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.
A plant pathway that initiates with the formation of citramalate from pyruvate and acetyl-CoA by citramalate synthase (CMS) is shown to contribute to the synthesis of α-ketoacids and important odor-active esters in apple (Malus × domestica) fruit. Microarray screening led to the discovery of a gene with high amino acid similarity to 2-isopropylmalate synthase (IPMS). However, functional analysis of recombinant protein revealed its substrate preference differed substantially from IPMS and was more typical of CMS. MdCMS also lacked the regulatory region present in MdIPMS and was not sensitive to feedback inhibition. 13C-acetate feeding of apple tissue labeled citramalate and α-ketoacids in a manner consistent with the presence of the citramalate pathway, labeling both straight- and branched-chain esters. Analysis of genomic DNA (gDNA) revealed the presence of two nearly identical alleles in “Jonagold” fruit (MdCMS_1 and MdCMS_2), differing by two nonsynonymous single-nucleotide polymorphisms (SNPs). The mature proteins differed only at amino acid 387, possessing either glutamine387 (MdCMS_1) or glutamate387 (MdCMS_2). Glutamate387 was associated with near complete loss of activity. MdCMS expression was fruit-specific, increasing severalfold during ripening. The translated protein product was detected in ripe fruit. Transient expression of MdCMS_1 in Nicotiana benthamiana induced the accumulation of high levels of citramalate, whereas MdCMS_2 did not. Domesticated apple lines with MdCMS isozymes containing only glutamate387 produced a very low proportion of 2-methylbutanol- and 2-methylbutanoate (2MB) and 1-propanol and propanoate (PROP) esters. The citramalate pathway, previously only described in microorganisms, is shown to function in ripening apple and contribute to isoleucine and 2MB and PROP ester biosynthesis without feedback regulation.