INHIBITION OF GIBBERELLIN-INDUCED ELONGATION, REORIENTATION OF CORTICAL MICROTUBULES AND CHANGE OF ISOFORM OF TUBULIN IN EPICOTYL SEGMENTS OF AZUKI-BEAN BY PROTEIN-KINASE INHIBITORS

When the segments of azuki bean were incubated with 3-indoleacetic acid (IAA) plus gibberellin A(3) (GA(3)), one isoform of a-tubulin, referred to as a(b) for convenience, was found to disappear after 12 h. The loss of this isoform was initially assumed to be the result of a shift in electrophoretic mobility due to post-translational modification, such as phosphorylation. This assumption led to an investigation of the effects of protein kinase inhibitors on GA(3)-induced phenomena. 6-Dimethylaminopurine (6-DMAP) prevented not only the disappearance of the a(b) isoform of tubulin but also simultaneously prevented the GA(3)-induced promotion of elongation. Moreover, the GA(3)-induced change in alignment of cortical MTs to the transverse orientation was also prevented and cortical MTs became aligned longitudinally or parallel to the cell axis. Although, in IAA-treated segments, 6-DMAP forced the longitudinal alignment of cortical MTs, no inhibition of IAA-induced elongation was detected. The disappearance of the a(b) isoform of tubulin was found only in the inner tissues of segments and not in epidermal tissue. Therefore, the change associated with this isoform seems not to be involved directly in GA(3)-induced promotion of elongation and the change in orientation of cortical MTs, but a protein kinase may still be a major participant in GA(3)-induced phenomena by alteration of other proteins.