Suppression of bolting in the Arabidopsis <i>hmg1</i> mutant under microgravity conditions in space – Possible involvement of lipid rafts

Abstract

3-Hydroxy-3-methylglutaryl-CoA reductase (HMGR) catalyzes the rate-limiting step of the mevalonate pathway for biosynthesis of isoprenoids, such as sterols. An Arabidopsis T-DNA insertion mutant for HMGR1 (hmg1) produced roots, hypocotyls, rosette leaves, and inflorescences after germination at onorbit 1 g in the Resist Tubule space experiment. The processes up to rosette leaf growth were normal in the hmg1 mutant under microgravity conditions; however, bolting of the inflorescence stem was strongly suppressed. The expression of a series of genes involved in sterol biosynthesis, including sterol glycosyltransferases implicated in producing lipid raft component steryl glycosides, and some of the raft-associated genes in the apical region of inflorescences in the α-tubulin 4 mutant (tua4) was suppressed under microgravity conditions. In addition, the fluorescence due to the raft marker remorin (REM1.3) was uniformly distributed along the rim of the cells of Arabidopsis hypocotyls in microgravity, as in the on-orbit control, showing the presence of lipid rafts in the plasma membrane under microgravity conditions. These results suggest that a severe reduction in sterol levels due to the hmg1 mutation, in combination with downregulation of the expression of sterol biosynthesis genes, causes the suppression of bolting in Arabidopsis under microgravity conditions in space and that lipid rafts may be involved in the bolting.