A novel in vitro infection system to study Phytophthora-host interactions

One of the most devastating plant diseases world wide is late blight on potato and tomato caused by the oomycete pathogen Phytophthora infestans. During the early biotrophic phase of infection, Phytophthora penetrates host tissue and thereafter forms specialized feeding structures called haustoria. Here, effectors produced by the pathogen, are transferred into the host cells to manipulate the host cell machinery thereby suppressing plant defense. Therefore, studying the interface between the host and the pathogen at the early stages of infection is of great interest. An important drawback when studying the Phytophthora-host interaction in leaves is the lack of synchronization of the infection process. For this purpose, a new in vitro infection system was established, in which MsK8 tomato cell suspensions were challenged with zoospores of different Phytophthora species. Here we show that P. infestans infects MsK8 cells in a similar fashion as tomato leaf tissue. In contrast, other Phytophthora species that are not pathogenic on tomato could not penetrate the MsK8 cells. Expression analyses of Phytophthora effector and tomato defense genes and various histological assays were performed to monitor Phytophthora-MsK8 interactions in more detail. The use of this novel infection system allows simplification and synchronization of the infection process, and is expected to provide a more detailed insight into Phytophthora-host interactions.