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.