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Plant hormones cytokinins (CKs) are one of the major mediators of physiological responses throughout plant life span. Therefore, a proper homeostasis is maintained by regulation of their active levels. Besides degradation, CKs are deactivated by uridine diphosphate glycosyltransferases (UGTs). Physiologically, CKs active levels decline in senescing organs, providing a signal to nutrients that a shift to reproductive tissues has begun. In this work, we show CK glucosides distribution in  Arabidopsis  leaves during major developmental transition phases. Besides continuous accumulation of  N -glucosides we detected sharp maximum of the glucosides in senescence. This is caused prevalently by  N 7-glucosides followed by  N 9-glucosides and specifically also by  trans -zeatin- O -glucoside ( t ZOG). Interestingly, we observed a similar trend in response to exogenously applied CK. In  Arabidopsis , only three UGTs deactivate CKs  in vivo : UGT76C1, UGT76C2 and UGT85A1. We thereby show that  UGT85A1  is specifically expressed in senescent leaves whereas  UGT76C2  is activated rapidly in response to exogenously applied CK. To shed more light on the UGTs physiological roles, we performed a comparative study on UGTs loss-of-function mutants, characterizing a true  ugt85a1-1  loss-of-function mutant for the first time. Although no altered phenotype was detected under standard condition we observed reduced chlorophyll degradation with increased anthocyanin accumulation in our experiment on detached leaves accompanied by senescence and stress related genes modulated expression. Among the mutants,  ugt76c2  possessed extremely diminished CK  N -glucosides levels whereas  ugt76c1  showed some specificity toward  cis -zeatin ( c Z). Besides  t ZOG, a broader range of CK glucosides was decreased in  ugt85a1-1.  Performing CK metabolism gene expression profiling, we revealed that activation of CK degradation pathway serves as a general regulatory mechanism of disturbed CK homeostasis followed by decreased CK signaling in all UGT mutants. In contrast, a specific regulation of  CKX7 ,  CKX1  and  CKX2  was observed for each individual UGT mutant isoform after exogenous CK uptake. Employing an  in silico  prediction we proposed cytosolic localization of UGT76C1 and UGT76C2, that we further confirmed by GFP tagging of UGT76C2. Integrating all the results, we therefore hypothesize that UGTs possess different physiological roles in  Arabidopsis  and serve as a fine-tuning mechanism of active CK levels in cytosol.

Cytokinin-Specific Glycosyltransferases Possess Different Roles in Cytokinin Homeostasis Maintenance