Chemotactic preferences and strain variation in the response of Phytophthora sojae zoospores to host isoflavones

The zoospores of Phytophthora sojae are chemotactically attracted to the isoflavones genistein and daidzein that are released by soybean roots, In this study we have examined the response of P. sojae zoospores to a wide range of compounds having some structural similarity to genistein and daidzein, including isoflavones, flavones, chalcones, stilbenes, benzoins, benzoates, benzophenones, acetophenones, and coumarins, Of 59 compounds examined, 43 elicited some response. A comparison of the chemotactic responses elicited by the various compounds revealed a primary role for the phenolic 4'- and 7-hydroxyl groups on the isoflavone structure. A few compounds acted as repellents, notably methylated flavones with a hydrophobic B ring, The chemotactic response to many of the analogs was markedly different among different strains of P. sojae.