MAP kinase kinase
kinase, CTR1, which interacts with the
receptors and also acts as a negative
regulator (Figure 3). The integral
membrane protein, EIN2, and the
transcription factors EIN3 and EIL1
are positive regulators of ethylene
signaling downstream of CTR1.
Current models propose that hormone
binding inactivates the receptors,
thus resulting in down-regulation of
CTR1 activity. Since the identifi cation
of CTR1, biologists have speculated
that a MAP kinase cascade may be
involved. Only recently, however, have
putative MAP kinase kinase and MAP
kinase components of the ethylene
pathway been identifi ed (Chang 2003).
Interestingly, these kinases appear to
positively regulate ethylene response,
suggesting that CTR1 must inhibit their
function. If so, this would represent
a novel twist on the traditional MAP
kinase signaling paradigm. Precisely
September 2004 | Volume 2 | Issue 9 | e311
DOI: 10.1371/journal.pbio.0020311.g002
Figure 2. The Ubiquitin-Mediated Proteolysis of Aux/IAA Proteins Regulates Auxin Response
(A) Wild-type Arabidopsis thaliana and the axr2-1 mutant. axr2-1 is a dominant gain-
of-function mutation in an Aux/IAA gene that confers reduced auxin response. The
mutant axr2-1 protein constitutively represses auxin response because it cannot be
targeted for proteolysis by the SCF
TIR1
ubiquitin ligase. The effect of the mutation
on AXR2 stability is shown in a pulse-chase experiment (inset). Wild-type and
axr2-1 seedlings were labeled with
35
S-methionine and AXR2/axr2-1 protein was
immunoprecipitated either immediately after the labeling period (t = 0) or following a
15-minute chase with unlabeled methionine (t = 15).
(B) A simplifi ed model for auxin response. In the absence of an auxin stimulus, Aux/
IAA proteins inhibit ARF transcriptional activity by forming heterodimers. Auxin
perception (by an unknown receptor) targets the Aux/IAA proteins to the SCF
TIR1
complex, resulting in their ubiquitination and degradation, thereby de-repressing the
ARF transcription factors. Among the ARF targets are the Aux/IAA genes themselves,
which produce nascent Aux/IAA proteins that restore repression upon the pathway in a
negative feedback loop.
PLoS Biology | www.plosbiology.org 1272
how the ethylene signal is transduced
to the EIN3 and EIL1 transcription
factors remains unclear. However,
the recent fi nding that ethylene
stabilizes these transcription factors,
which are targeted for degradation
by an SCF complex in the absence of
ethylene, clearly indicates a role for
the ubiquitin pathway (Guo and Ecker
2003; Potuschak et al. 2003). One of
the known targets for EIN3 is the ERF1
transcription factor, which activates
several genes involved in a subset of
ethylene responses.
Signal Integration and
Combinatorial Control
Long ago, plant physiologists noted
the apparent antagonistic interactions
between some of the phytohormones,
such as between auxin and cytokinin
in the regulation of root–shoot
differentiation and between GA and
ABA in germination. Other processes
are synergistically regulated by multiple
hormones. While it has long been
obvious that hormones do not function
in discrete pathways, but rather
exhibit extensive cross-talk and signal
integration with each other and with
environmental and developmental
signaling pathways, the molecular basis
for such coordinated regulation has
been unclear. Several recent fi ndings
have begun to elucidate the molecular
details of some of these events.
One example of such signal
integration was recently described
for the ethylene and JA pathways
(Lorenzo et al. 2003). Genetic studies
had previously implicated both
hormones as important regulators
of pathogen defense responses, as
well as of the wounding response
and other stress-related pathways.
Additionally, microarray analysis has
identifi ed a large number of genes
that are responsive to both hormones.
