Design of effective synbiotics against aboveground insect herbivory through characterization of host plant rhizosphere microbiota and metabolites.

Plants can cope with stresses via the so-called "cry for help" strategy, but how aboveground insect herbivores induce alterations in the rhizosphere microbiota through eliciting this plant-driven response remains unexplored. Here, we exposed cabbage plants to aboveground insect herbivory for five sequential planting rounds in the same soil. New cabbage plants, growing in the soils conditioned for five rounds showed a significant increase in resistance to aboveground insect herbivory. This effect was attributed to the accumulation of Pseudomonas in herbivore-conditioned soils. Pseudomonas could be enriched by application of amino acids, that were present at higher concentrations in the rhizosphere of cabbage plants suffering from insect herbivory. Notably, cabbage plants exhibited the highest resistance to insect herbivory following the application of a synbiotic, a combination of amino acids (prebiotics) and Pseudomonas spp. (probiotics). Moreover, we show that Pseudomonas activates the jasmonate signaling pathway in the plant. This activation occurred in SA-deficient but not in JA-deficient Arabidopsis thaliana mutants, and led to the induction of glucosinolates-based defenses that are well known to act against insect herbivory. Collectively, this work reveals a belowground "cry for help" response in plants induced by aboveground herbivory, which guided the development of a novel synbiotic for plant health maintenance.