Changes occurring in chloroplasts of Phaseolus following infection by Sclerotinia: a cytochemical study

Dramatic changes occur in the ultrastructure of chloroplasts within host tissues during the infection of Phaseolus by the plant pathogenic fungus Sclerotinia. Electron-opaque deposits develop initially in the peripheral region of the chloroplast stroma, in cells in advance of the hyphal front. As infection continues, the amount and intensity of deposition increases and spreads throughout the chloroplast. The deposits possess a high degree of structural integrity in the form of a crystalline square lattice with 10 nm periodicity. Enzyme digestion studies have been used to show that the deposits are proteinaceous. Similar deposits are also induced by treatment with oxalic and citric acids, but not by the potassium salts of these acids buffered at pH 7.2. It is suggested that a pH change within the chloroplasts, resulting from oxalic acid secretion by the fungus, is responsible for induction of protein deposition. The likelihood that the deposits are ribulose, 1,5-bisphosphate carboxylase-oxygenase is discussed.