Genomic insights on the contribution of introgressions from Xian/Indica to the genetic improvement of Geng/Japonica rice cultivars

Hybridization between Xian/indica (XI) and Geng/japonica (GJ) rice combined with utilization of plant ideotypes has greatly contributed to yield improvements in modern GJ rice in China over the past 50 years. To explore the genomic basis of improved yield and disease resistance in GJ rice, we conducted a large-scale genomic landscape analysis of 816 elite GJ cultivars representing multiple eras of germplasm from China. We detected consistently increasing introgressions from three XI subpopulations into GJ cultivars since the 1980s and found that the XI genome introgressions significantly increased the grain number per panicle (GN) and decreased the panicle number per plant. This contributed to the improvement of plant type during modern breeding, changing multi-tiller plants to moderate tiller plants with a large panicle size and increasing the blast resistance. Notably, we found that key gene haplotypes controlling plant architecture, yield components, and pest and disease resistance, including IPA1, SMG1, DEP3, Pib, Pi-d2, and Bph3, were introduced from XI rice by introgression. By GWAS analysis, we detected a GN-related gene Gnd5, which had been consistently introgressed from XI into GJ cultivars since the 1980s. Gnd5 is a GRAS transcription factor gene, and Gnd5 knockout mutants showed a significant reduction in GN. The estimated genetic effects of genes varied among different breeding locations, which explained the distinct introgression levels of XI gene haplotypes, including Gnd5, DEP3, etc., to these GJ breeding pedigrees. These findings reveal the genomic contributions of introgressions from XI to the trait improvements of GJ rice cultivars and provide new insights for future rice genomic breeding.