Lomasomes and Other Fungal Plasma Membrane Macroinvaginations Have a Tubular and Lamellar Genesis

Abstract

The plasma membrane of filamentous fungi forms large-sized invaginations, which are either tubes or parietal vesicles. Vesicular macroinvaginations at the ultrastructural level correspond to classical lomasomes. There is an assumption that vesicular macroinvaginations/lomasomes may be involved in macrovesicular endocytosis. The original aim of this study was to test for the presence of macroendocytosis in xylotrophic basidiomycetes using time-lapse and Z-stacks fluorescent microscopic technologies. However, the results were unexpected since most of the membrane structures labeled by the endocytic tracer (FM4-64 analog) are various types of plasma membrane macroinvaginations and not any endomembranes. All of these macroinvaginations have a tubular or lamellar genesis. Moreover, under specific conditions of a microscopic preparation, the diameter of the tubes forming the macroinvaginations increases with the time of the sample observation. In addition, the morphology and successive formation of the macroinvaginations mimic the endocytic pathway; these invaginations can easily be mistaken for endocytic vesicles, endosomes, and vacuole-lysosomes. The paper analyzes the various macroinvagination types, suggests their biological functions, and discusses some features of fungal endocytosis. This study is a next step toward understanding complex fungal physiology and is a presentation of a new intracellular tubular system in wood-decaying fungi.

Instantaneous change in hyphal diameter in basidiomycete fungi

“…It is obvious that the ability to instantly change the diameter of hyphae and the ability to form macroinvaginations of the plasma membrane are closely related and represent a single mechanism that is a powerful tool for the rapid adaptation of the fungus to various changes occurring in the substrate. Therefore, this work is a logical continuation of our previous studies of the fungal macroinvagination systems (Mazheika et al, 2020b, 2022). The goal of this work is a light microscopic quantitative and qualitative analysis using a large number of fungal cells (several tens of thousands) of the ability of three species of basidiomycetes from different systematic and ecological groups to change the thickness of their hyphae in response to various factors.…”

“…Thus, in S. hirsutum , hyperosmotic conditions cause a sharp shrinkage of the hyphae by an average of 12%; an hour’s preincubation in liquid medium slightly expands the hyphae and reduces the effect of hyperphase on mycelium; inhibitors of actin assembly may either slightly expand or shrink or have no effect on hyphae. At the same time, all of these factors increase the number of lomasomes (glomeruli and pendants; see Mazheika et al, 2022) in S. hirsutum cells on average by 2-3 times. Similarly, the listed factors increase the frequency of occurrence of some other macroinvaginations, hollow and larger in size than lomasomes – vesicles and tubes – but not as definitely as in the case of lomasomes (see Mazheika et al, 2022).…”

“…At the same time, all of these factors increase the number of lomasomes (glomeruli and pendants; see Mazheika et al, 2022) in S. hirsutum cells on average by 2-3 times. Similarly, the listed factors increase the frequency of occurrence of some other macroinvaginations, hollow and larger in size than lomasomes – vesicles and tubes – but not as definitely as in the case of lomasomes (see Mazheika et al, 2022). According to the logic outlined in the Introduction, hyphal shrinkage should lead to an increase in the number and/or size of macroinvaginations, expansion – to decrease.…”

“…In our previous work, we studied the influence of practically the same factors that are considered in this study on the system of macroinvaginations of S. hirsutum (Mazheika et al, 2022). Even earlier, the effect of F-actin assembly inhibitors on the macroinvagination system of R. solani was analyzed (Mazheika et al, 2020b).…”

“…Previously, the authors of this paper associated the macroinvaginations with endocytosis and divided fungi into two categories: with classical (patch-type) endocytosis with small endocytic pits, and with macrovesicular endocytosis, which is characterized by a larger range of sizes of primary invaginations of the plasma membrane (Mazheika et al, 2020a, b). However, recent detailed study of some species of wood-decaying basidiomycetes using time-lapse video and Z-stacks fluorescent microscopic technologies changed our view of the problem (detailed information in Mazheika et al, 2022). It turned out that the scission of macroinvaginations from the plasma membrane a phenomenon that exists, but not frequent in these fungi.…”