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Apical Dominance and Shoot Branching

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Apical Dominance and Shoot Branching Update on Apical Dominance and Shoot Branching Apical Dominance and Shoot Branching. Divergent Opinions or Divergent Mechanisms?1 Elizabeth Ann Dun2, Brett James Ferguson2, and Christine Anne Beveridge* Australian Research Council Centre of Excellence for Integrative Legume Research and School of Integrative Biology, The University of Queensland, St. Lucia, Queensland 4072, Australia Apical dominance is the term used to describe the differences between these and intact systems are sub- control of the shoot tip over axillary bud outgrowth stantial, giving good reason to consider that branching (e.g. Cline, 1997). It is best demonstrated via shoot tip control may differ in each case. Under natural condi- removal (decapitation), which leads to apical domi- tions, plant species with a poor response to decapita- nance. Indeed, decapitation has been widely used to tion would be likely to have a selective disadvantage. study bud outgrowth. In contrast, branching may also Therefore, apical dominance, however mediated, may occur in the presence of a vigorous shoot tip and be have evolved to ensure that bud outgrowth occurs modulated by signals emanating from the root and stem. after decapitation, allowing the plant to complete its Whereas the term apical dominance can be used to life cycle. In contrast, branches of intact plants with describe branching phenotypes, this may not be mean- vigorous shoot tip growth are generally not essential ingful in cases where shoot branching is not mediated for the life cycle, but rather serve to enhance vegetative predominantly by the shoot tip. Moreover, different proliferation and/or to generate multiple sites for seed hypotheses of branching control may be due to differ- production. Excessive branching may be costly and ent experimental systems and techniques rather than hence branching, especially in intact plants, is likely to divergent mechanisms of control between species. be carefully modulated in response to environmental Three hypotheses continue to arise that involve a role factors, such as light quality, nitrogen and carbon avail- for the plant hormone auxin. The classical hypothesis ability, and growth and development of other plant states that auxin acts to regulate shoot branching in parts. Consequently, it is reasonable to expect that dif- conjunction with secondary messengers, such as cyto- ferent regulatory systems may have evolved to control kinin (Sachs and Thimann, 1967; Bangerth, 1994; Li branching after decapitation compared with that in et al., 1995). The auxin transport hypothesis proposes intact plants. It is therefore important to distinguish that regulatory control is exerted by auxin movement experimental systems that focus on apical dominance in the auxin transport stream, as opposed to the actual using decapitation and in vitro techniques from those level of auxin (Morris, 1977; Bangerth, 1989; Li and that evaluate branching control using intact plants. A Bangerth, 1999). The bud transition hypothesis postu- further consideration for decapitation and in vitro lates that the bud enters different developmental stages studies is that these techniques not only involve the that have varying degrees of sensitivity or responses to removal of a substantial source of auxin, but also the long-distance signals, including auxin (Stafstrom and removal of a major sink for nutrients and energy, in Sussex, 1992; Shimizu-Sato and Mori, 2001; Morris addition to invoking stress and biophysical responses et al., 2005). Here, we address these hypotheses and not observed in intact plants. propose that several components of each can be in- If branching in decapitated or in vitro systems is corporated into one model of shoot branching. regulated somewhat differently from branching in Different shoot-branching hypotheses have arisen intact plants, it is reasonable to expect that studies fo- from experiments that have varied largely in terms of cused on one or the other will lead to different view- whether they focus on intact, decapitated, or in vitro points (Napoli et al., 1999). Often, the experimental systems. Although decapitated and in vitro systems approach implemented (decapitated, in vitro, or intact) are excellent tools for studying apical dominance, the is dictated by the plant species studied. For example, two species that have contributed widely to our cur- rent understanding of apical dominance, pea (Pisum 1 This work was supported by an Australian Postgraduate Award sativum) and Arabidopsis (Arabidopsis thaliana), have (scholarship to E.A.D.) and the Australian Research Council. 