The molecular basis of cytokinin action

Current understanding of cytokinin (CK) physiology at the cellular level results largely from the manipulation of endogenous CK levels by either application of exogenous CKs or the expression of CK biosynthetic transgenes, as well as the characterisation of single gene mutants. Cytokinins modulate changes in plant gene expression, which are in turn assumed to effect physiological and morphological changes with which CK action is associated. Presently, a major focus of investigation is elucidation of the biochemical events leading from the perception of CK to the manifestation of a response. Analysis of the expression patterns of CK-regulated genes and identification of their products provides one means of investigating CK action at the molecular level. Biochemical approaches have led to the identification of several soluble CK-binding proteins, although their functional roles in CK signalling largely remain uncertain. Conclusive identification of a bona fide CK receptor has yet to be achieved, although several potential candidates have been suggested. Pharmacological and molecular genetic strategies have implicated the involvement of signalling mechanisms likely to be involved in CK action. The apparent involvement of fluctuations in the concentration of intracellular Ca2+, changes in protein phosphorylation as well as DNA and/or protein methylation provide information concerning the types of proteins likely to be involved in the process. Dissection of CK signal transduction chains and elucidation of their interaction with other pathways that regulate plant growth and development is likely to be essential in understanding the mode of action of this poorly understood class of plant growth regulator. However, integration of this knowledge with an improved understanding of the mechanisms whereby overall hormone homeostasis is regulated at the metabolic level will be necessary for comprehensive appreciation of the influence of CKs on plant morphology and physiology.