Grain size (length, width, and thickness) determines grain weight and affects grain quality. Several major quantitative trait loci (QTLs) regulating grain size have been cloned while our understanding of the underlying mechanism on size of rice grains regulation remains fragmentary. Recently, a study conducted by China National Rice Research Institute (CNRRI) of Chinese Academy of Agricultural Sciences (CAAS) found a dominant QTL (GS2) can lead to larger cells and increased numbers of cells, which thus enhances grain weight and yield.

To understand the molecular basis of grain size, CNRRI scientists identified a number of large-grain lines, including a full-and-large grain line, Baodali (BDL, Oryza sativa ssp. indica). Morphological observation revealed that both grain length and width were significantly increased in BDL as compared to Zhonghua 11 (ZH11, Oryza sativa ssp. japonica). By using molecular markers to carry out primary QTL analysis, six QTLs were identified, which might correlate with grain length, weight, or size in BDL. Among these QTLs, a major grain-size QTL, named GRAIN SIZE ON CHROMOSOME 2 (GS2) explained 72% of the phenotypic variation in grain length and 36% of the variation in grain width. The spikelet hulls and brown grains of the F1 plants were larger than those of ZH11, suggesting that GS2 is a dominant locus regulating grain size.