Dalakouras, A. | Search bioRxiv

bioRxiv2022

Systemic silencing and DNA methylation of a host reporter gene induced by a beneficial fungal root endophyte

A growing body of evidence suggests that RNA interference (RNAi) plays a pivotal role in the communication between plants and pathogenic fungi, where a bi-directional cross-kingdom RNAi is established to the advantage of either the host or the pathogen. Similar mechanisms acting during plant association with non-pathogenic symbiotic microorganisms have been elusive to this date. Here, we report on an RNAi-based mechanism of communication between a beneficial fungal endophyte, Fusarium solani strain K (FsK) and its host plants. This soil-borne endophyte that confers resistance and/or tolerance to biotic and abiotic stress in tomato and, as shown in this study, promotes plant growth in Nicotiana benthamiana, is restricted to the root system in both host plants. We first showed that the fungus has a functional core RNAi machinery; double stranded RNAs (dsRNAs) are processed into short interfering RNAs (siRNAs) of predominantly 21-nt in size, which lead to the degradation of homologous mRNAs. Importantly, by using an RNAi sensor system, we demonstrated that root colonization of N. benthamiana by FsK led to the induction of systemic silencing and DNA methylation of a host reporter gene.. These data reflect a more general but so far unrecognized mechanism wherein root endophytes systemically translocate RNAi signals to the aboveground tissues of their hosts to modulate gene expression during symbiosis, which may be translated to the beneficial phenotypes. HighlightA root-restricted, beneficial fungal endophyte can induce systemic silencing and epigenetic modifications to its host plant.

plant biology↗

bioRxiv2021

High-pressure sprayed siRNA triggers influence the efficiency but not the profile of transitive silencing

In plants, small interfering RNAs (siRNAs) are a quintessential class of RNA interference (RNAi)-inducing molecules produced by the endonucleolytic cleavage of double stranded RNAs (dsRNAs). In order to ensure robust RNAi, siRNAs are amplified through a positive feedback mechanism called transitivity. Transitivity relies on RNA-DIRECTED-RNA POLYMERASE 6 (RDR6)-mediated dsRNA synthesis using siRNA-targeted RNA. The newly synthesized dsRNA is subsequently cleaved into secondary siRNAs by DICER-LIKE (DCL) endonucleases. Just like primary siRNAs, secondary siRNAs are also loaded into ARGONAUTE proteins (AGOs) to form an RNA-induced silencing complex (RISC) reinforcing the cleavage of the target RNA. Although the molecular players underlying transitivity are well established, the mode of action of transitivity remains elusive. In this study, we investigated the influence of primary target sites on transgene silencing and transitivity using the GFP-expressing Nicotiana benthamiana 16C line, high pressure spraying protocol (HPSP), and synthetic 22-nucleotide (nt) long siRNAs. We found that the 22-nt siRNA targeting the 3 of the GFP transgene was less efficient in inducing silencing when compared to the siRNAs targeting the 5 and middle region of the GFP. Moreover, sRNA sequencing of locally silenced leaves showed that the amount but not the profile of secondary RNAs is shaped by the occupancy of the primary siRNA triggers on the target RNA. Our findings suggest that RDR6-mediated dsRNA synthesis is not primed by primary siRNAs and that dsRNA synthesis appears to be generally initiated at the 3 end of the target RNA. SIGNIFICANCE STATEMENTThis work answers a long-standing question about the role of siRNA triggers in initiating transitive silencing. By using high-pressure spraying-mediated delivery of synthetic siRNAs, we provided experimental evidence that target position of 22-nt-long primary siRNAs influences the efficiency of RDR6-driven double stranded RNA (dsRNA) synthesis, but it does not change the profile of accumulating secondary siRNAs originating from processing of RDR6-produced dsRNA.

plant biology↗