Discoveries and Dilemmas Concerning Cytokinin Metabolism

Natural cytokinin compounds that function as plant hormones are adenine molecules connected to an N6 side chain. Metabolic pathways produce various structural modifications to the adenine moiety and/or the side chain that affect cytokinin function, activity, stability, and transport in plants. This review focuses on recent research that is elucidating the genes and biochemical pathways involved in cytokinin conjugation, deconjugation, and interconversion. One area of new discovery concerns the process of O-glucoside conjugate formation. Genes have been identified for O-glucosyltransferase enzymes in Phaseolus (ZOG1 and ZOX1) and maize (cisZOG1) and have been used to study enzyme activity, structure, and function in plant development. New information about the genetic basis of the adenine salvage pathway and its relationship to cytokinin interconversion and regulation has also become available. However, various dilemmas about cytokinin metabolism remain and need to be resolved using a combination of research approaches.