A saline-alkali tolerant strain of Citrobacter: application in promoting plant growth and mining of saline-alkali tolerance genes

Abstract:

[Background] Saline-alkali tolerant and plant growth-promoting bacteria are praised for the effects on improving the growth and enhancing the saline-alkali tolerance of crops under saline-alkali stress. [Objective] To acquire the microbial resources that possess both saline-alkali tolerance and growth-promoting effects and decipher the mechanism underlying saline-alkali tolerance, we aimed to isolate a bacterial strain from the saline soil in Yinchuan City, Ningxia Hui Autonomous Region, China. [Methods] The strain was identified by morphological and microscopic observation, Biolog Gen Ⅲ test, and 16S rRNA gene sequencing. The culture method was used to examine the saline-alkali tolerance and growth-promoting effect of the strain. The genome information of the strain was analyzed by functional gene annotation. [Results] Strain YS-AT2 could grow in the LB media with pH 8.0–12.0 and 0%–9% (W/V) NaCl, and showed the alkali reduction rate reaching 13% and above in the media at pH 8.5–9.5. Strain YS-AT2 could use 54 carbon sources and was sensitive to 20 chemical sensitivity test substances. According to 16S rRNA gene sequencing results, YS-AT2 belonged to the genus Citrobacter. strain YS-AT2 could promote the seed growth of Arabidopsis thaliana in a saline environment with NaHCO₃ (pH 9.5) and the germination of soybean seeds under 50–200 mmol/L NaCl or 50–200 mmol/L NaHCO₃ stress. Furthermore, it increased soybean plant height, fresh weight, dry weight, and root length by 19.84%, 18.75%, 10.31%, and 32.58%, respectively, in the soil with pH 8.2 and EC 450 μs/cm. Strain YS-AT2 carried multiple genes associated with saline-alkali tolerance. The saline-alkali tolerance of this strain was probably realized through the transport of saline-alkali substances, the transport and regulation of sodium and potassium ions, and the synthesis of high-concentration cell solutes. [Conclusion] We analyzed the growth-promoting effect of the saline-alkali tolerant strain Citrobacter YS-AT2 on A. thaliana and soybean and mined the genes associated with its saline-alkali tolerance. The findings not only provided a theoretical basis and strain resources for the development of biofertilizers with saline-alkali growth promoting function but also laid a theoretical foundation for deciphering the mechanism underlying the saline-alkali tolerance.

Key words:Citrobacter sp.;saline-alkali tolerance;plant growth-promoting effect;gene annotation