Resumen de Biological control and endophytism of the olive root bacterium Pseudomonas fluorescens PICF7
María Mercedes Maldonado González
Olive (Olea europaea L.) has always been a fundamental crop in the Mediterranean Basin. Driven by the fact, among others, that an increasing number of scientific reports highlight the benefits that olive oil consumption has for human health, olive tree cultivation has spread worldwide to other regions with Mediterranean-type climate. Two relevant pathogens affecting olive trees are the hemibiotrophic soil-borne fungus Verticillium dahliae and the bacterium Pseudomonas savastanoi pv. savastanoi (Psv), causal agents of Verticillium wilt of olive (VWO) and olive knot disease (OKD), respectively. Effective control of these pathogens must rely on integrated disease management strategies, with emphasis in preventive, cost-effective and environmentally-friendly measures. Among the available control tools, the use of microbial antagonists with biocontrol potential emerges as a promising strategy to implement either alone or in combination with other disease management measures. Moreover, the use of biological control agents (BCA) native from the host plant, and therefore adapted to the target ecological niche where their benefits can be deployed, is a reasonable approach. In previous studies, Pseudomonas fluorescens PICF7, an indigenous inhabitant bacterium of the olive rhizosphere, was confirmed as an effective BCA against the defoliating, highly-virulent pathotype of V. dahliae. This bacterial strain displays endophytic lifestyle in olive roots, produces the siderophore pyoverdine (Pvd), and is able to induce a broad range of defense responses in both roots and above-ground organs when colonizing olive roots. Even though the information so far gathered has increased in the last few years, the study of the PICF7- V. dahliae-olive tripartite interaction is complex and an in-depth knowledge on the relationships established among the three partners is needed. On the one hand, working with a woody, long-living plant such as olive poses difficulties inherent to the own characteristics of the plant. On the other hand, the genetic and molecular mechanisms underlying VWO biocontrol exerted by strain PICF7 are still unknown. To shed light for the first time on PICF7 traits involved in the control of V. dahliae as well as in the ability of this BCA to colonize the interior of olive roots, four PICF7 phenotypes were studied by mutant analysis. Thus, two mutants affected in traits usually related to colonization ability and biological control of plant pathogens (i.e. swimming motility and siderophore pyoverdine production) and two altered in nutritional requirements (i.e. growth delay in PDA medium and cysteine auxotrophy) were generated, characterized and evaluated in both in vitro and in in planta assays. In addition, this thesis also investigated whether the model plant Arabidopsis thaliana can be used to facilitate the identification of P. fluorescens PICF7 traits involved in both V. dahliae biocontrol and endophytic lifestyle. The aim was to compare the behavior of the same PICF7 mutant phenotypes in olive and Arabidopsis. Furthermore, since strain...
