Regulation of tomato HMG1 during cell proliferation and growth

The enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR, EC 1.1.1. 34) is encoded by a small multigene family in tomato (Lycopersicon esculent um Mill.) and catalyzes the synthesis of mevalonic acid (MVA): a committed step in the biosynthesis of sterols and isoprenoids. A chimeric HMG1::GUS r eporter gene fusion was used to analyze the regulation of HMG1 gene express ion in detail. HMG1 promoter 5' deletion mutants established the boundary o f a fully inducible promoter. In HMG1::GUS transgenic tomato plants, histoc hemical staining with 5-bromo-3-indolylglucuronide demonstrated that HMG1 w as primarily expressed in shoot and root meristems, and in young tomato fru it. This result was confirmed by both HMG1 in-situ hybridization and RNA ge l blot analysis. Tomato suspension cell experiments showed that steady-stat e HMG1 mRNA accumulated during lag and exponential growth phases, but not d uring the stationary phase. Transient expression of the HMG1::GUS in tissue culture cells treated with mevinolin indicated that HMG1 expression was su bject to feedback regulation by a biosynthetic product derived from MVA. Th ese results suggest that a primary, although not exclusive, role of HMG1 is to supply the MVA demand associated with cell division and growth.