The Curtain Model as an Alternative and Complementary to the Classic Turgor Concept of Filamentous Fungi

Turgor pressure, the intracellular hydrostatic pressure exerted by the cell protoplast on the cell wall, is of critical physiological importance for all organisms with the cell wall. However, in mycology, there is increasing evidence that turgor pressure is not always the main factor influencing the tension of the plasma membrane, the size and shape of non-apical fungal cells, and it is not even always the determining factor of apical growth. This paper characterizes various aspects of the curtain model, previously proposed to describe the regulation of plasma membrane tension in the hyphae of basidiomycetes. The current understanding of the four main components of the curtain model is outlined: the driving actin cytoskeleton, the elastic cell wall, tight adhesion of the plasma membrane to the cell wall, and macroinvaginations of the plasma membrane. All four elements, combined in the single curtain model, complement or replace the physiological functions of turgor and allow us to understand how a non-apical fungal cell maintains its integrity and physiological functionality under changing osmotic and other conditions. Further experimental confirmation of the curtain model is not only fundamentally important for mycology but also opens up new prospects in developing drugs that control the growth and pathogenicity of filamentous fungi.