A hypomorphic allele of the DJ-1 homolog CmYGP2 causes young yellow leaves that regreen in melon

A spontaneous virescent leaf mutant, Cmygp2 (originally designated MP), was identified in melon ( Cucumis melo L.). The mutant’s cotyledons and young true leaves are yellow but gradually turn green as the leaves mature, and adult plants set seeds normally. Genetic analysis revealed that this yellow-leaf trait is controlled by a single recessive nuclear gene. Through BSA-seq and fine mapping, the Cmygp2 locus was delimited to a 39.4-kb interval on chromosome 7 containing four annotated genes. Among these, only MELO3C016136 , which encodes a DJ-1 homolog, carried three polymorphisms: an 8-bp promoter deletion, a nonsynonymous A→G SNP in the sixth exon (Met281Val), and a synonymous C→T SNP. CmYGP2 transcript levels did not differ significantly between the mutant and the wild type. A natural variant (YM12) carrying the 8-bp promoter deletion but retaining the wild-type A allele exhibited normal green leaves, suggesting that the Met281Val substitution is the most likely causal variant. Subcellular localization showed that CmYGP2 is a chloroplast-localized protein, and VIGS-mediated silencing indicated that it is likely essential for leaf greening. Transcriptome analysis revealed that the photosynthesis–antenna proteins pathway (ko00196) was the most significantly enriched pathway; all 13 differentially expressed genes assigned to this pathway were down-regulated. This finding was consistent with a 56.9% reduction in total chlorophyll (Chl a + Chl b) content relative to the wild type (FP), with the mutant retaining 43.1% of the wild-type level, and with impaired photosynthetic efficiency (lower F v /F m and ETR) in the Cmygp2 mutant compared with the wild type (FP). Concurrently, multiple antioxidant-related pathways—including glutathione metabolism, the peroxisome pathway, and cysteine/methionine metabolism—were transcriptionally activated, whereas phenylpropanoid biosynthesis was reprogrammed, potentially forming a compensatory defense network. This study provides genetic evidence that a DJ-1 family protein is likely essential for chloroplast development in Cucurbitaceae. The virescent mutant can serve as a seedling-stage marker for hybrid purity identification.