A novel pathosystem between Phytophthora and the moss Physcomitrella patens gives insight into cellular plant defence responses

Oomycete pathogens of the genus Phytophthora cause enormous crop losses worldwide and are difficult to control. One of the strategies to combat pathogens is to improve the plant’s own immunity, which requires detailed insight into the cellular and molecular mechanisms underlying immunity. In general, live cell imaging of plant-pathogen interactions is hampered by the tissue complexity and multi-cell layered nature of the host. Therefore, we developed a novel Phytophthora pathosystem with Physcomitrella patens as host. The single-cell layered protonema of this moss is ideal for visualizing interactions with the pathogen over time using advanced fluorescence microscopy. Of four tested Phytophthora species, Phytophthora infestans and Phytophthora capsici were able to successfully penetrate moss cells and showed invasive hyphal growth and sporangia formation on moss tissue. Upon infection, several defence-related genes of Physcomitrella patens were upregulated and local cell death was induced. At a cellular level, we observed repositioning of the nucleus, accumulation of cytoplasm near the site of penetration and rearrangement of the actin cytoskeleton. Furthermore, penetration of Phytophthora was often blocked by the deposition of cell wall material in papilla-like structures, which is also a common defence response observed in higher plants. We will employ this novel pathosystem to obtain new insights in the regulation of cellular defence responses, e.g. the molecular pathways that regulate targeted exocytosis towards the site of pathogen attack.