Forbidden Fruit: Dominance Relationships and the Control of Shoot Architecture

Plants continually integrate environmental information to make decisions about their development. Correlative controls, in which one part of the plant regulates the growth of another, form an important class of regulatory mechanism, but their study has been neglected, and their molecular basis remains unclear. In this article, we examine the role of negative correlative controls or ‘dominance’ phenomena in the regulation of shoot architecture. Apical dominance, in which actively growing shoot branches inhibit the growth of other branches, is perhaps the most famous example of this. We discuss the recent progress made in understanding the mechanistic basis for apical dominance and three plausible models for shoot branching control. We then use the apical dominance paradigm to explore other dominance phenomena, including seed–seed inhibition (carpic dominance), seed-to-meristem inhibition, and the control of maternal senescence by seeds. We propose that apical and carpic dominance may share a common mechanistic basis rooted in auxin transport canalisation. Conversely, we conclude that seed-to-meristem inhibition and seed-driven senescence may not be ‘true’ correlative controls, but rather more complex phenomena in which seed set plays a permissive rather than instructive role. Overall, we attempt to develop a coherent framework for understanding the developmental and regulatory mechanisms that control shoot architecture and provide new insights into the end of flowering, fruiting, and growth.