Hypocotyls of Arabidopsis <i>hmg</i> mutants have an altered growth response to hypergravity

3-Hydroxy-3-methylglutaryl-CoA reductase (HMGR) catalyzes the rate-limiting step of mevalonate pathway for the biosynthesis of isoprenoids, such as sterols. To clarify the role of membrane sterols in plant response to the gravitational acceleration, we examined the growth behavior and cell wall properties of hypocotyls in Arabidopsis T-DNA insertion mutants for HMGR1 (hmg1) and HMGR2 (hmg2) grown under hypergravity conditions. Hypergravity suppresses elongation growth and stimulates lateral expansion of wild-type hypocotyls. Hypocotyl length was shorter in hmg mutants, whereas its diameter was larger than that in the corresponding wild-type at 1 G, and hypergravity did not modify those growth parameters. Hypergravity also decreased the cell wall extensibility in the wild-type hypocotyls. In hmg mutants, the cell wall extensibility was lower than that in wild-type hypocotyls under 1 G conditions, and hypergravity did not influence it. Additionally, the levels of cell wall polysaccharides per unit length of wild-type hypocotyls were increased by hypergravity. The polysaccharide levels in hmg mutant hypocotyls at 1 G were higher than those in the wild-type, and hypergravity did not further increase these levels. Thus, hmg mutant hypocotyls did not respond to hypergravity under the present experimental conditions. These results suggest that membrane sterols are responsible for the maintenance of normal growth anisotropy via the regulation of cell wall properties in plant organs, and are thereby involved in the growth response to the gravitational acceleration.