Cytokinins

Abstract

Cytokinin is one of the so-called classic plant growth phytohormones and functions to promote cell division and cell differentiation. Cytokinins are adenine derivatives that carry a variable side chain at the N6position of the purine. As a key growth-promoting phytohormone, cytokinin is involved in almost all aspects of plant growth and development. Cytokinin signaling is mediated by a two-component system involved in sequential transfer of phosphoryl groups from the receptors to downstream effectors. In Arabidopsis, upon binding cytokinin, the receptors, a small class of His kinases, are autophosphorylated at a highly conserved His residue. The phosphoryl group is subsequently transferred to PHOSPHOTRANSFER PROTEINS and then to RESPONSE REGULATORS, eventually activating the transcription of downstream effector genes, thereby turning on the signaling pathway. This chapter mainly summarizes recent progress and our current understanding on cytokinin metabolism, translocation, signal transduction, regulatory mechanisms, and physiological roles of cytokinin in plant growth and development.