In plants, 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR, EC 1.1.1.34) is a key enzyme regulating biosynthesis of phytosterols, plant growth regulators, carotenoids, antimicrobial defense compounds, and numerous other isoprenoids. To initiate molecular studies of HMGR in relation to defense responses in plants, we utilized yeast HMGR cDNA sequences to isolate tomato genomic sequences encoding HMGR. The nucleic acid sequence and gene structure was determined. The tomato HMGR gene (HMG2) contains four exons separated by three introns and encodes a polypeptide of 602 amino acid residues (about 64,714 Da). Two membrane-spanning regions are contained in the NH₂-terminus of the HMGR polypeptide. The COOH-terminus shares significant homology with HMGR sequences from different species. Genomic Southern hybridization analyses reveals that tomato contains 3 to 4 HMGR genes. The HMGz2 gene cross-hybridizes to mRNA of about 2.7 kb which is highly induced in tomato cells treated with fungal elicitors and in stems, leaves, or roots stressed by wounding suggesting that the HMGz2 is a defense-related gene in tomato. Hybridization with a gene specific probe indicates that the HMG2 gene is induced specifically during defense responses and is distinct from the gene(s) expressed during fruit development and ripening. / Ph. D.