Identification of a jasmonate-regulated allene oxide synthase that metabolizes 9-hydroperoxides of linoleic and linolenic acids.

Allene oxide synthase (AOS) is a cytochrome P-450 (CYP74A) that catalyzes the first step in the conversion of 13-hydroperoxy linolenic acid to jasmonic acid and related signaling molecules in plants. Here, we report the molecular cloning and characterization of a novel AOS-encoding cDNA (LeAOS3) from Lycopersicon esculentum whose predicted amino acid sequence classifies it as a member of the CYP74C subfamily of enzymes that was hitherto not known to include AOSs. Recombinant LeAOS3 expressed in Escherichia coli showed spectral characteristics of a P-450. The enzyme transformed 9- and 13-hydroperoxides of linoleic and linolenic acid to alpha-ketol, gamma-ketol, and cyclopentenone compounds that arise from spontaneous hydrolysis of unstable allene oxides, indicating that the enzyme is an AOS. Kinetic assays demonstrated that LeAOS3 was approximately 10-fold more active against 9-hydroperoxides than the corresponding 13-isomers. LeAOS3 transcripts accumulated in roots, but were undetectable in aerial parts of mature plants. In contrast to wild-type plants, LeAOS3 expression was undetectable in roots of a tomato mutant that is defective in jasmonic acid signaling. These findings suggest that LeAOS3 plays a role in the metabolism of 9-lipoxygenase-derived hydroperoxides in roots, and that this branch of oxylipin biosynthesis is regulated by the jasmonate signaling cascade.