Phytophthora sojae zoospores differ in chemotaxis to the root and root exudates of host soybean and nonhost common bean

Phytophthora root and stem rot of soybean is a destructive disease in many countries caused by the soil-borne pathogen Phytophthora sojae. The interaction between soil-borne pathogens and plant roots before invasion is a focus of interest for revealing host and nonhost resistance mechanisms to soil-borne pathogens. In the present study, soybean cvs. Sloan (susceptible), Williams 82 (resistant) and nonhost common bean cv. Yidianhong were used to determine the effect of roots, root exudates, and isoflavones, amino acids, sugars, and citric acid from the root exudates on the pre-infection behavior (taxis, encystment and cyst germination) of zoospores of P. sojae. The elongation zone of roots of both host soybeans attracted significantly more zoospores than did that of the nonhost bean, and that of the susceptible cultivar attracted significantly more zoospores than did that of the resistant cultivar. Similarly, the host soybean root exudates attracted zoospores and promoted zoospore encystment and cyst germination, but nonhost root exudates had no effect on zoospores. Thus, P. sojae apparently selects its host depending on the root exudates. Of the 26 total components detected from root exudates, the exudates from susceptible and resistant soybeans and nonhost common bean had 12, 17, and 25 components respectively. The differences between the host and nonhost were mainly in the type and concentration of amino acids. Most components were significant chemoattractants of P. sojae zoospores. However, when these attractants were mixed at their relative concentrations in the root exudates, the chemoattraction was significantly reduced compared to that of the sum of each components, and the reduction from strongest to weakest was nonhost common bean, resistant and susceptible soybeans, indicating that these components in the root exudates interact with each other, and the intensity of the interaction relates to the type and concentration of the components. This phenomenon is because some components may share the same receptor, thus causing an obstructive effect. This result may also be a reason that common bean is a nonhost of P. sojae. In addition, the relative local concentrations of citric acid in the root exudates of both nonhost cv. Yidianhong and resistant cv. Williams 82 were repellent to zoospores of P. sojae. Root exudates of susceptible cv. Sloan amended with two concentrations of citric acids especially reduced zoospore chemoattraction, indicating that citric acid in the root exudates also plays an important role in host and nonhost resistance to P. sojae.