Azospirillum brasilense and tomato exudate or cytidine increase phytopathogen resistance and modulate phyllosphere/rhizosphere

Tomatoes are an important crop worldwide and phytopathogens can cause devastating losses. This study describes a treatment, consisting of Azospirillum brasilense Sp7 and either tomato seedling exudate or the exudate compound cytidine. The combination of A. brasilense Sp7 with cytidine showed a remarkable reduction of 83.4% in disease severity of tomatoes challenged with Pseudomonas syringae pv. tomato DC3000. Replacing cytidine with exudate was less effective at 71%, but the reduction in disease severity was still larger than by A. brasilense Sp7 alone at 55%. This reduction in disease severity was not paralleled by a decrease in P. syringae in leaf homogenates. Cytidine caused a 6.7 fold increase in A. brasilense Sp7 16S rDNA in root homogenates. In phyllosphere and rhizosphere, treatments modulated the microbial composition. In the phyllosphere, specific associations between treatment groups and bacterial orders could be computed. In the rhizosphere, principal component analysis revealed that variation along PC1 was dominated by the presence or absence of A. brasilense. Intriguingly, the inoculant caused an increase in the abundance of other Azospirillales species.