Molecular Mechanisms of Cytokinin Signaling

Cytokinins regulate a myriad of plant growth and developmental processes. Recent molecular genetic studies in Arabidopsis have begun to unravel the molecular mechanisms underlying cytokinin perception and signal transduction. A family of cytokinin receptors has been identified, and these are homologous to bacterial two-component sensor kinases. Events immediately downstream of cytokinin binding are similar to the classic phosphorelay paradigm. The cytokinin signal appears to be transduced from the membrane-localized histidine kinase-like receptors into the nucleus via a transient translocation of the AHP proteins, which are Arabidopsis homologs of histidine phosphotransfer proteins. Once in the nucleus, the AHPs activate the type-B class of Arabidopsis response regulators (ARRs), which in turn activate the transcription of a second class of Arabidopsis response regulators, the type-A ARRs. A model of cytokinin signaling from perception at the plasma membrane to activation of gene expression in the nucleus is beginning to emerge.