ISOLATION OF A MONOCOT 3-HYDROXY-3-METHYLGLUTARYL COENZYME-A REDUCTASE GENE THAT IS ELICITOR-INDUCIBLE

The rice (Oryza sativa) phytoalexins, momilactones and oryzalexins, ar e synthesized by the isoprenoid pathway. An early step in this pathway , one that is rate-limiting in mammalian systems, is catalyzed by the enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR). A gene that encodes this enzyme has been isolated from rice, and found to con tain an open reading frame of 1527 bases. The encoded protein sequence of the rice HMGR appears to be conserved with respect to other HMGR p roteins, and 1 or 2 membrane-spanning domains characteristic of plant HMGRs are predicted by a hydropathy plot of the amino acid sequence. T he protein is truncated at its 5' end, and shows reduced sequence cons ervation in this region as compared to other plant sequences. The rice genome contains a small family of HMGR genes. The isolated gene, HMGR I, is expressed at low levels in both vegetative and floral organs of rice plants. It is not induced in plants by wounding, but is strongly and rapidly induced in suspension cells by a fungal cell wall elicito r from the pathogen Magnaporthe grisea, causal agent of ride blast dis ease. This suggests that HMGR I may be important in the induction of r ice phytoalexin biosynthesis in response to pathogen attack, and there fore may play a key role as a component of the inducible defense mecha nism in rice.