2 vastly different phenotypes (Fig. 1). Pea plants have These authors contributed equally to the paper. long internodes separating their axillary buds from * Corresponding author; e-mail [email protected]; fax 61–7– one another and the shoot tip. This phenotype has 3365–3550. The author responsible for distribution of materials integral to the allowed for experiments using a wide range of sys- findings presented in this article in accordance with the policy tems, primarily during the vegetative phase. In con- described in the Instructions for Authors (www.plantphysiol.org) is: trast, research using Arabidopsis has predominantly Christine Anne Beveridge ([email protected]). focused on in vitro studies following floral transition, www.plantphysiol.org/cgi/doi/10.1104/pp.106.086868 largely due to constraints imposed by its vegetative 812 Plant Physiology, November 2006, Vol. 142, pp. 812–819, www.plantphysiol.org Ó 2006 American Society of Plant Biologists Apical Dominance Figure 1. Comparable 3-week-old plants of pea (left) and Arabidopsis (right) exhibiting absolute apical dominance phenotypes. Important features and limitations that pertain to studies of shoot branching in these species are shown alongside. Plants were grown under an 18-h photoperiod. Bar 5 2 cm. rosette phenotype and its weak response to exogenous the epicotyl and root) play an important role in reg- auxin following decapitation (Cline, 1996; Cline et al., ulating shoot branching (Napoli, 1996; Foo et al., 2001; 2001). Turnbull et al., 2002). This supports the notion argued Mutants in several species have been essential for above that the control of branching exerted by the identifying mechanisms of shoot branching in intact shoot tip (apical dominance) is but one part of a reg- systems. A series of mutants in pea (rms), Arabidopsis ulatory system controlling branching in plants. (max), petunia (Petunia hybrida; dad), and rice (Oryza Several genes in different species have now been sativa; e.g. dwarf3) have been identified that show shown to affect the level of a graft-transmissible signal increased shoot branching in intact plants (e.g. Foo involved in branching inhibition (i.e. RMS1, RMS5, et al., 2005; Ishikawa et al., 2005; Snowden et al., 2005; MAX1, MAX3, MAX4, and DAD1; for review, see Bennett et al., 2006). Branching at vegetative nodes of McSteen and Leyser, 2005; Beveridge, 2006). For simplic- rms mutants of pea occurs during several stages of ity, we refer to this signal as the shoot multiplication sig- plant ontogeny and in the presence of a vigorous shoot nal (SMS; Beveridge, 2006). SMS, which is produced in tip (e.g. Beveridge et al., 2003). In contrast, elongation the shoot and rootstock, is neither the auxin indole-3- growth of axillary buds at rosette nodes in Arabidop- acetic acid (IAA) nor a well-known cytokinin (Beveridge sis is constrained by the floral transition. Phenotypic et al., 1997b; Morris et al., 2001); instead, molecular differences among intact max mutant and wild-type evidence indicates that it is possibly a carotenoid de- plants are not apparent at cauline nodes of the elon- rivative (Sorefan et al., 2003; Schwartz et al., 2004). gated inflorescence shoot (Stirnberg et al., 2002; Sorefan et al., 2003); yet, as mentioned above, physiological studies investigating Arabidopsis emphasize in vitro CLASSICAL HYPOTHESIS approaches using detached segments with cauline nodes. Typically, genetic differences revealed from obser- The classical hypothesis states that auxin content vations using isolated segments of these nodes are regulates shoot branching by influencing the levels, used to develop hypotheses regarding the regulation transport, and/or action of other signals required to of bud outgrowth at rosette nodes of the intact plant inhibit bud outgrowth (Snow, 1937; Hall and Hillman, (e.g. Stirnberg et al., 2002; Sorefan et al., 2003; Bainbridge 1975; Morris, 1977; Bangerth, 1989). Candidates for this et al., 2005; Bennett et al., 2006). auxin-regulated signal include cytokinin (Fig. 2a; e.g. Grafting studies using rms mutants of pea (e.g. Sachs and Thimann, 1967; Li et al., 1995) and/or, as Beveridge et al., 1994, 1997b) were the first to show proposed more recently, SMS (Fig. 2b; Foo et al., 2005). that wild-type rootstocks can completely restore the The bulk of evidence supporting the classical hy- branching phenotype of mutant shoots to that of the pothesis of auxin control comes from decapitation wild type. These and other studies also highlighted studies. For example, in various legumes, decapitation the point that tissues other than the shoot tip (such as leads to increased levels of endogenous cytokinins in Plant Physiol. Vol. 142, 2006 813 Dun et al. Figure 2. Model for regulation of stages of bud outgrowth. The classical hypothesis is illustrated in blue (a, b, c, and d); the auxin transport hypothesis (Bennett et al., 2006) is illustrated in orange (e and f); an alternative interpretation
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