Single and co-inoculation of Bacillus subtilis and Azospirillum brasilense on Lycopersicon esculentum: Effects on plant growth and rhizosphere microbial community

In this study inoculation of the tomato (Lycopersicon esculentum) with the microbial pest control agent (MPCA) Bacillus subtilis strain 101 was analyzed in combination with the plant growth promoting rhizobacterium (PGPR) Azospirillum brasilense Sp245. This study aims to evaluate whether the co-inoculation of these two species would enhance the efficacy and reliability of healthy effects elicited on crops by each microbe. Data showed that the combination of the two rhizobacteria had no synergistic or comparable effects on plant biomass, with respect to their single applications. In vitro tests and cellular analysis of root tips revealed a growth inhibition of the primary root, when they are co-present on the root system, which is not related to a reduced persistence in the rhizosphere of one or both bacteria. Moreover, B. subtilis causes alterations on the root system architecture, suggesting a different mode of action in regulating root growth development with respect to A. brasilense. These results, along with rhizosphere microbial community fingerprinting, suggest that plant-growth reduction and root-architectural alterations, elicited by the microbial combination, may involve independent signalling pathways of the two bacterial species.