Transcriptome analysis of genes and pathways associated with salt tolerance in alfalfa under non-uniform salt stress

Soil salinity of fields is often non-uniform. To obtain a better understanding of molecular response to non-uniform salt stress, we conducted transcriptomic analysis on the leaves and roots of alfalfa grown under 0/0, 200/200, and 0/200 mM NaCl treatments. A total of 233,742 unigenes were obtained from the assembled cDNA libraries. There were 98 and 710 unigenes identified as significantly differentially expressed genes (DEGs) in the leaves of non-uniform and uniform salt treatment, respectively. Furthermore, there were 5178 DEGs in the roots under uniform salt stress, 273 DEGs in the non-saline side and 4616 in the high-saline side roots under nonuniform salt stress. Alfalfa treated with non-uniform salinity had greater dry weight and less salt damage compared to treatment with uniform salinity. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis of the DEGs in roots revealed that both sides of the non-uniform salinity were enriched in pathways related to "phenylpropanoid biosynthesis" and "linoleic acid metabolism"; and "MAPK signaling pathway-plant" was also indicated as a key pathway in the high-saline roots. We also combined a set of important salt-response genes and found that roots from the non-saline side developed more roots with increased water uptake by altering the expression of aquaporins and genes related to growth regulation. Moreover, the hormone signal transduction and the antioxidant pathway probably play important roles in inducing more salt-related genes and increasing resistance to non-uniform salt stress on both sides of the roots.