A chlorophyllide a oxygenase gene YGL2 regulates leaf color and cold tolerance in melon (Cucumis melo L.)

• A novel yellow-green leaf mutant ( ygl2 ) in melon is linked to the chlorophyll a oxygenase gene ( CmCAO ). • The ygl2 mutant exhibits pleiotropic effects, including impaired chloroplast development and disruptions in chlorophyll metabolism and photosynthesis. • Loss of CmCAO function enhances sensitivity to cold stress by elevating ROS accumulation and membrane damage. Chlorophyll a oxygenase (CAO) facilitates the conversion of chlorophyll a into chlorophyll b , playing a crucial role in plant growth and grain yield. However, the identification and functional characterization of CAO gene in melon has not been documented till now. In this study, we characterize a yellow-green leaf2 ( ygl2 ) mutant in melon. Compared to wild-type plants, ygl2 exhibits a lower growth and photosynthetic rate, reduced chlorophyll content, and abnormal chloroplast ultrastructure. Through map-based cloning and subsequent verification, we identify variations in the exons of the chlorophyll a oxygenase gene CmCAO , as responsible for the mutant phenotype. YGL2, identified as CmCAO, is located within chloroplasts and demonstrates a high expression in juvenile leaves. In addition, the ygl2 mutant displays reduced enzyme activity and altered expression of genes involved in chlorophyll metabolism, photosynthesis, stress response, and immunology. Furthermore, the ygl2 mutant exhibits enhanced sensitivity to cold stress by increasing reactive oxygen species (ROS) production, malondialdehyde (MDA) accumulation and electrolyte leakage. These results indicate that YGL2 has pleiotropic effects in melon.