Visual characterization of the extracellular matrix of Cochliobolus heterostrophus and a mutant strain with a modified matrix

Abstract

Two layers of extracellular matrix (ECM) and a film secreted outside the layers were visualized on germlings of Cochliobolus heterostrophus Drechsler grown on glass slides, cellophane membranes, and the surface of maize leaves. A mutant of C. heterostrophus, less virulent than the wild type, possessed the inner layer of ECM and the film, but not the outer layer. Using cytochemical and morphological methods, we explored the hypothesis that the reduced virulence of the mutant in leaves was due to the absence of the outer layer of the ECM. All ECMs were characterized using ruthenium red fixation, cryopreservation, immunocytochemistry, and colloidal gold labeling, before being examined with light and electron microscopy. With immunocytochemistry, antigens were localized in islands stained with ruthenium red within the scaffolding of the outer layer of the wild-type ECM on leaf surfaces and within the leaf. In the mutant, antigens were localized in the film on leaf surfaces. Comparisons between leaves infected by the two strains showed hyphae to be enclosed within material interpreted to be host response within intercellular spaces of leaves infected by the mutant, but not the wild type.Key words: Cochliobolus, cytochemistry, extracellular matrix, microscopy, mutant, virulence.