Effect of non-pathogenic bacteria on plant pathogen virulence and the plant immune system

The development of so-called biocontrol agents is in the spotlight to solve agricultural losses caused by foliar pathogens. Biocontrol agents can protect plants via direct antagonistic interactions or by modulating host immune networks to induce plant resistance indirectly. The focus of this thesis lies on screening potential microbial biocontrol agents against model pathogens and dissecting the underlying mechanisms behind such protection. I have employed an in planta assay to inoculate gnotobiotic Arabidopsis with individual strains from a diverse set of bacteria prior to infection with either the model biotrophic pathogen Pseudomonas syringae DC3000 or the model necrotrophic pathogen Botrytis cinerea. The protective ability of each bacterial strain was determined by examining the plant phenotype. The direct antagonistic interactions were suggested on a bacterial population level. In addition, a protoplast-based assay was established to investigate the potential link between the reduction in disease phenotype and the potential modulation of the plant immune responses to bacteria. As a result, Acidovorax sp. leaf 84, Pedobacter sp. leaf 194, Plantibacter sp. leaf 1 and Pseudomonas citronellolis P3B5 in addition to Sphingomonas species showed a striking ability to protect plants from developing severe disease symptoms caused by biotrophic pathogen Pseudomonas syringae DC3000 infection. Arthrobacter sp. leaf 145, Pseudomonas syringae DC3000, Pseudomonas syringae B728a, Pantoea vagans PW, Pantoea vagans C9-1, Pantoea agglomerans 299R, Rhodococcus sp. leaf 225, Sphingomonas sp. leaf 17, Sphingomonas phyllosphaerae FA2 and Sphingomonas melonis FR1 showed plant-protection in reducing plant mortality rate infected by necrotrophic pathogen Botrytis cinerea. Interestingly, the mode of Pseudomonas syringae DC3000 induced disease suppression seems to be different based on the bacterial strains used: A. sp. leaf 84 and P. citronellolis P3B5 significantly reduced the Pseudomonas syringae DC3000 population size in planta while P. citronellolis P3B5, P. syringae B728a, P. vagans C9-1, P. agglomerans 299R, S. sp. leaf 17 and S. phyllosphaerae FA2 induced a strong plant immune response in the protoplast assay. This suggests A. sp. leaf 84 may only depend on direct antagonistic interactions to diminish pathogen proliferation while the other protective strains can employ plant immune systems to confer their protection. Moreover, a few protective strains such as P. syringae B728a against Botrytis induced salicylic acid signalling responses in the protoplast assay, which infers the positive effect of salicylic acid signalling on defence against necrotrophic pathogens. This study has successfully screened the biocontrol agents against model pathogens and furthers our understanding of biocontrol activity, thereby aiding to achieve improved disease control