Protoplasma (2002) 219: 13–22 Summary. A host-free system was established to induce the early development of the obligate biotrophic pathogen Plasmopara viti- cola, the downy mildew of grapevine. This system was used to study cytoskeletal responses during encystation and germ tube formation. During these processes, both the actin and the tubulin cytoskeleton show a stage-specific pattern of distribution. Elimination of the cytoskeleton by the actin drug latrunculin B and the microtubule drug ethyl-N-phenyl-carbamate did not affect the release of mobile zoospores from the sporangia, nor the encystation process, but effi- ciently inhibited the formation of a germ tube. The data are dis- cussed with respect to a role of both actin and microtubules for the establishment of the cell polarity guiding the emergence and the growth of the germ tube. Keywords: Plasmopara viticola; Latrunculin B; Ethyl-N-phenyl- carbamate; Cytoskeletal response; Polarity; Germ tube. Introduction Plasmopara viticola is one of the most important pathogens of Vitis vinifera in moderate climates. Its infection cycle is characterized by fast transitions between different developmental stages (for detailed description, see Müller and Sleumer 1934). For suc- cessful infection, the zoospores of P. viticola have to reach the appropriate host cell, the guard cells on the lower surface of the leaf.They subsequently encyst and develop a germ tube that penetrates into the respira- tory cavity. The germinating spore is thus behaving as a typical tip-growing cell. In general, tip growth requires a vigorous transport of vesicles, a polarity guiding this transport, and the allocation of resources to the growing tip. The cytoskeleton can meet at least the first two of these requirements for tip growth. Microtubules and fila- mentous actin (F-actin) are both polar structures that can serve as tracks for the transport of vesicles carry- ing cell wall components or enzymes to the actively growing zone (Gow 1995, Heath 1995). Furthermore, there exist molecular motors, such as kinesins and dyneins for microtubules and myosins for F-actin, that can move along these tracks and carry specific cargoes (for a review, see Nick 1999). Numerous examples demonstrate that cell polarity can be disturbed by treatment with cytoskeletal drugs. For instance, the axis of root growth in Arabidopsis thaliana (Baskin et al. 1994) and the induction of polarity by light in zygotes of fucoid algae (Kropf et al. 1998) are affected upon the elimination of F-actin or microtubules. In the oomycete Phytophthora cinnamomi, zoosporogenesis and the correct distribution of vesicles and organelles have been attributed to microtubules (Hyde and Hardham 1993), and the participation of the cytoskele- ton in sporangium development, zoosporogenesis, or hyphal tip morphogenesis has been described for other oomycetes as well (e.g., Harold and Harold 1992, Temperli et al. 1990, Heath et al. 2000). The third prerequisite of tip growth is the continu- ous allocation of energy and genetic information into the growing region over a distance that is too large to be bridged by mere diffusion. Again, it is the cytoskeleton that drives this process: Microtubules seem to be involved in nuclear movement in Phy- tophthora cinnamomi (Hyde and Hardham 1993), PROTOPLASMA © Springer-Verlag 2002 Printed in Austria Cytoskeletal responses during early development of the downy mildew of grapevine (Plasmopara viticola) M. Riemann 1, *, C. Büche 2 , H.-H. Kassemeyer 2 , and P. Nick 1 1 Institut für Biologie II, Albert-Ludwigs-Universität Freiburg, Freiburg 2 Staatliches Weinbauinstitut, Freiburg Received October 4, 2001 Accepted October 23, 2001 * Correspondence and reprints: Institut für Biologie II, Albert- Ludwigs-Universität Freiburg, Schänzlestrasse 1, 79104 Freiburg, Federal Republic of Germany.