Citramalate synthase yields a biosynthetic pathway for isoleucine and straight- and branched-chain ester formation in ripening apple fruit

Significance Fruit aroma influences herbivory and food choice by humans, ultimately affecting seed dispersal and plant reproductive success. Despite the significance of scent, our understanding of the biosynthesis of odor-active volatiles is incomplete. Herein, we detail a plant pathway that uses pyruvate and acetyl-CoA to form citramalic acid and, through a series of recursive reactions that bypass regulation at threonine deaminase, enables 1-C α-ketoacid elongation and synthesis of isoleucine and straight and branched chain esters. The initiating enzyme, citramalate synthase, is a neofunctionalized form of 2-isopropylmalate synthase that is insensitive to feedback inhibition. Engagement of the “citramalate pathway” in ripening fruit provides for an elevated and persistent production of isoleucine and volatile esters as fruit tissues ripen, age, and senesce.