Changes in membrane lipid composition in azuki bean epicotyls under hypergravity conditions: Possible role of membrane sterols in gravity resistance

Seedlings of azuki bean (Vigna angularis Ohwi et Ohashi) were cultivated under hypergravity conditions, and changes in membrane lipid composition in their epicotyls were analyzed. Under hypergravity conditions at 300g, the levels of total sterols, phospholipids, and fatty acids per fresh weight were kept higher, as compared with 1g controls. In particular, sterol levels were prominently increased by hypergravity. On the other hand, hypergravity did not clearly influence the levels of each phospholipid and glycolipid class, or their fatty acid compositions. Thus, the effect of hypergravity on membrane lipid metabolism was specific for sterol biosynthesis. In various regions of azuki epicotyls, high growth rate was associated with high sterol levels. Hypergravity suppressed elongation growth and stimulated lateral expansion of azuki epicotyls. In the presence of lovastatin, an inhibitor of sterol biosynthesis, at 30 μM, such changes in growth parameters occurred even under 1g conditions, suggesting that lovastatin made epicotyls hypersensitive to the gravitational force. These results support the hypothesis that membrane sterols are involved in maintenance of normal growth capacity of plant organs against gravity.