Plant and Cell Physiology, 1995, Vol. 36, No. 1 173-181
© 1995
A Possible Double Role for Brassinolide in the Reorientation of Cortical Microtubules in the Epidermal Cells of Azuki Bean Epicotyls
Department of Biology, Faculty of Science, Osaka University Toyonaka, Osaka, 560 Japan
Brassinolide, at 10–8M or higher, enhanced the elongation of epicotyl segments from azuki bean seedlings that was induced by IAA, but it did not enhance the increase in fresh weight of the segments, an indication that brassinolide suppressed the lateral expansion of the segments. The additional elongation caused by brassinolide was completely prevented in the presence of 10–5 M cremart, which disrupted the cortical microtubules (MTs) in epidermal cells in the segments, and in the presence of 10–6M 2,6-dichlorobenzonitrile, an inhibitor of the synthesis of cellulose. Brassinolide at 10–7M, applied together with IAA, increased the percentage of epidermal cells with transversely oriented cortical MTs. Brassinolide appears to enhance the longitudinal expansion and suppress the lateral expansion of epicotyl cells by organizing cortical MTs transversely to the cell axis and, thereby, causing the deposition of cellulose microfibrils in the same orientation.
Brassinolide by itself, at 10–8M or higher, induced the elongation of epicotyl segments and the elongation caused by brassinolide was partially prevented by 10–5M cremart, results that suggest that brassinolide regulates cell expansion via at least two processes, an MT-dependent process and an MT-independent process. Brassinolide by itself increased the percentage of epidermal cells with transversely oriented cortical MTs. Since, in azuki bean epicotyls, the percentage of cells with transverse MTs is increased only by the combination of auxin and gibberellin but not by either alone, brassinolide applied alone seems to play a double role, similar to that of auxin and of gibberellin, in organizing cortical MTs.
(Received September 2, 1994; Accepted November 16, 1994)
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