The molecular basis of cytokinin function.

Cytokinins are a class of phytohormones that regulate a wide variety of physiological and developmental processes such as shoot and root growth. Cytokinin signaling relies on a phosphorelay mechanism similar to the prokaryotic two-component system. Although the principal components mediating this cascade have been identified, only recently have we begun to understand the molecular basis of cytokinin action. For example cytokinins control cell differentiation rate during root meristem development by suppressing both auxin signaling and transport, whereas at early stages of embryo development auxin counteracts cytokinin signaling to establish the embryonic root stem-cell niche. The antagonistic interaction between cytokinins and auxin seems to also occur in other developmental processes, such as lateral root emergence and leaf initiation. Copyright 2009 Elsevier Ltd. All rights reserved.