In many plant species only a small proportion of buds yield branches. Both the timing and extent of bud activation are tightly regulated to produce specific branching architectures. For example, the primary shoot apex can inhibit the activation of lateral buds. This process is termed apical dominance and is dependent on the plant hormone auxin moving down the main stem in the polar auxin transport stream. We use a computational model and mathematical analysis to show that apical dominance can be explained in terms of an auxin transport switch established by the temporal precedence between competing auxin sources. Our model suggests a mechanistic basis for the indirect action of auxin in bud inhibition and captures the effects of diverse genetic and physiological manipulations. In particular, the model explains the surprising observation that highly branched Arabidopsis phenotypes can exhibit either high or low auxin transport.auxin transport canalization ͉ dynamic system ͉ MAX ͉ shoot branching ͉ simulation model

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.

Expansins are a group of extracellular proteins that directly modify the mechanical properties of plant cell walls, leading to turgor-driven cell extension. Within the completely sequenced Arabidopsis genome, we identified 38 expansin sequences that fall into three discrete subfamilies. Based on phylogenetic analysis and shared intron patterns, we propose a new, systematic nomenclature of Arabidopsis expansins. Further phylogenetic analysis, including expansin sequences found here in monocots, pine (Pinus radiata, Pinus taeda), fern (Regnellidium diphyllum, Marsilea quadrifolia), and moss (Physcomitrella patens) indicate that the three plant expansin subfamilies arose and began diversifying very early in, if not before, colonization of land by plants. Closely related "expansin-like" sequences were also identified in the social amoeba, Dictyostelium discoidium, suggesting that these wall-modifying proteins have a very deep evolutionary origin.The availability of information from genome sequencing programs now offer a new route to understanding multigene families within and across different species. Several recent studies have demonstrated the usefulness of phylogenetic analysis to complement parallel investigations of gene function in vivo (Sanderfoot et al., 2000; Kellogg, 2001; Li et al., 2001; Ross et al., 2001). The present analysis makes use of the completely sequenced Arabidopsis genome (The Arabidopsis Genome Initiative, 2000), together with comprehensive searches of GenBank and expressed sequence tag (EST) databases (maintained at the National Center for Biotechnology Information, NCBI), to determine the phylogeny of the plant cell wall protein, expansin.Expansins play a variety of roles in vivo by modifying the cell wall matrix during growth and development (for review, see Cosgrove, 2000a; Darley et al., 2001). Initially identified by their unique ability to induce the pH-dependent extension of plant cell walls in vitro (McQueen-Mason et al., 1992), expansins appear to increase polymer mobility in the cell wall, allowing the structure to slide apart during extension (McQueen-Mason et al., 1993; McQueenMason and Cosgrove, 1994, 1995; Whitney et al., 2000). To date, expansin remains the only protein to demonstrate cell wall extension in vitro and in vivo. In addition to roles in plant cell growth, expansins are also now believed to play key roles in the early development of leaf primordia (Fleming et al., 1997), fruit softening (Civello et al., 1999; Rose et al., 2000), plant reproduction (Cosgrove et al., 1997), and wall disassembly (Cho and Cosgrove, 2000).Growing tissues from a wide range of plants, including dicotyledons (Rayle and Cleland, 1977), grasses (Kutschera, 1994), gymnosperms (Kim et al., 2000), and green algae (Metraux and Taiz, 1977), have been shown to undergo acid-induced extension. As it is now generally accepted that expansins are the chief agents responsible for acid-induced extension, these data suggest that expansins may be found in all land plants and probably algae. In suppor...

The degree of shoot branching is strongly affected by environmental conditions, such as nutrient availability. Here we demonstrate that nitrate limitation reduces shoot branching in Arabidopsis (Arabidopsis thaliana) both by delaying axillary bud activation and by attenuating the basipetal sequence of bud activation that is triggered following floral transition. Ammonium supply has similar effects, suggesting that they are caused by plant nitrogen (N) status, rather than direct nitrate signaling. We identify increased auxin export from active shoot apices, resulting in increased auxin in the polar auxin transport stream of the main stem, as a likely cause for the suppression of basal branches. Consistent with this idea, in the auxin response mutant axr1 and the strigolactone biosynthesis mutant more axillary growth1, increased retention of basal branches on low N is associated with a failure to increase auxin in the main stem. The complex interactions between the hormones that regulate branching make it difficult to rule out other mechanisms of N action, such as up-regulation of strigolactone synthesis. However, the proposed increase in auxin export from active buds can also explain how reduced shoot branching is achieved without compromising root growth, leading to the characteristic shift in relative biomass allocation to the root when N is limiting.

Studies of apical dominance have benefited greatly from two-branch assays in pea and bean, in which the shoot system is trimmed back to leave only two active cotyledonary axillary branches. In these two-branch shoots, a large body of evidence shows that one actively growing branch is able to inhibit the growth of the other, prompting studies on the nature of the inhibitory signals, which are still poorly understood. Here, we describe the establishment of two-branch assays in Arabidopsis, using consecutive branches on the bolting stem. As with the classical studies in pea and bean, these consecutive branches are able to inhibit one another's growth. Not only can the upper branch inhibit the lower branch, but also the lower branch can inhibit the upper branch, illustrating the bi-directional action of the inhibitory signals. Using mutants, we show that the inhibition is partially dependent on the MAX pathway and that while the inhibition is clearly transmitted across the stem from the active to the inhibited branch, the vascular connectivity of the two branches is weak, and the MAX pathway is capable of acting unilaterally in the stem.