Cytokinin signaling: two-components and more.

The hormone cytokinin is perceived via a phosphorelay that is similar to the two-component systems with which bacteria sense and respond to environmental stimuli. Characterization of two-component elements in Arabidopsis, maize and rice shows that cytokinin responses are mediated via partially redundant two-component protein families: histidine kinases, histidine phosphotransfer proteins and response regulators, as well as by novel players, such as the cytokinin response factors. Recent studies in the model plant Arabidopsis have revealed that cytokinin regulates these signaling components through a variety of mechanisms, including modulating transcription, controlling phosphorelay and regulating protein localization and stability. Genetic analyses of cytokinin signaling components have clarified the roles of cytokinin signaling in development and revealed novel functions for this 'classic' phytohormone.