Surface glycoproteins : evidence that they may function as the race specific phytoalexin elicitors of Phytophthora megasperma f.sp. glycinea

Abstract

Glycoproteins were extracted from isolated cell walls of Phytophthora megasperma f.sp. glycinea (formerly P. megasperma var. sojae) with 0·1 n NaOH at 0·C and elicited glyceollin in soybean hypocotyls with the same specificity as the fungus races from which they were obtained. Fractionation of the crude extracts on DEAE Bio-Gel and Bio-Gel A-5m columns showed that specific elicitor activity was associated with the presence of high molecular weight glycoproteins detected by SDS gel electrophoresis. The glycoproteins appeared to contain only glucose and mannose as neutral sugars. The elicitor activity of the glycoproteins was not diminished by boiling at 100°C or pronase treatment, but was destroyed by periodate, thus indicating that the carbohydrate portions are important for activity. The glycoproteins were the only concanavalin A reactive species detected in the crude cell wall extracts, and fluorescein labelled concanavalin A was hapten-specifically bound to living hyphae of the fungus and to native but not NaOH-extracted isolated cell walls. Therefore it was concluded that the glycoproteins are present at the surface of the fungus cell wall. Tunicamycin, which inhibits the glycosylation of eucaryote surface glycoproteins, was a potent inhibitor of mycelial growth of the fungus. The data supported the hypothesis that race specificity in the soybean— P. megasperma f.sp. glycinea system may be determined by specific plant recognition of fungus surface glycoproteins.