Homogalacturonan (HG) methyl-esterified modification pathway genome-wide expression analysis in two texture types of oriental melon fruits ‘HDB’ (crisp) and ‘HPM’ (mealy) at different developmental stages revealed that the Golgi S-adenosyl-L-methionine transporter gene CmGoSAMT1 and the pectin methyltransferase genes CmPMT1 and CmPMT15 were critical functional genes for HG methyl-esterification in melon. However, overexpression of CmGoSAMT1 cannot significantly alter fruit hardness, whereas overexpression of CmPMT1 and CmPMT15 appears to cause the fruit to synthesize more highly methyl-esterified HG, which are separated from each other due to the ‘steric effect’ of methyl groups on the C-6 carboxyl of D-GalA, and cannot be cross-linked by Ca2+ to form an ‘egg-box’ matrix making it more accessible to de-methylating and degrading enzymes (CmPMEs, CmPGs, and CmPLs), thus fruit susceptible to softening. In addition, a transcription factor CmbZIP11 co-expressed with CmPMT1 and CmPMT15 was verified, which could activate the expression of CmPMT1 and CmPMT15 to regulate the methyl-esterification of HG, thereby affecting fruit softening.

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