End product of the ESCRT-pathway – EVs
In recent years, EVs have gained prominence in the field of biology due to their ubiquitous nature in organisms in all domains of life and their composition, thus making them appealing for better understanding plant-microbe interactions (Kalluri and LeBleu, 2020). As a result, EVs have been characterized from cells across a spectrum of microbes including plant pathogenic fungi (e.g., Penicillium digitatum ), bacteria (e.g., Xanthomonas campestris pv., campestris) and protozoans (e.g., Trypanosoma cruzi ) (Chatterjee and Das, 1967; Raiborg et al., 2003; Sidhu et al., 2008; Torrecilhas et al., 2009; Coelho and Casadevall, 2019; Rybak and Robatzek, 2019; Costa et al., 2021). Presently, EVs are better characterized in human bacterial pathogens than any other microbes whereby they contribute to trafficking and delivery of effector proteins which induce host immune response (Rybak and Robatzek, 2019).
Studies on fungal EVs, especially those of plant pathogens, are gradually gaining momentum. Recently, a study by Costa et al., (2021) has demonstrated that EVs are used by filamentous fungal pathogens to export the phytotoxic compound, tryptoquialanines A, during plant infection and consequently cause tissue damage on citrus seeds. This is consistent with several other studies showing that EVs derived from fungal pathogens exert toxic effects on plant tissues (e.g., Silva et al., 2014; Bleackley et al., 2020). In other words, EVs may trigger a type of immune response in plants, which leads to hypersensitivity and subsequent discoloured areas on the plant leaf. This suggests the potential use of pathogen-derived EVs as plant improvement agents by serving as immune boosters or biological biomarkers.