In rice,the spikelet is the basic unit of inflorescence,and its development is important for determining the grain yield and quality.We reported a rice spikelet mutant multi-floret spikelet 4(mfs4)which resulted in the production of extra floral organs or a whole extra floret,and elongated sterile lemmas.The results suggested that the mutation of the MFS4 gene interfered with spikelet meristem determinacy and floral organ identity.In addition,the plant height and the grain length and width in the mfs4 mutant were all less than those in the wild type.Using the bulked segregant analysis method,the MFS4 gene was localized in a 557-kb region on the long arm of chromosome 1.Sequence analysis showed that there was a C-base deletion at the open reading frame of LOC_Os01g67430.Further tests indicated that a wild type copy of LOC Os01 g67430 was able to reverse the mfs4 defects,which indicated that LOC_Os01g67430 was the MFS4 gene.The MFS4 gene encodes a lipase located in the mitochondria and is expressed strongly in the young inflorescence.qRT-PCR results showed that the expression of some genes that were known to regulate spikelet meristem determinacy and grain size were decreased in the mfs4 mutant,which indicated that the MFS4 gene regulates spikelet meristem determinacy and grain size by modulating the expression of these genes.