An haustoria inducer for the root parasite agalinis purpurea

Abstract Host recognition by the angiosperm root parasite Agalinis purpurea occurs through the development of haustoria, organs specialized for the attachment and penetration of host roots. Axenic cultures of Agalinis do not develop haustoria, but low molecular weight compounds produced by host plants induce haustoria development. Structure-activity studies have shown that highest levels of haustoria induction are afforded by flavonoids bearing specifically substituted methoxyphenol functionality. This paper examines Lespedeza sericea to determine the extent of this structure-activity relationship among hosts of Agalinis . A pentacyclic triterpene has been isolated from Lespedeza roots as a haustoria inducer, and extensive NMR and mass spectral experiments led to its characterization as soyasapogenol B (3β,22β,24-trihydroxy-olean-12-ene). The activity of soyasapogenol B is lower than the phenolic inducers, and modification of hydroxyl substitutents on soyasapogenol B abolishes activity. This haustoria inducer appears to be preferentially secreted from the roots of Lespedeza , but not in quantities sufficient to account for all haustoria inducing activity exhibited by the crude root exudate.