Soybean Isoflavones Trigger a Calcium Influx in Phytophthora sojae

Both the motile zoospores and the hyphal germ tubes of Phytophthora sojae respond chemotropically to the soybean isoflavones daidzein and genistein. The role of Ca2+ in the cellular response to these host signals was investigated by using X-ray microanalysis of cells to monitor net changes in cellular levels of Ca2+ and by quantifying the effects of exogenous Ca2+ and daidzein on the developmental fate of encysted zoospores. Confirmation that isoflavones trigger a net influx of Ca2+ into the cell was demonstrated by X-ray microanalysis of individual encysted zoospores. Zoospores exposed to 10 mM Ca2+ and 1 μM daidzein at the time of encystment formed cysts that contained more Ca2+ than zoospores exposed to Ca2+ alone. The magnitude of internal Ca2+ stores appears to be a determining factor affecting the developmental fate of P. sojae cysts.