Marker-assisted introgression of the salinity tolerance locus Saltol in temperate japonica rice

A Marker-Assisted Backcross (MABC) procedure was adopted in rice to transfer the Saltol locus conferring salt tolerance at the seedling stage, from a donor indica line (IR64-Saltol) to two renowned Italian japonica varieties, Vialone Nano and Onice. Forward and background selections were accomplished by using polymorphic KASP markers and a final evaluation of the genetic background of the selected lines was conducted using 15,580 SNP markers obtained with Genotyping by Sequencing (GBS). Three MABC generations, followed by two selfing generations, allowed the identification of the introgression lines having up to 100% recovery of the recurrent parent (RP) genome (based on the KASP markers data) or 98.97% (based on GBS data) in a time frame of three years. The comparison on the efficiency between the two genotyping procedures (96 KASP markers and SNP markers from GBS) conducted on the selected BC3F4 lines suggested that a chip of 96 polymorphic KASP markers can efficiently assist the background selection procedure. Introgression lines with the highest RP recovery and their recurrent parents were evaluated for agronomical-phenological relevant traits in field under non-salinized conditions. The results obtained supported the effectiveness of the MABC procedure adopted for recovering of the RP favorable traits and allowed the selection of three introgression lines (VN1, VN4 and O1) exploitable for commercial use. A physiological characterization was conducted on these three lines by evaluating their salt tolerance in hydroponic conditions. When comparing the three selected lines to their RP, they scored lower Standard Evaluation System (SES) scores, reduced electrolyte leakage (EL) values, limited negative effects on both relative water content (RWC) and shoot/root fresh weight ratio, as well as a better ability to maintain a relatively low the Na+/K+ molar ratio in the blade leaves, indicating an improved salinity tolerance.