Parasitic plants in the Orobanchaceae family, such as Striga spp., are a global agricultural problem due to their destructive effects on crop production. The parasitic plants form invasive organs, the haustoria, which penetrate the host tissue, reach the host steel, and connect their vasculatures to that of the hosts to obtain nutrients and water. Haustorium induction is triggered by host-derived signals, collectively called as haustorium-inducing factors (HIFs). HIFs include quinones and their precursor phenolics, such as 2,6-dimethoxy-p-benzoquinones (DMBQ) and syringic acid. HIF treatments induce the formation of prehaustoria, early haustorial structures with divided and expanded root cells. The haustorium induction process is critical for parasitism, but remains poorly understood. To elucidate the mechanisms of haustorium induction, we have performed a forward genetic screen using a model parasitic species, Phtheirospermum japonicum. We have identified two mutants with opposite phenotypes; one forms spontaneous prehaustoria without external HIFs, and another does not form prehaustoria in response to HIFs. Analysis of these mutants revealed that modification of monolignol precursors or their derivatives is crucial for distinguishing self and non-self. Moreover, a significant overlap of quinone-triggered immunity and HIF signaling pathways suggests co-option of the immunity pathway for host recognition in parasitic plants.
