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6 Apical dominance and vascularization
6.1 Organ communication
The hormonal signals that induce vascular differentiation are the controlling signals
that synchronize plant development, organ growth regulation, and feedback cross talks
between the shoot and the root. These signals regulate organ development by
promoting or inhibiting plant organ growth and therefore should be clarified in order to
understand the regulation of vascular differentiation. There is a continuous positive
hormonal feedback communication between the shoot apices and the root tips that
synchronizes plant development; each plant pole sends its growth promoting hormonal
signal to the opposite side of the plant, informing the other plant pole about its activity
and quantity. The major shoot signal is auxin produced in the apical bud and young
leaves; while the basic root tip signals produced in the root cap are cytokinins. Auxin
promotes the initiation and development of the roots, while cytokinins from the root tips
promote the growth and development of the shoot organs.
The auxin flows polarly from the young leaves to root tips and induces continuous
vascular tissues along the plant, forming the pathways for further signal flows; while
cytokinins from the root tips are transported upward, promotes bud and leave
development, cell division activity in the vascular system, activating the meristematic
cambium and secondary vascular differentiation along the plant (Sachs 1981; Aloni
1995; Scarpella and Helariutta 2010).
Conversely, as will be clarified below, due to inside organ competition, identical
organs may cause inhibition, when one of them becomes dominant and retards the
others, which is well studied in shoots, and known as shoot apical dominance.
6.2 Regulation of shoot apical dominance
Apical dominance is the control exerted by the apical bud over the outgrowth of the
lateral buds (Thimann and Skoog 1933; Sachs 1991), which is most easily
demonstrated by shoot decapitation that results in the prompt outgrowth of the axillary
buds.
(1) Auxin produced in the apical bud and its young leaves is the primary signal
that inhibits the growth of the axillaries. This phenomenon of apical bud control is
replaceable by auxin application after shoot decapitation (Thimann and
Skoog 1933, 1934; Thimann et al. 1971). The auxin produced in the shoot apical bud
induces well-developed vascular tissues which supply the apical bud; while the
inhibited lateral buds cannot develop their supporting vascular tissues and therefore
remain suppressed.
Understanding the control mechanism of apical dominance is important for
agriculture implications to determine plant architecture by pruning trees for increasing
fruit productivity and quality, or for ornamental purposes. Apical dominance is an
adaptive mechanism which gives the plant an advantage in competition for light, as it
enables plants to concentrate their energy, nutrients and growth to promote the
optimal fast growth of the main stem, and successfully compete with neighboring
plants. On the other hand, the release from apical dominance is a plant survival
mechanism, which enables recovery and regeneration of the shoot after a damage to
the main stem.
In addition to the classic model that the auxin indole-3-acetic acid is produced in
the shoot’s apical bud and transported down the stem, where it inhibits axillary bud
growth (Thimann and Skoog 1933, 1934), shoot apical dominance is also regulated by
the following major signals: strigolactone, cytokinin, gibberellin and sugar availability,
and is clarified below.
Shoot branching evolved independently in flowering plant sporophytes
and moss gametophytes (the haploid growth phase). Lateral branches of the
gametophytic shoots of the moss Physcomitrella, arise by re-specification of epidermal
cells into branch initials. Interestingly, like in sporophytic branching of higher plants,
the ancient hormonal signals, namely, auxin, cytokinin and strigolactone regulate
branch development in moss (Coudert et al. 2015). The size of the apical inhibition
zone significantly increased by an auxin application, the branch number was strongly
reduced and branch initiation in the branching zone significantly decreased,
suggesting that auxin can act as a global suppressor of shoot branching of mosses
and higher plants. However, the apical dominant mechanisms are different, lateral bud
inhibition in higher plants is regulated by PIN-mediated basipetal auxin transport,
whereas in the moss Physcomitrella, the mechanism is less developed and a bi-
directional transport occurs likely through plasmodesmata (Coudert et al. 2015).
(2) Strigolactone, is an important hormonal signal involved in promoting shoot
apical dominance by preventing axillary bud development. The strigolactone is a root
hormone which moves upward to the shoot apex through the xylem (Kohlen et al.
2011), where it inhibits axillary bud development by down-regulating the cytokinin
biosynthetic genes (IPT). Auxin from the apical bud promotes strigolactone activity in
nodes, where the strigolactone inhibits the activation of cytokinin producing genes (El-
Showk et al. 2013). Additionally, the strigolactone can inhibit axillary growth by
triggering rapid depletion of the auxin efflux protein PIN1 from the plasma membrane
of parenchyma cells, located between the axillary buds and the vascular system of the
stem (Shinohara et al. 2013), namely, preventing polar auxin flow from the axillary
buds to the stem, which prevents vascular development from the inhibited buds into
the stem.
