Studies of 3-hydroxy-3-methylglutaryl coenzyme A reductase expression during Zea mays seed development.

Abstract

The first committed step in isoprenoid biosynthesis is the conversion of 3-hydroxy-3-methylglutaryl coenzyme A to mevalonate, which is catalyzed by the enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR). To begin to understand the role of isoprenoid synthesis in maize, the expression of 3-hydroxy-3-methylglutaryl coenzyme A reductase was examined. This study shows that the specific activity of the rate-limiting enzyme, HMGR, is regulated during Zea mays development. High levels of activity were correlated with stages of rapid, mitotic divisions. During seed development, HMGR specific activity is differentially regulated in the embryo versus the endosperm. During the transition from mitotic division to cell expansion in the seed, HMGR activity may represent a unique biochemical marker. The HMGR activity does not appear to be regulated at the levels of mRNA or protein accumulation. HMGR specific activity is inversely correlated with endogenous abscisic acid levels in Zea mays endosperm. HMGR activity was increased in the endosperm of three vivipary mutants, vp2, vp5, and vp7, which are defective in abscisic acid biosynthesis by the zygote. Examination of vp8 mutants revealed that high HMGR specific activity also was correlated with reductions in endogenous levels of maternally-derived abscisic acid. Abscisic acid appears to regulate HMGR throughout late embryogenesis and maturation. In addition, the HMGR specific activity was increased in the vp1 mutant, which is defective in an abscisic acid response element. There was no correlation between HMGR protein levels and specific activity patterns in these mutants. Exogenous abscisic acid inhibits HMGR specific activity in maize roots. Application of abscisic acid results in decreased HMGR transcript levels, and the inhibition of activity is protein kinase-dependent. Although abscisic acid is a negative regulator of HMGR, there appear to be other signals which set the overall pattern and level of HMGR activity and override the influence of the phytohormone. Examination of HMGR specific activity in vivipary embryos, demonstrated that signals which set the developmental stage of the tissue, also dictate the patterns of enzyme expression. What signals determine the developmental expression of HMGR remains to be determined.