Control of Outgrowth and Dormancy in Axillary Buds

Abstract

The shoot system has an important role in generating a large variety of diverse plant forms (Steeves and Sussex, 1989). The overall architecture of the shoot system is derived from the activity of the primary shoot apical meristem (SAM), arising during embryogenesis, together with the activity of the additional meristems subsequently formed after seed germination. The primary SAM provides the main axis of the plant body. Plant architecture is further modified by shoot branching that results from the activity of the additional meristems. The complexity of the branching pattern depends on the temporal and spatial development of these branches. These characteristics, although they are plastic in their response to environmental cues, are genetically determined. The developmental program that specifies branching patterns in different plant species is fundamentally important for generating species-specific plant forms.The shoot branching process generally involves two developmental stages: the formation of axillary meristems in the leaf axils and the growth of axillary buds. In many plant species, the growth of axillary meristems is inhibited by the primary shoot or primary inflorescence. This phenomenon is generally known as apical dominance. The plant hormones auxin and cytokinin are thought to have a major role in controlling this process (Phillips, 1975;Cline, 1994;Tamas, 1995;Napoli et al., 1999). Auxin has an inhibitory effect on the growth of axillary buds, whereas cytokinin promotes axillary bud outgrowth. The mechanisms of axillary bud outgrowth depend on the ratio of these two hormones rather than the absolute levels of either hormone.A variety of experimental approaches have been used to examine the mechanisms controlling dormancy and outgrowth of axillary buds. These range from physiological studies, such as measurement and exogenous application of plant hormones, to analyses of transgenic plants overexpressing hormone biosynthetic genes to alter endogenous hormone levels. Isolation and characterization of mutations that cause alterations in shoot branching patterns are powerful approaches. These molecular genetic approaches combined with the conventional physiological studies, such as grafting experiments, revealed that not only do auxin and cytokinin function to control the growth of axillary buds, but other factors and/or signals also have important roles. More recently, the genes expressed in dormant axillary buds were isolated and characterized.This review focuses on recent findings uncovered by physiological, genetic, and molecular studies and approaches to investigate the control of shoot branching, apical dominance, and dormancy in plants. DEVELOPMENT AND POTENTIAL OF AXILLARY MERISTEMS

Auxin Acts in Xylem-Associated or Medullary Cells to Mediate Apical Dominance

“…However, more recent studies have suggested that cytokinin synthesized locally in the node may play a role in the regulation of branching. After decapitation, a pea adenylate IPT gene was found to be upregulated in nodes from which branches would grow out (Shimizu-Sato and Mori, 2001). Therefore the increased cytokinin content of uninhibited buds could be caused by the uptake of cytokinin synthesized within the node, with auxin acting to control uptake and/or synthesis.…”

Seed Production Affects Maternal Growth and Senescence in Arabidopsis

“…In Arabidopsis, dormancy occurs e.g. in axillary buds that are suppressed by apical dominance (Shimizu-Sato and Mori, 2001). Previous work has established patterns of gene expression changes in lateral buds upon release of apical dominance, and identified two expression markers that are highly expressed in dormant buds, the DORMANCY-ASSOCIATED PROTEIN1 and 2 (AtDRM1 and AtDRM2) genes (Tatematsu et al, 2005).…”

Photomorphogenesis in Dracaena draco

“…Also known as ''release of suppressed/axillary buds'', it comes with ageing, trauma and bending, affecting the tree growth form (e.g. Brown 1974;Shimizu-Sato and Mori 2001). It is envisaged that in a tree, active apical meristem releases auxin that inhibits the growth of lateral buds (apical dominance) so when the apex is removed (by flowering or pruning), the apical dominance stops and lateral buds awake (Brown 1974;Cline 1997)…”