Haustoria regulation in the facultative parasitic plant Phtheirospermum japonicum

Sammanfattning: Parasitic plants are important agricultural pests that lead to considerable yield losses annually. Parasitic plants that completely rely on their hosts for their survival are known as obligatory parasites, whereas those independent of their hosts but parasitize under the right conditions are known as facultative parasitic plants. All parasitic plants form a multicellular organ termed the haustorium via which they invade their host, establish vascular connections and uptake water and nutrients. Despite recent advances in our understanding of parasitic plants, the mechanisms of haustoria regulation remain largely unknown. Here we aimed to identify how the environmental nutrient status affects haustoria formation in the facultative parasitic plant Phtheirospermum japonicum. We showed that P. japonicum inhibits haustoria formation in response to the macronutrient nitrogen and that this nitrogen-mediated haustoria inhibition is mediated by the hormone abscisic acid (Paper I). Further investigation of haustorium regulatory mechanisms demonstrated that P. japonicum harbors a haustoria autoregulation mechanism (AOH) that utilizes a long-distance signalling mechanism that involves small mobile peptides (Paper II). Lastly, we investigated the role of cell wall modifying enzymes in haustoria and xylem bridge formation. We identified two genes encoding cell wall modifying enzymes, one pectin methylesterase and one pectin methylesterase inhibitor, that are involved in both haustoria and xylem bridge formation in P. japonicum (Paper III). Overall this thesis contributes to our understanding of haustoria regulation in response to nitrogen availability or pre-existing established haustoria.