Parasitic plants in the Orobanchaceae use chemicals released from host-plant roots to direct developmental processes crucial to their heterotrophic lifestyle. An illustrative example is the development of haustoria; parasite root organs that function in host attachment and penetration, and in the establishment of a physiological conduit through which host resources are robbed. The facultative parasite Triphysaria develops haustoria only in the presence of host roots or host root factors. An in vitro assay was used to identify several phenolic derivatives that induce haustorium formation; the activity of multiple signalling molecules is consistent with a redundancy of active molecules in the rhizosphere triggering haustorium development. Haustorium-inducing factors are structurally related to phytotoxic allelochemicals released by some plants to inhibit the growth of neighbouring plants. We used genomic approaches to demonstrate that similar genetic pathways are up-regulated in parasitic roots upon contact with host plants as are regulated in response to allelochemical exposure. A parasite quinone oxidoreductase was identified that has properties suggesting that it functions in both allelochemical detoxification and haustorium signal transduction. These and other mechanistic similarities between allelopathic toxicity and haustorium signal transduction support the hypothesis that parasitic plants have recruited allelotoxin defence mechanisms for host-plant recognitio
