Structure and development of the dolipore septum inPisolithus tinctorius

Summary

The structure and development of dolipore septa and associated clamp connections are described for the ectomycorrhizal holobasidiomycetePisolithus tinctorius, following freeze-substitution of growing hyphae. Septa in the main hypha and clamp are formed synchronously and are completed within a few minutes. They are produced by a furrowing of the plasma 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 parenthesome capping it, indicating that this is organised late in septal pore development. At maturity, the pore is surrounded by a dome-shaped, perforate parenthesome on each side and is filled with filamentous electron-opaque material which spreads laterally over the adjacent septal membrane. Filaments radiate from this material to contact the parenthesome. The entire structure is interpreted as a co-ordinated whole, with the radiating filaments anchoring and supporting the parenthesome, so that its shape, position and orientation in relation to the pore entrance are maintained. Similarly the rough endoplasmic reticulum (ER) parallel to the septum also appears anchored to the plasma membrane by short fine filaments. Continuities between the lumen of this ER and the parenthesome could not be found, and the evidence indicates that the rim of the parenthesome is anchored to the plasma membrane rather than to the ER. Septal structure and development are discussed in relation to symplastic continuity in the hyphae and cell-to-cell transport across the dolipore.