Cytokinin Oxidase and the Degradative Metabolism of Cytokinins

Abstract

Over the last 16 years considerable knowledge has accumulated about the metabolism of naturally occurring and synthetic cytokinins. In the case of naturally occurring cytokinins, many of the metabolites identified after the external application of a cytokinin to a variety of plant tissues have been subsequently identified as endogenous compounds. Thus, a comprehensive picture now exists of the metabolic reactions that may be involved in the regulation of cytokinin levels in plants. A basic biochemical model for the control of cytokinin levels would envisage a dynamic balance between biosynthesis, the anabolic reactions leading to the various cytokinin conjugates and the catabolic reactions leading to loss of cytokinin activity. Whilst it is still unclear as to the precise function of cytokinin conjugation, i.e. to what extent it leads to the formation of storage, transported or inactivated forms of the cytokinins; the degradative metabolism of cytokinins via N⁶ side chain cleavage is clearly a process whereby plant tissues may regulate the levels of biologically active cytokinins.