Cytokinin Oxidase and the Regulation of Cytokinin Degradation

Abstract

Cytokinin degradation is an important component process in the metabolic network that controls the levels of cytokinin metabolites and their distribution in plant systems. The existence in plant tissues of enzymatic machinery capable of degrading cytokinins to inactive products was demonstrated in the earliest studies of cytokinin metabolism. In 1962 McCalla et al. observed that the metabolic products formed when the cytokinin N ⁶-benzyladenine-8- ¹⁴ C was supplied to senescing leaves of cocklebur (Xanthium pensylvanicum) included the degradation products adenosine, guanosine, and inosine monophosphates (AMP, GMP, IMP), urea, and several ureides. ¹ In the 30 years that have elapsed since this initial study, the metabolism of cytokinins has been examined in a wide variety of plant systems, using a number of different cytokinins as radioactive tracers. As illustrated in Table 1, degradation has been observed to be a significant component of cytokinin metabolism in many plant tissues. Adenine, adenosine, and AMP have been the degradation products most frequently identified.