Molecular Specificity of Haustorial Induction in Agalinis purpurea (L.) Raf. (Scrophulariaceae)*

An exogenous signal normally contained in host root exudate is required for initiation of the haustorium by the root parasite Agalinis purpurea (L.) Raf. (Scrophulariaceae). Two flavonoids that induce haustoria have been isolated from gum tragacanth and a number of structural analogues have been synthesized. The results show that a high degree of molecular specificity is required for haustorial induction. Both isolated flavonoids contain substituted 3-methoxyphenol functionality, and synthetic analogues have shown that 4-substituted 3-methoxyphenol functionality is critical for high levels of haustorial induction. These data provide a model for understanding host recognition at the level of haustorial induction in parasitic angiosperms.