Rice is one of the three most important food crops in the world. Increasing rice yield is an effective way to ensure food security. Grain size is a key factor affecting rice yield; however, the genetic and molecular mechanisms regulating grain size have not been fully investigated. In this study, we identified a rice mutant, wide grain 4-D (wg4-D), that exhibited a significant increase in grain width and a decrease in grain length. Histological analysis demonstrated that WG4 affects cell expansion thereby regulating grain size. MutMap-based gene mapping and complementary transgenic experiments revealed that WG4 encodes an alpha-tubulin, OsTubA1. A SNP mutation in WG4 affected the arrangement of cortical microtubules and caused a wide-grain phenotype. WG4 is located in nuclei and cytoplasm and expressed in various tissues. Our results provide insights into the function of tubulin in rice and identifies novel targets the regulation of grain size in crop breeding.

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 indica cultivars, 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 qPLN10 for 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-TAW1 showed increased panicle length (14.0%), number of secondary branches (20.9%) and GNP (22.0%), and the final grain yield per plant of NIL-TAW1 was increased by 18.6%. Transgenic experiments showed that an appropriate expression level of TAW1 was necessary for panicle development. Haplotype analysis suggested that the favorable F771-type (Hap 13) of TAW1 was introduced from aus accessions and had great potential value in high-yield breeding both in indica and japonica varieties. Our results provide a promising genetic resource for rice grain yield improvement.