Hoyerová Klára, Hošek Petr
Institute of Experimental Botany of the Czech Academy of Sciences, Prague, Czechia.
Front Plant Sci. 2020 Jun 5;11:741. doi: 10.3389/fpls.2020.00741. eCollection 2020.
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.
细胞分裂素（CK）-葡萄糖苷是许多物种中最丰富的CK代谢产物组；然而，它们的生理作用长期以来仅被视为不可逆的CK储存形式。最近，一项全面的筛选表明，与苔藓、藻类和真菌不同，只有维管植物会形成CK-葡萄糖苷。因此，作为无生物活性的CK缀合物的CK-葡萄糖苷的形成代表了一种进化上较新的CK碱基失活机制。尽管CK-葡萄糖苷本身由于无法激活CK感知系统而没有生物活性，但关于CK-葡萄糖苷代谢的新数据表明，玉米素（tZ）N7-和N9-葡萄糖苷会被代谢，有效释放出游离的CK碱基，这些碱基很可能是许多生物测定中观察到的生物活性的原因。此外，CK-葡萄糖苷的亚细胞定位及其在木质部中的丰度都表明它们可能通过质膜运输，也表明其作为CK运输形式的作用。鉴定负责tZ N7-和N9-葡萄糖苷水解的酶，以及发现假定的CK-葡萄糖苷质膜转运体，将揭示CK代谢相互转化全貌及其生理重要性的重要部分。
