The initial focus of our group was on molecular plant pathology, i.e. on understanding the molecular basis of plant/pathogen-interactions, with the long term goal to use this knowledge for the development of novel, environment friendly and consumer safe strategies for disease protection in crop plants.
Pathogenicity factors and resistance mechanisms in cereal rust fungi
We have a longstanding interest in cereal rust diseases. Rust fungi are highly specialised, obligately biotrophic plant pathogens responsible for immense losses in crop harvests, especially in cereals such as wheat, but also in many dicots such as soybean, coffee, or groundnut. One of the potentially most devastating crop plant diseases is wheat stem rust caused by Puccinia graminis f.sp. tritici. Tremendous losses in the 1950s due to massive rust epidemics in the US and Canada triggered modern plant breeding and the Green Revolution which has supplied us with sufficient food for more than 50 years. Today, however, global climate change, dwindling water resources, and exhausted soils are beginning to take their toll on agricultural food production. And concomitantly, old threats such as wheat stem rust are resurfacing. A new race of this fungus, Ug99, has emerged in East Africa and is now on its way around the globe. Ug99 has broken the highly successful rust resistance gene Sr31 which has protected all major wheat cultivars worldwide for more than 30 years. Already, wheat harvests in Uganda and Kenya have drastically decreased by more than 50 %, and the fungus has now reached South Africa and Iran and is imminently threatening the large wheat producers Pakistan and India. Within a few years only, Ug99 has further evolved, picking up additional virulences and, thus, breaking reserve resistance genes. The Global Rust Initiative has been set up to try and remedy this potentially highly dangerous situation. As our group possesses the possibly unique ability to grow the rust fungus in axenic culture (i.e. without a host plant), we are supporting this strategy by trying to find molecular tools to analyse the pathogenicity and virulence factors of Ug99, e.g. by setting up a transformation system for the fungus. This work was supported by our previous experience in genetically dissecting a highly effective form of adult plant resistance of wheat against wheat yellow rust, performed in collaboration with partners in South Africa and Australia.
Polyphenoloxidases in the disease resistance of dandelion
A second project in Molecular Phytopathology concerns the stunning disease resistance of dandelion, Taraxacum officinale. This sturdy weed knows almost no diseases unless it is an old plant or one growing under poor conditions. This is all the more surprising given that North of the Alps, dandelion exists as clonal populations of mostly triploid plants. This genetic uniformity is thought to be one of the main reasons for the high disease susceptibility of modern, high yielding crop varieties, yet dandelion is highly resistant. Using molecular genetic and biochemical approaches, we are investigating the molecular basis of this resistance. We had suspected polyphenoloxidases (PPO) as a potential resistance factor and using i.a. a gene knock-down strategy, we were able to at least partially verify this hypothesis. In the process, we had developed a protocol for the heterologous over-expression of plant PPO genes in E. coli, yielding active recombinant enzymes for the detailed characterization of their properties. This gives us the unique opportunity to study structure-function relationships of PPOs and their different domains. As dandelion possesses an unusually large PPO gene family of so far 13 genes, we are currently trying to understand this biodiversity by analyzing their substrate specificities and the possible physiological roles of their products.
Cellular priming as a mechanism in induced plant disease resistance
Another research project at the crossroads of Molecular Phytopathology and Renewable Resources deals with bacterial and algal polysaccharides with the potential to induce disease resistance in crop plants. It is well known that Plant Growth Promoting Rhizobacteria (PGPR) can induce a state of induced disease resistance (as compared to genetically determined resistance) in many plants. We have pinpointed the molecular trigger for this reaction induced by Pantoea agglomerans to an exopolysaccharide (EPS) secreted by this bacterium. This EPS (but not that of other bacteria such as E. coli) has the ability to prime cereal cells, i.e. to induce a state of alert with highly increased sensitivity to pathogens and their elicitors triggering resistance reactions. We have found a similar effect when we treat cereal plants or cells with the polysaccharide ulvan isolated from the green alga Ulva fasciata, and we are currently trying to identify the biologically active components of both priming inducing polysaccharides and their molecular modes of action. Of course, we were fascinated by the idea that the long-known but poorly understood plant protective activities of chitosans might also be based at least partially on cellular priming. After many years of intensive research, we have now identified specific paCOS (partially acetylated chitosan oligosaccharides) which indeed have cellular priming activity, and we are currently exploring their potential as biostimulants, i.e.as agriculturally useful plant strengtheners.