A cytochemical investigation of the host-parasite interface inPisum sativum infected by the downy mildew fungusPeronospora pisi

Abstract

A cytochemical study involving enzymic digestions, chemical extractions and specific staining methods was made of the host-parasite interface in downy mildew infected pea plants. Enzymic hydrolysis revealed both the penetration and extrahaustorial matrices to have a proteinaceous component, possibly glycoprotein, while the extrahaustorial matrix had cellulose as an additional constituent. Intense silver proteinate staining of both matrices following a prolonged incubation in thiocarbohydrazide indicated the presence of complex carbohydrates. Carbohydrates were confirmed as constituents of both matrices following the complete suppression of silver staining using the aldehyde blocking agents dimedone and sodium borohydride. Both matrices also exhibited a marked affinity for phosphotungstic acid. Following acetylation a complete elimination of phosphotungstate staining resulted, again indicating carbohydrate as a constituent of both matrices. Digestion of the fungal cell wall using an enzyme which hydrolyses β-1,3-glucans showed that these carbohydrates are important in its construction. However such enzyme treatment did not affect the structural integrity of either the penetration or extrahaustorial matrix. The extrahaustorial membrane exhibited negligible staining with phosphotungstic-chromic acid treatment, while the host plasmalemma showed a positive but variable staining reaction. A very intense staining reaction characterized the fungal plasmalemma after staining with either phosphotungstic-chromic acid, phosphotungstic acid or silver proteinate.