Transformation of the obligate biotrophic rust fungus Uromyces fabae

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Obligate biotrophic fungi, like the rust fungi, represent a major class of pathogens especially of economically important crops world wide. Yet, research on rust fungi and other obligate biotrophs has been hampered by the absence of a system for stable transformation. We have established Biolistics as a delivery tool for transgenes into the legume rust Uromyces fabae. Endogenous regulatory elements derived from the plasma membrane ATPase gene (Uf- PMA1) are used to drive expression of color and selection markers. GUS, GFP and DsRed have been established as color markers, while a mutated form of the endogenous gene succinate dehydrogenase 1 (SucDH1(H254Y)) is used as selection marker. The system uses the fungicide Carboxin as selective agent and allows a selection of transformants directly on the host plant Vicia faba. However, so far the transgene is lost after a couple of rounds of propagation. We are in the process of establishing a second transformation method based on Agrobacterium tumefaciens mediated gene delivery. This system uses the same control elements, markers and selection procedure as the biolistic system, but has the advantage of an increased stability of the transgenes. Future work will therefore center on the improvement of the A. tumefaciens mediated technique and on the extension of the method to other rust fungi.