Intraspecific signalling regulates protohaustorium formation in the parasitic plant Phtheirospermum japonicum

Mass occurrences of parasitic plants from the Orobanchaceae family pose significant challenges in agricultural settings. The impact of parasitic weeds such as Striga and Rhamphicarpa increases markedly with parasite density, often causing yield losses approaching 100% under heavy infestation (Parker 2012; Rodenburg et al. 2016). The hemiparasite Rhamphicarpa fistulosa, for example, substantially reduces host-plant biomass, with crop losses rising alongside parasite density (Kabiri et al. 2017). Similarly, populations of Rhinanthus minor form “shifting clouds”, where dense infestations exert stronger growth-reducing effects on host plant communities (Cameron et al. 2009). For photoautotrophic hemiparasites, enhanced host suppression may improve aboveground light capture, whereas higher parasite densities may intensify belowground competition for host access. However, whether parasitic plants also increase their physiological virulence under high-density conditions remains unknown. This thesis investigates how conspecific density influences haustorium formation in the facultative hemiparasite Phtheirospermum japonicum. Using protohaustorium formation, the early developmental stage of the infection organ as a quantitative proxy for virulence, it was found that seedlings grown at high conspecific density consistently produced more protohaustoria than those at low-density, across diverse haustorium-inducing factors (HIFs) including quinone, phenolics, peptide, and natural hosts. This enhancement was independent of nutrient resource and host identity, indicating the involvement of a distinct, density-dependent mechanism of protohaustoria formation rather than specific host-derived cues. Biochemical and metabolomic analyses demonstrated that this effect is mediated by a diffusible, low-molecular-weight (