Characterization of an auxin-inducible 1-aminocyclopropane-1-carboxylate synthase gene, VR-ACS6, of mungbean (Vigna radiata (L.) Wilczek) and hormonal interactions on the promoter activity in transgenic tobacco

A genomic clone for VR-ACS6, an isozyme of auxin-inducible ACC synthase of mungbean, was isolated, and its promoter activity was examined in transgeni c tobacco. The clone contained 1,612 bp long 5' untranscribed region and it s coding sequence consisted of three exons and two introns, Genomic Souther n hybridization indicated that VR-A CS6 is a single copy gene. The transcri ption initiation site was a cytosine present at 231-base upstream the trans lation start codon. The VR-ACS6 promoter contained DNA sequences homologous to various functionally identified auxin-responsive elements. To demonstra te hormonal response of the promoter region, transgenic tobacco plants carr ying the 1,719 bp VR-ACS6 promoter/-glucuronidase (GUS) fusion gene were ge nerated. Strong GUS expression occurred by auxin treatment of leaves of To transformants and hypocotyls of T-1 etiolated seedlings. Magnitude of the r esponse to auxin was dose-dependent, and the increased GUS activity was det ected at 0.1 mu M and higher concentrations of IAA. Other plant hormones di d not induce GUS activity, but greatly modified the response to auxin. Cyto kinin enhanced the IAA-induced expression of GUS reporter gene, whereas ABA and ethylene suppressed the expression. These characteristics of VR-ACS6 p romoter activity in transgenic tobacco are in good accordance with the expr ession patterns of the gene in mungbean hypocotyls, Histochemical staining showed that GUS activity was evident in both etiolated and light grown seed lings treated with IAA. Cytokinin enhanced the intensity of auxin-induced G US stain and also expanded the stained area, whereas ABA and ethylene reduc ed both intensity and area of the stain.