Biochar-based Bacillus subtilis inoculant for enhancing plants disease prevention: microbiota response and root exudates regulation

Plants regulate root exudates to form the composition of rhizosphere microbial community and resist disease stress. Many studies advocate intervention with biochar (BC) and exogenous microbe to enhance this process and improve plant defenses. However, the mechanism by which BC mediates exogenous microorganisms to enhance root exudate-soil microbial defensive feedback remains unclear. Here, a BC-based Bacillus subtilis SL-44 inoculant (BC@SL) was prepared to investigate the defensive feedback mechanism for plants, which enhanced plant growth and defense more than BC or SL-44 alone. BC@SL not only strengthened the direct inhibition of Rhizoctonia solani Rs by solving the problem of reduced viability of a single SL-44 inoculant but also indirectly alleviated the Rs stress by strengthening plant defensive feedback, which was specifically manifested by the following: (1) increasing the root resistance enzyme activities (superoxide dismutase up to 3.5 FC); (2) increasing the abundance of beneficial microbe in soil (0.38-16.31% Bacillus); and (3) remodeling the composition of root exudates (palmitic acid 3.95-6.96%, stearic acid 3.56-5.93%, 2,4 tert-butylphenol 1.23-2.62%, increasing citric acid 0.94-1.81%, and benzoic acid 0.97-2.13%). The mechanism reveals that BC@SL can enhance the positive regulatory effect between root exudates and microorganisms by optimizing their composition. Overall, BC@SL is a stable and efficient new solid exogenous soil auxiliary, and this study lays the foundation for the generalization and application of green pesticides.