Mutation of Mg-chelatase subunit I causes distinct phenotypes of chlorophyll accumulation and seedling etiolation in watermelon (Citrullus lanatus) and melon (Cucumis melo).

Chlorophyll biosynthesis is essential for plant growth and development, as it directly modulates photosynthetic capacity. Here, we identified yellow mutants with chlorophyll deficiency and abnormal thylakoid stacking-disturbed in watermelon (lethal) and melon (non-lethal), and mapped the traits to SNPs (H359R in watermelon, T193A in melon) in ChlI, which encodes Mg-chelatase subunit I. Suppression of ClChlI in watermelon and CmChlI in melon caused reduced chlorophyll accumulation and leaf yellowing. Moreover, the mutated ChlI exhibited altered autologous protein structures, and weakened interaction ability with itself and ChlD, especially the mutated ClChlI. Additionally, compared to melon mutant, the watermelon mutant had significantly lower abundances of chlorophyll biosynthesis- and photosynthesis-related proteins and metabolites. In summary, our findings lay a solid groundwork for further investigations into the structure-function relationship of the ChlI, and confers to the precise genetic improvement of crop agronomic traits.