Identification of Rice Large Grain Gene GW2 by Whole-Genome Sequencing of a Large Grain-Isogenic Line Integrated with Japonica Native Gene and Its Linkage Relationship with the Co-integrated Semidwarf Gene d60 on Chromosome 2

Genetic analysis of InochinoIchi, an exceptionally large grain rice variety, was conducted through five continuous backcrosses with Koshihikari as a recurrent parent using the large grain F-3 plant in Koshihikari x Inochinoichi as a nonrecurrent parent. Thorough the F-2 and all BCnF(2) generations, large, medium, and small grain segregated in a 1:2:1 ratio, indicating that the large grain is controlled by a single allele. Mapping by using simple sequence repeat (SSR) and single nucleotide polymorphism (SNP) markers with small grain homozygous segregants in the F-2 of Nipponbare x Inochinoichi, revealed linkage with around 7.7 Mb markers from the distal end of the short arm of chromosome 2. Whole-genome sequencing on a large grain isogenic Koshihikari (BC4F2) using next-generation sequencing (NGS) identified a single nucleotide deletion in GW2 gene, which is located 8.1 Mb from the end of chromosome 2, encoding a RING protein with E3 ubiquitin ligase activity. The GW2-integrated isogenic Koshihikari showed a 34% increase in thousand kernel weight compared to Koshihikari, while retaining a taste score of 80. We further developed a large grain/semi-dwarf isogenic Koshihikari integrated with GW2 and the semidwarfing gene d60, which was found to be localized on chromosome 2. The combined genotype secured high yielding while providing robustness to withstand climate change, which can contribute to the New Green Revolution.