Mutation in CmPPR confers a yellow-leaf phenotype and chloroplast defects in melon (Cucumis melo L.)

• The yellow-leaf trait in melon was controlled by a single recessive gene, designated CmPPR . • MELO3C021959 is the causal gene, encoding a P-type PPR protein ( CmPPR ), for Cmyl-1 . Leaf color variation in melon ( Cucumis melo L.) serves as a valuable model for investigating chlorophyll biosynthesis, chloroplast development and photosynthesis owing to its obvious morphological differences. In this study, a stable yellow-leaf mutant Cmyl-1 was characterized, exhibiting reduced chlorophyll and carotenoid contents, impaired chloroplast ultrastructure, and retarded plant growth. Genetic segregation analysis indicated that the yellow-leaf trait was regulated by a single recessive gene, and the green leaf showed complete dominance over the yellow leaf. Genetic mapping confined the Cmyl-1 locus to a 78.45-kb region between the CAPS markers 1-CAPS and 21-CAPS on chromosome 11. Genomic and sequencing results revealed a 12-bp insertion in the promoter and a single C to T transition in the exon of MELO3C021959 in Cmyl-1 ; the latter caused a Pro-to-Leu amino acid change. MELO3C021959 is presumed to be the candidate gene, encoding a P-type PPR protein ( CmPPR ), which localizes to the cytoplasm and chloroplasts, and possibly to the cell membrane. Silencing of CmPPR in cucumber via VIGS further confirmed its role as the causal gene for the Cmyl-1 phenotype. RNA-seq analysis further demonstrated significant downregulation of chloroplast-associated genes in the Cmyl-1 mutant. The results found in this study will not only contribute to melon genetic breeding, but also provide a useful insight into the molecular understanding of leaf color development in Cucurbitaceae crops.