Flavonoids promote haustoria formation in the root parasite Triphysaria versicolor

Parasitic plants in the Scrophulariaceae develop infective root structures called haustoria in response to chemical signals released from host-plant r oots. This study used a simple in vitro assay to characterize natural and s ynthetic molecules that induce haustoria in the facultative parasite Triphy saria versicolor. Several phenolic acids, flavonoids, and the quinone 2,6-d imethoxy-p-benzoquinone induced haustoria in T. versicolor root tips within hours after treatment. The concentration at which different molecules were active varied widely, the most active being 2,6-dimethoxy-p-benzoquinone a nd the anthocyanidin peonidin. Maize (Zea mays) seeds are rich sources of m olecules that induce T. versicolor haustoria in vitro, and chromatographic analyses indicated that the active molecules present in maize-seed rinses i nclude anthocyanins, other flavonoids, and simple phenolics. The presence o f different classes of inducing molecules in seed rinses was substantiated by the observation that maize kernels deficient in chalcone synthase, a key enzyme in flavonoid biosynthesis, released haustoria-inducing molecules, a lthough at reduced levels compared with wild-type kernels. We discuss these results in light of existing models for host perception in the related par asitic plant Striga.