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 domi- nance. 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 mean- ingful in cases where shoot branching is not mediated predominantly by the shoot tip. Moreover, different hypotheses of branching control may be due to differ- ent 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 cyto-