Auxin and its henchmen: Hormonal cross talk in root growth and development

Abstract

Among the intrinsic growth regulators, auxin occupies a central role in plant growth and development. Auxin has been implicated in the regulation of many developmental processes at different scales, including cell division, cell differentiation, organogenesis, and morphogenesis. This is for instance evident in the sometimes similar and sometimes divergent phenotypes of auxin pathway mutants with regard to root system architecture, root meristem maintenance and lateral root organogenesis. Interestingly, other hormone pathways often similarly affect root system development, which has given rise to the notion of hormone pathway cross talk and/or synergy. Indeed, in recent years a few examples of mechanistically defined hormone pathway interactions have been identified. Most of these inputs appear to ultimately converge on the modulation of auxin levels, transport, or signaling and can explain aspects of context-specific auxin action. In this chapter, we will discuss examples of interaction between the pathways of auxin and other hormones, notably brassinosteroid, cytokinin, ethylene, and gibberellin, with a focus on root growth and development.