The tripartite interaction olive-Pseudomonas fluorescens PICF7-Verticillium dahliae as study system to unravel biocontrol performance of an endophytic bacterium and plant genetic responses to root colonization

Abstract

One of our research lines focuses on biological control as a sustainable, environmentally-friendly measure to confront Verticillium wilt of olive (Olea europaea L.) (VWO), caused by the soil-borne fungus Verticillium dahliae, within an integrated disease management strategy. A number of Pseudomonas spp. strains, natural colonizers of olive roots, were shown as efficient biological control agents against VWO. Endophytic lifestyle in olive root tissues was demonstrated under different experimental conditions for one of the most promising strains, Pseudomonas fluorescens PICF7. Interestingly, root hairs were shown to play a key role in the endophytic colonization of root tissues by this rhizobacteria. Surface colonization of intact olive roots and endophytic establishment seem to be required for the effective control of VWO by strain PICF7. However, mechanism(s) underlying PICF7-mediated biocontrol of V. dahliae and endophytism remain poorly understood. Mutant analysis has revealed that bacterial traits such as siderophore pyoverdine production and swimming motility are involved neither in biocontrol performance nor in endophytic colonization. On the other hand, colonization of olive roots by PICF7 triggers a broad range of transcriptomic changes, mostly related to defense responses to different (a)biotic stresses at both local (roots) and systemic (stems) level. These responses, which seem to be delicately modulated along time, could explain (i) biocontrol effectiveness of strain PICF7 and (ii) how olive tissues recognize and ‘tolerate’ this bacterium as a non-hostile invader. Another research line is to explore olive roots as the source of culturable (endo)rhizobacteria with potential as biocontrol agents against different soil-borne pathogens.