Transcriptional and post-transcriptional activation of genes in salt-tolerant alfalfa cells

Salt-tolerant cell lines of alfalfa (Medicago sativa L.) selected in this laboratory showed increased mRNA accumulation for both nuclear- and chloroplast-encoded genes involved in photosynthesis as well as in several non-photosynthetic related functions. The basis for this constitutive and salt-dependent gene activation was investigated by measuring both nuclear and plastid run-on transcription from the salt-sensitive parent line and from selected salt-tolerant lines. Plastids from tolerant cells showed a 2.5-fold increase in transcription rate over those from sensitive cells and a 4.5-fold increase if isolated from tolerant cells grown in salt. Nuclei isolated from salt-tolerant cells grown on normal medium showed higher transcription of the photosynthesis-related genes rbcS, cab1 and cab4 than those from salt-sensitive cells, confirming that the salt-tolerant cells had acquired altered transcriptional regulation of these genes. However, the major salt-induced increase in steady-state mRNA accumulation, from photosynthesis-related and other genes (alfin1, pA18 and histone H3cI and H3cII genes), was not reflected in run-on assays from these same cells. These results indicated that salt-dependent post-transcriptional mRNA stabilization led to the steady-slate mRNA accumulation. The mRNA stabilization appears to be transcript specific, since transcripts of a constitutively-expressed gene (Msc27) remained unaffected by growth of the tolerant cells in 171 mM NaCl.