Interaction of Cytokinin and Ethylene in the Regulation of Primary Root Growth and Development

Abstract

Cytokinin and ethylene inhibit growth of the primary root through their effects on cell proliferation and cell expansion, doing so independently as well as cooperatively. Here we provide an overview of the cytokinin and ethylene signal transduction pathways. We discuss points of crosstalk between the pathways based on the ability of cytokinin to induce ethylene biosynthesis, the ability of ethylene to signal through the multi-step phosphorelay that mediates cytokinin signal transduction, and the ability of both hormones to cross-regulate gene expression for key elements in each other’s biosynthesis and signaling pathways. The ability of these two hormones to regulate auxin activity plays a major role by which they inhibit primary root growth. To this end, mechanisms by which these two hormones regulate rootward and shootward auxin transport to control cell proliferation in the root meristem and cell expansion within the elongation zone, respectively, are discussed. Auxin-independent mechanisms to regulate root growth by these hormones are also considered. A model is proposed that provides a framework for the interaction of these hormones in the regulation of primary root growth.