Ultrastructural patterns of beech leaf degradation by Sporotrichum pulverulentum

Abstract

Degradation of beech leaves by the white rot fungus Sporotrichum pulverulentum was investigated for 4 weeks under laboratory conditions. Observations by transmission electron microscopy revealed that the degradation patterns depended on the nature of the foliar tissues and on the stage of decay. Fibres, parenchyma cells and specific zones of the sclerenchyma tissue, corresponding to the cells located between vessels, were strongly degraded, while vessels and epidermal cells were more resistant to degradation. During the early stages of degradation, a selective removal of components from the lignin and hemicellulose-rich layers was observed in the cell walls of sclerenchyma fibres and parenchyma cells. At a more advanced stage of decay, a simultaneous disappearance of all cell wall layers (primary wall, secondary wall, middle lamella and cell corners) occurred, irrespective of cell type. In parenchyma cells, removal of the intracellular brown pigments occurred prior to degradation of the cell walls, while in epidermis, the cell walls were altered first. In sclerenchyma cells surrounding the vessels, hyphae were found to be closely associated with decayed areas, while in all the other tissues, there was no contact of the fungus with lignocellulosic compounds and polyphenols, suggesting a diffusion of the fungal degradative enzymes. A fibrillar mucilagenous matrix, often detected between hyphae and cell walls, might make this diffusion easier. Cellulase activity was detected in the decayed tissues by the release of reducing sugars localized in the vicinity of the leaf cell walls and often somewhat distant from the hyphae. The role of the fungal enzymes involved in degradation of cell wall components and brown pigments in beech leaves is discussed.