EVs and plant-fungal interactions – the plant host viewpoint
To survive, plants must respond quickly to its pathogens. As a result, certain intercellular changes, such as organelle rearrangement and cytoskeleton structural changes, may form a physical barrier that prevents infection at the site of attack. (Frey and Robatzek, 2009). Previously, plant EVs were reported to accumulate at the site of fungal infection and, thus, believed to be involved in trafficking molecules associated with defence between the plasma membrane and cell wall (Snetselaar and Mims, 1994; An et al., 2006; Samuel et al., 2015). Additionally, researchers have established that the secretion of plant EVs is enhanced during infection, which suggests involvement in some of the intercellular modifications (An et al., 2006; Wang et al., 2014 and Rutter and Innes, 2017). The cargo of plant EVs is sorted and packaged in response to the type of infection or injury, and it is significantly distinct to the cargo of fungal EVs secreted during infection (Samuel et al., 2015).
Knowledge on plant EVs is limited. Nonetheless, plant EVs have been reported to facilitate trafficking and delivery of large quantities of proteins lacking signal peptides to the extracellular environment (Regente et al., 2012; Pompa et al., 2017). Recent studies show that plants release EVs enriched with cell wall remodelling and defence related proteins in response to biotic or abiotic stress (Regente et al., 2009; Prado et al., 2014; Regente et al., 2017; Rutter and Innes, 2017; Baldrich et al., 2019). Rutter and Innes, (2018) also suggest that plant EVs presumably ferry RNAs into pathogens, thus demonstrating their potential as mediators of RNAs interspecies transmitters. Additionally, Baldrich et al., (2019) reported that EVs of A. thaliana convey diverse classes of sRNAs (i.e., microRNAs and small interfering RNAs) including a class of “tinyRNAs” (10-17 nt) whose role is not yet known but suspected to be associated with disintegrated artifacts of sRNA production. Koch et al., (2020) further demonstrate that EVs isolated from A. thaliana apoplastic fluids and leaf extracts enclosed transgene-derived sRNAs believed to be HIGs (Host induced silencing) related, which are transported during host-pathogen interactions. Defence genes targeted by HIGS have been reported to reduce B. cinerea virulence through silencing of its Dicer-like 1 andDicer-like 2 genes (Wang et al., 2016).