Optimization of fermentation conditions of Bacillus paramycoides SH-1464 with growth-promoting activity

Abstract:

[Background] Plant growth-promoting rhizobacteria (PGPR) has great potential in improving plant resistance and regulating soil nutrient status. [Objective] To provide efficient microbial resources for the development and utilization of microbial fertilizer, we isolated and screened out the strains with strong growth-promoting properties from the rhizosphere soil of tobacco. [Methods] The culturable bacteria in the rhizosphere soil of tobacco were isolated by the dilution-plate coating method. The strains with strong phosphorus-solubilizing activity were screened by PVK medium, and the phosphorus-solubilizing activity of each strain was determined by molybdenum-antimony resistance colorimetry. Salkowski’s colorimetric solution was used to determine the production of indole acetic acid (IAA). With the phosphorus-solubilizing amount and IAA yield as indicators, we optimized the fermentation conditions by single factor tests to obtain the optimal fermentation scheme for phosphorus solubilization and IAA production. The growth-promoting activity of the strain was verified by seed germination and seedling growth experiments. The strain was identified based on the morphological, physiological, and biochemical characteristics and 16S rRNA gene sequence. [Results] A total of 127 strains were isolated from the rhizosphere soil of tobacco. Ten of the strains had the ability to solubilize inorganic phosphorus, among which SH-1464 had the highest phosphorus-solubilizing activity with a diameter of 16.11 mm and an amount of 119.22 mg/L. Strain SH-1464 had strong IAA production of 39.88 μg/mL. The optimal fermentation conditions of SH-1464 for solubilizing phosphorus was 10 g/L glucose as the carbon source, 0.5 g/L peptone as the nitrogen source, 2.5 g/L Ca₃(PO₄)₂ as the inorganic phosphorus source, initial pH 7.5, temperature of 30 ℃, and rotation speed of 200 r/min. The optimum conditions of strain SH-1464 for producing IAA were 10 g/L maltose, 5 g/L yeast powder, 10 g/L NH₄NO₃, 2.5 g/L KCl, 0.1 g/L l-tryptophan, pH 6.5, 30 ℃, and 180 r/min. The phosphorus-solubilizing amount and IAA yield of the strain were compared before and after optimization. After optimization, the phosphorus-solubilizing amount and IAA yield of the strain was 225.9 mg/L and 75.3 μg/mL, which increased by 84.5% and 88.8%, respectively, compared with those before optimization. The inoculation of SH-1464 increased the germination rate of tomato seeds by 84.5%, plant height by 55.1%, stem circumference by 32.5%, root length by 16.4%, root weight by 149%, aboveground part fresh weight by 172.2%, and aboveground part dry weight by 203.6%. Strain SH-1464 was identified as Bacillus paramycoides (GenBank accession number: ON417365, deposit number: CCTCCM 2022924). In addition, the strain had the abilities of producing protease and cellulase, fixing nitrogen, and secreting siderophore. [Conclusion] Strain SH-1464 had the abilities of solubilizing inorganic phosphorus, fixing nitrogen, and producing IAA, protease, cellulase, and siderophore. It significantly improved the germination rate of tomato seeds and the seedling agronomic traits such as plant height, stem circumference, root length, root weight, and fresh and dry weight of aboveground part. This strain has a wide application prospect in the development and utilization of microbial agents.