Effects of genome doubling on expression of genes regulating grain size in rice

Rice is one of the most important staple crops. It has been the major focus in breeding program to improve grain yield. A unique feature of tetraploid rice is the increased grain size and weight compared to diploid. Therefore, investigating the effects of genome doubling on expression of genes regulating grain size is important for yield improvement in rice breeding program. In this study, we analyzed differential expression of six genes regulating grain size in young panicles of various developmental stages between diploid and tetraploid rice. Transgenic approaches were employed to explore the dosage effects on gene expression and grain size. The results showed that genome duplications did not influence the developmental patterns of rice growth, but enhanced plant height, leaf width and grain size. The grain length and width in Indica tetraploid increased significantly, but the grain length showed more obvious change than width in Japonica tetraploid. The expression levels were affected not only by the developmental stages, but also by genetic background. Upon genome doubling, the positive regulation gene GS5 and HGW expression levels were generally higher in tetraploid than the corresponding diploid. Negative regulation gene GS3 in Indica tetraploid tended to be down-regulated or silenced, but increased in Japonica tetraploid. Another negative regulation GW2 was up-regulated in Indica tetraploid and silenced in Japonica tetraploid. The extra copies of GW2 in diploid transgenic lines exerted a gene dosage effect that resulted in the higher expression level than that of wild type diploid and tetraploid, which causes small grain formation in transgenic lines. Our results will help to understand the function of genes regulating the grain size in the diploid and tetraploid, and provide a theoretical basis for yield improvement.