Functional Analysis of α-DOX2, an Active α-Dioxygenase Critical for Normal Development in Tomato Plants

Plant alpha-dioxygenases initiate the synthesis of oxylipins by catalyzing the incorporation of molecular oxygen at the alpha-methylene carbon atom of fatty acids. Previously, alpha-DOX1 has been shown to display alpha-dioxygenase activity and to be implicated in plant defense. In this study, we investigated the function of a second alpha-dioxygenase isoform, alpha-DOX2, in tomato (Solanum lycopersicum) and Arabidopsis (Arabidopsis thaliana). Recombinant Sl alpha-DOX2 and At alpha-DOX2 proteins catalyzed the conversion of a wide range of fatty acids into 2(R)-hydroperoxy derivatives. Expression of Sl alpha-DOX2 and At alpha-DOX2 was found in seedlings and increased during senescence induced by detachment of leaves. In contrast, microbial infection, earlier known to increase the expression of alpha-DOX1, did not alter the expression of Sl alpha-DOX2 or At alpha-DOX2. The tomato mutant divaricata, characterized by early dwarfing and anthocyanin accumulation, carries a mutation at the Sl alpha-DOX2 locus and was chosen for functional studies of alpha-DOX2. Transcriptional changes in such mutants showed the up-regulation of genes playing roles in lipid and phenylpropanoid metabolism, the latter being in consonance with the anthocyanin accumulation. Transgenic expression of At alpha-DOX2 and Sl alpha-DOX2 in divaricata partially complemented the compromised phenotype in mature plants and fully complemented it in seedlings, thus indicating the functional exchangeability between alpha-DOX2 from tomato and Arabidopsis. However, deletion of At alpha-DOX2 in Arabidopsis plants did not provoke any visible phenotypic alteration indicating that the relative importance of alpha-DOX2 in plant physiology is species specific.