Accumulation of photosynthesis gene transcripts in response to sodium chloride by salt-tolerant alfalfa cells

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Abstract
A cell line from diploid alfalfa (Medicago sativa L.), HG2-N1, which is stably tolerant to 1% (0.171 M) NaCl in the medium and noticeably more green than the parent line (HG2), was investigated to determine if increased chlorophyll accumulation was related to chloroplast development toward greater photosynthetic activity and increases in chloroplast-gene expression in this cell line. The ctDNA (chloroplast-DNA) copy number in HG2-N1 was found to be increased by 50% from that of the salt-sensitive parent HG2 line. RNA accumulation, as detected by hybridization to a nuclear actin gene, appeared comparable in HG2 and HG2-N1. In contrast, mRNA levels of the plastid-encoded psbA gene were increased in the salt-tolerant HG2-N1 grown on normal medium. The mRNA levels of HG2-N1 were further increased in response to added NaCl in the medium. The mRNA levels from other chloroplast genes necessary for photosynthesis (psbD, psaB, atpB, rbcL), as well as from several nuclear genes (pCab4, pCab1, rbcS) encoding polypeptides participating in photosynthesis, also increased in the salt-tolerant HG2-N1 when it was grown in the presence of NaCl whereas actin-mRNA and chloroplast-rRNA levels remained comparable under growth conditions ± NaCl. Several independently isolated salt-tolerant cell lines showed a similar response to salt, as monitored by accumulation of rbcL mRNA, indicating that transcript accumulation for photosynthesis genes is correlated with the salt response in these salt-tolerant alfalfa cell lines.