(3) Cytokinin (CK) produced primary in root tips, is known to promote the
outgrowth of axillary buds (Sachs and Thimann 1967; Thimann et al. 1971). Tanaka et
al. (2006) demonstrated that auxin negatively regulates local CK biosynthesis in
the nodes along the stem by controlling the expression level of the pea (Pisum
sativum) gene adenosine phosphate-isopentenyltransferase (PsIPT), which encodes a
key enzyme in CK biosynthesis. Expression of PsIPT was repressed by the application
of the auxin IAA. These results indicate that in apical dominance one role of auxin is to
repress local biosynthesis of CK in the stem nodes. After decapitation, the CKs, which
are thought to be derived from the root caps, can also be locally synthesized in the
nodes along the stem, rather than only in the root tips. Müller et al. (2015) confirmed
that shoot decapitation promotes cytokinin biosynthesis in the stem, suggesting that
CK acts to overcome auxin-mediated bud inhibition, allowing buds to escape apical
dominance under favorable conditions, such as high nitrate availability; conditions
known to stimulate CK production in the root, which is transported upward to the shoot
(Geßler et al. 2004; Miyawaki et al. 2004) (see chapters 3.3 and 4).
(4) Gibberellin (GA) acts as a positive regulator in promoting shoot branching in
the woody plant Jatropha curcas and other trees like papaya, indicating that the
regulatory control of shoot branching in some perennial woody plants may be more
complicated. Ni et al. (2015) showed that GA and CK synergistically promote lateral
bud outgrowth. Treatment with paclobutrazol, an inhibitor of de novo GA biosynthesis,
significantly reduced the promotion of bud outgrowth by CK, suggesting that GA is
required for CK-mediated axillary bud outgrowth (Ni et al. 2015).
(5) Sugar availability regulates shoot apical dominance as has been proposed
by the auxin-directed nutrient hypothesis (see Cline 1991), suggesting that auxin
originating in the growing shoot apex directs nutrient transport to the actively growing
apex supplied with well-developed vascular tissues, but away from the inactive lateral
buds which do not grow out because of insufficient nutrients. This idea was supported
by Mason et al. (2014) findings in Pisum sativum showing that sugar demand, not
auxin, is the initial regulator of apical dominance; supporting the idea that apical
dominance is controlled by the shoot tip’s strong demand for sugars arriving via the
phloem, which inhibits axillary bud outgrowth by substantially limiting the amount of
sugar available for the axillary buds. After the loss of the shoot tip, sugars are rapidly
redistributed over large distances and accumulate in axillary buds within a timeframe
that correlates with bud release. Barbier et al. (2015) confirmed the importance of
sucrose as an early modulator of the hormonal mechanism controlling bud outgrowth
in Rosa hybrida shoots.
Aloni et al. (2003, 2006b) found that from their early developmental stage, the leaf
and flower primordia are loaded with conjugated auxin that can release the bioactive
IAA when the tissues above (or in flowers beside, see chapter 8.3) them mature or
experimentally removed. As will be discussed in chapters 7 and 8 on leaves and
flowers, the concept of shoot apical dominance can be applied to understand flower-
organ development and organized vascularization in leaves and flowers, as will be
clarify in the two following chapters (7 and 8).
6.3 Cytokinin-dependent root apical dominance
In tissue cultures, applications of elevated-auxin concentrations inhibit shoot-organ
formation and promote root development, while high-cytokinin concentrations inhibit
root formation and promote shoot development (Skoog and Miller 1965; Taiz and
Zeiger 2006; Evert and Eichhorn 2013). The auxin from the apical bud and young
leaves which inhibits shoot axillary buds is the promoting signal for root initiation and
growth (Casimiro et al. 2001; Bhalerao et al. 2002). On the other hand, with a similar
biological behavior, but in the opposite direction, the cytokinin produced in the main
root tip inhibits the initiation of lateral roots (Rani Debi et al. 2005; Laplaze et al.
2007; Chang et al. 2013; Márquez et al. 2019), and is the promoting signal for shoot
development (Aloni et al. 2006a). Therefore, the concept of apical dominance that
originally was developed for understanding shoot development and architecture
(Thimann and Skoog 1933, 1934) can properly be used for roots (Böttger 1974; Aloni
2006a) as a general concept for both shoots and roots.
Actively growing primary roots of dicot plants may exhibit root apical dominance,
where the primary root inhibits lateral root initiation (Zhang and Hasenstein
1999; Lloret and Casero 2002). The phenomenon of root apical dominance can be
compared with the well-known shoot apical dominance; in both cases the actively
growing leader inhibits lateral identical organ initiation and development (Aloni et al.
2006a). In lettuce (Lactuca sativa) seedlings, the removal of the primary root tip
stimulates rapid formation of lateral-root primordia, which normally are not produced
by the intact lettuce root (Zhang and Hasenstein 1999). In maize (Zea mays), cytokinin
application prevented lateral root initiation in the primary root initiation zone, and the
inhibitory effect of CK occurs in the earliest stages of lateral-root development
(Márquez et al. 2019).
