Soil microorganisms with growth-promoting activities in plants, such as rhizobacteria, can improve plant health in many ways. These beneficial organisms may confer to plants broad-spectrum resistance by modifying their physiological status and affecting direct and indirect plant defences against insect herbivores. Here, we evaluated the role of the plant growth-promoting rhizobacterium (PGPR) Bacillus amyloliquefaciens on the interactions of tomato plants, Solanum lycopersicum L., the specialist herbivore Tuta absoluta (Meyrick), and the predator Macrolophus basicornis (Stål) through a combination of chemical and behavioural approaches. Our results showed that rhizobacterial colonization of tomato roots increased the endogenous level of jasmonic acid compared to control plants (mock). Volatile analysis revealed that PGPR-treated plants also increased their emission of several terpenoid compounds. Moreover, rhizobacterial inoculation significantly affected plant biomass, increasing root and shoot dry weight, but did not affect the germination rate. Tomato plants were more resistant to T. absoluta when inoculated with the rhizobacterium. The rhizobacterial treatment resulted in decreased larval and pupal weight compared to mock plants. Tuta absoluta laid significantly fewer eggs on PGPR-treated plants compared to untreated plants. In two-choice oviposition experiments, significantly higher number of eggs were laid on untreated plants compared to PGPR-treated plants. In terms of indirect defence, PGPR-treated plants were more attractive to M. basicornis than untreated plants, however, when the predators were given a choice between PGPR-treated and T. absoluta-damaged plants, they preferred the latter. Overall, our results indicate that B. amyloliquefaciens modulates tomato defence against T. absoluta by modifying plant chemistry and enhanced attraction of predators.

Bottom-up effects of the rhizobaterium Bacilus amyloliquefaciens on tritrophic interactions

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