Structure and function of rust haustoria

K. Mendgen, C. Struck, M. Hahn, and R. T. Voegele
Lehrstuhl für Phytopathologie, Fachbereich Biologie der Universität Konstanz, 78434 Konstanz, Germany
Fachbereich Biologie, Phytopathologie, Universität Kaiserslautern, 67653 Kaiserslautern

Rust fungi are among the most common plant diseases infecting more then 8000 monocot and over 40 000 dicot species. These parasites are obligate biotrophs. They differentiate infection structures such as appressoria over stomatal openings to enter the leaf and develop haustoria within host cells. High-pressure freezing and freeze substitution of haustoria revealed a matrix between the plasma membranes of host and parasite that serves as apoplastic space probably used for nutrient transfer. Gene expression in haustoria of Uromyces fabae, the broad bean rust fungus, is differentially regulated. Some 20 % of the mRNA population is found mainly in haustoria, and not at all or only at very low levels in spores and the other early infection structures, respectively.
Highly expressed genes were sequenced and the function of the most abundant proteins was elucidated. Immunolabelling showed that the fungus secretes invertase into the extrahaustorial matrix. An H+-ATPase is located in the fungal plasma membrane and pumps protons into this fungus-plant interface. A proton-hexose cotransporter pumps glucose and fructose into the haustoria. Within the haustorium, hexoses are metabolized, but are also converted to mannitol and other hexitols. These compounds are likely to serve as storage metabolites, in osmoprotection and/or protection from free radicals.
We characterized several amino acid transporters active in haustoria by heterologous expression in yeast and Xenopus oocytes. These transporters are specific for amino acids exhibiting only low concentrations in the plant apoplast.
Our data indicate that the rust haustorium is one, if not the main source and distributor of amino acids and carbohydrates for the rust fungus. Its specific role makes it a target to control rust diseases.
References
1. Voegele, R.T ., Struck, C., Hahn, M., Mendgen, K. (2001) The role of haustoria in sugar supply during infection of Vicia faba by the rust fungus Uromyces fabae. Proc. Natl. Acad. Sci. U.S.A. 98, 8133 - 8138
2. Hahn, M, Mendgen, K. (2001) Signal and nutrient exchange at biotrophic plant-fungus interfaces. Curr. Op. in Plant Biol. 4, 322-327
3. Struck, C., Ernst, M., Hahn, M. (2002) Characterization of a developmentally regulated amino acid transporter (AAT1p) of the rust fungus Uromyces fabae. Mol. Plant Pathol. 3, 23-30
E-mail: Kurt.W.Mendgen@uni-konstanz.de