| Summary: | Abstract:: Grain size is frequently selected during domestication and breeding. It influences the preferences of consumers, thus affecting the commercial value of rice. In present study, a mutant named as SLENDER GRAIN3 (sg3) was identified from cultivar Zhenong 41 (Oryza sativa L. ssp. indica) with ethyl methanesulfonate (EMS) treatment. Histological analysis showed that the slender grain of sg3 mutant resulted from increased cell division longitudinally and decreased cell division horizontally. Compared with the wild type Zhenong 41, starch granules in sg3 mutant were more closely packed, thus decreasing the chalkiness. Moreover, grain yield per plant in sg3 mutant was improved by 14%. By map-based cloning, SG3 was located on the long arm of chromosome 3 with a physical distance of 82 kb, and a 9-bp deletion in the 5⿲-UTR of LOC_Os03g27110 was identified, which upregulated the expression level significantly. Moreover, a molecular marker for SG3 was developed to identify the grain size during the early generation breeding in rice. The novel factor SG3 regulated the grain size mainly through changing the cell division and the endosperm formation in rice. Key words: grain size, gene mapping, phenotype analysis, rice
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