Characterization of RCI‐1, a chloroplastic rice lipoxygenase whose synthesis is induced by chemical plant resistance activators

A full‐length lipoxygenase cDNA (RCI‐1) has been cloned from rice (Oryza sativa) whose corresponding transcripts accumulate in response to treatment of the plants with chemical inducers of acquired resistance such as benzo(1,2,3)thiadiazole‐7‐carbothioic acid S‐methyl ester (BTH), 2,6‐dichloroisonicotinic acid (INA), and probenazol. In contrast, RCI‐1 transcript levels did not increase after inoculation with compatible and incompatible races of the rice blast fungus Magnaporthe grisea and the nonhost pathogen Pseudomonas syringae pv. syringae. RCI‐1 transcript levels also increased after exogenous application of jasmonic acid, but not upon wounding. Dose–response and time course experiments revealed a similar pattern of transcript accumulation and lipoxygenase activity in BTH‐treated rice leaves. Enzymatic analysis of recombinant RCI‐1 protein produced in Escherichia coli revealed that 13‐hydroperoxy‐octadecanoic acids were the predominant reaction products when either linoleic or linolenic acid used as a substrate. The RCI‐1 sequence features a putative chloroplast targeting sequence at its N‐terminus. Indeed, a protein consisting of the putative chloroplast transit peptide fused to green fluorescent protein was exclusively localized in chloroplasts, indicating that RCI‐1 is a chloroplastic enzyme.