Cytokinins (CK) are a class of plant growth substances (phytohormones) that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence. . Their regulation can take the form of biosynthesis, import, conjugation, and degradation. Homeostasis regulation by irreversible degradation is carried out by cytokinin oxidases which form adenine-like compounds resulting from the cleavage of the N6-isopentenyl-side chain. The cytokinins degradation pathway consists of five different degradation reactions that can be localized to either the endoplasmic reticulum or the vacuole. Cytokinin oxidase was found to catalyze two such reactions: the conversion of N6-dimethylallyladenine into 3-methyl-2-butenal and adenine and the conversion of trans-zeatin into 3-methyl-4-trans-hydroxy-2-butenal and adenine. It has not yet been determined if cytokinin oxidase also catalyzes the other three reactions in the cytokinins degradation pathway: the conversion of isopentenyl adenosine into adenosine and 3-methyl-2-butenal, the conversion of cis-zeatin into 3-methyl-4-cis-hydroxy-2-butenal and adenine, and the conversion of trans-zeatin riboside into 3-methyl-4-trans-hydroxy-2-butenal and adenosine. As such, the enzyme of these reactions are coloured orange in the image.

Cytokinins Degradation References