Lomasomes and other fungal plasma membrane macroinvaginations have a tubular and lamellar genesis

The plasma membrane of filamentous fungi forms large invaginations (>100-200 nm in diameter or length), which are especially pronounced in xylotrophic basidiomycetes. Typically, such macroinvaginations are either tubules or parietal vesicles. Vesicular macroinvaginations at the electron microscopic level correspond to classical lomasomes. There is an assumption that vesicular macroinvaginations/lomasomes may be involved in macrovesicular endocytosis, i.e., be scissored from the plasma membrane and be transported to the cytoplasm. The original aim of this study was to test for the presence of macroendocytosis in xylotrophic fungi using a fluorescent styryl label and time-lapse and Z-stacks microscopic technologies. However, the results were unexpected. It was found that most of the membrane structures labeled in fungal cells by the styryl fluorophore (analogue of FM4-64) is various types of macroinvaginations of the plasma membrane. All of these macroinvaginations are or go through a tube or lamella stage. Moreover, under specific conditions of a microscopic preparation, different types of invaginations are formed at different times - the longer the sample observation time, the greater may be the diameter of the tubes that form macroinvaginations. In addition, the morphology and successive formation of macroinvaginations mimics the endocytic pathway: different types of the invaginations can easily be mistaken for primary endocytic vesicles, endosomes, and vacuole-lysosomes. The paper qualitatively and quantitatively analyzes various types of macroinvaginations of the plasma membrane, suggests their biological functions, and discusses the presence and features of classical and macrovesicular endocytosis in fungi. This study is another step towards understanding the complex physiology of filamentous fungi associated with changes in plasma membrane tension, rapid changes in hyphae size, and endocytosis. In addition, this work is a presentation of a new tubular system in wood-decaying fungi, which takes its place among the already described tubular systems: vacuolar, mitochondrial, and endoplasmic-reticular.