A quinone signaling inhibitor enables functional dissection of haustorium-inducing factors in Orobanchaceae parasitic plants

Parasitic weeds in the Orobanchaceae family pose a major threat to crop production worldwide. Parasitic plants develop specialized invasive structures called haustoria, which penetrate host tissues to establish connections and absorb nutrients. The formation of prehaustoria, early-stage haustorial structures, is triggered by host-derived haustorium-inducing factors (HIFs), such as 2,6-dimethoxy-1,4-benzoquinone (DMBQ) and syringic acid. Since prehaustorium formation is a critical initial step in parasitism, its inhibition represents a promising strategy for controlling parasitic weeds. In this study, we performed a chemical screening to identify inhibitors of prehaustorium formation and discovered a compound, designated Haustorium INhibiting Compound 55 (HINC55), that effectively inhibits prehaustorium formation in the parasitic plants Striga (Striga hermonthica) and Phtheirospermum japonicum. Notably, HINC55 suppressed prehaustorium induction by quinones and phenolics, but not by cytokinins in Striga. Furthermore, HINC55 inhibited DMBQ-induced stomata closure in both Arabidopsis (Arabidopsis thaliana) and P. japonicum, suggesting that HINC55 functions as an inhibitor of plant quinone signaling. We used HINC55 to evaluate the composition of HIFs in host root exudates. HINC55 partially suppressed prehaustorium formation in Striga and almost completely in P. japonicum when induced by host root exudates, reflecting the broader HIF responsiveness of Striga. Transcriptome analysis further confirmed the stronger suppression in P. japonicum in response to rice (Oryza sativa) root exudate than in Striga. Overall, HINC55 serves as a tool for investigating plant quinone signaling and dissecting host-parasite chemical communications, as well as a compound for developing novel strategies to control parasitic weeds.