Abstract
cis-Zeatin (cZ) is generally regarded as a cytokinin with little or no activity, compared with the highly active trans-zeatin (tZ). Although recent studies suggested possible roles for cZ, its physiological significance remains unclear. In our studies with rice (Oryza sativa), cZ inhibited seminal root elongation and up-regulated cytokinin-inducible genes, and its activities were comparable to those of tZ. Tracer experiments showed that exogenously supplied cZ-riboside was mainly converted into cZ derivatives but scarcely into tZ derivatives, indicating that isomerizations of cZ derivatives into tZ derivatives are a minor pathway in rice cytokinin metabolism. We identified three putative cZ-O-glucosyltransferases (cZOGT1, cZOGT2, and cZOGT3) in rice. The cZOGTs preferentially catalyzed O-glucosylation of cZ and cZ-riboside rather than tZ and tZ-riboside in vitro. Transgenic rice lines ectopically overexpressing the cZOGT1 and cZOGT2 genes exhibited short-shoot phenotypes, delay of leaf senescence, and decrease in crown root number, while cZOGT3 overexpressor lines did not show shortened shoots. These results propose that cZ activity has a physiological impact on the growth and development of rice.
Highlights
To test whether cZ is an active cytokinin in rice, bioactivities of cZ and tZ were compared in root growth assays using Arabidopsis and rice seedlings
When we monitored the isotope labels until 24 h after feeding, the proportions remained essentially unchanged (Supplemental Tables S1 and S2). These results suggested that isomerization, if any occurs, is negligible compared with other reactions of cytokinin metabolism such as glucosylation and phosphorylation
The results suggested that cZOGTs catalyze O-glucosylation in a Mg2+-dependent and ATP-independent manner, using UDP-Glc as a Glc donor, at optimum pH values of 6.0 for cZOGT1 and cZOGT3 and of 7.0 for cZOGT2 (Supplemental Fig. S2)
Summary
Transgenic rice lines ectopically overexpressing the cZOGT1 and cZOGT2 genes exhibited short-shoot phenotypes, delay of leaf senescence, and decrease in crown root number, while cZOGT3 overexpressor lines did not show shortened shoots. These results propose that cZ activity has a physiological impact on the growth and development of rice. Natural cytokinins detected in plants are N6-substituted adenine derivatives and are structurally classified into isoprenoid cytokinins or aromatic cytokinins carrying an isoprene-derived or aromatic N6 side chain, respectively (Mok and Mok, 2001; Sakakibara et al, 2006). While tZ is considered a highly active cytokinin and has been the main focus of cytokinin research, cZ is regarded as a derivative of low activity
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