The metabolism of cytokinin

Since their first discovery in 1955, much work has been carried out on cytokinins. This has led to a better understanding of the structure and function of these plant hormones. Cytokinins play many roles in plants ranging from long distance signalling, cell differentiation through to regulation of senescence. This section of plant biology advice takes a look into the metabolism of cytokinins.

Classifications of cytokinin metabolism

There are two major classifications of cytokinin metabolism:

1. Modification of the moiety of adenine
2. Modification of cytokinin side chains

It is thought that many of the steps of cytokinin metabolism are shared with those of the purine salvage pathway, and that the flow of cytokinin nucleotides to the active nucleobases has a circular nature. This is supported by the fact that applying cytokinins exogenously to plants results in their incorporation into into plant nucleotides and nucleosides. This is thought to be because many isoenzymes that are encoded by genes in the purine salvage pathway are able to recognise and interact with cytokinins in addition to adenine.

Cytokinin glycosylation and the control of cytokinin homoeostasis

Alterations of cytokinin moiety and side chains by glycosylation play a major role in the metabolism of cytokinins. Glycosylation has been shown to affect the moiety of cytokinins by interacting at the N³, N⁷ and N⁹ positions. With regards to the side chains glycosylation has been demonstrated to affect the hydroxyl groups of trans-zeatin , cis-zeatin and dihydrozeatin.

It is known that the levels of cytokinins in plants are kept at homoeostasis by control of the release rate of cytokinin conjugates and the rate of the degradation of cytokinins. The interactions of glycosides on cytokinins enable the inactivation of cytokinins (The interactions that are catalysed by b-glucosidease lead to either reversible o-glucosylation or a non-reversible cleaving N-glucosylation). Cytokinin oxidases and dehydrogenase are involved in the degradation of cytokinin side chains. Another way in which the quantity of cytokinin in the plant system is by phosphoribosylation.