The demonstration that spray-induced gene silencing (SIGS) can confer strong disease resistance, bypassing the laborious and time-consuming transgenic expression of double-stranded (ds)RNA to induce the gene silencing of pathogenic targets, was ground-breaking. However, future field applications will require fundamental mechanistic knowledge of dsRNA uptake, processing, and transfer. There is increasing evidence that extracellular vesicles (EVs) mediate the transfer of transgene-derived small interfering (si)RNAs in host-induced gene silencing (HIGS) applications. In this study, we establish a protocol for barley EV isolation and assess the possibilities for EVs regarding the translocation of sprayed dsRNA from barley (Hordeum vulgare) to its interacting fungal pathogens. We found barley EVs that were 156 nm in size, containing predominantly 21 and 19 nucleotide (nts) siRNAs, starting with a 5′-terminal Adenine. Although a direct comparison of the RNA cargo between HIGS and SIGS EV isolates is improper given their underlying mechanistic differences, we identified sequence-identical siRNAs in both systems. Overall, the number of siRNAs isolated from the EVs of dsRNA-sprayed barley plants with sequence complementarity to the sprayed dsRNA precursor was low. However, whether these few siRNAs are sufficient to induce the SIGS of pathogenic target genes requires further research. Taken together, our results raise the possibility that EVs may not be mandatory for the spray-delivered siRNA uptake and induction of SIGS.

Small (s)RNA molecules are crucial factors in the communication between hosts and their interacting pathogens, where they function as effectors that can modulate both host defense and microbial virulence/pathogenicity through a mechanism termed cross-kingdom RNA interference (ckRNAi). Consistent with this recent knowledge, sRNAs and their doublestranded (ds)RNA precursors have been adopted to control diseases in crop plants through transgenic expression (host-induced gene silencing, HIGS) or exogenous application (sprayinduced gene silencing, SIGS). While these strategies proved to be effective, the mechanism of RNA transfer at the plant -pathogen interface is widely unresolved. Here we show that extracellular vesicles (EVs) purified from Arabidopsis (Arabidopsis thaliana) leaf extracts and apoplastic fluids contain transgene-derived sRNAs. EVs from plants expressing CYP3RNA, a 791 nt long dsRNA, which was originally designed to target the three CYP51 genes of the fungal pathogen Fusarium graminearum, contain CYP3RNA-derived small interfering (si)RNAs as shown by RNA sequencing (RNA-seq) analysis. Notably, the EVs cargo retained the same CYP3RNA-derived siRNA profile as the respective leaf extracts, suggesting that there was no selective uptake of specific artificial sRNAs into EVs. In addition, mutants of the ESCRT-III complex were impaired in HIGS further indicating that endosomal vesicle trafficking supports transfer of transgene-derived siRNAs between donor host cells and recipient fungal cells.Further supporting the relevance of EV-mediated transport of sRNA, we demonstrate that HIGS plants, expressing a 100 nt dsRNA-target-sequence identified via EV-sRNA-seq of CYP3RNA Arabidopsis, confers strong resistance to F. graminearum. Together, these findings support the view that EVs are key mediators in the transport of HIGS-related sRNAs to reduce the virulence of interacting fungal pathogens during host-pathogen interaction.(MVBs) fuse with the PM [33][34][35][36][37]. In mammals, exosomes mediate intercellular communication by shuttling proteins, lipids and RNAs, where RNA molecules remain functional after delivery and can elicit effects in the recipient cell [38][39][40].Based on this knowledge, we speculated that HIGS-and SIGS-related RNAs also are loaded into plant vesicles and transferred by EVs that cross the plant-fungus interface. It was reasoned already more than 50 years ago that a fusion of plant MVBs with the PM may result in the release of small vesicles into the extracellular space [41]. Consistent with this early observation, biogenesis of immune-related MVBs [29,30] and the release of their cargo via EVs also is inherent to the plant secretory pathway [27]. Transmission electron microscopy (TEM) revealed proliferation of MVBs next to plant cell wall papillae in response to infection with powdery mildew fungus and the release of para-mural vesicles. The authors proposed that released vesicles might be similar to exosomes in animal cell thus anticipating that exosomes exist in plants [29,30]. Immun...

