Biosynthesis of Oxylipins by Rhizoctonia solani with Allene Oxide and Oleate 8S,9S-Diol Synthase Activities.

Abstract

Oxylipin biosynthesis by fungi is catalyzed by both the lipoxygenase (LOX) family and the linoleate diol synthase (LDS) family of the peroxidase-cyclooxygenase superfamily. Rhizoctonia solani, a pathogenic fungus, infects staple crops such as potato and rice. The genome predicts three genes with 9-13 introns, which code for tentative dioxygenase (DOX)-cytochrome P450 fusion enzymes of the LDS family, and one gene, which might code for a 13-LOX. The objective was to determine whether mycelia or nitrogen powder of mycelia oxidized unsaturated C18 fatty acids to LDS- or LOX-related metabolites. Mycelia converted 18:2n-6 to 8R-hydroxy-9Z,12Z-octadecadienoic acid and to an α-ketol, 9S-hydroxy-10-oxo-12Z-octadecenoic acid. In addition to these metabolites, nitrogen powder of mycelia oxidized 18:2n-6 to 9S-hydroperoxy-10E, 12Z-octadecadienoic, and 13S-hydroperoxy-9Z,11E-octadecadienoic acids; the latter was likely formed by the predicted 13-LOX. 18:1n-9 was transformed into 8S-hydroperoxy-9Z-octadecenoic and into 8S,9S-dihydroxy-10E-octadecenoic acids, indicating the expression of 8,9-diol synthase. The allene oxide, 9S(10)epoxy-10,12Z-octadecadienoic acid, is unstable and decomposes rapidly to the α-ketol above, indicating biosynthesis by 9S-DOX-allene oxide synthase. This allene oxide and α-ketol are also formed by potato stolons, which illustrates catalytic similarities between the plant host and fungal pathogen.