Characterization of 3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase Activity during Maize Seed Development, Germination, and Seedling Emergence

Abstract

Seed development in maize (Zea mays) can be divided into three phases based upon morphology and regulation of seed storage protein synthesis (Randolph, 1936;Kiesselbach, 1949;Jones and Brenner, 1987; Kriz, 1989;Ober et al., 1991). During embryogenesis, crucial morphological events occur as evidenced by rapid cell growth, mitotic division, and differentiation. During maturation, the embryo and endosperm enlarge through cell expansion, and storage metabolites are synthesized and deposited (Randolph, 1936;Burr and Burr, 1976;Bewley and Marcus, 1990;Wallace et al., 1990;Ober et al., 1991). The onset of dormancy occurs at approximately 25 DAP. During this phase, the activities of most metabolic pathways decrease, and the seed undergoes desiccation.There is evidence that isoprenoid compounds are necessary during seed development and may function to regulate specific developmental processes; these compounds include membrane sterols and ABA (Gage et al., 1989; Hole et al., 1989;Belefont and Fong, 1991 also occur during seed development (Bewley and Marcus, 1990;Belefont and Fong, 1991). In spite of these isoprenoid requirements, there has been little investigation into how the synthesis of these compounds is regulated during seed development and seedling emergence. The enzyme HMGR catalyzes the conversion of HMG to MVA, the precursor to a11 isoprenoids. This reaction has been shown to be the rate-limiting step in mammalian isoprenoid biosynthesis (Bach et al., 1990). In plants, these isoprenoids include sterols, natural rubber, phytoalexins, carotenoid pigments, and growth regulators such as cytokinins, GA3, and ABA. These compounds play critica1 roles in metabolism, growth, and development at specific times and in specific plant tissues (Bach, 1987;Gray, 1987;Stermer and Bostock, 1987; Bach et al., 1990). It follows that the regulated synthesis of isoprenoids, and thus, the regulation of HMGR, is also vital to plant development. Although HMGR activity has been investigated in a variety of young plant tissues (Brooker and Russell, 1975;Russell, 1985; Bach, 1987;Stermer and Bostock, 1987; Narita and Gruissem, 1989; Bach et al., 1990;Ji et al., 1992), there has been no investigation of HMGR activity during seed development. To begin to understand the role of the regulation of isoprenoid synthesis during seed development, we have addressed the question of whether HMGR specific activity is rcgulated during normal maize development. We have examined microsomal HMGR specific activity in maize embryos and endosperm during seed development and during subsequent seedling emergence. MATERIALS AND METHODS Plant MaterialSeeds of Zea mays line FunkF(G4343) were backcrossed for four generations to provide the final inbred line used in this study. Seeds from field plants were harvested at various days after pollination; the pericarp was removed, and embryos and endosperm were excised. Embryos and endosperm were analyzed for HMGR specific activity.Dry FunkF seeds from the spring 1991 planting were used for the germination study. Whole ...

Expression of the Arabidopsis HMG2 gene, encoding 3-hydroxy-3-methylglutaryl coenzyme A reductase, is restricted to meristematic and floral tissues.

“…Analysis of HMGR activity during maize seed development has shown that levels are high in the endosperm and embryo during stages of rapid mitotic divisions (Moore and Oishi, 1993). Maize endosperm HMGR activity decreases in parallel to mitotic activity (Moore and Oishi, 1993). The restricted expression of the HMGP-GUS fusions in the dividing endosperm cells might reflect a role for HMGR in cell division similar to that previously discussed for the meristematic cells.…”

“…Expression of HMGR genes in rapidly dividing tissues has also been reported in other plants (Aoyagi et al, 1993;Cramer et al, 1993;Moore and Oishi, 1993). The expression of HMGP in meristematic tissues might be related primarily to the high rate of sterol biosynthesis required to sustain the active synthesis of membranes in actively dividing cells.…”

“…This latter possibility is supported by the observation that, in the presence of high concentrations of mevinolin (up to 50 pM), Arabidopsis seed germinated normally and the developing seedlings grew well for 2 to 3 days (A. Boronat and T.J. Bach, unpublished results). The presence of a pool of isoprenoid compounds or intermediates in quiescent maize seed has also been proposed (Moore and Oishi, 1993).…”

“…However, GUS activity declined when the embryo started the cellular divisions leading to the globular stage. Analysis of HMGR activity during maize seed development has shown that levels are high in the endosperm and embryo during stages of rapid mitotic divisions (Moore and Oishi, 1993). Maize endosperm HMGR activity decreases in parallel to mitotic activity (Moore and Oishi, 1993).…”

3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase Activity in the Endosperm of Maize vivipary Mutants

“…Plant microsomal HMGR has been shown to be regulated by light (Brooker and Russell, 1979;Ji et al, 1992;Moore and Oishi, 1993), phytohormones (Brooker and Russell, 1979;Russell and Davidson, 1982), wounding, fungal infection, and fungal elicitors (Stermer and Bostock, 1987). Results from our studies of HMGR activity during seed development suggest that this enzyme and its regulation are important during seed development.…”