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. 2019 Oct 10;14(10):e0223707.
doi: 10.1371/journal.pone.0223707. eCollection 2019.

Metabolomic analysis of the occurrence of bitter fruits on grafted oriental melon plants

Shuangshuang Zhang  1   2 , Lanchun Nie  1   2   3 , Wensheng Zhao  1   2   3 , Qiang Cui  1   3 , Jiahao Wang  1   3 , Yaqian Duan  1   3 , Chang Ge  1   2
Affiliations

Metabolomic analysis of the occurrence of bitter fruits on grafted oriental melon plants

Shuangshuang Zhang et al. PLoS One. .

Abstract

Grafting has been widely applied to melon (Cucumis melo L.) production to alleviate obstacles of continuous cropping and control soil-borne diseases. However, grafting often leads to a decline of fruit quality. For example, sometimes bitter fruits are produced on grafted plants. However, the underlying physiological mechanism still remains unclear. This study investigated the effects of different rootstocks on the taste of fruits of the Balengcui, an oriental melon cultivar, during summer production. The results showed that all grafted plants with Cucurbita maxima Duch. rootstocks produced bitter fruits, while non-grafted plants and plants grafted onto muskmelon rootstocks produced no bitter fruits. Liquid chromatography-mass spectrometry and metabonomic analysis were performed to investigate the mechanism underlying the occurrence of bitter fruits. Metabolite comparisons of fruits from plants grafted onto Ribenxuesong rootstocks both with non-grafted plants and plants grafted onto muskmelon rootstocks showed that 17 metabolites including phospholipids, cucurbitacins and flavonoids, exhibited changes. The three Cucurbitacins, Cucurbitacin O, Cucurbitacin C, and Cucurbitacin S, increased dramatically. The 10 phospholipids PS(18:1(9Z)/18:2(9Z,12Z)), PS(P-18:0/15:0), PA(18:1(11Z)/18:1(11Z)), PE(16:0/18:0), PS(O-16:0/17:2(9Z,12Z)), PI(16:0/18:2(9Z,12Z)), PA(15:0/22:6(4Z,7Z,10Z,13Z,16Z,19Z)), PS(P-16:0/17:2(9Z,12Z)), PS(22:0/22:1(11Z)), and PA(17:1(9Z)/0:0)) were significantly decreased, while two PA (16:0/18:2 (9Z, 12Z) and 16:0/18:1 (11Z)), two flavonoids (pelargonidin 3-(6''-malonylglucoside)-5-glucoside and malvidin 3-rutinoside) significantly increased in fruits of plants grafted onto Cucurbita maxima Duch. rootstocks. These metabolites were involved in the glycerophospholipid metabolic pathway, the mevalonate pathway, and the phenylpropanoid pathway. In summary, these results showed that the bitter fruits of grafted Balengcui were caused by Cucurbita maxima Duch. rootstocks. Phospholipids, cucurbitacins, and flavonoids were the key contributors for the occurrence of bitter fruits in Balengcui melon after grafting onto Cucurbita maxima Duch. rootstocks.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Base peak intensity (BPI) chromatograms of melon fruits from non-grafted plants and plants grafted onto either muskmelon or pumpkin rootstocks.
(A) BPI chromatogram in positive mode. (B) BPI chromatogram in negative mode.
Fig 2
Fig 2. Classification of identified fruit metabolites.
Different classifications of fruit metabolites are shown as different colors: Black, fatty acyls. Blue, glycerophospholipids. Orange, sterol lipids. Gray, polyketides. Green, glycerolipids. Purple, prenol lipids. Red, sphingolipids. Light blue, steroids and steroid derivatives. Yellow, carboxylicacids and derivatives. Dark red, organooxygen compounds. Pink, amino acids. Light green, other metabolites.
Fig 3
Fig 3. PCA score plot of melon fruits from non-grafted plants and plants grafted onto muskmelon or pumpkin rootstocks.
R2X = 0.763, Q2 = 0.219.
Fig 4
Fig 4. OPLS-DA score plot of melon fruits from non-grafted plants and plants grafted onto muskmelon or pumpkin rootstocks.
(A) OPLS-DA score plot of melon fruits from non-grafted plants and from plants grafted onto pumpkin rootstocks. R2X = 0.586, R2Y = 0.955, Q2 = 0.873. (B) OPLS-DA score plot of melon fruits from plants grafted onto muskmelon rootstocks and plants grafted onto pumpkin rootstocks. R2X = 0.598, R2Y = 0.972, Q2 = 0.881.
Fig 5
Fig 5. Significantly changed fruit metabolites identified via comparison between plants grafted onto pumpkin rootstocks and non-grafted plants or plants grafted onto muskmelon rootstocks.
Seventeen significantly changed fruit metabolites were identified including phospholipids (A), cucurbitacin (B), and flavonoids (C). Numbers 1–12 represent PS(18:1(9Z)/18:2(9Z,12Z)), PS(P-18:0/15:0), PA(18:1(11Z)/18:1(11Z)), PE(16:0/18:0), PA(16:0/18:1(11Z)), PA(16:0/18:2(9Z,12Z)), PS(O-16:0/17:2(9Z,12Z)), PI(16:0/18:2(9Z,12Z)), PA(15:0/22:6(4Z,7Z,10Z,13Z,16Z,19Z)), PS(P-16:0/17:2(9Z,12Z)), PS(22:0/22:1(11Z)), PA(17:1(9Z)/0:0), respectively. Numbers 13–15 represent Cucurbitacin C, Cucurbitacin S, and Cucurbitacin O, respectively. Numbers 16 and 17 represent pelargonidin 3-(6''-malonylglucoside)-5-glucoside and malvidin 3-rutinoside, respectively. Lowercase letters represent P < 0.05 and capital letters represent P < 0.01. The letter before "/" indicates the significance between plants grafted onto pumpkin rootstocks and non-grafted plants. The letter after "/" indicates the significance between plants grafted onto pumpkin rootstocks and muskmelon rootstocks.
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