2.1. Plant Materials and Crossing Scheme. The scheme for constructing the plant materials used in this study is summarized in Figure 1. A highly-salt tolerant FL478 (IR66946-3R-178-1-1) was used as the donor of Saltol QTLs, whereas BT7 (O. sativa spp. indica), a popular growing
Vietnamese elite cultivar with high quality and popularly grown in the Red River Delta of Vietnam, was used as the recipient parent. A total of 477 SSR markers distributed in
the 12 chromosomes including foreground, recombinant, and background markers were screened. For the MABC scheme, BT7 was crossed with FL478 to obtain F1 seeds (Figure 1). F1s were backcrossed with BT7 to obtain a large number of BC1F1 seeds. In the BC1F1generation, individual plants that were heterozygous at the Saltol locus were identified reducing the population size for further screening (foreground selection). From the individual plants that were heterozygous for Saltol, those that were homozygous for the recipient allele at one marker locus (RM10825) distally
franking the Saltol locus (i.e., recombinant) were identified. We termed this as “recombinant selection” [18]. Some used markers are shown in detail in Table 1. From these recombinant plants, individuals with the fewest number of markers from the donor genome were selected (background selection). In the second and third BC generations, the same strategy was followed for selection of individual plants with the desired allele combination at the target loci
including selection for recombinants between Saltol and the nearest proximal marker locus (RM10694) and suitable genomic composition at the nontarget loci and crossed with the recipient parent to develop the next generation. The selected BC2 and BC3 plants were self-pollinated for further analyses.