Use of a 15N-tracer Method as a Tool to Indicate the Assimilation of Elicitin-Sterol Complexes by Phytophthorasojae

The mechanism of sterol acquisition and transport in oomycetes of the genus Phytophthora is suspected to involve secreted class-I elicitins, but little information is available to confirm this. The objective of this study was to determine if class-I elicitins of Phytophthorasojaepromote assimilation into this organism when a phytosterol is amended into its growth medium using a qualitative 15N tracer method. Two P. sojae growth experiments were conducted involving 15N labeled mycelium or 15N spent broth containing putative P. sojaeelicitins in the absence or presence of a phytosterol (stigmasterol). Mycelium was harvested after 0, 2, 3 or 6 days of growth and analyzed for d15N by Elemental Analysis-Isotopic Ratio Mass Spectrometry (EA-IRMS). Results from the two experiments demonstrated that P. sojae assimilated putative elicitins from the spent broth more rapidly when stigmasterol was present in the growth medium. These results provide evidence for the involvement of secreted class-I elicitins in sterol acquisition by P. sojae.