Polyketide Synthase-Like Functionality Acquired by Plant Fatty Acid Elongase
Fatty acid elongation (FAE) typically proceeds through a four-step cycle of condensation, reduction, dehydration, and reduction to yield very long-chain fatty acids. Here, we describe a variation of this pathway in Orychophragmus limprichtianus, whose seed oil was found to contains previously unknown C24-C28 keto-hydroxy fatty acids that account for [~]25% of total fatty acids. Our studies revealed that these compounds are produced through an endoplasmic reticulum-localized discontinuous elongation process in which a 3-keto hydroxy intermediate bypasses full reduction and is extended through a polyketide synthase-like mechanism. Transcriptomic and functional assays identified two divergent enzymes, OlFAE1-2 and the low-activity ketoreductase OlKCR1-1, as central to this process. Protein modeling and mutant analysis suggest that specific amino acid substitutions underlie the altered activity of OlKCR1-1, enabling accumulation of keto intermediates. A portion of these unusual fatty acids accumulated in estolides, complex triacylglycerols that expand the chemical repertoire of seed oils. Together, our findings reveal unexpected flexibility in plant fatty acid elongation and provide new tools for engineering plants and microbes to produce renewable oils with tailored industrial functions.