PDC1, a pyruvate/α‐ketoacid decarboxylase, is involved in acetaldehyde, propanal and pentanal biosynthesis in melon (Cucumis melo L.) fruit

Abstract

Plant pyruvate decarboxylases (PDC) catalyze the decarboxylation of pyruvate to form acetaldehyde and CO 2 and are well known to play a key role in energy supply via fermentative metabolism in oxygen-limiting conditions. In addition to their role in fermentation, plant PDCs have also been hypothesized to be involved in aroma formation although, to date, there is no direct biochemical evidence for this function. We investigated the role of PDCs in fruit volatile biosynthesis, and identified a melon pyruvate decarboxylase, PDC1, that is highly expressed in ripe fruits. In vitro biochemical characterization of the recombinant PDC1 enzyme showed that it could not only decarboxylate pyruvate, but that it also had significant activity toward other straight-and branched-chain a-ketoacids, greatly expanding the range of substrates previously known to be accepted by the plant enzyme. RNAi-mediated transient and stable silencing of PDC1 expression in melon showed that this gene is involved in acetaldehyde, propanal and pentanal production, while it does not contribute to branched-chain amino acid (BCAA)-derived aldehyde biosynthesis in melon fruit. Importantly, our results not only demonstrate additional functions for the PDC enzyme, but also challenge the long standing hypothesis that PDC is involved in BCAA-derived aldehyde formation in fruit.

From Central to Specialized Metabolism: An Overview of Some Secondary Compounds Derived From the Primary Metabolism for Their Role in Conferring Nutritional and Organoleptic Characteristics to Fruit

“…Cyclization of lycopene in which lycopene ε-cyclase and lycopene β-cyclase are involved gives rise to α-carotene and β-carotene (orange pigments), while following hydroxylation by two non-heme carotene hydroxylases (BCH1 and BCH2) and two heme hydroxylases (CYP97A and CYP97C) produces yellow xanthophylls (Sun et al, 2018). Oxidative cleavage by carotenoid cleavage dioxygenases (CCDs) and non-enzymatic cleavage of carotenoid molecules between the C9 and C10 position, yield to apocarotenoid formation (also called norisoprenes), including phytohormones and volatile compounds such as α- and β-ionone, 6-methyl-5-hepten-2-one, or geranylacetone that play an important role in the aroma of fruits like tomato, melon, or apricot (Beltran and Stange, 2016; Hou et al, 2016; Tieman et al, 2017; Wang et al, 2019). Despite the intensive research in this field, little is known about the regulation of carotenoid metabolism.…”

“…Fatty acids serve as precursors for straight chain esters, while branched chain esters originate from branched chain amino acids. In addition, aromatic esters (such as benzyl acetate) derive from phenylalanine (Wang et al, 2019).…”

“…Furthermore, it seems that the regulation of volatile synthesis occurs downstream of their precursor supply, at the level of α-keto acid precursors (Kochevenko et al, 2012). Indeed, Wang et al (2019) showed that the decarboxylation of some α-keto acids by a pyruvate decarboxylase isoform ( PDC1 ) highly expressed in melon fruit was a limiting step in the synthesis of ethyl and pentyl esters. In particular, the decarboxylation of 2-oxobutanoate by PDC1 leads to propanal synthesis, which is an intermediate in the formation of several straight chain esters (Wang et al, 2019).…”

Melon ethylene-mediated transcriptome and methylome dynamics provide insights to volatile production

“…While the final steps in volatile ester biosynthesis involving AHD and AAT are largely understood, earlier steps in aldehyde formation remain uncertain. Recently CmPDC1 was found to mediate acetaldehyde biosynthesis in ripening melon fruit though additional factors are certainly involved (Wang et al, 2019). We demonstrate here that one such factor is CmTHA1 as demonstrated by +2 m/z labeling of acetaldehyde originating from threonine (Fig.…”

“…Acetaldehyde is synthesized from pyruvate, the end product of glycolysis via pyruvate decarboxylase (PDC) (Tietel et al, 2011). Recently CmPDC1, a ripening induced melon pyruvate decarboxylase has been shown to be involved in acetaldehyde biosynthesis and downstream ester accumulation (Wang et al, 2019). Acetaldehyde has been shown to be a limiting factor in ethanol and derived ester levels in feijoa and strawberry (Pesis and Avissar, 1990; Pesis et al, 1991).…”