Chemotactic responses of Phytophthora sojae zoospores to amino acids and sugars in root exudates

Chemotaxis, encystment, cyst germination, and germ tube growth of Phytophthora sojae zoospores in response to common amino acids and sugars in root exudates (l-glutamate, l-aspartate, l-asparagine, l-alanine, l-serine, l-valine, l-histidine, and l-hydroxyproline; l-glucose, l-sucrose, l-fructose, l-arabinose, and l-galactose were examined. All 8 l-amino acids and 5 l-sugars stimulated zoospore chemotaxis, encystment and cyst germination but did not stimulate subsequent growth of the germ tube in P. sojae. Germ tube growth was significantly retarded in response to each of the test compounds. These findings suggest that chemotaxis, encystment and cyst germination in P. sojae zoospores are receptor-mediated and that the zoospores did not take up and utilize the test amino acids and sugars as nutrients. Different amino acids and sugars affected zoospore chemotaxis to different degrees. Maximum promotion or inhibition occurred at specific concentrations for each compound.