Interaction between endophytic Proteobacteria strains and Serendipita indica enhances biocontrol activity against fungal pathogens

Aims Plants host communities of fungal and bacterial endophytes, establishing a complex network of multipartite interactions, but the mechanisms whereby they interact are poorly understood. Some fungi, such as the beneficial mycorrhiza-like fungusSerendipita(=Piriformospora)indica, can be helped by bacteria for establishment, survival and colonization. Although this fungus harbors aRhizobiumas an endofungal bacterium, we hypothesized that other bacteria might also establish associations with the fungus and combiningS. indicawith bacteria might enhance plant growth and health. Methods The interactions amongS. indicaand four endophytic Proteobacteria belonging toMethylobacterium,Tardiphaga,RhodanobacterandTrinickiaspp. were characterized in vitro and for their effect on tomato growth and biocontrol ofFusarium oxysporumandRhizoctonia solani. Possible mechanisms behind these interactions were described based on genome and microscopic analyses, using fungal and bacterial strains tagged with fluorescent markers. Results All bacteria stimulatedS. indicagrowth in vitro. Moreover, several of the bacteria stimulated growth of tomato plants, but co-inoculations withS. indicaand bacteria did not perform better than single inoculations. Contrarily, combinations ofS. indicaand bacteria significantly reduced disease progression of fungal pathogens. These microbes seem to cooperate in the process of root colonization for instance by increasing fungal sporulation and hyphae expansion, showing multipartite interaction between microbes and plants. Interestingly, the strain ofTrinickiainternally colonizes spores ofS. indicaas an endofungal bacterium during in vitro-co-culturing, suggesting further that the fungus might acquire formerly unrecognized genera of bacteria and genome analysis of the bacteria revealed many genes potentially involved in fungal and plant growth stimulation, biocontrol and root colonization, highlighting putative mechanisms of plant-fungal-bacterial interaction. Conclusions Our study represents an important step towards unraveling the complex interactions among plants,S. indica, endophytic bacteria and fungal pathogens, and indicates that adding bacteria to fungal inoculum could have a remarkable impact on the plant-S. indicasymbiosis.