STRUCTURE AND DEVELOPMENT OF THE DOLIPORE SEPTUM IN PISOLITHUS-TINCTORIUS

The structure and development of dolipore septa and associated clamp c onnections are described for the ectomycorrhizal holobasidiomycete Pis olithus tinctorius, following freeze-substitution of growing hyphae. S epta in the main hypha and clamp are formed synchronously and are comp leted within a few minutes. They are produced by a furrowing of the pl asma membrane and concurrent wall deposition. Fine filaments occur in a ring adjacent to the deposited septum. Radial and parallel filaments , that occur in a complex arrangement around the apex of the membrane infolding, are likely to be instrumental in bringing about cytokinesis . The pore opening is reduced to about 140 nm and there is still no pa renthesome capping it, indicating that this is organised late in septa l pore development. At maturity, the pore is surrounded by a dome-shap ed, perforate parenthesome on each side and is filled with filamentous electron-opaque material which spreads laterally over the adjacent se ptal membrane. Filaments radiate from this material to contact the par enthesome. The entire structure is interpreted as a co-ordinated whole , with the radiating filaments anchoring and supporting the parentheso me, so that its shape, position and orientation in relation to the por e entrance are maintained. Similarly the rough endoplasmic reticulum ( ER) parallel to the septum also appears anchored to the plasma membran e by short fine filaments. Continuities between the lumen of this ER a nd the parenthesome could not be found, and the evidence indicates tha t the rim of the parenthesome is anchored to the plasma membrane rathe r than to the ER. Septal structure and development are discussed in re lation to symplastic continuity in the hyphae and cell-to-cell transpo rt across the dolipore.