ABSTRACT
The true extent of the ability of plants to metabolize cytokinins was first revealed by D. S. Letham’s group in the middle 1970s following the feeding of labeled cytokinins to blue lupin plants and the separation of numerous labeled, but uncharacterized, compounds. The enzyme systems catalyzing the interconversions of cytokinin base-riboside-nucleotide appear to be the same as those for adenine-adenosine-adenosine monophosphate interconversions. Oxidative side chain cleavage to produce adenine, adenosine, and adenine nucleotides is the predominant fate of exogenously supplied zeatin, zeatin riboside, and isopentenyladenine, in various plant tissues and results in irreversible destruction of cytokinin activity. In some plants metabolism is similar, irrespective of the supplied cytokinin, whereas in others it varies depending on the substrate. Qualitative changes in the endogenous cytokinin complement during leaf development have been reported on numerous occasions. The challenge for future studies is to relate the wealth of information on the metabolism of externally applied compounds to the dynamics of endogenous cytokinins.
