Response of Phytophthora sojae zoospores to the rootlet and root exudates of host soybean and non-host common bean and host selectivity

Abstract:

[Background] Phytophthora root rot as a devastating soybean disease has been reported in the United States, Canada and many other countries, its pathogen Phytophthora sojae is a typical soil-borne pathogen. In recent years, the interaction between soil-borne pathogens and plant roots has become the main direction for studying the host selectivity mechanism of soil-borne pathogens. [Objective] To study the effects of Phytophthora sojae zoospores to the rootlet and root exudates of host soybean and non-host common bean, illustrate the relationship between these effects and P. sojae selecting its host. [Methods] The soybean susceptible cultivar Sloan and resistant cultivar Williams82 as well as non-host common bean Yidianhong of P. sojae were cultivated by the method of situ soil culture. The pre-infection behavior of the single zoospore of P. sojae to the rootlet of host soybean and non-host common bean was determined. The root exudates of host soybean and non-host common bean were collected and the chemotaxis of P. sojae zoospores to the root exudates was measured, including taxis of the zoospores to the root exudates, the promotion of the root exudates on the zoospores encystment, and on the germination and the growth of the germ tubes. [Results] The single zoospore had strong chemotaxis to the host rootlet. It encysted and germinated rapidly on the elongation zone of the root tip after several exploratory contacts along the root surface. The tip of the germ tube attached to the rootlet surface. The difference between the susceptible and resistant cultivar was that the germ tube on the rootlet of susceptible cultivar is shorter and thicker. However, the single zoospore had no chemotaxis to the rootlet of non-host common bean rootlet. It ran away from the rootlet after first visit of the rootlet surface, then encysted and germinated in a random growth direction at the location of approximately 75 μm far away from the rootlet. The substantial difference in the behavior which the P. sojae zoospores infected the rootlet of host soybean and non-host common bean had been repeated in the experiments with root exudates, that is P. sojae zoospores had strong chemotaxis to host root exudates. The host root exudates could effectively attract zoospores and promote encystment of the zoospores and germination of cysts, but inhibit the growth of the germ tube, whilst the root exudates of non-host common bean had no effect on the zoospores. [Conclusion] P. sojae selects its host depending on the root exudates, which will provide a theoretical basis for further understanding the host selection mechanism of P. sojae.

Key words:Phytophthora sojae, Zoospores, Root exudates, Chemotaxis, Host selection