UC Berkeley

The plant microbiome is known to benefit host health in numerous ways, including providing protection against pathogens. Here, we provide evidence that tomato seed-associated microbiota play an important role in early seedling health. To test the importance of seed epiphytic communities for seedling susceptibility to the common bacterial pathogen, Pseudomonas syringae pv. tomato, we transplanted naturally occurring seed epiphytic microbial communities back onto seeds prior to germination and compared disease susceptibility to those that were not reinoculated after surface sterilization. We found that the epiphytic microbiome can protect seedlings against Pseudomonas syringae pv. tomato establishment and disease, and we further show that this protective effect is not due to a genotype by microbiome interaction. Using 16S amplicon sequencing, we found that these microbiomes were dominated by species of Pantoea, and we took a culturing approach to show that these isolates, including both Pantoea agglomerans and Pantoea dispersa, are sufficient to protect against Pseudomonas syringae pv. tomato. By varying concentration of first the pathogen and then the protective symbiont, we then examined the dose response of protection and found that applying a higher concentration of protective inoculum to the seeds does not correlate to a lessened Pseudomonas syringae pv. tomato load but does reduce disease burden. Instead, the most protective dose of Pantoea in terms of Pseudomonas syringae pv. tomato growth corresponds to the original density at which protective bacteria were found on seeds. Overall, our findings contribute to a broader understanding of the importance of vertically transmitted plant-associated microbes, with implications for the design and efficacy of biocontrol agents.