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Abstract
Cellular interactions between the ascomycete Cymadothea trifolii and Trifolium repens (white clover) were analyzed using high-pressure freezing and freeze substitution. Cymadothea trifolii, a biotrophic leaf pathogen, forms a unique structure within its own hyphae, presumably for nutrient uptake from its host. This structure, called an interaction apparatus, consists of long, thin, often net-like cisternae surrounded by a membrane continuous with the fungal plasma membrane. The plant plasmalemma opposite the interaction apparatus invaginates to produce a host bubble. The interaction apparatus and host bubble are apoplastic and are linked by a tube with an electron dense sheath that may channel nutrients from the host to the pathogen. Within the tube, the cell walls of host and parasite appear altered. The interaction apparatus and host bubble may be analogous to haustoria in other obligately biotrophic fungi while the electron dense sheath of the tube may be equivalent to the haustorial neckband.
We would like to express our gratitude to R. Kirschner for specimens of C. trifolii, H. Schwarz for his help with cryofixation and freeze substitution and Y. Stierhof and U. Nehls for many inspiring discussions. The valuable technical assistance of E. Wagner-Eha, F. Albrecht and H. Steigerle is appreciated greatly. We also thank the reviewers for their helpful suggestions. This work was financed by a DFG-fellowship to U. Simon (Graduate College “Infection Biology” 685).