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.