R.T. Voegele(1), S. Wirsel(2), C. Struck(1), M. Hahn(3), and K. Mendgen(1)
(1) Phytopathologie, Fachbereich Biologie, Universität Konstanz, 78457 Konstanz, Germany
(2) Institut für Pflanzenzüchtung und Pflanzenschutz, Landwirtschaftliche Fakultät, Universität Halle, 06099 Halle, Germany
(3) Phytopathologie, Technische Universität Kaiserslautern, 67663 Kaiserslautern, Germany

Obligate biotrophic parasites are among the most serious plant pathogens world wide. Yet, very little is known about the molecular mechanisms underlying this specific life style. This is mainly due to the fact that these organisms cannot be grown in culture, at least not to a point equivalent to the true biotrophic phase. In order to develop new strategies in disease suppression it would be highly desirable to understand how these pathogens manage to maintain a symbiosis-like, prolonged association with their hosts. Obligate biotrophs are best exemplified by the powdery mildew - and the rust fungi. Both classes of fungi are characterized by the formation of specially differentiated hyphae, so called haustoria, which differentiate within the host cell. We have chosen the model systems Uromyces fabae / Vicia faba and U. striatus / Medicago truncatula to study the role of these structures in maintaining the biotrophic life style. Based on the isolation of haustoria and the construction of a haustorium-specific cDNA-library, we were able to compare this blue print of the parasitic phase with other stages of pathogen development at a molecular level. This comparison enabled us to identify genes which are preferentially or exclusively linked to the parasitic growth phase. We were able to show that rust haustoria are small molecular power stations involved in biosynthetic processes and that they are the exclusive sites for the uptake of carbohydrates. However, the capacity of haustoria stretches beyond simple uptake and metabolic duties. A fungal invertase is expressed in early stages of fungal growth long before the hexose transporter is present. This might be a way for the fungus to pre-condition colonized tissue for a source-sink conversion. Support for this hypothesis stems from the influence of the parasite on the expression of various host genes, including invertases, even in plant organs far remote from the site of infection. Another interesting aspect of haustorial function is the production of large quantities of sugar alcohols. These alcohols act as mobile carbohydrate storage compounds. However, they are also excreted into the apoplast where they presumably act as scavengers of reactive oxygen species. A further focal point of our research regarding the suppression of host plant defense responses is the elucidation of the role of specific rust transferred proteins (RTPs) which are secreted from the fungus into the infected host cell. Although known from bacterial pathogens, this represents a novelty for fungal pathogens. However, a link to biotrophy remains to be established.