Isolation and identification of a maize growth-promoting bacterial strain with straw-decomposing capacity and optimization of fermentation conditions

Abstract:

[Background] Shajiang black soil is characterized by heavy texture, poor structure, and low fertility. The long-term straw returning in the area with Shajiang black soil leads to stunted growing of crops and slow straw decomposition. [Objective] To accelerate straw decomposition and increase crop yield, we screened a strain producing both carboxymethyl cellulase (CMCase) and indoleacetic acid (IAA) from Shajiang black soil. [Methods] The strain was identified based on physiological and biochemical properties and 16S rRNA gene sequence analysis. Shake flask of straw decomposition and maize pot experiments were carried out to verify the straw-decomposing, growth-promoting, and soil-improving effects of the strain. Single factor tests were then carried out to optimize the conditions for strain growth and IAA production. Green fluorescent protein (GFP) gene was introduced into the strain cells by electroporation and verified by PCR combined with agarose gel electrophoresis. The colonization rate was determined by fluorescence quantitative experiment. [Results] The straw-decomposing and maize growth-promoting strain was screened out and named as MC29, which belonged to Cellulosimicrobium cellulans. It presented the CMCase activity of 13.32 U/mL and the IAA production of 8.63 mg/L. Compared with the control, the application of MC29 increased the straw decomposition rate by 24.8% and the soil and plant analyzer development (SPAD) value, total plant weight, root surface area, and root length of maize by 7.6%, 21.3%, 30.9%, and 18.3%, respectively. Furthermore, the strain increased the soil available nitrogen, available phosphorus, and available potassium by 68.1%, 5.8%, and 6.0%, respectively. The optimum growth conditions of strain MC29 were pH 7.0, liquid-loading volume of 25/250 mL, maltose as carbon source, and yeast powder as nitrogen source, while the optimum conditions for IAA production were pH 7.0, liquid-loading volume 50/250 mL, fructose as carbon source, and potassium nitrate as nitrogen source. The strain MC29-GFP was successfully constructed, which showed the colonization amount of 2.8×10⁵ to 9.5×10⁵ copies/g 15 days after inoculation in the Shajiang black soil, demonstrating good colonization ability. [Conclusion] C. cellulans MC29 has the ability of decomposing straw and promoting maize growth, which plays a role in the development of multifunctional straw-decomposing agent and microbial fertilizer and the yield improvement of maize. The good and stable colonization ability lays a foundation for the practical application of strain MC29 in Shajiang black soil.