A Possible Double Role for Brassinolide in the Reorientation of Cortical Microtubules in the Epidermal Cells of Azuki Bean Epicotyls |
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Authors: | Mayumi Keiko; Shibaoka Hiroh |
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Institution: | Department of Biology, Faculty of Science, Osaka University Toyonaka, Osaka, 560 Japan |
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Abstract: | Brassinolide, at 108M or higher, enhanced the elongationof epicotyl segments from azuki bean seedlings that was inducedby IAA, but it did not enhance the increase in fresh weightof the segments, an indication that brassinolide suppressedthe lateral expansion of the segments. The additional elongationcaused by brassinolide was completely prevented in the presenceof 105 M cremart, which disrupted the cortical microtubules(MTs) in epidermal cells in the segments, and in the presenceof 106M 2,6-dichlorobenzonitrile, an inhibitor of thesynthesis of cellulose. Brassinolide at 107M, appliedtogether with IAA, increased the percentage of epidermal cellswith transversely oriented cortical MTs. Brassinolide appearsto enhance the longitudinal expansion and suppress the lateralexpansion of epicotyl cells by organizing cortical MTs transverselyto the cell axis and, thereby, causing the deposition of cellulosemicrofibrils in the same orientation. Brassinolide by itself, at 108M or higher, induced theelongation of epicotyl segments and the elongation caused bybrassinolide was partially prevented by 105M cremart,results that suggest that brassinolide regulates cell expansionvia at least two processes, an MT-dependent process and an MT-independentprocess. Brassinolide by itself increased the percentage ofepidermal cells with transversely oriented cortical MTs. Since,in azuki bean epicotyls, the percentage of cells with transverseMTs is increased only by the combination of auxin and gibberellinbut not by either alone, brassinolide applied alone seems toplay 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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