A mutation in the promoter of the yellow stripe-like transporter gene in cucumber results in a yellow cotyledon phenotype

Leaf color mutants in higher plants are considered to be ideal materials for studying the chlorophyll biosynthesis, photosynthesis mechanism and chloroplast development. Herein, we identified a spontaneous mutant, yc412, in cultivated cucumber that exhibited yellow cotyledons. The yellow-lethal mutant was diagnosed with an abnormal chloroplast ultrastructure, and reduced photosynthetic capacity and pigment content. Through bulked segregant analysis-based whole-genome sequencing and fine genetic mapping, we narrowed the yellow cotyledons (yc) locus to a 96.8 kb interval on chromosome 3. By resequencing and molecular cloning, we showed that Csyc is a potential candidate gene, which encodes a yellow stripe-like (YSL) transporter. The T to C mutation in the promoter region of Csyc caused the yellow cotyledon phenotype in yc412. Compared to YZU027A (WT), the expression of Csyc was significantly downregulated in the cotyledons of yc412. Silencing of Csyc in cucumber via virus-induced gene silencing resulted in chlorotic leaves, mainly by suppressing the chlorophyll content. Furthermore, a comparative transcriptome analysis revealed that chloroplast-related genes and chlorophyll biosynthesis genes were significantly downregulated in yc412 cotyledons. Our results provide new insights into the molecular function of the YSL transporter in plant chloroplast development and chlorophyll synthesis.