Transcriptional Regulation of Genes Encoding Key Enzymes of Abscisic Acid Metabolism During Melon (Cucumis melo L.) Fruit Development and Ripening

The ‘Elizabeth’ melon (Cucumis melo L. cv. Elizabeth) is regarded as a climacteric fruit, but it has characteristics of both climacteric and nonclimacteric fruits during the ripening stage. To clarify whether the ethylene-independent pathways during ripening were related to abscisic acid (ABA), four cDNAs that encode for key enzymes of ABA metabolism (CMNCEDs, CMCYP707A1, and CMBG1) were cloned and their transcription expression levels were analyzed. The levels of endogenous ABA and ethylene production in fruit ripening and the effect of exogenous ABA or nordihydroguaiaretic acid (NDGA) treatment on fruit senescence post-harvest were also investigated. The results showed that the ABA levels in young fruit were initially high then gradually decreased. At 20 days after fruit setting (DAFS) there was a rapid increase that peaked at 30 DAFS. Ethylene production, 1-aminocyclopropane-1-carboxylic acid (ACC) content, and ACC oxidase (ACO) activity were all at low levels at the early stages of fruit setting, and their peaks were observed 5 days after the ABA peak. The CMNCED3 gene and the β-glucosidase gene CMBG1 had similar expression patterns, with expression peaks that occurred at 30 DAFS, which was consistent with the accumulation of ABA. The expression of CMNCED2 was relatively high during the early stages and no peak value occurred in the ripening stage. The CMCYP707A1 gene was expressed mainly at the late ripening stage. The exogenous ABA treatment promoted fruit ripening and softening by the upregulation and expression of the ethylene synthesis genes CmACS1 and CmACO1 and the cell wall catabolic enzyme gene CmPG1. The effect was proportional to the concentration of ABA, but too much ABA (>500 μmol/L) had a negative effect on fruit development. The NDGA treatment also had a negative effect on fruit ripening and development. ABA and ethylene have a synergistic effect on the regulation of melon fruit ripening. ABA mainly acted in the early stages of development, whereas ethylene played a major role in later stages of ripening. The endogenous ABA levels were maintained in a dynamic balance of biosynthesis and catabolism, which were regulated by CMNCEDs, CMBG1, and CMCYP707A1.