Ultrastructure and immunocytochemistry of degradation in spruce and ash sapwood by the brown rot fungus Postia placenta: Characterization of incipient stages of decay and variation in decay process

• Degradation of spruce and ash wood by the brown rot fungus Postia placenta was investigated using TEM immunocytochemistry.
• Incipient degradation was reflected by loss of pectins and hemicelluloses in compound/cell corner middle lamella regions.
• Decay patterns differed significantly between spruce (softwood) and ash (hardwood) as attack progressed.
• Degradation in middle lamellae and S3 layers at incipient stages differed from previous concepts reported for brown rot fungi.

Aspects of the degradation of non-cellulosic polysaccharides (pectins, hemicelluloses) in spruce and ash sapwood by the brown rot fungus Postia placenta was investigated using transmission electron microscopy coupled with immunocytochemistry. Degradation of non-cellulosic polysaccharides and changes in ultrastructure were initiated in the compound middle lamella (CML)/middle lamella cell corner (MLcc) regions of xylem cells; sites that were remote from hyphal colonization in the cell lumen. Degradation of the S3 layer was also detected in both wood species at early stages of decay. Apart from early stages of attack, the degradation patterns differed greatly between spruce and ash. Spruce showed three patterns of hemicellulose degradation in the secondary cell walls of tracheids, while ash showed a single pattern in fibers. Spruce parenchyma cells also showed lower resistance to decay than tracheids, whereas ash parenchyma cells showed higher resistance than fibers. Together, degradation of xylem cells by P. placenta differs between spruce (softwood) and ash (hardwood) even though they show similar initial degradation in CML/MLcc regions and the S3 layer. The overall degradation patterns observed, particularly in spruce differ from previous ideas and generally accepted decay patterns produced by brown rot fungi in wood.