Early endocytosis as a key to understanding mechanisms of plasma membrane tension regulation in filamentous fungi

Background Information Two main systems regulate plasma membrane tension (PMT) and provide a close connection between the protoplast and the cell wall in fungi: turgor pressure and the actin cytoskeleton. These systems work together with the plasma membrane focal adhesion to the cell wall and their contribution to fungal cell organization and physiology has been partially studied. However, it remains controversial in model filamentous ascomycetes and oomycetes and even less investigated in filamentous basidiomycetes. Early endocytosis can be used to research the mechanisms regulating PMT since the dynamics of early endocytosis is largely dependent on this tension. Results This study examined the effects of actin polymerization inhibitors and hyperosmotic shock on early endocytosis and cell morphology in two filamentous basidiomycetes. The main obtained results are: (i) the depolymerisation of F‐actin leads to the fast formation of endocytic pits while inhibiting of their scission from the plasma membrane and (ii) the moderate hyperosmotic shock does not affect the dynamics of early endocytosis. These and our other results have allowed suggesting a curtain model for the regulation of PMT in basidiomycetes. Conclusions and significance According to the proposed curtain model, the PMT in many non‐apical cells of hyphae is more often regulated not by turgor pressure but by a system of actin driver cables that are associated with the proteins of the focal adhesion sites. The change in PMT occurs similar to the movement of a curtain along the curtain rod using the curtain drivers. This model addresses the fundamental properties of the fungal structure and physiology. It requires confirmation including the currently technically unavailable high‐quality labelling of the actin cytoskeleton of the basidiomycetes. Research article: Is turgor pressure equally important for cell organization in all organisms with a cell wall? It is known that not all cells in filamentous fungi and oomycetes have significant turgor pressure. In this work, it was shown that not only turgor, but mainly the actin cytoskeleton, which interacts with the sites of focal adhesion of the plasma membrane to the cell wall, regulates the plasma membrane tension, the dynamics of early endocytosis and plays a significant role in maintaining the shape and other properties of the cellular protoplast in filamentous basidiomycetes.