Nutrient and dose dependent microbiome-mediated protection against a plant pathogen. Berg & Koskella
Plant-associated microbial communities can promote plant nutrient uptake, growth, and resistance to pathogens. Host resistance to infection can increase directly through commensal-pathogen interactions or indirectly through priming of host defenses, and the specific mechanisms are currently best described for rhizosphere bacteria. For example, Arabidopsis plants infected with the foliar pathogen, Pseudomonas syringae pathovar tomato (Pst), increase their root secretion of malate, which attracts Bacillus subtillis to the roots and leads to a stronger host immune response against Pst. While there are numerous examples of individual defensive symbionts, it is less clear whether this type of protection can be an emergent property of whole microbial communities. In particular, relatively little is known about if and how the presence of phyllosphere (above ground) microbial communities can increase host resistance against pathogens. In this study, we examined the ability of augmented tomato phyllosphere microbiomes to confer resistance against the causal agent of bacterial speck, Pst. Across 5 independent experiments, the phyllosphere microbiome was found to decrease pathogen colonization. Furthermore, the dose of commensal bacteria applied affected the degree of protection conferred, and although the effect is dependent on microbial composition, it is not clearly related to overall diversity. Finally, our results suggest that microbial resource community dynamics play an important role in protection, as the addition of fertilizer abolished the observed microbiome-mediated protection. Together, these results have clear relevance to microbiome-mediated protection within agricultural settings, and the use of plant probiotics to increase disease resistance.