Expression of Brassica juncea 3-hydroxy-3-methylglutaryl CoA synthase is developmentally regulated and stress-responsive

3-Hydroxy-3-methylglutaryl-coenzyme A synthase (HMGS) is an enzyme in meval onate biosynthesis. In plants, investigations have focused on HMG CoA reduc tase (HMGR) and less is known of the preceding enzyme, HMGS. To understand the regulation of HMGS, we have isolated a Brassica juncea cDNA encoding HM GS, BjHMGS1, for use as a hybridization probe in Northern blot analyses. Bj HMGS is expressed in all plant organs and shows developmental regulation in flower, seed and seedling, with highest expression in early development. I n seedlings, expression is highest in young hypocotyls and is induced durin g the greening of etiolated cotyledons. BjHMGS is down-regulated by abscisi c acid, osmotic stress and dehydration, the effects of which arrested seedl ing growth. Thus BjHMGS expression shows correlation with rapid cell divisi on and growth, like HMGR. This is not unexpected, as mevalonate is the prec ursor to many essential isoprenoid compounds, including sterols for membran e biogenesis. Wounding, methyl jasmonate or salicylic acid induce BjHMGS ex pression, suggesting that, like HMGR, HMGS is involved in defence. As in an imals, coordinated regulation of HMGS with HMGR occurred in B. juncea upon germination and in response to salicylic acid. HMGS assays confirmed that E scherichia coli-expressed recombinant BjHMGS1 shows HMGS activity that is i nhibited by F244, a specific inhibitor of HMGS. Southern blot analysis reve aled gene families encoding HMGS in Brassica species and a summation of hom ologous genes in the fusion amphidiploid genome of B. juncea, a bi-parental species derived from diploids B. nigra and B. campestris.