Shoot branching

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Abstract

The mature form of a plant shoot system is an expression of several genetically controlled traits, many of which are also environmentally regulated. A major component of this architectural variation is the degree of shoot branching. Recent results indicate conserved mechanisms for shoot branch development across the monocots and eudicots. The existence of a novel long-range branch-inhibiting signal has been inferred from studies of branching mutants in pea and Arabidopsis.

Introduction

Changes in plant architecture have been central to the domestication of wild species; for example, the ‘Green Revolution’ saw the introduction of wheat varieties that had modified architecture that increased yield and improved resistance to wind and rain damage [1]. The architecture of the shoot system affects a plant’s light-harvesting potential, the synchrony of flowering and seed set, and ultimately the reproductive success a plant. Flowering plants display a remarkable range of inflorescence architectures, which are defined by the degree of branching, internodal elongation and shoot determinancy. In this review, we discuss only the role of axillary meristems in influencing shoot architecture.

Shoot branches arise from axillary shoot meristems, which form in the axils of leaves on the primary shoot axis. After initiation, axillary meristems produce a few leaves to form a bud. At this stage, the bud may either develop as a vegetative branch immediately or remain dormant indefinitely until outgrowth is triggered. Hence, the degree of branching depends not only on the establishment of an axillary meristem but also upon its subsequent activity. In this review, we focus on recent studies that illustrate the importance of auxin, cytokinin and an as yet unidentified substance in regulating axillary meristem activity. We also discuss the genetic regulation of axillary meristem initiation, highlighting recent studies that have identified common components from eudicots and monocots.

Section snippets

Axillary meristem initiation

Mutants that have altered patterns of shoot branching have been described in several species, including tomato, pea, maize and Arabidopsis. They fall into three classes on the basis of whether they affect meristem initiation (e.g. revoluta, pinhead, lateral suppressor [ls] and blind/torosa), meristem outgrowth (e.g. more axillary growth [max], ramosus [rms] and decreased apical dominance [dad]) or both (e.g. supershoot/bushy and Teosinte branched1 [Tb1]) [2]. In this section, we highlight the

Axillary meristem activity

During vegetative development in Arabidopsis, axillary meristems are initiated and released in an acropetal gradient at a distance from the SAM. The converse is true following floral transition, when axillary meristems are released in a basipetal sequence in close proximity to the SAM 14., 15.. Arabidopsis has a monopodial growth habit, which is characterised by the SAM’s remaining active throughout the life span of the plant. In other species, such as tomato and Petunia, a sympodial growth

Site of auxin action

The weight of evidence shows that auxin does not act directly in the bud to inhibit outgrowth. It must, therefore, act elsewhere to control the synthesis, transport or metabolism of one or more second messengers. Two sites for auxin action have been proposed in two recent studies. First, auxin resistant1 (axr1) mutations result in increased shoot branching [21]. The axr1-12 mutation promotes the growth of axillary buds. Analysis of the response of axr1-12 buds to apically applied auxin in

Second messengers for auxin action

In addition to several potential sites of action for auxin, there are several proposed second messengers that could relay the auxin signal into the bud. Each of these is also likely to have auxin-independent roles in regulating bud growth. The possible role of cytokinin has already been mentioned above. It is clear that cytokinin can promote bud outgrowth; for example, the application of basal cytokinin can overcome the effects of apical auxin in the Arabidopsis excised node assay [19].

Conclusions

This review focusses on recent developments in understanding the control of shoot branching. Two major themes are apparent. First, it is clear that the use of a range of different approaches and techniques offers the best opportunity for progress. Grafting techniques and classical physiological approaches have been successfully combined with molecular genetic and transgenic approaches. This trend will no doubt continue with the addition of high-throughput genomics approaches, which have already

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • of special interest

  • ••

    of outstanding interest

Acknowledgements

We would like to acknowledge funding from the Biotechnology and Biological Sciences Research Council (BBSRC) of the UK. We thank members of the Leyser lab for critical reading of the manuscript and helpful discussions.

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