The ERF1 transcription factor was
recently found to be an intersection
point for these two signaling pathways
(Lorenzo et al. 2003). Like ethylene,
JA rapidly induces ERF1 expression,
and treatment with both hormones
synergistically activates ERF1. Induction
of ERF1 by both hormones alone or in
combination is dependent upon both
signaling pathways, and constitutive
overexpression of ERF1 rescues the
defense-response defects of both
ethylene- and JA-insensitive mutants.
These fi ndings suggest that ERF1
represents one of the fi rst signaling
nodes identifi ed in the complex web of
hormonal cross-talk.
The auxin and BR pathways also
appear to converge and mutually
regulate some developmental
processes. Both hormones promote cell
expansion, and microarray studies have
revealed that as many as 40% of all BR-
induced genes are also up-regulated by
auxin (Goda et al. 2004; Nemhauser
et al. 2004). BR is perceived by the cell
surface receptor kinase BRI1 (Wang
and He 2004). The SHAGGY/GSK3-
type kinase BIN2 acts as a negative
regulator of the pathway downstream
of the receptor. In the absence of a
BR signal, BIN2 phosphorylates the
transcription factors BES1 and BZR1,
targeting them for proteolysis by the
26S proteasome. Upon a BR stimulus,
BIN2 is inactivated, allowing BES1 and
BZR1to accumulate in the nucleus,
where they are presumably involved in
regulating BR-responsive genes.
Using combined genetic,
physiological, and genomic
approaches, Nemhauser and colleagues
(2004) were able to demonstrate that
auxin and BR regulate Arabidopsis
hypocotyl (embryonic stem) elongation
in a synergistic and interdependent
fashion. Elevating endogenous auxin
levels rendered plants more sensitive to
BR application in hypocotyl elongation
assays, and this response was dependent
upon both the auxin and BR signaling
pathways. Genetic studies suggest that
the convergence of these two pathways
occurs at a late point in hormone
signaling, perhaps at the promoters
of the many genes responsive to both
hormones. In support of this notion,
bioinformatic analysis identifi ed
distinct sequence elements that were
enriched specifi cally in the promoters
of auxin-induced, BR-induced, and
auxin/BR-induced genes.
Many Unanswered Questions
While great strides have been made
in recent years in understanding the
molecular basis of phytohormone
action, many fundamental questions
remain. Receptors and other upstream
signaling components remain to be
identifi ed for the majority of the
phytohormones. Equally important
are the elucidation of hormonal
networks and the integration of these
networks with the morphogenetic
program, such that our understanding
of hormone action can be placed in a
developmental context.
Acknowledgments
The author wishes to thank members
of his lab for helpful comments on
this manuscript. Work in the author’s
September 2004 | Volume 2 | Issue 9 | e311
DOI: 10.1371/journal.pbio.0020311.g003
Figure 3. A Model for the Arabidopsis Ethylene
Response Pathway
Ethylene is perceived by a family of
two-component receptors containing a
consensus (unshaded) or degenerate
(shaded) HK domain (H). Three of
the receptors also contain a C-terminal
receiver domain (R). The receptors
negatively regulate ethylene response
together with CTR1 in a complex on
the endoplasmic reticulum membrane.
Perception results in reduced receptor
and CTR1 activities and activation of
a MAP kinase kinase, which transmits
the signal through the EIN2 membrane
protein, ultimately resulting in the
activation of a transcriptional cascade
in the nucleus. The EIN3 and EIL1
transcription factors regulate primary
response genes including ERF1, which
activates a subset of secondary ethylene-
induced genes involved in defense
responses. EIN3/EIL1 abundance is
regulated in an ethylene-dependent
manner by SCF complexes containing
F-box proteins encoded by the ethylene-
induced genes EBF1 and EBF2. Positive-
and negative-acting components of the
pathway are indicated in green and
red, respectively. Solid lines indicate
regulation that is likely to be through
direct interactions. Dotted lines indicate
speculative interactions based on genetic
studies.
PLoS Biology | www.plosbiology.org 1273
laboratory on auxin response is
supported by National Institutes of
Health grant GM067203 and the
Mcknight Foundation.
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