The world population has been continuously increasing and has led to the growing demand for rice. It is therefore important to pay as much attention to the enhancement of grain yield as well as grain quality. Grain size is one of the major factors determining grain yield and quality. A large number of genes are known to be involved in regulation of grain size. However, the influence of their haplotype combination is still largely unknown. Of the previously characterized genes, we especially focused on the six genes (GS3, GS5, GS6, GW2, qSW5/GW5, and GW8/OsSPL16) to expand our understanding of regulation of grain size and to develop a regression equation model that can be used for molecular rice breeding. A total of 215 rice germplasms, which originated or developed from 28 rice-consuming countries, were used in this study. The genotyping analysis revealed that different alleles of the six genes were widely distributed in our germplasm collection and also showed significant associations with the differences in grain size. Interestingly, we found that the relatively small number of haplotype combinations preserved in diverse rice germplasms and showed significant associations with the differences in grain size. In addition, we also found that a single gene-specific allelic variation plays an important role in regulation of grain size in the presence of a certain type of haplotype combination. Based on these results, we developed a regression equation model for prediction of rice grain size. We expect that our model can be used for rice molecular breeding to develop new rice varieties having a grain size in a particular range.

Increasing demand on cereal grains to meet the population growth requires more production of rice as well as other cereals, which is supposed to rise up to 40% until 2030. Grain size and shape are critical factors determining grain yield. Several genes on grain shape and size have been reported, and of them, seven cloned genes were chosen for haplotype analysis, such as GS3, GW2, qSW5, GS6, GW8, GS5, and TGW6. We genotyped 218 rice varieties which had diverse grain size and shape originating from 25 countries. As for GS3 and qSW5, PCR markers were developed for point mutation and deletion, respectively. For the other five genes in which functional SNPs were reported, we designed primer sets to distinguish a fuctional allele for each gene. GW2 and TGW6 had two alleles, while the other five genes displayed three alleles which were evenly distributed throughout the population. Analysis on the relationship between haplotype of the genes and grain phenotype is in progress. We expect that desirable allelic combination of genes would assure the optimal grain size and shape for higher grain yield and market quality. This work was supported by a grant from the Next-Generation BioGreen 21 Program (Plant Molecular Breeding Center No.PJ009076), Rural Development Administration, Republic of Korea.