Functional characterization of Pseudomonas soli VMAP1 as a biocontrol agent against Xanthomonas vesicatoria in tomato plants

Although several Pseudomonas spp. have recognised biocontrol properties, the ecological and functional potential of Pseudomonas soli remains largely unexplored. Here, the 5.6 Mb genome of the strain VMAP1 was sequenced and annotated to confirm its taxonomic identity and investigate genes associated with biocontrol traits. Genome mining and targeted in vitro and in planta assays were combined to assess its biocontrol potential. The genome harbours genes linked to the synthesis of bioactive compounds, as well as to regulatory and secretion systems homologous to those involved in biocontrol in other Pseudomonas spp. Functionally, VMAP1 exhibited motility and tolerance to changing environmental conditions. It also produced outer membrane vesicles and bioactive compounds including hydrogen cyanide, xantholysins and pseudopyronines. Diffusible VMAP1-derived metabolites impaired motility and biofilm formation in phytopathogen Xanthomonas vesicatoria, without inhibiting its growth. They also elicited plant defence responses, namely callose deposition and stomatal closure. Our results are consistent with earlier observations of a 75% reduction in the severity of bacterial spot in tomato plants treated with diffusible VMAP1-derived metabolites. They also offer novel insights into VMAP1’s ability to suppress the pathogen and induce host immune responses, and thus into its potential as a novel biocontrol agent for sustainable tomato production.