N-Glucosylation of Cytokinins by Glycosyltransferases of Arabidopsis thaliana

Abstract

Cytokinins are plant hormones that can be glucosylated to form O-glucosides and N-glucosides. The glycoconjugates are inactive and are thought to play a role in homeostasis of the hormones. Although O-glucosyltransferases have been identified that recognize cytokinins, the enzymes involved in N-glucosylation have not been identified even though the process has been recognized for many years. This study utilizes a screening strategy in which 105 recombinant glycosyltransferases (UGTs) of Arabidopsis have been analyzed for catalytic activity toward the classical cytokinins: trans-zeatin, dihydrozeatin, N 6 -benzyladenine, N 6 -isopentenyladenine, and kinetin. Five UGTs were identified in the screen. UGT76C1 and UGT76C2 recognized all cytokinins and glucosylated the hormones at the N 7 and N 9 positions. UGT85A1, UGT73C5, and UGT73C1 recognized trans-zeatin and dihydrozeatin, which have an available hydroxyl group for glucosylation and formed the O-glucosides. The biochemical characteristics of the N-glucosyltransferases were analyzed, and highly effective inhibitors of their activities were identified. Constitutive overexpression of UGT76C1 in transgenic Arabidopsis confirmed that the recombinant enzyme functioned in vivo to glucosylate cytokinin applied to the plant. The role of the N-glucosyltransferases in cytokinin metabolism is discussed.

Transcription Analysis of Arabidopsis Membrane Transporters and Hormone Pathways during Developmental and Induced Leaf Senescence

“…The role of CKX proteins in senescence is as yet unclear, which is illustrated by the observation that, unexpectedly, overexpression of CKX genes may result in retardation of leaf senescence . Four of the five active glycosyltransferases from Arabidopsis (Hou et al, 2004) are weakly expressed and barely regulated. However, the O-glycosyltransferase AtUGT85A1 shows a remarkable activation in all three senescence conditions (Fig.…”

Metabolism of acetaminophen (paracetamol) in plants—two independent pathways result in the formation of a glutathione and a glucose conjugate

“…2.4) is a key mechanism in the metabolic homoeostasis of plant cells. Potential acceptor molecules can be plant hormones (Hou et al 2004), endogenous synthesized secondary metabolites (Richman et al 2005) as well as xenobiotics (Messner et al 2003). With their multiple functions and the variety of possible substrates, glycosyltransferases play a major role in buffering the impacts of either biotic or abiotic challenges on the plant (Bowles et al 2005).…”

Functional genomics of family 1 glycosyltransferases in Arabidopsis

“…A number of genes have been reported to be involved in CK metabolism: 7 IPTs (Kakimoto 2001; Takei et al 2001a), 2 CYP735As (Takei et al 2004), 7 CKXs (Schmulling et al 2003) and 5 UGTs (Hou et al 2004) that have CK O-GT/N-GT activity. However, no significant correlations were observed by transcriptome coexpresssion analysis (all datasets, ver.…”

“…UGT recombinant proteins of 105 of these genes were used in a comprehensive in vitro screen to identify CK UGTs (Hou et al 2004) and ABA UGTs (Lim et al 2005) (Table 1). Five UGTs were shown to have CK UGT activity (Hou et al 2004).…”

“…UGT recombinant proteins of 105 of these genes were used in a comprehensive in vitro screen to identify CK UGTs (Hou et al 2004) and ABA UGTs (Lim et al 2005) (Table 1). Five UGTs were shown to have CK UGT activity (Hou et al 2004). Eight UGTs showed UGT activity for (Ϯ)-ABA, but only UGT71B6 had enantioselective UGT activity towards the naturally occurring form, (ϩ)-ABA (Lim et al 2005).…”