Abstract
Lipoxygenases (LOXs) play important role in the synthesis of volatile organic compounds (VOCs), which influence the aroma of fruit. In this study, we elucidate that there is a positive relationship between LOXs activity and VOC production in melon (Cucumis melo), and CmLOX genes are involved in fruit aroma generation in melon. To this end, we tested four aroma types of melon that feature a thin pericarp: two aromatic cultivars of the oriental melons (C. melo var. makuwa Makino), ‘Yu Meiren’ (YMR) and ‘Cui Bao’ (CB); a non-aromatic oriental pickling melon (C. melo var. conomon), ‘Shao Gua’ (SHAO); and a non-aromatic snake melon (C. melo L. var. flexuosus Naud), ‘Cai Gua’ (CAI). A principal component analysis (PCA) revealed that the aromas of SHAO and CAI are similar in nature because their ester contents are lower than those of YMR and CB. Ethyl acetate, benzyl acetate, (E, Z)-2, 6-nonadienal and menthol are four principal volatile compounds that affect the aromatic characteristics of these four types of melons. The LOX activity and total ester content in YMR were the highest among the examined melon varieties. The expression patterns of 18 CmLOX genes were found to vary based on the aromatic nature of the melon. Four of them were highly expressed in YMR. Moreover, we treated the fruit disks of YMR with LOX substrates (linoleic acid and linolenic acid) and LOX inhibitors (n-propyl gallate and nordihydroguariaretic acid). Substrate application promoted LOX activity and induced accumulation of hexanal, (2E)-nonenal and straight-chain esters, such as ethyl acetate. In contrast, LOX inhibitors decreased the levels of these compounds. The effect of CmLOXs in the biosynthesis of esters in melons are discussed.
Highlights
The melon (Cucumis melo) is an important alternative model plant for studying fruit ripening characteristic, such as aromatic traits [1]
The solids content (SSC) directly correlated with fruit development and were maximized at 32 days after pollination (DAP) in YMR (13.83%), 32 DAP in Cui Bao’ (CB) (11.8%), 28 DAP in Shao Gua’ (SHAO) (4.3%), and 28 DAP in Cai Gua’ (CAI) (3.67%)
Especially straight-chain esters, as important volatile organic compounds (VOCs) in aromatic melons
Summary
The melon (Cucumis melo) is an important alternative model plant for studying fruit ripening characteristic, such as aromatic traits [1]. Most volatile aroma compounds are produced during the maturation period, and emission of volatile compounds is considered a sign of fruit maturity [5,6]. The volatile aroma compounds of melons have been thoroughly investigated, and approximately 240 volatile compounds have been identified in different melon varieties, including volatile aldehydes, alcohols, and an especially large number of esters [12,13,14,15]. Climacteric melons exhibit greater aroma intensity and contain significantly higher levels of volatile esters than non-climacteric melons [17,18]. Esters, which include branchedchain esters and straight-chain esters, are the most abundant aroma-relevant volatile compounds and considered key contributors to the unique aroma of melons[21,22,23,24,25]. Straight-chain esters are synthesized from the lipoxygenase (LOX) pathway [28,29]
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