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Cytokinins are master regulators of plant growth and development. The glucosyltransferase UGT76C1 capable of N-glucosylation of different cytokinins at the N7- and N9-position was previously identified in Arabidopsis thaliana, but its physiological relevance in plants remains unclear. In the present work, we investigated the physiological characteristics of UGT76C1 mutant (ugt76c1) and its overexpressors. Under normal growth conditions, although ugt76c1 plants and UGT76C1 overexpressors did not display obvious phenotypic alteration, ugt76c1 plants significantly reduced the accumulation of cytokinin N-glucosides, whereas UGT76C1 overexpressors increased cytokinin N-glucosides. Unexpectedly, the concentrations of free forms of cytokinins (mainly trans-zeatin and N6-isopentenyladenine) were comparable to those of the wild type. Upon application of exogenous cytokinin, the mutant showed the same tendency of more sensitive cytokinin response in primary root elongation, chlorophyll retention and anthocyanin accumulation. In contrast, overexpressors showed a tendency of less sensitive cytokinin response in most tests. Furthermore, cytokinin-related genes were investigated for their expression; and the expression levels of AHK3, ARR1, CYP735A2 and LOG2 noticeably changed in ugt76c1 plants, suggesting that plants employ a set of cytokinin regulation mechanisms to coordinate the loss-of-function of UGT76C1. Tissue-specific expression of UGT76C1 showed a high level of expression in germinating seeds and young seedlings. Taken together, our data suggest that the glucosyltransferase UGT76C1 could finely modulate cytokinin responses in planta via N-glucosylation of cytokinins.

glucosyltransferase, Arabidopsis thaliana, cytokinins, root elongation, chlorophyll retention

Publication DOI: 10.1016/j.plaphy.2013.01.012
Journal NLM ID: 9882449
Publisher: Elsevier Science for Société Française De Physiologie Végétale
Correspondence: bkhou@sdu.edu.cn
Institutions: The Key Laboratory of Plant Cell Engineering and Germplasm Innovation, Education Ministry of China, School of Life Science, Shandong University, No. 27 Shanda South Road, Jinan, Shandong 250100, PR China, Plant Productivity Systems Research Group, RIKEN Plant Science Center, 1-7-22, Suehiro, Tsurumi, Yokohama 230-0045, Japan
Methods: biological assays, genetic manipulations, analytical procedures

Cytokinins are a class of phytohormones that play a crucial role in plant growth and development. The gene UGT76C2 encoding cytokinin N-glucosyltransferase of Arabidopsis thaliana has been previously identified. To determine the in planta role of UGT76C2 in cytokinin metabolism and response, we analyzed the phenotypes of its loss-of-function mutant (ugt76c2) and its overexpressors. The accumulation level of the cytokinin N-glucosides was significantly decreased in ugt76c2, but substantially increased in UGT76C2 overexpressors compared with the wild type. When treated with exogenously applied cytokinin, ugt76c2 showed more sensitivity and UGT76C2 overexpressors showed less sensitivity to cytokinin in primary root elongation, lateral root formation, Chl retention and anthocyanin accumulation. Under normal growth conditions ugt76c2 had smaller seeds than the wild type, with accompanying lowered levels of active and N-glucosylated cytokinin forms. The expression levels of cytokinin-related genes such as AHK2, AHK3, ARR1, IPT5 and CKX3 were changed in ugt76c2, suggesting homeostatic control of cytokinin activity. Studies of spatiotemporal expression patterns showed that UGT76C2 was expressed at a relatively higher level in the seedling and developing seed. In their entirety, our data, based mainly on this comparison and opposite phenotypes of knockout and overexpressors, strongly suggest that UGT76C2 is involved in cytokinin homeostasis and cytokinin response in planta through cytokinin N-glucosylation.

Arabidopsis thaliana, overexpression, cytokinin, cytokinin glucosylation, N-glucosyltransferase, T-DNA mutant

Publication DOI: 10.1093/pcp/pcr152
Journal NLM ID: 9430925
Publisher: Tokyo: Oxford University Press
Correspondence: bkhou@sdu.edu.cn
Institutions: The Key lab of plant cell engineering and germplasm innovation, Education Ministry of China, School of Life Science, Shandong University, No.27 Shanda South Road, Jinan, Shandong, 250100, PR China, Plant Productivity Systems Research Group, RIKEN Plant Science Center, 1-7-22, Suehiro, Tsurumi, Yokohama, 230-0045 Japan
Methods: biological assays, genetic manipulations, analytical procedures

Trans-zeatin is a kind of cytokinins that plays a crucial role in plant growth and development. The master trans-zeatin O-glucosyltransferase of Arabidopsis thaliana, UGT85A1, has been previously identified through biochemical approach. To determine the in planta role of UGT85A1 gene, the characterization of transgenic Arabidopsis plants overexpressing UGT85A1 was carried out. Under normal conditions, transgenic Arabidopsis did not display clearly altered phenotypes. A remarkable alteration is that the accumulation level of the trans-zeatin O-glucosides was significantly increased in UGT85A1 overexpressing transgenic Arabidopsis, while other forms of cytokinins kept the similar concentrations compared to the wild type. When treated with exogenously applied trans-zeatin, UGT85A1 overexpressing Arabidopsis showed much less sensitivity to trans-zeatin in primary root elongation and lateral root formation. Meanwhile, the chlorophyll content of detached leaves of transgenic Arabidopsis was much lower than wild type. Studies of spatial-temporal expression patterns showed that UGT85A1 was mainly expressed in the early seedlings and developing seeds. Analysis of subcellular localization suggested that UGT85A1 was localized to cytoplasm and nucleus. Taken together, our data suggest that overexpression of Arabidopsis glucosyltransferase UGT85A1 influences trans-zeatin homeostasis and trans-zeatin responses likely through O-glucosylation in planta.

glucosyltransferase, Arabidopsis, cytokinin, trans-zeatin, transgenic plant

Publication DOI: 10.1007/s00425-012-1818-4
Journal NLM ID: 1250576
Publisher: Berlin, New York, Springer
Correspondence: bkhou@sdu.edu.cn
Institutions: The Key Lab of Plant Cell Engineering and Germplasm Innovation, Education Ministry of China, Jinan, 250100, Shandong, People’s Republic of China, School of Life Science, Shandong University, No. 27 Shanda South Road, Jinan, 250100, Shandong, People’s Republic of China, Plant Productivity Systems Research Group, RIKEN Plant Science Center, 1-7-22, Suehiro, Tsurumi, Yokohama, 230-0045, Japan
Methods: biological assays, genetic manipulations, analytical procedures