Molecular cloning and mRNA expression analysis of the first rice jasmonate biosynthetic pathway gene allene oxide synthase

The octadecanoid pathway metabolite jasmonic acid (JA) plays a vital role in rice (Oryza sativa L. cv. Nipponbare) defense/stress response(s). However, genes involved in its biosynthesis remain unidentified. Here, we cloned a novel rice cDNA highly homologous to the allene oxide synthase (EC 4.2.1.92) AOS gene, the first committed step in JA biosynthesis, showing significant similarity at the amino acid level with a related monocotyledoneous barley AOS. OsAOS is a novel member of the cytochrome P450 CYP74A subfamily and exists as a single copy gene in the rice genome. An examination of its steady state mRNA level in two-week-old seedling leaves revealed that OsAOS does not express constitutively in healthy leaves, and shows a weak responsiveness to cut. Signaling components of defense/stress pathways, in particular JA itself, strongly up-regulated the OsAOS transcript, whereas salicylate, ethylene, abscisic acid, and hydrogen peroxide were not so effective. Copper, a heavy metal also significantly enhanced the OsAOS expression. Protein phosphatase inhibitors proved to be the most potent in up-regulating the OsAOS mRNA level, suggesting the involvement of phosphorylation/dephosphorylation events in its regulation. Moreover, the inducible nature of OsAOS was influenced by light signal(s). Blast pathogen (Magnaporthe grisea) specifically elicited the accumulation of OsAOS mRNA in leaves in an incompatible versus compatible interaction, a first demonstration of pathogen responsiveness for any AOS gene to date. These results strongly suggest the importance of OsAOS in rice defense/stress response pathway(s).