A PATHWAY FOR THE BIOSYNTHESIS OF STRAIGHT AND BRANCHED, ODD-LENGTH AND EVEN-LENGTH, MEDIUM-CHAIN FATTY-ACIDS IN PLANTS

Pathways and enzymes of fatty acid synthase-mediated, long-even-chain (generally C-16-C-20) fatty acid synthesis are well studied, and general metabolism involved in short chain (C-4-C-7) fatty acid biosynthesis is also understood. In contrast, mechanisms of medium chain (C-8-C-14) fatty acid synthesis are unclear. Recent work suggests involvement of chain-elongation terminating thioesterases in medium-chain fatty acid formation in oilseeds and animals. We have shown that iso and anteiso-branched and straight, odd- and even-length, short-chain fatty acids esterified in plant-trichomegland-produced sucrose esters are synthesized by using carbon skeletons provided by modified branched chain amino acid metabolism/catabolism. The principal enzymes involved are those catalyzing leucine biosynthesis in all organisms and those leading to short-chain alcohols in mutant yeasts and alkyl acids in Clostridium species (products often serving as mammalian pheromones). Here we provide evidence that C-10-C-12 straight medium-chain and C-10-C-12 branched medium-chain acyl acids of tomato, C-6-C-8 straight-chain acyl acids of Petunia, and C-6 and C-8 branched acyl acids of Nicotiana glutinosa are formed by alpha-ketoacid elongation without participation of fatty acid synthase-mediated reactions or independent thioesterases. This different metabolism suggests greater integration of amino acid and fatty acid metabolism than previously considered and provides other avenues to study and manipulate not only straight even length but also odd- and even-length straight and branched medium-chain fatty acid biosynthesis.