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Effects of hypergravity on expression of XTH genes in azuki bean epicotyls.

Hypergravity produced by centrifugation caused inhibition of elongation growth and a decrease in the cell wall extensibility in azuki bean epicotyls (Vigna angularis Ohwi et Ohashi). Also, hypergravity increased the molecular mass of xyloglucans, whereas it decreased xyloglucan-degrading activity in epicotyls. When the expression profiles of three xyloglucan endotransglucosylase/hydrolase (XTH) genes, VaXTHS4, VaXTH1 and VaXTH2, were analyzed under hypergravity conditions, the expression of VaXTHS4, which shows only hydrolase activity, was downregulated in proportion to the logarithm of the magnitude of gravity (R = -0.94). However, the gene expression of VaXTH1 or VaXTH2, which shows only transglucosylase activity, was not affected by gravitational conditions. When the seedlings that had been grown at 1 g were transferred to hypergravity conditions at 300 g, the downregulation of VaXTHS4 expression was detected within 1 h. By removal of hypergravity stimulus, VaXTHS4 expression was increased within 1 h. These results suggest that azuki bean epicotyls promptly regulate the expression level of only VaXTHS4 in response to gravity stimuli. The regulation of xyloglucan-hydrolyzing activity as a result of changes in VaXTHS4 expression may be involved in the regulation by gravity of molecular mass of xyloglucans, leading to modifications of cell wall mechanical properties and cell elongation. Lanthanum and gadolinium, potential blockers of mechanosensitive calcium ion permeable channels (mechanoreceptors), nullified the suppression of VaXTHS4 expression, suggesting that mechanoreceptors are responsible for inhibition by hypergravity of VaXTHS4 expression.