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Grain weight and grain number are two important traits directly determining grain yield in rice. To date, a lot of genes related to grain weight and grain number have been identified; however, the regulatory mechanism underlying these genes remains largely unknown. In this study, we studied the biological function of OsSPL18during grain and panicle development in rice. Knockout (KO) mutants of OsSPL18exhibited reduced grain width and thickness, panicle length and grain number, but increased tiller number. Cytological analysis showed that OsSPL18regulates the development of spikelet hulls by affecting cell proliferation. qRT-PCR and GUS staining analyses showed that OsSPL18was highly expressed in developing young panicles and young spikelet hulls, in agreement with its function in regulating grain and panicle development. Transcriptional activation experiments indicated that OsSPL18 is a functional transcription factor with activation domains in both the N-terminus and C-terminus, and both activation domains are indispensable for its biological functions. Quantitative expression analysis showed that DEP1, a major grain number regulator, was significantly down-regulated in OsSPL18 KOlines. Both yeast one-hybrid and dual-luciferase (LUC) assays showed that OsSPL18 could bind to the DEP1promoter, suggesting that OsSPL18regulates panicle development by positively regulating the expression of DEP1. Sequence analysis showed that OsSPL18contains the OsmiR156kcomplementary sequence in the third exon; 5ʹ RLM-RACE experiments indicated that OsSPL18could be cleaved by OsmiR156k. Taken together, our results uncovered a new OsmiR156k-OsSPL18-DEP1pathway regulating grain number in rice.

Grain number per panicle (GNP) is a complex trait controlled by quantitative trait loci (QTL), directly determining grain yield in rice. Identifying GNP-associated QTL is desirable for increasing rice yield. A rice chromosome segment substitution line (CSSL), F771, which showed increased panicle length and GNP, was identified in a set of CSSLs derived from a cross between two indicacultivars, R498 (recipient) and WY11327 (donor). Genetic analysis showed that the panicle traits in F771 were semidominant and controlled by multiple QTL. Six QTL were consistently identified by QTL-seq analysis. Among them, the major QTL qPLN10for panicle length and GNP was localized to a 121-kb interval between markers N802 and N909 on chromosome 10. Based on quantitative real-time PCR and sequence analysis, TAWAWA1(TAW1), a known regulator of rice inflorescence architecture, was identified as the candidate gene for qPLN10. A near-isogenic line, NIL-TAW1, was developed to evaluate its effects. In comparison with the recurrent parent R498, NIL-TAW1showed increased panicle length (14.0%), number of secondary branches (20.9%) and GNP (22.0%), and the final grain yield per plant of NIL-TAW1was increased by 18.6%. Transgenic experiments showed that an appropriate expression level of TAW1was necessary for panicle development. Haplotype analysis suggested that the favorable F771-type (Hap 13) of TAW1was introduced from ausaccessions and had great potential value in high-yield breeding both in indicaand japonicavarieties. Our results provide a promising genetic resource for rice grain yield improvement.