Free bioactive CK can be visualized by the expression of ARR5::GUS (a CK-
activated promoter sequence of an Arabidopsis response regulator fused to β-
glucuronidase), which reflects the sites of the transcriptional activation of this CK-
sensitive promoter. The construct reacts with free bioactive CK in a concentration-
dependent manner (D'Agostino et al. 2000; Aloni et al. 2004, 2005). This construct
shows that the cap of the primary root of a soil-grown Arabidopsis plant produces
elevated concentrations of free bioactive CK (Fig. 6.1A), which is usually much higher
than free CK concentrations of lateral roots (Fig. 6.1B, C) (Aloni et al. 2005). When
primary root tips are excised, one or more lateral roots become dominant and their tip
may start to produce higher concentrations of free CK (Forsyth and Van Staden
1981; Lloret and Casero 2002).
Fig. 6.1
Root apical dominance may occur in wild-type plants with an actively growing
primary root that inhibits lateral root initiation by the root-cap-synthesized CK, and their
lateral roots develop further away from the tip of the main root. By contrast, a low CK
content in CK-deficient transgenic plants (overexpressing the CYTOKININ
OXIDASES/DEHYDROGENASES (CKX) genes that catalyze irreversible degradation
of the cytokinins; Werner et al. 2001, 2003), or almost CK insensitivity [in the double
and triple loss-of-function CK receptor, of the ahk (Arabidopsis histidine kinase)
mutants, which are almost insensitive to cytokinin], result in the formation of lateral
roots closer to the root tip and an increase in root branching (Schmülling 2002).
Emphasizing and extend the previously formulated concept of root apical dominance
(Böttger 1974; Zhang and Hasenstein 1999; Lloret and Casero 2002) by focusing on
CK mediation (Aloni et al. 2006a). The evidence that the root cap predominantly
produces CK by expressing the IPT genes (Miyawaki et al. 2004; Takei et al. 2004;
Sakakibara et al. 2006; Ruffel et al. 2011) and that the highest concentration of free
CK in a root is found in the root tip (Aloni et al. 2004, 2005) justify a more precise term
for this instance of apical dominance, namely, cytokinin-dependent root apical
dominance. From an ecological point of view, this CK-dependent root apical
dominance gives priority to the primary root in competition with its own lateral roots as
well as neighboring root systems and enables the main root to reach water in deeper
soil layers faster, which might be vital for plants before the dry season. CK regulates
root architecture by balancing the promoting role of IAA on lateral root development
(see chapter 9). CK produced in the active root cap (see chapter 2, Fig. 2.2B) of a
primary root is the hormonal signal which enables maximum development of an
actively growing primary root by retarding lateral root initiation (see chapter 9, Fig.
9.2). This reduces the quantity of lateral roots, their development and requirements of
which would be at the expense of the primary root growth.
Summary
A continuous positive hormonal feedback communication occurs between the
auxin-producing young leaves and the cytokinin-producing root tips that
synchronize plant development. Auxin promotes the development of the root
system, and cytokinin stimulates the growth of the shoot organs.
Actively growing leader, of either the shoot apical bud or the primary root tip,
inhibits lateral identical organ initiation and development, which prevents
competition of similar organ. This behavior of the leader stimulated the developed
of the apical dominance concept.
Shoot apical dominance is the control imposed by the apical bud to inhibit the
outgrowth of lateral buds. Auxin produced in the apical bud is the primary signal
that prevents the development of axillary buds.
In addition to the primary role of auxin in inhibiting axillary buds, shoot apical
dominance is also regulated by strigolactone, cytokinin, gibberellin and sugar
availability.
Root apical dominance is regulated by the production of high-concentrations of
cytokinin in the root cap of the primary root, which inhibits the initiation and growth
of lateral roots.
References and recommended reading
Papers of particular interest for suggested reading have been highlighted (with *)
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Aloni R, Aloni E, Langhans M, Ullrich CI (2006b) Role of auxin in regulating
Arabidopsis flower development. Planta 223: 315-328.
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of root gravitropism. Planta 220: 177-182.
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vascular differentiation and leaf morphogenesis in Arabidopsis. Planta
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Legend for the Figure
Fig. 6.1 GUS expression patterns visualizing free cytokinin (CK)
in the root tip of CK-responsive ARR5::GUS transformant of
Arabidopsis thaliana plants grown protected from wind,
demonstrating the high-CK concentrations in the main root
compared with much lower CK production in the lateral roots. A,
main root of soil-grown wind-protected plant at 35 days after
germination, with strong ARR5::GUS expression reflecting
massive CK production and accumulation due to wind protection
in the entire elongating zone (arrow), in the vascular cylinder,
cortex, epidermis and root hairs. The concentration of GUS
expression in the cortex decreases gradually. B and C, lateral
roots and main root grown on MS basal medium in closed boxes.
B, Due to typically low CK production in a lateral root,
ARR5::GUS expression was only observed in the root cap, with
CK export restricted to the base of the central vascular tissue
(arrowhead). The periphery of the root cap was almost free of
GUS expression (arrow). C, CK production in the tip of a lateral
root (short arrow), and almost absent from its vascular cylinder,
cortex and epidermis (arrowhead); while strong CK accumulation
is observe along the entire axis of the main root (large arrow).
Bars = 25 µm (B), 50 µm (A, C), from Aloni et al. (2005).
