Hormonal control of shoot branching

Shoot branching is the process by which axillary buds, located on the axil of a leaf, develop and form new flowers or branches. The process by which a dormant bud activates and becomes an actively growing branch is complex and very finely tuned. Bud outgrowth is regulated by the interaction of environmental signals and endogenous ones, such as plant hormones. Thus these interacting factors have a major effect on shoot system architecture. Hormones known to have a major influence are auxin, cytokinin, and a novel, as yet chemically undefined, hormone. Auxin is actively transported basipetally in the shoot and inhibits bud outgrowth. By contrast, cytokinins travel acropetally and promote bud outgrowth. The novel hormone also moves acropetally but it inhibits bud outgrowth. The aim of this review is to integrate what is known about the hormonal control of shoot branching in Arabidopsis, focusing on these three hormones and their interactions.     

Auxin, cytokinin and the control of shoot branching

BACKGROUND: It has been known for many decades that auxin inhibits the activation of axillary buds, and hence shoot branching, while cytokinin has the opposite effect. However, the modes of action of these two hormones in branching control is still a matter of debate, and their mechanisms of interaction are equally unresolved. SCOPE: Here we review the evidence for various hypotheses that have been put forward to explain how auxin and cytokinin influence axillary bud activity. In particular we discuss the roles of auxin and cytokinin in regulating each other's synthesis, the cell cycle, meristem function and auxin transport, each of which could affect branching. These different mechanisms have implications for the main site of hormone action, ranging from systemic action throughout the plant, to local action at the node or in the bud meristem or leaves. The alternative models have specific predictions, and our increasing understanding of the molecular basis for hormone transport and signalling, cell cycle control and meristem biology is providing new tools to enable these predictions to be tested.     

MAX1 and MAX2 control shoot lateral branching in Arabidopsis

Plant shoots elaborate their adult form by selective control over the growth of both their primary shoot apical meristem and their axillary shoot meristems. We describe recessive mutations at two loci in Arabidopsis, MAX1 and MAX2, that affect the selective repression of axillary shoots. All the first order (but not higher order) axillary shoots initiated by mutant plants remain active, resulting in bushier shoots than those of wild type. In vegetative plants where axillary shoots develop in a basal to apical sequence, the mutations do not clearly alter node distance, from the shoot apex, at which axillary shoot meristems initiate but shorten the distance at which the first axillary leaf primordium is produced by the axillary shoot meristem. A small number of mutant axillary shoot meristems is enlarged and, later in development, a low proportion of mutant lateral shoots is fasciated. Together, this suggests that MAX1 and MAX2 do not control the timing of axillary meristem initiation but repress primordia formation by the axillary meristem. In addition to shoot branching, mutations at both loci affect leaf shape. The mutations at MAX2 cause increased hypocotyl and petiole elongation in light-grown seedlings. Positional cloning identifies MAX2 as a member of the F-box leucine-rich repeat family of proteins. MAX2 is identical to ORE9, a proposed regulator of leaf senescence ( Woo, H. R., Chung, K. M., Park, J.-H., Oh, S. A., Ahn, T., Hong, S. H., Jang, S. K. and Nam, H. G. (2001) Plant Cell 13, 1779-1790). Our results suggest that selective repression of axillary shoots involves ubiquitin-mediated degradation of as yet unidentified proteins that activate axillary growth.     

The control of shoot branching: an example of plant information processing

Throughout their life cycle, plants adjust their body plan to suit the environmental conditions in which they are growing. A good example of this is in the regulation of shoot branching. Axillary meristems laid down in each leaf formed from the primary shoot apical meristem can remain dormant, or activate to produce a branch. The decision whether to activate an axillary meristem involves the assessment of a wide range of external environmental, internal physiological and developmental factors. Much of this information is conveyed to the axillary meristem via a network of interacting hormonal signals that can integrate inputs from diverse sources, combining multiple local signals to generate a rich source of systemically transmitted information. Local interpretation of the information provides another layer of control, ensuring that appropriate decisions are made. Rapid progress in molecular biology is uncovering the component parts of this signalling network, and combining this with physiological studies and mathematical modelling will allow the operation of the system to be better understood.     

Regulation of shoot branching by auxin

The idea that apically derived auxin inhibits shoot branching by inhibiting the activity of axillary buds was first proposed 70 years ago, but it soon became clear that its mechanism of action was complex and indirect. Recent advances in the study of axillary bud development and of auxin signal transduction are allowing a better understanding of the role of auxin in controlling shoot branching. These studies have identified a new role for auxin early in bud development as well as some of the second messengers involved in mediating the branch-inhibiting effects of auxin.     

