Only with highly diverse immune gene repertoires can plant populations cope with myriad pathogen pressures. In Capsella, we have documented intense selection for immune gene diversity. At the same time, a near-clonal A. thaliana lineage has successfully spread throughout North America, populating a wide variety of habitats in the presence of its major microbial pathogens, Hyaloperonospora arabidopsidis and Albugo laibachii. Using wild isolates collected on field trips, we are addressing the molecular evolution of both host and pathogens using genome-sequencing based approaches, and use pathogen testing to reveal whether the uniform A. thaliana lineage is less adapted to local pathogen strains than accessions from the species’ native range.

Another major effort in this direction is to understand how microbes shape the immune gene repertoire of A. thaliana. We are using natural populations to establish the links that exist between microbiome composition and genetic diversity of the host. This will be a crucial step in identifying causal agents that drive differentiation of the plant immune system in the wild. In addition, we are using local A. thaliana populations from stands around Tübingen to investigate the dynamics of plant pathogen spread in natural host populations, taking into account that pre-existing microbiomes might alter host susceptibility.

• Microbiome analyses in natural plant populations
• Molecular co-evolution of Arabidopsis and its pathogens
• Effect of microbiome composition on plant colonization by pathogens