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 non-uniform 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.

Effects of non-uniform root zone salinity on growth, ion regulation, and antioxidant defense system in two alfalfa cultivars.

A split-root system was established to investigate the effects of uniform (0/0, 50/50, and 200/200 mM salt [NaCl]) and non-uniform (0/200 and 50/200 mM NaCl) salt stress on growth, ion regulation, and the antioxidant defense system of alfalfa (Medicago sativa) by comparing a salt-tolerant (Zhongmu No.1) and salt-sensitive (Algonquin) cultivar. We found that non-uniform salinity was associated with greater plant growth rate and shoot dry weight, lower leaf Na+ concentration, higher leaf potassium cation (K+) concentration, lower lipid peroxidation, and greater superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6), and peroxidase (EC 1.11.1.7) activities, compared to uniform salt stress in both alfalfa cultivars. Under non-uniform salinity, a significant increase in Na+ concentration and Na+ efflux and a decline in K+ efflux in the no-saline or low-saline part of the roots alleviated salt damage. Our results also demonstrated that proline and antioxidant enzymes accumulated in both the no- or low-saline and high-saline roots, revealing that osmotic adjustment and antioxidant defense had systemic rather than localized effects in alfalfa plants, and there was a functional equilibrium within the root system under non-uniform salt stress. The salt-tolerant cultivar Zhongmu No.1 exhibited greater levels of growth compared to Algonquin under both uniform and non-uniform salt stress, with Na+ tolerance and efflux abilities more effective and greater antioxidant defense capacity evident for cultivar Zhongmu No.1.