S3.4: The effect of parasitic plants on host immunity against microbial pathogens

The Orobanchaceae family is one of the species-richest lineages of parasitic plants and contains notoriously difficult to control weeds of crops, such as witchweeds (Striga spp.) and broomrapes (Orobanche spp. and Phelipanche spp.). All parasites in the Orobanchaceae family infect the root of a host plant. Parasitism is obligatory for some species, while others can propagate, at least under laboratory conditions, without a host. Non-weedy Phtheirospermum japonicum (Pj) belongs to the latter group and has emerged as one of the major model systems to study parasitism on a molecular level (see also project M3.4).
Phtheirospermum japonicum is a generalist and able to parasite many different plant species. It also thrives on the model plant Arabidopsis thaliana, which facilitates genetic analysis of host responses to parasitism (figure, left panel).



Surprisingly, parasitism is not a one-way street: The haustorium (H), the infection organ of parasitic plants, produces signals that move to the host shoot. Cytokinins (CKs) produced by PjIPT1a in the tip of the haustorium induce CK responses in the host plants, independently of host CK-biosynthesis genes (figure, right panel). Pj-induced CK-signaling triggers hypertrophic growth in the host, a symptom frequently observed in plants infected by parasitic plants.
Alongside CKs, other signals may travel to the parasitized host. The nature of these signals and if these signals interfere with host responses to other organisms, including microbial pathogens, is currently unknown. Project S3.4 aims to systemically study how these signals are generated, how they are perceived, and to what extent they compromise host immune responses to other pathogens. To unravel these complex trophic networks, we are collaborating with our partner labs from Prof. Xin Li from UBC Vancouver and Prof. Volker Lipka from the University of Göttingen.

References:
Greifenhagen, A., Braunstein, I., Pfannstiel, J., Yoshida, S., Shirasu, K., Schaller, A., and Spallek, T. (2021). The Phtheirospermum japonicum isopentenyltransferase PjIPT1a regulates host cytokinin responses in Arabidopsis. New Phytol. 232: 1582–1590. doi: 10.1111/nph.17615
Spallek, T., Melnyk, C.W., Wakatake, T., Zhang, J., Sakamoto, Y., Kiba, T., Yoshida, S., Matsunaga, S., Sakakibara, H., and Shirasu, K. (2017). Interspecies hormonal control of host root morphology by parasitic plants. Proc. Natl. Acad. Sci. 114: 5283–5288. doi: 10.1073/pnas.1619078114