Fusarium graminearum (Fg) is a necrotrophic fungal pathogen that causes devastating diseases on its crop hosts barley and wheat. Recently, small RNAs (sRNAs) were identified as mobile communication signals between eukaryotes and their pathogens, symbionts or parasites. It has been shown that pathogens secrete sRNAs as effectors to suppress plant immunity and plants use endogenous sRNAs to resist infection, a phenomenon termed cross-kingdom RNAi; ckRNAi. However, little is known about the transport of fungus- or plant produced sRNAs to silence genes that contribute to immunity. Extracellular vesicles (EVs) are predicted playing a key role in the bidirectional transfer of sRNAs that mediate ckRNAi. To address this knowledge gap, we investigated the effects of EVs isolated from barley and Fg on their counterparts during plant-fungal interaction. Towards this, we developed a protocol for the isolation of EVs from Fg liquid cultures and assessed how Fg EVs contribute to fungal pathogenesis in barley using infiltration assays. To test the interdependence of EVs during Barley-Fg interaction, we treated Fg cultures with barley EVs. We found that infiltration of Fg EVs caused host specific phytotoxic effects in barley and barley EVs impaired Fg growth. Of note, Fg cultures showed an increase in purple pigmentation upon inoculation with barley EVs, suggesting a stress-induced premature formation of fruiting bodies. Together, our results demonstrate that EVs contribute to the Barley-Fg interaction, however, further studies are needed to unravel the nature of EV cargoes (e.g. protein and/or sRNA) responsible for affecting its plant/fungus counterpart.

Numerous reports have shown that incorporating a double-stranded RNA (dsRNA)-expressing transgene into plants or applying dsRNA by spraying it onto their leaves successfully protects them against invading pathogens exploiting the mechanism of RNA interference (RNAi). How dsRNAs or siRNAs are transferred between donor host cells and recipient fungal cells is largely unknown. It is speculated that plant extracellular vesicles (EVs) function as RNA shuttles between plants and their pathogens. Recently, we found that EVs isolated from host-induced gene silencing (HIGS) or spray-induced gene silencing (SIGS) plants contained dsRNA-derived siRNAs. In this study, we evaluated whether isolated EVs from dsRNA-sprayed barley (Hordeum vulgare) plants affected the growth of the phytopathogenic ascomycete Fusarium graminearum. Encouraged by our previous finding that dropping barley-derived EVs on F. graminearum cultures caused fungal stress phenotypes, we conducted an in vitro growth experiment in microtiter plates where we co-cultivated F. graminearum with plant EVs isolated from dsRNA-sprayed barley leaves. We observed that co-cultivation of F. graminearum macroconidia with barley EVs did not affect fungal growth. Furthermore, plant EVs containing SIGS-derived siRNA appeared not to affect F. graminearum growth and showed no gene silencing activity on F. graminearum CYP51 genes. Based on our findings, we concluded that either the amount of SIGS-derived siRNA was insufficient to induce target gene silencing in F. graminearum, indicating that the role of EVs in SIGS is minor, or that F. graminearum uptake of plant EVs from liquid cultures was inefficient or impossible.

Incorporating a double-stranded RNA (dsRNA)-expressing transgene into plants or applying dsRNA by spraying it onto plant leaves successfully protects plants against invading pathogens with RNA interference (RNAi). How dsRNAs or siRNAs are transferred between donor host cells and recipient fungal cells is largely unknown It is speculated that plant extracellular vesicles (EVs) function as RNA shuttles between plants and their interacting pathogens. Recently, we found that EVs isolated from HIGS or SIGS plants contained dsRNA-derived siRNAs. In this study, we evaluated whether isolated EVs from RNA-sprayed barley ( Hordeum vulgare ) plants affected the growth of the phytopathogenic ascomycete Fusarium graminearum ( Fg ). Encouraged by our previous finding that dropping barley-derived EVs on Fg cultures caused fungal stress phenotypes, we conducted an in vitro growth experiment in microtiter plates where we co-cultivated Fg with plant EVs isolated from dsRNA-sprayed barley leaves. We observed that co-cultivation of Fg macroconidia with barley EVs did not affect fungal growth. Furthermore, plant EVs containing SIGS-derived siRNA appeared not to affect Fg growth and showed no gene silencing activity on FgCYP51 genes. We conclude that either the amount of spray-derived sRNA was insufficient to induce target gene silencing (SIGS) in Fg or Fg uptake of plant EVs from liquid cultures was inefficient or impossible.

The demonstration that spray-induced gene silencing (SIGS) can confer strong disease resistance bypassing the laborious and time-consuming transgenic expression of double-stranded (ds)RNA to induce gene silencing of pathogenic targets was groundbreaking. However, future field applications will require fundamental mechanistic knowledge on dsRNA uptake, processing, and its transfer. There is increasing evidence that extracellular vesicles (EVs) mediate the transfer of transgene-derived small interfering (si)RNAs in host-induced gene silencing (HIGS) applications. Here, we examined the role of EVs regarding the translocation of sprayed dsRNA from barley (Hordeum vulgare) to the target fungusFusarium graminearum. We found barley EVs with 156 nm in size containing predominantly 21 and 19 nucleotide (nt) siRNAs starting with a 5’-terminal Adenine. Notably, barley EVs contain less siRNA compared to EVs isolated from transgenic HIGS Arabidopsis plants. Together our results further underpin mechanistic differences between HIGS and SIGS applications and a minor role of EVs in SIGS.