Signaling organogenesis in parasitic angiosperms: Xenognosin generation, perception, and response

Parasitic strategies within the angiosperms generally succeed by tightly co upling developmental transitions with host recognition signals in a process referred to as xenognosis. Within the Scrophulariaceae, Striga asiatica is among the most studied and best understood parasitic member with respect t o the processes of host recognition. Specific xenognosins regulate seed ger mination, the development of the host attachment organ, the haustorium, and several later stages of host-parasite integration. Here we discuss the sig nals regulating the development of the haustorium, the critical vegetative/ parasitic transition in the life cycle of this obligate parasite. We provid e evidence for the localized production of H2O2 at the Striga root tip and suggest how this oxidant is used to exploit host peroxidases and cell wall pectins to generate a simple benzoquinone signal. This benzoquinone xenogno sin proves to be both necessary and sufficient for haustorial induction in cultured seedlings. Furthermore, evidence is provided that benzoquinone bin ding to a redox active site completes a "redox circuit" to mediate signal p erception. This redox reaction regulates the time-dependent expression of s pecific marker genes critical for the development of the mature host attach ment organ. These studies extend the emerging series of events necessary fo r the molecular regulation of organogenesis within the parasitic plants and suggest novel signaling features and molecular mechanisms that may be comm on across higher plants.