CmbZIP11 regulates <i>CmPMT1</i> / <i>15</i> affecting homogalacturonan methyl-esterification and fruit softening in melon

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 <i>CmGoSAMT1</i> and the pectin methyltransferase genes <i>CmPMT1</i> and <i>CmPMT15</i> were critical functional genes for HG methyl-esterification in melon. However, overexpression of <i>CmGoSAMT1</i> cannot significantly alter fruit hardness, whereas overexpression of <i>CmPMT1</i> and <i>CmPMT15</i> 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 <i>C</i>-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 <i>CmPMT1</i> and <i>CmPMT15</i> was verified, which could activate the expression of <i>CmPMT1</i> and <i>CmPMT15</i> to regulate the methyl-esterification of HG, thereby affecting fruit softening.