Up-regulation of expression of tubulin genes and roles of microtubules in hypergravity-induced growth modification in Arabidopsis hypocotyls

We examined the roles of microtubules in gravity-induced modification of growth and development in plants by analyzing the expression levels of the α- and β-tubulin gene family and growth behavior of Arabidopsis hypocotyls treated with the microtubule-disrupting reagents colchicine, oryzalin, and propyzamide. Expression of the majority of the examined α- and β-tubulin genes was up-regulated by hypergravity at 300 g, although the extent was variable among genes, indicating that up-regulation of the expression of tubulin genes is the universal response of Arabidopsis hypocotyls to hypergravity. Hypergravity suppressed elongation growth by decreasing the cell-wall extensibility, whereas it stimulated lateral thickening of hypocotyls. By treatment with colchicine, oryzalin, and propyzamide, the elongation growth was suppressed, lateral thickening was stimulated, and the cell-wall extensibility of hypocotyls decreased dose-dependently even under 1 g conditions. The degree of hypergravity-induced changes decreased with increasing concentration of microtubule-disrupting reagents. As a result, hypergravity affected neither the length, the thickness, nor the cell-wall extensibility of hypocotyls in the presence of high concentrations of microtubule-disrupting reagents. Moreover, colchicine-treated seedlings showed helical growth even under 1 g conditions, and this phenotype was intensified under hypergravity conditions. These results suggest that the up-regulation of the expression of tubulin genes is involved in gravity-induced modification of microtubule dynamics, which may play an important role in the resistance of plant organs to the gravitational force and maintenance of normal growth phenotype.