Update on Apical Dominance and Shoot Branching Apical Dominance and Shoot Branching. Divergent Opinions or Divergent Mechanisms?

Apical dominance is the term used to describe the control of the shoot tip over axillary bud outgrowth (e.g. Cline, 1997). It is best demonstrated via shoot tip removal (decapitation), which leads to apical dominance. Indeed, decapitation has been widely used to study bud outgrowth. In contrast, branching may also occur in the presence of a vigorous shoot tip and be modulated by signals emanating from the root and stem. Whereas the term apical dominance can be used to describe branching phenotypes, this may not be meaningful in cases where shoot branching is not mediated predominantly by the shoot tip. Moreover, different hypotheses of branching control may be due to different experimental systems and techniques rather than divergent mechanisms of control between species. Three hypotheses continue to arise that involve a role for the plant hormone auxin. The classical hypothesis states that auxin acts to regulate shoot branching in conjunction with secondary messengers, such as cytokinin (Sachs and Thimann, 1967; Bangerth, 1994; Li et al., 1995). The auxin transport hypothesis proposes that regulatory control is exerted by auxin movement in the auxin transport stream, as opposed to the actual level of auxin (Morris, 1977; Bangerth, 1989; Li and Bangerth, 1999). The bud transition hypothesis postulates that the bud enters different developmental stages that have varying degrees of sensitivity or responses to long-distance signals, including auxin (Stafstrom and Sussex, 1992; Shimizu-Sato and Mori, 2001; Morris et al., 2005). Here, we address these hypotheses and propose that several components of each can be incorporated into one model of shoot branching. Different shoot-branching hypotheses have arisen from experiments that have varied largely in terms of whether they focus on intact, decapitated, or in vitro systems. Although decapitated and in vitro systems are excellent tools for studying apical dominance, the differences between these and intact systems are substantial, giving good reason to consider that branching control may differ in each case. Under natural conditions, plant species with a poor response to decapitation would be likely to have a selective disadvantage. Therefore, apical dominance, however mediated, may have evolved to ensure that bud outgrowth occurs after decapitation, allowing the plant to complete its life cycle. In contrast, branches of intact plants with vigorous shoot tip growth are generally not essential for the life cycle, but rather serve to enhance vegetative proliferation and/or to generate multiple sites for seed production. Excessive branching may be costly and hence branching, especially in intact plants, is likely to be carefully modulated in response to environmental factors, such as light quality, nitrogen and carbon availability, and growth and development of other plant parts. Consequently, it is reasonable to expect that different regulatory systems may have evolved to control branching after decapitation compared with that in intact plants. It is therefore important to distinguish experimental systems that focus on apical dominance using decapitation and in vitro techniques from those that evaluate branching control using intact plants. A further consideration for decapitation and in vitro studies is that these techniques not only involve the removal of a substantial source of auxin, but also the removal of a major sink for nutrients and energy, in addition to invoking stress and biophysical responses not observed in intact plants. If branching in decapitated or in vitro systems is regulated somewhat differently from branching in intact plants, it is reasonable to expect that studies focused on one or the other will lead to different viewpoints (Napoli et al., 1999). Often, the experimental approach implemented (decapitated, in vitro, or intact) is dictated by the plant species studied. For example, two species that have contributed widely to our current understanding of apical dominance, pea (Pisum sativum) and Arabidopsis (Arabidopsis thaliana), have vastly different phenotypes (Fig. 1). Pea plants have long internodes separating their axillary buds from one another and the shoot tip. This phenotype has allowed for experiments using a wide range of systems, primarily during the vegetative phase. In contrast, research using Arabidopsis has predominantly focused on in vitro studies following floral transition, largely due to constraints imposed by its vegetative 1 This work was supported by an Australian Postgraduate Award (scholarship to E.A.D.) and the Australian Research Council. 2 These authors contributed equally to the paper. * Corresponding author; e-mail c.beveridge@uq.edu. au; fax 61–7– 3365–3550. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Christine Anne Beveridge (c.beveridge@uq.edu.au). www.plantphysiol. org/cgi/doi/10.1104/pp.106. 086868