New Insights Into the Metabolism and Role of Cytokinin -Glucosides in Plants.

Cytokinin (CK) -glucosides are the most abundant group of CK metabolites in many species; however, their physiological role was for a long time perceived as irreversible storage CK forms only. Recently, a comprehensive screen showed that only vascular plants form CK -glucosides in contrast to mosses, algae, and fungi. The formation of CK -glucosides as biologically inactive CK conjugates thus represents an evolutionarily young mechanism for deactivation of CK bases. Even though CK -glucosides are not biologically active themselves due to their inability to activate the CK perception system, new data on CK -glucoside metabolism show that -zeatin (tZ) N7- and N9-glucosides are metabolized , efficiently releasing free CK bases that are most probably responsible for the biological activities observed in a number of bioassays. Moreover, CK -glucosides' subcellular localization as well as their abundance in xylem both point to their possible plasma membrane transport and indicate a role also as CK transport forms. Identification of the enzyme(s) responsible for the hydrolysis of tZ N7- and N9-glucosides, as well as the discovery of putative CK -glucoside plasma membrane transporter, would unveil important parts of the overall picture of CK metabolic interconversions and their physiological importance.