Objectives Bacterial wilt is a soil-borne disease with high temperature and humidity, which is caused by Ralstonia solanacearum. Arbuscular mycorrhiza (AM) fungi have the potential to inhibit specific pathogens in soil by regulating the microbial community of the host rhizosphere. However, the effects of AM fungi on soil bacterial community infected by Ralstonia solanacearum is still unclear.
Methods Pot culture method was used in the research, the test plant was tomato (Solanum lycopersicum), the AM fungus was F. mosseae M47V, and the pathogen was Ralstonia solanacearum QL-RS 1115 (GenBank: GU390462). Mycorrhizal seedlings were prepared by inoculating tomato seeds with F. mosseae M47V after 5 days of germination, and non-mycorrhizal seedlings without AM fungus inoculation. Half of the mycorrhizal and non-mycorrhizal tomato seedlings of 30-days-old were inoculated with pathogen, while the other half consisted of only AM treatments. At 1 and 14 days after inoculation, tomato plants and rhizosphere soil were sampled. Real-time PCR was used to analyze the number of bacterial species in tomato rhizosphere, and 16S rRNA high-throughput sequencing was used to explore the diversity and structural stability of soil bacterial community.
Results On the 1st day of pathogen inoculation, the Chao1 index, Shannon index and Simpson index of bacterial community structure in rhizosphere soils under the two QL-RS 1115 inoculation treatments reduced significantly (P<0.05), the number of nodes and connections of co-occurrence network significantly decreased, and the degree of modularity decreased. The simplified co-occurrence network indicated that the stability of bacterial community structure decreased. On the 14th day of pathogen inoculation, the beneficial bacteria such as Acinetobacter, Sphingomonas, Lysobacter, and Pseudomonas were enriched in rhizosphere soil, the number of nodes and connections of bacterial co-occurrence network increased, and the degree of modularity increased, indicating that the stability of bacterial community was restored. Compared with non-mycorrhizal seedlings treatments, both the mycorrhizal seedlings with and without pathogen showed significantly lower abundance of pathogen in rhizosphere soil (P<0.05). AM fungi significantly increased the Chao1 index and Shannon index (P<0.05), and increased the relative abundance of Flavobacterium, Flavisolibacter, Cytophaga, and Bryobacter in the rhizosphere soil of tomato infected with pathogen. At the same time, the number of nodes and connections of the co-occurrence network were increased, and the benign interaction between bacterial species in tomato rhizosphere was promoted, and the complexity of the bacterial network was improved.
Conclusions Tomato infected with Ralstonia solanacearum induces the enrichment of beneficial bacteria, including Acinetobacter, Sphingomonas, Lysobacter, and Pseudomonas in the rhizosphere to restore the microbial diversity and stability. Inoculation of AM fungi significantly decreases the relative abundance of Ralstonia solanacearum but increases that of Flavobacterium, and further improves the diversity and stability of bacterial community in the rhizosphere soil of tomato, so enhances the resistance to Ralstonia solanacearum.
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