Systemic spreading of exogenous applied RNA biopesticides in the crop plant Hordeum vulgare

Background: Small (s) RNA molecules are crucial factors in the communication between hosts and their interacting pathogens/pests that can modulate both host defense and microbial virulence/pathogenicity known as crosskingdom RNA interference (ckRNAi). Consistent with this, sRNAs and their double-stranded (ds) RNA precursors have been adopted to control plant diseases through exogenously applied RNA biopesticides, known as spray-induced gene silencing (SIGS). While RNA spray proved to be effective, the mechanisms underlying the transfer and uptake of SIGS-associated RNAs are inadequately understood. Moreover, the use of the SIGS-technology as a biopesticide will require the systemic spreading of dsRNA/siRNA signals. Results: The integration of our findings strongly support the notion of long-distance spreading of SIGS-associated dsRNA and/or siRNA. In summary, our findings support the model that SIGS involves: (i) uptake of sprayed dsRNA by the plant (via stomata); (ii) transfer of apoplastic dsRNAs into the symplast (DCL processing into siRNAs); (iii) systemic translocation of siRNA or unprocessed dsRNA via the vascular system (phloem/xylem); (iv) uptake of apoplastic dsRNA or symplastic dsRNA/siRNA depending on the lifestyle/feeding behavior of the pathogen/pest. Conclusions: Our findings are significant contributions to our mechanistic understanding of RNA spray technology, as our previous data indicate that SIGS requires the processing of dsRNAs by the fungal RNAi machinery.