Changes in Free Amino Acid Content in ‘Jonagold’ Apple Fruit as Related to Branched-chain Ester Production, Ripening, and Senescence

Abstract

<jats:p>The relationship among the free amino acid content, the expression of genes related to branched-chain amino acid metabolism {branched-chain aminotransferase [BCAT], α-keto acid decarboxylase [pyruvate decarboxylase (PDC)], and threonine deaminase [TD]}, and the production of branched-chain (BC) esters during ripening and senescence in ‘Jonagold’ apple fruit (<jats:italic>Malus ×domestica</jats:italic>) was studied. Eighteen amino acids were measured by liquid chromatography coupled with tandem mass spectrometry. The content for all amino acids changed with developmental stage and some shared similar patterns of accumulation/diminution. The pattern for isoleucine differed from all other amino acids, increasing more than 20-fold during the ripening process. The onset of the increase was concomitant with the onset of increasing ethylene and BC ester production and the content remained elevated even during senescence. The elevated isoleucine levels are consistent with an increase in the flux through the pathway leading to the formation and degradation of the isoleucine precursor α-keto-β-methylvalerate, which is used for production of BC esters containing 2-methylbutanol and 2-methylbutanoate moieties. Unexpectedly, the content of threonine, the amino acid from which isoleucine is thought to be derived in plants, did not change in concert with isoleucine, but rather declined somewhat after ripening was well underway. Patterns in the expression of some, but not all, of the putative BCAT and PDC genes appeared to reflect the rise and fall in ester formation; however, the expression of putative TD genes did not change during ripening. The patterns in gene expression and amino acid content are interpreted to suggest that the synthesis of α-keto-β-methylvalerate and isoleucine during apple ripening may depend on an as yet uncharacterized pathway that bypasses threonine, similar to the citramalate pathway found in some bacteria.</jats:p>

Metabolic profiling reveals ethylene mediated metabolic changes and a coordinated adaptive mechanism of ‘Jonagold’ apple to low oxygen stress

“…E). The accumulation of isoleucine in the control apples is in agreement with previous reports (Magné et al , Defilippi et al , Sugimoto et al ). Valine accumulated significantly in the 1‐MCP treated but not in the control apples (Fig.…”

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

“…Tissue‐specific expression analysis revealed that PDC1 was expressed at a higher level in reproductive tissues compared with vegetative tissues, with an induction of expression in ripe fruit (Figure a), suggesting a role in ripening‐associated processes. PDCs identified in other fruits such as strawberry and apple also exhibit a ripening‐related expression pattern (Aharoni et al ., ; Moyano et al ., ; Sugimoto et al ., ). By contrast, PDC2 expression was low in fruit and highest in anther, pistil, ovary and root tissues (Figure b).…”

“…The general metabolic route for BCAA‐derived volatile formation in plants was thought to proceed in a similar way as that in bacteria or yeast, where BCAAs undergo an initial transamination leading to the formation of the corresponding branched‐chain α‐ketoacid, and decarboxylation followed by reduction and esterification giving rise to branched‐chain aldehydes, alcohols and esters, respectively (Hazelwood et al ., ). However, recent investigations in tomato fruit suggest that branched‐chain α–ketoacids, rather than BCAAs are the precursors of branched‐chain volatiles (Sugimoto et al ., ; Kochevenko et al ., ).…”

“…An alternative function to energy metabolism has been suggested because of the broad substrate specificity exhibited by some yeast PDCs (Vuralhan et al ., ; Gocke et al ., ; Romagnoli et al ., ). Specifically, plant PDCs have previously been hypothesized to have activity not only toward pyruvate to form acetaldehyde, but also toward other α‐ketoacids including branched‐chain α‐ketoacids, to form longer chain aldehydes in the context of aroma volatile biosynthesis (Schaffer et al ., ; Ortiz et al ., ; Sugimoto et al ., ). However, to date, there is no evidence for the involvement of PDCs in fruit aroma formation.…”

Pitaya: a potential plant resource of citramalic acid

“…The important function of citramalic acid has been reported and realized in plant. It participates in the tricarboxylic acid (TCA) cycle (Sugimoto et al, 2011), and is highly suitable for using as a ligand in coordination chemistry (Aakeroy et al, 2015). Moreover, citramalic acid is involved in the branched-chain amino acid biosynthesis pathway (Leroy et al, 2015) and is expected to act as an acid catalyst for the transesterification reaction (Ilham & Saka, 2009).…”