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 that have the cell wall. However, in mycology there is increasing evidence that turgor pressure is not the only and not always the main factor influencing, for example, the tension of the plasma membrane, the size and shape of non-apical fungal cells, and 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 are 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, summed in the single curtain model, complement or replace the physiological functions of turgor, and allow imagining 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 of fundamental importance for mycology, but also opens up new prospects in the development of drugs that control the growth and pathogenicity of filamentous fungi.