Abstract
Salinity is one of the strongest abiotic factors in nature and has harmful effects on plants and microorganisms. In recent years, the degree of soil salinization has become an increasingly serious problem, and the use of plant growth-promoting rhizobacteria has become an option to improve the stress resistance of plants. In the present study, the salt tolerance mechanism of the rhizosphere bacterium Rahnella aquatilis JZ-GX1 was investigated through scanning electron microscopy observations and analysis of growth characteristics, compatible solutes, ion distribution and gene expression. In addition, the effect of JZ-GX1 on plant germination and seedling growth was preliminarily assessed through germination experiments. R. aquatilis JZ-GX1 was tolerant to 0–9% NaCl and grew well at 3%. Strain JZ-GX1 promotes salt tolerance by stimulating the production of exopolysaccharides, and can secrete 60.6983 mg/L of exopolysaccharides under the high salt concentration of 9%. Furthermore, the accumulation of the compatible solute trehalose in cells as the NaCl concentration increased was shown to be the primary mechanism of resistance to high salt concentrations in JZ-GX1. Strain JZ-GX1 could still produce indole-3-acetic acid (IAA) and siderophores and dissolve inorganic phosphorus under salt stress, characteristics that promote the ability of plants to resist salt stress. When the salt concentration was 100 mmol/L, strain JZ-GX1 significantly improved the germination rate, germination potential, fresh weight, primary root length and stem length of tomato seeds by 10.52, 125.56, 50.00, 218.18, and 144.64%, respectively. Therefore, R. aquatilis JZ-GX1 is a moderately halophilic bacterium with good growth-promoting function that has potential for future development as a microbial agent and use in saline-alkali land resources.
Highlights
Soil salinization is a worldwide soil degradation problem that causes damage to resources and harms environmental and economic development (Zhang et al, 2010)
R. aquatilis JZGX1 was able to grow on plates containing 0–9% NaCl, indicating that strain JZ-GX1 is a moderately halophilic bacterium
On the 1st day, strain JZ-GX1 primarily relied on proline to resist salt stress, while trehalose was relied upon at later stage, indicating that were selective in the accumulation of soluble solutes. These results suggest that strain JZ-GX1 may alleviate salt stress by regulating the content of compatible solutes in vivo, and the accumulation of trehalose is the primary protective mechanism used by this bacterium against high salt stress
Summary
Soil salinization is a worldwide soil degradation problem that causes damage to resources and harms environmental and economic development (Zhang et al, 2010). Most halotolerant and moderately halophilic bacteria use a second strategy: the accumulation of compatible solutes These complex solutes (including amino acids, carbohydrates and their derivatives, sugars and polyols) can be absorbed from the medium or synthesized by organisms themselves (Shivanand and Mugeraya, 2011). In cells, these solutes accumulate at high concentrations to maintain the balance between intracellular and extracellular osmotic pressure (Sabine et al, 2018), which is the most widely used salt tolerance strategy in bacteria, eukaryotes and some methanogenic bacteria (Lai and Gunsalus, 1992; Gupta et al, 2015). EPSs form a layer around cells to reduce cell damage under abiotic stress conditions, and the production of EPSs is essential for adhesion of other organisms, biofilm formation and nutrient absorption (Sutherland, 2001; Nichols et al, 2005)
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