Fig. 1: Proposed β-diketone-forming pathways in barley.

A Pathways leading from acyl precursors (top left gray box) to various wax components (other gray boxes). Common wax components are formed by acyl-Acyl Carrier Protein (ACP) and acyl-CoA elongation (top row). The branch pathways leading to β-diketones and associated 2-alkanol esters (bottom half of scheme) proceed via central 3-ketoacid intermediates: diketone metabolism hydrolase (DMH) intercepts 3-ketoacyl-ACPs of plastidial Fatty Acid Synthase (FAS) or 3-ketoacyl-CoA intermediates of ER-bound Fatty Acid Elongase (FAE). The 3-ketoacids are either converted into 2-alkanol esters (left side; long-dashed arrows) or into β-diketones (right side). Two reaction paths from 3-ketoacids to β-diketones are feasible: (i) The elongation hypothesis (short-dashed arrows) involves activation by a Long-Chain Acyl-CoA Synthetase (LACS), condensation with a C₂ unit (from malonyl-CoA) by diketone metabolism polyketide synthase (DMP), further elongations catalyzed by FAE(s), and head group removal either by a thioesterase (TE) and a decarboxylase or by CER3/CER1-like enzymes. (ii) Alternatively, the head-to-head condensation hypothesis (solid arrows) predicts that DMP condenses fatty acyl-CoA starters and 3-ketoacid extenders. A diketone metabolism cytochrome-P450 (DMC) enzyme then likely hydroxylates β-diketones to produce hydroxy-β-diketones. B List of β-diketones in barley waxes, with isomers color-coded by diketo-group position. For isomers with n ≠ m two alternative names counting from either terminus are given. The chain length of the ketoacyl generating each β-diketone is given in orange. The acyl-CoA chain length required for head-to-head condensation to each β-diketone and the number of malonyl extender units required for the alternative elongation pathway are given in blue. Dashes indicate β-diketone isomers which cannot be synthesized by elongation. KAS β-ketoacyl-ACP synthase, WS wax ester synthase, CER3 ECERIFERUM 3, CER1 ECERIFERUM 1.