Chemoattraction of zoospores of the soybean pathogen, Phytophthora sojae, by isoflavones

The main infective agents in plant diseases caused by species of the fungal genera Phytophthora and Pythium are zoospores that are attracted chemotactically to plant surfaces. Here we demonstrate that the simple isoflavones daidzein and genistein, which occur in soybean root exudates, are highly effective chemoattractants for zoospores of Phytophthora sojae, an economically important pathogen of soybeans. When added to suspensions of actively swimming zoospores, daidzein and genistein also cause rapid encystment and germination. The isoflavones are active at concentrations down to 10 nM but are inactive with zoospores of several other species of Phytophthora and Pythium, non-pathogenic on soybeans. Daidzein and genistein are also inducers of nodulation genes in Bradyrhizobium japonicum, the nitrogen-fixing bacterial symbiont of soybeans. Thus, both the pathogen and the symbiont identify their host by recognizing the same chemical signals.