Allelic differentiations and effects of the Rf3 and Rf4 genes on fertility restoration in rice with wild abortive cytoplasmic male sterility

To reveal the allelic differentiations at the two genes for fertility restoration (Rf) on chromosomes 1 (Rf3) and 10 (Rf4), 15 chromosome single segment substitution lines (SSSLs) with the Rf3 locus and 18 SSSLs with the Rf4 locus were crossed with Bobai A (BbA), a cytoplasmic male sterility line with wild abortive type of cytoplasm (WA-CMS), respectively. Based on the pollen and seed fertility of the F₁ hybrids, the Rf3 and Rf4 genes were each classified into four alleles, namely Rf3-1, Rf3-2, Rf3-3, and Rf3-4 for Rf3, and Rf4-1, Rf4-2, Rf4-3, and Rf4-4 for Rf4. Out of the 33 SSSLs, an SSSL W23-19-06-06-11 carrying the genotype Rf3-4Rf3-4/Rf4-4Rf4-4 possessed the strongest restoring ability for BbA. To determine the genetic effects of Rf3 and Rf4 for WA-CMS, one BC₃F₂ population possessing the genetic background of W23-19-06-06-11 was generated from the cross between W23-19-06-06-11 and BbA by backcrossing and marker-assisted selection. In the BC₃F₂ population, the plants carrying the Rf3Rf3/Rf4Rf4, Rf3Rf3/rf4rf4, and rf3rf3/Rf4Rf4 genotypes were selected and their phenotyping for pollen and spikelet fertility were evaluated. The result showed that under the genetic background of SSSL W23-19-06-06-11, the effect of Rf4 appeared to be slightly larger than that of Rf3 and their effects were additive for WA-CMS system. These studies will lead to the transfer of Rf genes into adapted cultivars through marker-assisted selection in active hybrid rice breeding programs.