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

Abstract

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 a llene o xide s ynthase (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).