THE MOLECULAR-BASIS OF CYTOKININ ACTION

Current understanding of cytokinin (CK) physiology at the cellular lev el results largely from the manipulation of endogenous CK levels by ei ther application of exogenous CKs or the expression of CK biosynthetic transgenes, as well as the characterisation of single gene mutants. C ytokinins 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 C K to the manifestation of a response. Analysis of the expression patte rns of CK-regulated genes and identification of their products provide s one means of investigating CK action at the molecular level. Biochem ical approaches have led to the identification of several soluble CK-b inding proteins, although their functional roles in CK signalling larg ely remain uncertain. Conclusive identification of a bona fide CK rece ptor has yet to be achieved, although several potential candidates hav e been suggested. Pharmacological and molecular genetic strategies hav e implicated the involvement of signalling mechanisms likely to be inv olved in CK action. The apparent involvement of fluctuations in the co ncentration of intracellular Ca2+, changes in protein phosphorylation as well as DNA and/or protein methylation provide information concerni ng the types of proteins likely to be involved in the process. Dissect ion of CK signal transduction chains and elucidation of their interact ion with other pathways that regulate plant growth and development is likely to be essential in understanding the mode of action of this poo rly understood class of plant growth regulator. However, integration o f this knowledge with an improved understanding of the mechanisms wher eby overall hormone homeostasis is regulated at the metabolic level wi ll be necessary for comprehensive appreciation of the influence of CKs on plant morphology and physiology.