Pathogen invasion indirectly changes the composition of soil microbiome via shifts in root exudation profile

Plant-derived root exudates modulate plant-microbe interactions and may play an important role in pathogen suppression. Root exudates may, for instance, directly inhibit pathogens or alter microbiome composition. Here, we tested if plants modulate their root exudation in the presence of a pathogen and if these shifts alter the rhizosphere microbiome composition. We added exudates from healthy and Ralstonia solanacearum-infected tomato plants to an unplanted soil and followed changes in bacterial community composition. The presence of pathogen changed the exudation of phenolic compounds and increased the release of caffeic acid. The amendment of soils with exudates from the infected plants led to a development of distinct and less diverse soil microbiome communities. Crucially, we could reproduce similar shift in microbiome composition by adding pure caffeic acid into the soil. Caffeic acid further suppressed R. solanacearum growth in vitro. We conclude that pathogen-induced changes in root exudation profile may serve to control pathogen both by direct inhibition and by indirectly shifting the composition of rhizosphere microbiome.