Automated electron tomography of the septal pore cap in Rhizoctonia solani

Dolipore septa and septal pore caps (SPCs) in filamentous basidiomycetes ma y play an important role in maintaining the integrity of hyphal cells. We h ave investigated the ultrastructure of the dolipore septum and the SPC in R hizoctonia solani hyphal cells after high-pressure freezing, freeze substit ution, and Spurr embedding. We visualized the SPC with associated cell ultr astructures in three dimensions by automated electron tomography of thick-s ectioned cells, followed by 3D tomographic reconstructions. Using these met hods we were able to document the passage of mitochondria through the SPC, small tubular membranous structures at the entrance of the septal pore chan nel, filamentous structures connecting the inner side of the SPC with pore- plugging material, thin filaments anchoring the pore-plugging material with the plasma membrane, small vesicles attached to the plugging material, and tubular endoplasmic reticulum continuous with the base of the SPC. We hypo thesize that the SPC, the filamentous structures, the plugging material, an d the endoplasmic reticulum act in a coordinated fashion to maintain cellul ar integrity, intercellular communication, and the transport of solutes and cell organelles in the filamentous fungus R. solani. (C) 2000 Academic Pre ss.