2. Materials and methods
2.1. Rice materials
A total of 96 rice accessions were collected by Bangladesh Institute of Nuclear Agriculture (BINA) and used in this study. They included 86 landraces from southern Bangladesh, 9 indica varieties and lines, and salt tolerant Nona Bokra, the donor of SKC1, was used as the tolerant control (Table S1).
2.2. Screening for salinity tolerance
Hydroponic system based on the IRRI protocol [16] was used in the glasshouse at BINA to evaluate the salt tolerance responses of rice genotypes at the seedling stage. Three replications of 20 plants were tested under salt stress of 12 dS m− 1. The modified standard evaluation score (SES) of IRRI [17] was used to assess visual symptoms of salt toxicity 21 days after sowing. Binadhan-8 was used as a second tolerant control and Binadhan-7 was the susceptible control.
2.3. Marker genotyping
DNA was extracted from 6–8 individuals in each accession following the method of Zheng et al. [18]. To facilitate marker-assisted selection (MAS), sequence tagged site (STS) markers, rather than single nucleotide polymorphism (SNP) markers, were developed based on Insertion/Deletions (InDels) between the Nipponbare and 9311 genome sequences at the SKC1 (Chr. 1) [6], SalT (Chr. 1) [8] and DST (Chr. 3) [19] loci. Primers ( Table 1) were designed using Oligo 7.0 software. Eight STS markers were developed, 2 for SKC1, 3 for SalT and 3 for DST ( Table 1). Wn11463 and Wn11466 were designed based on 4 bp and 17 bp InDels downstream of SKC1 (LOC_Os01g20160); Wn13900 was based on a 4 bp InDel upstream of SalT (LOC_Os01g24710); Wn13902 and Wn13903 were based on 7 bp and 8 bp InDels in the SalT coding region; Th32637 was based on a 3 bp InDel upstream in DST (LOC_Os03g57240); and Th32638 and Th32369 were based on 12 bp and 18 bp InDels in the coding region of DST.