EVs in fungal biofilm biology
Membrane vesicles play a significant role in the formation and maintenance of fungal biofilms (Zarnowski et al., 2018; Zarnowski et al., 2022). Biofilms are complex structures composed of communities of sessile cells embedded in a self-produced extracellular matrix (ECM) (Ramage et al., 2009; Harding et al., 2009). Formation of biofilms generally involves a succession of phases commencing with adherence of cells to surfaces, followed by the growth of cells which are covered by the ECM, leading up to maturation and dispersal of cells (Chandra et al., 2001; Harding et al., 2009). Nonetheless, the exact mechanisms by which membrane vesicles contribute to biofilm formation are not fully understood, but several mechanisms have been proposed. One proposed mechanism is that membrane vesicles are involved in the secretion of extracellular matrix (ECM) components, such as polysaccharides and proteins, which are necessary for the formation of the biofilm structure (Harding et al., 2009; Di Martino, 2018). Vesicles are also known to transport enzymes that are involved in the degradation of host tissues, which can create a suitable environment for biofilm formation (Zarnowskiet al., 2018). Another proposed mechanism is that membrane vesicles are involved in intercellular communication within the biofilm. Studies have shown that vesicles can transfer signaling molecules, such as lipopeptides and small RNAs, between cells, which can coordinate the behavior of the cells within the biofilm (Leone et al., 2017). Additionally, membrane vesicles are also thought to play a role in the development of antifungal resistance. Vesicles have been found to sequester drugs and protect the biofilm cells from their effects, and also to detoxify drugs, breaking them down before they reach the cells ( et al., 2018; Zarnowski et al., 2022).
EVs are also involved in development of biofilms of some bacterial and yeast species (i.e., Toxoplasma gondii etc. ) (Li et al., 2018, Zarnowski et al., 2022). Studies on the role of EV in biofilms in plant filamentous plant fungi have not been reported, although in yeasts (e.g., C. albicans ) a number of studies have been conducted. Recent studies have shown that the structural complexity of biofilms (i.e., high cell density, ECM etc.) leads to production of EVs which can enhance drug resistance and are unique to planktonic (Free-living) cells (Bielska and May, 2019, Honorato et al., 2021, Zarnowski et al., 2022). The cargo of C. albicans biofilm EVs ferry compounds including cell-wall degrading enzymes as well as mannan and glucan (Zarnowski et al., 2014, Mitchell et al., 2015, Garcia-Ceron et al., 2021). Compatible with the assembly of the ECM, the delivery mannan-glucan complex by EVs is critical for drug resistance (Zarnowski et al., 2014, Mitchell et al., 2015). ESCRT-I genes including Hse1 and Vps27 were found enclosed in secreted EV cargo of C. albicans . These ESCRT-I containing EVs are reported to restore the biofilm matrix architecture and quantities of the key mannan–glucan components, an indication that they may function in biofilm EV production and promotion of matrix biogenesis (Zarnowski et al., 2018). EVs are also believed to control morphogenesis in C. albicans as their presence hinders the process of biofilm formation and dimorphic transition (Honorato et al., 2021). For instance, EVs containing RNA are assumed to actively take part in the dimorphic transition of Pichia fermentans (Leone et al., 2017). Overall, membrane vesicles are involved in multiple aspects of biofilm formation, including the secretion of matrix components, intercellular communication, and drug resistance.