| Summary: | Leaf color mutants are the ideal materials to study the regulation mechanisms of chlorophyll biosynthesis and chloroplast development or as markers for crop breeding. In this study, we identified a virescent true leaf mutant <i>se59</i> from the ethyl methane sulfonate (EMS)-induced mutant lines of cucumber (<i>Cucumis sativus</i> L.). The <i>se59</i> mutant showed normal cotyledons, but the true leaf displays light green at early growth stage, which can recover normal green later. The <i>se59</i> locus was controlled by a single recessive nuclear gene. The grana stacking in the chloroplasts of <i>se59</i> decreased significantly, and both the photosynthetic ability and the photosynthetic pigment contents of the <i>se59</i> were significantly lower than those of wild type. The results of BSA-seq and genotyping showed that an Invertase/Pectin Methyl Esterase Inhibitor (INV/PMEI) protein encoded by <i>CsSE59</i> is a candidate gene for the virescent true leaf mutant. The expression level of <i>CsSE59</i> in stem, leaf and root is high. Based on the transcriptome analysis of the first true leaf of <i>se59</i> mutant, the expression levels of 17 leaf color related genes changed significantly, suggesting <i>CsSE59</i> may regulate virescent true leaf by interacting with some of these genes in cucumber. The identification of <i>CsSE59</i> is helpful to clarify the role of INV/PMEI in chloroplast development and to understand the mechanisms of leaf color variation in cucumber.
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