Introduction
In 2015, a satellite workshop at the International Symposium on Extracellular Vesicles (ISEV) in Sao Paulo, Brazil, emphasized on the potential significance of EVs in cross-kingdom communication in hosts, microbes and parasites (1). As an important conclusion of this meeting, the necessity to develop robust protocols for EV isolation and the identification of suitable biomarkers for the different types of EVs was pointed out, in order to advance the study of specific roles of EVs in host-pathogen interactions. In August 2018, around 50 scientists from all over the world came together for a mini-symposium on EVs in inter-organismal communication in Munich, at the Ludwig-Maximilians University. The scientific discussions included new findings of EVs discovered in plant, fungal, oomycete, nematode and bacterial systems, as well as exchange of current opinions on how to integrate such discoveries into the current concepts of EVs in inter-species and cross-kingdom communication (2). As two important goals for the future, the participants defined a better understanding of EV biology across kingdoms, and the extension of our knowledge on how EVs participate in plant immunity and in plant-pathogen interaction.
Plants recognise by pattern recognition receptors (PRRs) pathogen molecular patterns located at the cell periphery, which activate cellular immune signalling. Host-adapted plant pathogens secrete small peptides, called effectors that interact with, and sometimes enter, plant cells to suppress host immunity. In defence, plants evolve nucleotide-binding domain leucine-rich repeat containing (NLR) receptors that perceive pathogen effectors and mount an effector-triggered immunity. To escape recognition, pathogens co-evolve their effector repertoires, leading to a continual molecular arms race between pathogens and host plants (3). During plant-pathogen interactions, pathogen effectors and plant antimicrobial molecules must be transported through the extracellular space between plant and pathogen cells, without getting degraded or diluted, to be functional. Extracellular vesicles (EVs) fit perfectly to take over this function (Figure 1).
