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Key message: qGSN5, a novel quantitative trait locus coordinating grain size and grain number in rice, was fine-mapped to an 85.60-kb region. GS3 may be a suppressor of qGSN5. Grain size and grain number are two factors that directly determine rice grain yield; however, the underlying genetic mechanisms are complicated and remain largely unclear. In this study, a chromosome segment substitution line (CSSL), CSSL28, which showed increased grain size and decreased grain number per panicle, was identified in a set of CSSLs derived from a cross between 93-11 (recipient) and Nipponbare (donor). Four substitution segments were identified in CSSL28, and the substitution segment located on chromosome 5 was responsible for the phenotypes of CSSL28. Thus, we defined this quantitative trait locus (QTL) as grain size and grain number 5 (qGSN5). Cytological and quantitative PCR analysis showed that qGSN5 regulates the development of the spikelet hull by affecting cell proliferation. Genetic analysis showed that qGSN5 is a semi-dominant locus regulating grain size and grain number. Through map-based cloning and overlapping substitution segment analysis, qGSN5 was finally delimited to an 85.60-kb region. Based on sequence and quantitative PCR analysis, Os05g47510, which encodes a P-type pentatricopeptide repeat protein, is the most likely candidate gene for qGSN5. Pyramiding analysis showed that the effect of qGSN5 was significantly lower in the presence of a functional GS3 gene, indicating that GS3 may be a suppressor of qGSN5. In addition, we found that qGSN5 could improve the grain shape of hybrid rice. Together, our results lay the foundation for cloning a novel QTL coordinating grain size and grain number in rice and provide a good genetic material for long-grain hybrid rice breeding. [ABSTRACT FROM AUTHOR]