Control of cell expansion and cortical microtubule arrangement by plant hormones

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1 Relationship between gibberellin (GA)-induced stimulation and ABA-induced inhibition of hypocotyl elongation and changes in the pattern of cortical microtubule (MT) arrangement was examined using light-grown cucumber seedlings. In the tangential surface of epidermal cells, the predominant pattern of MT arrangement was oblique, while in the radial surface of epidermal cells and cortex cells, MT arrangement was mostly transverse. GA increased and ABA decreased cells with transversely arranged MT. GA and ABA counteracted to each other. The frequency of the occurrence of cells with trasversely arranged MT in the tangential surface of the epidermis was found to be highly correlated to the growth rate of the hypocotyl, suggesting that the epidermis is an essential tissue for hypocotyl elongation.2 Relationship between the growth and the pattern of MT arrangement in the tangential surface of epidermal cells of the coleoptile was examined using D_8, a putative GA-receptor dwarf mutant of maize, and its normal sib. The length of the coleoptile as well as the cell length in D_8 seedlings was shorter than those in normal seedlings. In D_8, cells with obliquely and longitudinally arranged MT are dominant, while in normal those with transversely arranged MT are dominant. Treatment with uniconazole, a GA biosynthesis inhibitor, inhibited the growth of the coleoptile and increased cells with obliquely and longitudinally arranged MT in both D_8 and normal. The growth of the coleoptile seems to be controlled by endogenous GA through its control of MT arrangement.3 Brassinolide, a new plant hormone, also had an effect on MT arrangement similar to that of GA in the epidermal cells of light-grown cucumber hypocotyls.4 The in vitro formation of microtubules from tubulins extracted from etiolated pea seedlings was stimulated by GA and brassinolide.