Ultrastructure of sclerotia of Rhizoctonia solani Kühn invaded and decayed by soil microorganisms

Abstract

Ultrastructural changes of sclerotia of Rhizoctonia solani Kühn undergoing microbial breakdown in soil were studied with two isolates differing in survival ability. Sclerotial cells of a short-lived isolate (F-15) lost their cytoplasmic contents more rapidly than those of long-lived isolate (B-5) and only the cell walls remained 8 months after burial in soil. In isolate F-15, incomplete septa lacking the complex septal pore apparatus were frequently found in empty cells. Bacterial-like organisms were detected in abundance within the amorphous layer surrounding sclerotial cells. They eroded the amorphous layer and the cell wall matrix. Disappearance of the cell wall resulted at least partially from exolysis caused by these microorganisms which were also found within empty cells. Penetration of the cell wall and sporulation by soil fungi were also frequently observed within sclerotial cells of F-15. Eventually, the empty sclerotial cell invaded by soil microorganisms collapsed and became loosely attached to one another. In contrast, the occurrence of bacterial-like organisms and microbial invasion were not as frequent with the sclerotia of the long-lived isolate (B-5). Instead, its sclerotia were characterized by many intact cells with thickened cell walls. Resistance of sclerotia of B-5 to microbial breakdown may be a result of thick cell walls or the ability of the fungus to repair decomposing cell walls.