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Elucidating the biosynthesis of polyketides accumulating in barley (Hordeum vulgare) cuticular wax Ruiz Orduna, Alberto

Abstract

The epicuticular waxes covering the uppermost organs of the barley plant, including spikes, peduncles and flag leaf sheaths, are dominated by β-diketones and related hydroxy-β-diketones. Cultivars characterized by higher amounts of these compounds display improved resistance to water loss, therefore resulting in higher production yields. It was recently shown that biosynthesis of the different β-diketones and hydroxy-β-diketones relies on (1) a hydrolase known to participate in the formation of β-keto fatty acids, (2) a type-III polyketide synthase involved in β-diketone formation, and (3) a cytochrome P450 believed to perform the final hydroxylation of β-diketones. So far, only the hydrolase has been partially characterized, and the mechanism through which these enzymes form the different β-diketone-related compounds remains to be demonstrated. Here, a detailed chemical analysis of the β-diketone-related compounds found in the wax of barley cv. Morex spikes has revealed the presence of a novel β-diketone whose structure cannot be explained with the currently assumed biosynthesis pathways. In addition, a natural isotope abundance analysis of the predominant barley β-diketone revealed ¹³C enrichment also conflicting with previous biosynthesis hypotheses and prompting detailed studies of the polyketide synthase involved in β-diketone formation. This barley diketone metabolism polyketide synthase, DMP, was characterized in yeast and in vitro, revealing that it catalyzes the formation of β-diketones via head-to-head condensation between β-ketoacids and fatty acyl-CoAs. In the light of further results confirming that the first pathway enzyme, diketone metabolism hydrolase (DMH), delivers mainly C₁₆ β-ketoacid as substrate for DMP, a revised pathway for β-diketone biosynthesis in barley may now be proposed. Overall, the contribution of this research to the knowledge of barley β-diketones biosynthesis represents an important step towards the breeding of new cultivars with enhanced drought resistance and higher yield production. Interestingly, it seems very likely that current findings for β-diketone biosynthesis can be extrapolated from barley to wheat as a very closely related species, ultimately also enabling the breeding of stress-resistant wheat cultivars.

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