Growth promotion and mitigation of salt stress in wheat seedlings by a Kushneria bacterium

Globally, soil salinization is a major land degradation process, taking more than 1 million hectares of farmland out of production per year, threatening food security. In salt-affected soils, crop growth, development, and yield dramatically decrease due to salt toxicity in plants, reduced soil fertility and water availability to plants, and altered hydraulic properties of the soil. The rhizospheric microbiome is closely related to crop stress tolerance. Plants recruit specific groups of microbes in the rhizosphere, which provide nutrients and plant hormones that promote plant growth and stress tolerance. Kushneria indalinina JP-JH is a salt-tolerant and auxin-secreting bacteria strain isolated from the rhizosphere of Suaeda salsa. In this study, the effects of K. indalinina JP-JH on the growth and salt tolerance of wheat seedlings were investigated. The experiment was conducted using the wheat variety ‘Xiaoyan 60’ in a hydroponic system with Hoagland nutrient solution. The seedlings were inoculated with K. indalinina JP-JH under no salt (0 mmol∙L⁻¹), low salt (200 mmol∙L⁻¹), and high salt (400 mmol∙L⁻¹) conditions. Seeds without bacterial inoculation were prepared under the same conditions and used as control treatments. Plant growth parameters and organic acids contents in the root exudates were analyzed after 40 days of incubation. The results showed that the fresh weight of wheat seedlings in the treatments with K. indalinina JP-JH inoculation was significantly higher than that of the uninoculated group at all three salt concentrations. K. indalinina JP-JH inoculation significantly increased the plant dry weight of the whole plant under both salt-free and low salt-stressed conditions but had no significant effect on dry weight under high salt-stressed conditions. Meanwhile, K. indalinina JP-JH inoculation significantly increased plant height under high salt-stressed conditions but had no significant effect under salt-free and low salt-stressed conditions. Dry weight and plant height were negatively related to the salt concentration in the inoculated and uninoculated treatments. The concentrations of six organic acids (oxalic acid, tartaric acid, malic acid, citric acid, fumaric acid, and succinic acid) secreted from roots were determined for treatments with different salt concentrations, with or without bacterial inoculation. K. indalinina JP-JH inoculation significantly increased the contents of oxalic acid and tartaric acid in the root exudates under salt-stressed conditions (both low and high salt concentrations) but had no significant effect on organic acids secretion under salt-free conditions. This suggests that oxalic acid and tartaric acid may play important roles in regulating plant-microbe interactions and improving the salt tolerance of wheat variety ‘Xiaoyan 60’. Taken together, our results indicate that K. indalinina JP-JH promotes plant growth and development and improves the salt tolerance of wheat at seedling stage, which may be related to alterations in the root exudation potential and plant-microbe interactions. This study produced new experimental data on the mechanisms of microorganisms that promote plant growth and salt tolerance and provided biological resources for developing biofertilizers to enhance wheat growth under salt stress conditions.