Cloning and Characterization of the Abscisic Acid-Specific Glucosyltransferase Gene from Adzuki Bean Seedlings

Zheng-Jun Xu , Masatoshi Nakajima , Yoshihito Suzuki , and Isomaro Yamaguchi ^*

Bio-oriented Technology Research Advancement Institution, Tokyo 105--0001, Japan (Z.J.X.); and Department of Applied Biological Chemistry, The University of Tokyo, Bunkyo-ku, Tokyo 113--8657, Japan (M.N., Y.S., I.Y.)

^* Corresponding author; email: aisomar{at}mail.ecc.u-tokyo.ac.jp.

The glycosylated forms of abscisic acid (ABA) have been identified from many plant species and are known to be the forms of ABA-catabolism, although their (physiological) roles have not yet been elucidated. ABA-glucosyltransferase (-GTase) is thought to play a key role in the glycosylation of ABA. We isolated an ABA-inducible GTase gene from UDP-GTase homologs obtained from adzuki bean (Vigna angularis) seedlings. The deduced amino acid sequence (accession no. AB065190) showed 30% to 44% identity with the known UDP-GTase homologs. The recombinant protein with a glutathione S-transferase-tag was expressed in Escherichia coli and showed enzymatic activity in an ABA-specific manner. The enzymatic activity was detected over a wide pH range from 5.0 to 9.0, the optimum range being between pH 6.0 and 7.3, in a citrate and Tris-HCl buffer. The product from racemic ABA and UDP-D-glucose was identified to be ABA-GE by gas chromatography/mass spectrometry. The recombinant GTase (rAOG) converted 2-trans-(+)-ABA better than (+)-S-ABA and (-)-R-ABA. Although trans-cinnamic acid was slightly converted to its conjugate by the GTase, (-)-PA was not at all. The mRNA level was increased by ABA application or by water stress and wounding. We suggest that the gene encodes an ABA-specific GTase and that its expression is regulated by environmental stress.

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