Biosynthesis of barley wax β-diketones: a type-III polyketide synthase condensing two fatty acyl units
Yulin Sun, Alberto Ruiz Orduna, Zhonghang Zhang, Sarah J. Feakins, Reinhard Jetter
Abstract
Abstract The surface coatings of cereal plants are dominated by waxy β-diketones crucial for drought resistance and, therefore, grain yield. Here, barley ( Hordeum vulgare ) wax analyses reveal β-diketone and associated 2-alkanol ester profiles suggesting a common C 16 3-ketoacid precursor. Isotope analysis further shows that the major (C 31 ) diketone is synthesized from two plastidial C 16 acyl units. Previous studies identified a gene cluster encoding enzymes responsible for β-diketone formation in barley, but left their biochemical functions unknown. Various assays now characterize one of these enzymes as a thioesterase producing long-chain (mainly C 16 ) 3-ketoacids, and another one as a polyketide synthase (PKS) condensing the 3-ketoacids with long-chain (mainly C 16 ) acyl-CoAs into β-diketones. The two enzymes are localized to the plastids and Endoplasmic Reticulum (ER), respectively, implying substrate transfer between these two sub-cellular compartments. Overall, our findings define a two-step pathway involving an unprecedented PKS reaction leading directly to the β-diketone products.