The molecular and genetic regulation of shoot branching

The architecture of flowering plants exhibits both phenotypic diversity and plasticity, determined, in part, by the number and activity of axillary meristems and, in part, by the growth characteristics of the branches that develop from the axillary buds. The plasticity of shoot branching results from a combination of various intrinsic and genetic elements, such as number and position of nodes and type of growth phase, as well as environmental signals such as nutrient availability, light characteristics, and temperature (Napoli et al., 1998; Bennett and Leyser, 2006; Janssen et al., 2014; Teichmann and Muhr, 2015; Ueda and Yanagisawa, 2019). Axillary meristem initiation and axillary bud outgrowth are controlled by a complex and interconnected regulatory network. Although many of the genes and hormones that modulate branching patterns have been discovered and characterized through genetic and biochemical studies, there are still many gaps in our understanding of the control mechanisms at play. In this review, we will summarize our current knowledge of the control of axillary meristem initiation and outgrowth into a branch.

The key regulatory genes and the role of multiple plant hormones coordinate the process of axillary meristem initiation and subsequent growth into a branch.     

Overexpression of a NAC-domain protein promotes shoot branching in rice

For a better understanding of shoot branching in rice (Oryza sativa), a rice activation-tagging library was screened for mutations in tiller development. Here, an activation-tagging mutant Ostil1 (Oryza sativa tillering1) was characterized, which showed increased tillers, enlarged tiller angle and semidwarf phenotype. Flanking sequence was obtained by plasmid rescue. RNA-interfering and overexpression transgenic rice plants were produced using Agrobacterium-mediated transformation. The mutant phenotype was cosegregated with the reallocation of Ds element, and the flanking region of the reallocated Ds element was identified as part of the OsNAC2 gene. Northern analysis showed that expression of OsNAC2 was greatly induced in the mutant plants. Transgenic rice overexpressing the OsNAC2 resulted in recapture of the mutant phenotype, while downregulation of OsNAC2 in the Ostil1 mutant through RNA interfering (RNAi) complemented the mutant phenotype, confirming that the Ostil1 was caused by overexpression of OsNAC2. Overexpression of OsNAC2 regulates shoot branching in rice. Overexpression of OsNAC2 contributes tiller bud outgrowth, but does not affect tiller bud initiation. This suggests that OsNAC2 has potential utility for improving plant structure for higher light-use efficiency and higher yield potential in rice.     

Understanding shoot branching by modelling form and function

Shoot branching plays a pivotal role in the development of the aboveground plant structure. Therefore, to understand branching in relation to the environment, it is not only necessary to integrate the knowledge on mechanisms that regulate branching at multiple levels of biological organisation, but also to include plant structure explicitly. To this end, we propose the application of an established methodology called functional-structural plant modelling.     

Enhancements in sucrose biosynthesis capacity affect shoot branching in Arabidopsis

We previously demonstrated that transgenic tobacco plants expressing cyanobacterial fructose-1,6-/sedoheptulose-1,7-bisphosphatase in the cytosol increased the number of lateral shoots and leaves at elevated CO₂ levels. These findings suggest that alterations in carbon partitioning affect the development of shoot branching. In order to elucidate the underlying mechanisms at the molecular level, we generated transgenic Arabidopsis plants overexpressing cyanobacterial fructose-1,6-bisphosphatase-II in the cytosol (AcF). At elevated CO₂ levels, the number of lateral shoots was significantly increased in AcF plants. Sucrose and hexose levels were also higher in AcF plants than in wild-type plants. The expression levels of MAX1, MAX4, YUCCA8, YUCCA9, and BRC1, which are involved in auxin or strigolactone biosynthesis and responses, were lower in AcF plants than in wild-type plants. These results suggest that alterations in sugar partitioning affect hormone metabolism and responses, resulting in enhanced shoot branching.     

The histone methyltransferase SDG8 regulates shoot branching in Arabidopsis

Histone lysine methylation is an evolutionally conserved modification involved in determining chromatin states associated with gene activation or repression. Here we report that the Arabidopsis SET domain group 8 (SDG8) protein is a histone H3 methyltransferase involved in regulating shoot branching. Knockout mutations of the SDG8 gene markedly reduce the global levels of histone H3 trimethylation at lysines 9 and 36 as well as dimethylation at lysine 36. The sdg8 mutants produce more shoot branches than wild-type plants. The expression of SPS/BUS (supershoot/bushy), a repressor of shoot branching, is decreased in sdg8 mutants, while UGT74E2 (UDP-glycosyltransferase 74E2), a gene associated with increased shoot branching, is up-regulated in sdg8 mutants. The altered expression of SPS/BUS and UGT74E2 correlates with changed histone H3 methylation at these loci. These results suggest that SDG8 regulates shoot branching via controlling the methylation states of its target genes.