Abstract: 【Objective】 The present study was conducted aiming at genetic identification and candidate gene analysis of yellow-green leaf mutant 507ys. 【Method】 A yellow-green leaf mutant, designated as 507ys, was isolated from the progeny of a japonica rice cv. Nipponbare treated with ethyl methanesulfonate. Phenotypes of the 507ys mutant were observed and its main agronomic traits were analyzed under field conditions in Chengdu, Sichuan. After the 507ys mutant was crossed with normal green varieties, leaf colour phenotypes of the F1 progenies and the segregation ratio of yellow-green and green leaf plants in the F2 populations were investigated. Genetic mapping of the mutant gene was conducted using 360 yellow-green leaf individuals from the F2 mapping population of 507ys/Minghui63. Putative genes in the fine mapped region were analyzed, and the candidate genes in the mutant and its wild-type were sequenced, respectively. Alignment of the deduced amino acid sequences of homologous OsCAO1 proteins was conducted. In addition, contents of photosynthetic pigments in the 507ys mutant and its wild-type were determined by spectrophotometer, and their chlorophyll compositions were well-examined by high-performance liquid chromatography (HPLC) so that the candidate gene resulting in the 507ys mutant phenotype could be further identified.【Result】 The whole plant of the mutant exhibited yellow-green leaf trait throughout the growing period. Compared with its wild-type parent Nipponbare, the contents of chlorophyll and carotenoid decreased by 52.1% and 58.1%, and plant height, the number of productive panicles per plant, the number of spikelets per panicle and seed setting rate reduced by 8.3%, 51.0%, 7.4% and 11.6%, respectively. All F1 plants generated by crossing yellow-green leaf mutant 507ys with normal green varieties Nipponbare and Minghui 63 showed normal green leaf. The segregation ratios of normal leaf plants and yellow-green leaf plants in two F2 populations both 3:1, indicating the 507ys mutant phenotype is controlled by a single recessive nuclear gene. The mutant gene was finally mapped to a region of 60.2 kb between SSR marker RM333 and InDel marker L3 on the long arm near the end of chromosome 10, in which thirteen predicted genes were annotated in the Rice Genome Annotation Project. Sequencing analysis of these candidate genes between the mutant and its wild-type revealed the single base change (G2198A) in the coding region of the OsCAO1 (LOC_Os10g41780) gene for chlorophyllide a oxygenase resulted in a missense mutation (E353K) in the encoded product. HPLC analysis of chlorophyll composition indicated the 507ys mutant accumulated only chlorophyll a and no chlorophyll b.【Conclusion】 The 507ys mutant gene was allelic to OsCAO1 gene. A point mutation in exons of OsCAO1 gene in the 507ys mutant makes chlorophyllide a oxygenase inactive, resulting in the block in chlorophyll b biosynthesis.

Key words: rice , yellow-green leaf mutant , OsCAO1 , genetic identification , candidate gene analysis