Abstract
The effects of salt stress on the growth, nodulation, and nitrogen (N) fixation of legumes are well known, but the relationship between symbiotic nitrogen fixation (SNF) driven by rhizobium–legume symbiosis and salt tolerance in Medicago truncatula is not well studied. The effects of the active nodulation process on salt stress tolerance of Medicago truncatula were evaluated by quantifying the compatible solutes, soluble sugars, and antioxidants enzymes, as well as growth and survival rate of plants. Eight weeks old plants, divided in three groups: (i) no nodules (NN), (ii) inactive nodules (IN), and (iii) active nodules (AN), were exposed to 150 mM of NaCl salt stress for 0, 8, 16, 24, 32, 40, and 48 h in hydroponic system. AN plants showed a higher survival rate (30.83% and 38.35%), chlorophyll contents (37.18% and 44.51%), and photosynthesis compared to IN and NN plants, respectively. Improved salt tolerance in AN plants was linked with higher activities of enzymatic and nonenzymatic antioxidants and higher K+ (20.45% and 39.21%) and lower Na+ accumulations (17.54% and 24.51%) when compared with IN and NN plants, respectively. Additionally, higher generation of reactive oxygen species (ROS) was indicative of salt stress, causing membrane damage as revealed by higher electrolyte leakage and lipid peroxidation. All such effects were significantly ameliorated in AN plants, showing higher compatible solutes (proline, free amino acids, glycine betaine, soluble sugars, and proteins) and maintaining higher relative water contents (61.34%). This study advocates positive role of Rhizobium meliloti inoculation against salt stress through upregulation of antioxidant system and a higher concentration of compatible solutes.
Highlights
Global changes have increased abiotic stresses and mounted pressure on agriculture to produce more food from the existing land area to feed the ever-increasing human population
Plants showed a similar growth pattern among three nodulation groups (NN, inactive nodules (IN), and active nodules (AN)), and no significant differences were observed in shoot and root dry biomass and nitrogen content before salt stress treatment (Table 1)
The plants having active nodules produced in response to inoculation with Rhizobium meliloti strain maintained better growth and were better able to withstand salt stress compared to other plant groups (NN and IN)
Summary
Global changes have increased abiotic stresses and mounted pressure on agriculture to produce more food from the existing land area to feed the ever-increasing human population To overcome this challenge, there is a need to bring in large-area cultivation of problematic soils for crop production, such as salt-affected soils, which cover around 20%. Peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), and superoxide dismutase (SOD) are enzymes responsible for the scavenging of ROS and mainly exist as isoenzymes in different cell organelles such as mitochondria and chloroplasts [6,7] Osmolytes accumulation, such as those of proline, glycine betaine, free amino acids, soluble sugars, and proteins, is another metabolic phenomenon to overcome salt-induced stress in plants [8]
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