Advances in cytokinin signaling.

Cytokinins are essential plant hormones that control various processes in plants' development and response to external stimuli. The Arabidopsis cytokinin signal transduction pathway involves hybrid histidine protein kinase sensors, phosphotransfer proteins, and regulators as transcription activators and repressors in a phosphorelay system. Each step is executed by components encoded by multigene families. Recent findings have revealed new functions, new feedback loops, and connections to other signaling pathways.