Organic acids and root exudates of Brachypodium distachyon: Effects on chemotaxis and biofilm formation of endophytic bacteria.

Root colonization by plant growth-promoting (PGP) bacteria could not be useful without the beneficial properties of the bacterium itself. Thus, it is necessary to evaluate the bacterial capacity to form biofilms and establish a successful interaction with the plant roots. We assessed the ability of growth-promotion bacterial strains to form biofilm, and display chemotactic behaviour in response to organic acids or to root exudates of the model plant Brachypodium distachyon. This assessment was based on the evaluation of single strains of bacteria and of multispecies consortium. The strains coexisted together and formed biofilm under biotic (living root) and abiotic (glass) surfaces. Citric acid stimulated biofilm formation in all individual strains, indicating a strong chemotactic behaviour towards organic acids. Recognizing that the transition from single strains of bacteria to a "multicellular" system wouldn't happen without the presence of adhesion, alginate and exopolysaccharides (EPS) contents, they were evaluated. The EPS amounts were comparable in single strains and consortium forms. Alginate production increased 160% in consortium subjected to drought stress (10% PEG). These findings demonstrated that bacteria-bacteria interaction is the hub of various factors that would not only affect their relation, but further could indirectly affect the balanced plant-microbe relation, and that root exudates could be very selective in recruiting highly qualified multispecies consortium.