Cloning of novel rice allene oxide cyclase (OsAOC):: mRNA expression and comparative analysis with allene oxide synthase (OsAOS) gene provides insight into the transcriptional regulation of octadecanoid pathway biosynthetic genes in rice

Here we cloned a novel rice (japonica-type cv. Nipponbare) cDNA highly homologous to allene oxide cyclase (AOC, EC 5.3.99.6; jasmonate biosynthetic pathway), named OsAOC. OsAOC showed significant similarity at amino acid level with the available AOCs only from the dicotyledonous tomato and Arabidopsis. Examination of the steady-state mRNA level in an early time course experiment revealed that OsA OC expressed very weakly in healthy leaves, and showed responsiveness to wounding by cut within 30 min, followed by an increase with time till 120 min. Interestingly, jasmonic acid (JA) treatment itself potently enhanced its expression over the control (wounded leaves), suggesting feedback activation of OsA OC. Other signalling components, salicylic acid (SA), ethylene (using ethephon, ET), abscisic acid (ABA), and hydrogen peroxide (H2O2), also strongly and transiently up-regulated the OsAOC transcript. Moreover, heavy metals and protein phosphatase (PP) inhibitors were potent in inducing the OsAOC transcript. A protein synthesis inhibitor, cycloheximide (CHX), alone or in combination with JA and SA further enhanced its expression. suggesting the involvement of a de novo synthesized negative regulator in its regulation. A long-term time course experiment revealed that wounding by cut was the most effective in inducing the expression of OsAOC starting at 3 h after treatment, whereas all the above pharmacological agents showed induction at 3-12 h, after which the OsA OC expression declined under light. whereas this expression was substantially suppressed under darkness. Additionally, chitosan (CT), a fungal elicitor also transiently up-regulated the OsA OC expression. Intriguingly, OsA OC mRNA expression manifested a clear rhythmicity under light/ dark cycle. Finally, parallel investigations of allene oxide synthase (OsA OS, accession number AY062258) and its comparison with OsAOC indicated their differential response to various environmental cues. These results strongly suggest a role for OsAOC (and OsAOS) in rice plant defense/stress response pathway(s). (C) 2003 Elsevier Science Ireland Ltd. All rights reserved.