Rhizobacteria prime the activation of plant defense and nutritional responses to suppress aphid populations on barley over time

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Interactions between plants and soil microbes are widespread and are documented to modulate plant-insect herbivore interactions. Still, it remains unclear how these shape the overall plant defense responses and the mechanisms in suppressing insect populations. We performed bioassays and a time-series global gene expression analysis of barley (Hordeum vulgare) plants to study the underlying molecular pathways induced by two rhizobacteria, Acidovorax radicis or Bacillus subtilis, against Sitobion avenae aphids. Root inoculation by A. radicis or B. subtilis suppressed aphid populations on barley. Analysis of differentially expressed genes and co-expressed gene modules revealed a combination of rhizobacteria and aphid-induced plant responses. Aphid feeding triggered distinct plant responses in rhizobacteria-inoculated barley compared to uninoculated control plants, in phytohormone, glutathione, and phenylpropanoid pathways within 24 h. By day 7, stronger responses were observed in phenylpropanoid and nutrient pathways. By day 21, changes occurred in flavonoid pathways and genes related to aphid-induced tissue damage and repair. Our study suggests that rhizobacteria inoculation of barley against aphids is dynamic and acts through several molecular pathways to modulate and prime plant resistance (defenses) and tolerance (nutrition and growth) to aphids. Future research holds promise for exploiting these interactions for crop protection and pest management in agriculture.