Azospirillum baldaniorum Sp245 exploits Pseudomonas fluorescens A506 biofilm to overgrow in dual-species macrocolonies.

Biofilms are essential for plant-associated bacteria to colonize their host. In this work, we analysed the interaction of Azospirillum baldaniorum Sp245 and Pseudomonas fluorescens A506 in mixed macrocolony biofilms. We identified certain culture conditions where A. baldaniorum Sp245 exploits P. fluorescens A506 to boost its growth. Azospirillum growth increased proportionally to the initial number of pseudomonads building the biofilm, which in turn were negatively affected in their growth. Physical contact with P. fluorescens A506 was essential for A. baldaniorum Sp245 growth increase. Biofilm ultrastructure analysis revealed that Pseudomonas produces a thick structure that hosts Azospirillum cells in its interior. Additional experimentation demonstrated that Azospirillum growth boost is compromised when interacting with biofilm-deficient Pseudomonas mutants, and that a low oxygen concentration strongly induce A. baldaniorum Sp245 growth, overriding Pseudomonas stimulation. In this line, we used a microaerophilia reporter strain of A. baldaniorum Sp245 to confirm that dual-species macrocolonies contain a higher number of cells under microaerophilic conditions. Taking all the results into consideration, we propose that A. baldaniorum Sp245 can benefit from P. fluorescens A506 partnership in mixed biofilms by taking advantage of the low oxygen concentration and scaffold made up of Pseudomonas-derived matrix, to expand its growth.

Biocontrol Traits Correlate With Resistance to Predation by Protists in Soil Pseudomonads.

Root-colonizing bacteria can support plant growth and help fend off pathogens. It is clear that such bacteria benefit from plant-derived carbon, but it remains ambiguous why they invest in plant-beneficial traits. We suggest that selection via protist predation contributes to recruitment of plant-beneficial traits in rhizosphere bacteria. To this end, we examined the extent to which bacterial traits associated with pathogen inhibition coincide with resistance to protist predation. We investigated the resistance to predation of a collection of Pseudomonas spp. against a range of representative soil protists covering three eukaryotic supergroups. We then examined whether patterns of resistance to predation could be explained by functional traits related to plant growth promotion, disease suppression and root colonization success. We observed a strong correlation between resistance to predation and phytopathogen inhibition. In addition, our analysis highlighted an important contribution of lytic enzymes and motility traits to resist predation by protists. We conclude that the widespread occurrence of plant-protective traits in the rhizosphere microbiome may be driven by the evolutionary pressure for resistance against predation by protists. Protists may therefore act as microbiome regulators promoting native bacteria involved in plant protection against diseases.

Friends or foes in the rhizosphere: traits of fluorescent Pseudomonas that hinder Azospirillum brasilense growth and root colonization.

Bacteria of the Azospirillum and Pseudomonas genera are ubiquitous members of the rhizosphere, where they stimulate plant growth. Given the outstanding capacity of pseudomonads to antagonize other microorganisms, we analyzed the interaction between these two bacterial groups to identify determinants of their compatibility. We could establish that, when in direct contact, certain Pseudomonas strains produce lethality on Azospirillum brasilense cells using an antibacterial type 6 secretion system. When analyzing the effect of Pseudomonas spp. diffusible metabolites on A. brasilense growth on King's B medium, we detected strong inhibitory effects, mostly mediated by siderophores. On Congo Red medium, both inhibitory and stimulatory effects were induced by unidentified compounds. Under this condition, Pseudomonas protegens CHA0 produced a Gac/Rsm-regulated antibiotic which specifically inhibited A. brasilense Sp7 but not Sp245. This effect was not associated with the production of 2,4-diacetylphloroglucinol. The three identified antagonism determinants were also active in vivo, producing a reduction of viable cells of A. brasilense in the roots of wheat seedlings when co-inoculated with pseudomonads. These results are relevant to the understanding of social dynamics in the rhizosphere and might aid in the selection of strains for mixed inoculants.