The role of phosphoinositides in plant-pathogen-interactions

Plants respond to infection by pathogenic microbes (bacteria, fungi or oomycetes) with the induction of defense mechanisms, such as the secretion of antimicrobial substances by the host plant. Plant defense responses can be partially suppressed by effector proteins inserted into the host cells by the pathogen. To insert effectors into host cells, pathogenic fungi and oomycetes can utilize the endocytotic machinery of the plant. Endocytosis is also important for the cycling of plasma membrane receptors for pathogen-associated molecular patterns (PAMPs) of bacteria or eukaryotic pathogens, and thus, both endocytosis and exocytosis are important components of plant-pathogen-interactions. In all eukaryotes endocytosis and exocytosis are controlled by phosphoinositides (PIs), a class of regulatory phospholipids, raising the question whether PIs partake in membrane budding or vesicle fusion during infection of plant cells by microbial pathogens and subsequent defense responses. To date PIs have not systematically been studied with regard to possible roles in the plant-pathogen field. The proposed project aims at providing a systematic analysis of the roles of PIs in the induction of plant pathogen-defenses. Dynamic changes of PIs upon pathogen infection are tested by a combined biochemical and cell biological approach in different plant host models and using different pathogens. So far, dynamic changes in PI-levels upon infection have been observed in Arabidopsis thaliana infected with the necrotrophic fungus, Botrytis cinerea, and in Nicotiana tabacum infected with the bacterium, Xanthomonas campestris pv. vesicatoria (Xcv). First cell biological analyses of the infection of onion (Allium cepa) epidermal cells by the fungus, Colletotrichum graminicola, suggest localized accumulation of PIs around the sites of appressoria formation. The data support the notion that PIs contribute to the manifestation of plant defense responses, such as the localized deposition of callose to reinforce cell walls at sites of infection. Transgenic Arabidopsis plants with modified PI-metabolism are being generated to be tested for altered susceptibility to pathogen infection. To furthermore elucidate a possible role of pathogen effector proteins in modulating host PI-metabolism, selected pathogen effectors are being tested for their impact on the plant PI-system and on PI-controlled processes, including endocytosis and exocytosis. The information gained in this project will aid the understanding of plant surface-recognition and transmembrane signaling as well as provide a biologically relevant framework for the study of PI-controlled targeted secretion and localized vesicle internalization in plants.