Leaf color mutants are common in higher plants that can be used as markers in crop breeding or as an important tool in understanding regulatory mechanisms in chlorophyll biosynthesis and chloroplast development. In virescent leaf mutants, young leaves are yellow in color, which gradually return to normal green when the seedlings grow large. In the present study, we conducted phenotypic characterization and genetic mapping of the cucumber virescent leaf mutant 9110Gt conferred by the v-1 locus. Total chlorophyll and carotenoid content in 9110Gt was reduced by 44% and 21%, respectively, as compared with its wild type parental line 9110G. Electron microscopic investigation revealed fewer chloroplasts per cell and thylakoids per chloroplast in 9110Gt than in 9110G. Fine genetic mapping allowed for the assignment of the v-1 locus to a 50.4 kb genomic DNA region in chromosome 6 with two flanking markers that were 0.14 and 0.16 cM away from v-1, respectively. Multiple lines of evidence supported CsaCNGCs as the only candidate gene for the v-1 locus, which encoded a cyclic-nucleotide-gated ion channel protein. A single nucleotide change in the promoter region of v-1 seemed to be associated with the virescent color change in 9110Gt. Real-time PCR revealed significantly lower expression of CsaCNGCs in the true leaves of 9110Gt than in 9110G. This was the first report that connected the CsaCNGCs gene to virescent leaf color change, which provided a useful tool to establish linkages among virescent leaf color change, chloroplast development, chlorophyll biosynthesis, and the functions of the CsaCNGCs gene.
Keywords: chloroplast development; cucumber; map-based cloning; virescent leaf.