Interactions between key bacterial microbiota and soil properties in healthy soils suppress tobacco bacterial wilt

Infections with Ralstonia solanacearum have caused considerable agricultural and economic losses. Complex interactions among pathogens, the soil microbiome and soil properties determine health status; however, their interactions have not been explored thoroughly. Here, we collected soils from local naturally healthy and diseased tobacco fields, used Illumina amplicon sequencing to analyze the soil microbial community, determined the physicochemical properties of the soils, and explored the disease-suppressive mechanism of the healthy soil. We found that the microbial structure significantly differed and that the microbial network was more complex in healthy soils than in diseased soils. The key indicator species of the microbial community are also different; those of healthy soil included Sphingomonas and Pseudomonas , which are potentially beneficial bacteria. We further isolated culturable bacterial strains from healthy soil and obtained two indicator strains, Pseudomonas sp. strain R3 and Sphingomonas sp. strain S75. With respect to the soil physicochemical properties, hydrolysable nitrogen was closely related to the microbial community, positively correlated with the indicators of healthy soils and negatively correlated with Ralstonia solanacearum . We further investigated the inhibitory effects of healthy soil indicator species and hydrolysable nitrogen on tobacco bacterial wilt in laboratory and pot experiments, and the results revealed that the greenhouse experiment demonstrated that higher nitrate nitrogen availability enhanced the disease suppression provided by the inoculated indicator strains. We demonstrated that healthy soils presented relatively low pathogen abundance and had more beneficial bacterial indicator species. Hydrolysable nitrogen was the soil property most strongly associated with bacterial community composition and pathogen abundance. In greenhouse experiments, increased nitrate nitrogen enhanced the disease-suppressive effects of the beneficial indicator strains. However, the direct effect of hydrolysable nitrogen on microbial interactions requires further investigation.