Photosynthetic responses and identification of salt tolerance genes in a chromosome segment substitution line of 'Khao dawk Mali 105' rice

The rice chromosome segment substitution lines (CSSLs) with a 'Khao Dawk Mali 105' ('KDML105') genetic background and the putative drought tolerance genetic region of DH212 chromosome 1, were evaluated for salt tolerance. Lines CSSL11, CSSL12, and CSSL16 containing the full putative salt tolerance genetic region were compared with the parental lines, 'KDML105' and DH212. The CSSL16 plants were significantly more salt-tolerant than the 'KDML105' rice plants. Therefore, the CSSL16 and 'KDML105' genome sequences were compared to identify additional potential introgressed regions. The major regions that differed between the two lines were located on chromosomes 1, 4, and 7. To identify candidate “major effect” genes, the different loci between CSSL16 and 'KDML105' plants were subjected to a co-expression network analysis, which detected 10 major hub genes. Six of these genes were determined to function in chloroplasts. The PsbS1 expression patterns were similar in CSSL16 and DH212 plants, but differed from that of 'KDML105' plants. Rice PsbS1 is a homolog of Arabidopsis thaliana CP22, which encodes a protein that binds to chlorophylls a and b. CP22 is localized in photosystem II and is responsible for non-photochemical quenching activities. Thus, the salt tolerance of knocked-out Arabidopsis thaliana mutant lines lacking CP22 was assessed. A mutation in CP22 increased the susceptibility of the mutant lines to salt stress. The cp22 mutants also exhibited decreased quantum yield efficiency of photosystem II (Fv/Fm) and inhibited growth relative to the wild-type plants grown under salt stress conditions. These results suggest that rice PsbS1 is important for the adaptability and maintenance of photosynthetic activities in salt-stressed rice plants.