Bidirectional communication between pathogenic microbes and their plant hosts via small (s)RNA-mediated cross-kingdom RNA interference (ckRNAi) is a key element for successful host colonisation. Whether mutualistic fungi of the Serendipitaceae family, known for their extremely broad host range, use sRNAs to colonize plant roots is still under debate. To address this question, we developed a pipeline to validate the accumulation, translocation, and activity of fungal sRNAs in post-transcriptional silencing of Arabidopsis thaliana genes. Using stem-loop RT-qPCR, we detected the expression of a specific set of Serendipita indica (Si)sRNAs, targeting host genes involved in cell wall organization, hormonal signalling regulation, immunity, and gene regulation. To confirm the gene silencing activity of these sRNAs in plant cells, SisRNAs were transiently expressed in protoplasts. Stem-loop PCR confirmed sRNAs expression and accumulation, while qPCR validated post-transcriptional gene silencing of their predicted target genes. Furthermore, Arabidopsis ARGONAUTE 1 immunoprecipitation (AtAGO1-IP) revealed the loading of fungal SisRNAs into the plant RNAi machinery, suggesting the translocation of SisRNA from the fungus into root cells. In conclusion, this study provides a blueprint for rapid selection and analysis of sRNA effectors and further supports the model of cross-kingdom communication in the Sebacinoid symbiosis.
RNA interference (RNAi) is a crucial mechanism that can contribute to immunity against infectious microbes through the action of DICER-LIKE (DCL) and ARGONAUTE (AGO) proteins. In the case of the fungal pathogen Botrytis cinerea and the oomycete Hyaloperonospora arabidopsidis, plant DCL and AGO proteins have proven roles as negative regulators of immunity, suggesting functional specialization of these proteins. To address this aspect in a broader taxonomic context, we characterized the colonization pattern of an informative set of DCL and AGO loss-of-function mutants in Arabidopsis thaliana upon infection with a panel of pathogenic microbes with different lifestyles, and a fungal mutualist. Our results revealed that AGO1 and AGO4 function as positive regulators of immunity to a bacterial and a fungal pathogen, respectively. Additionally, AGO2 and AGO10 positively modulated the colonization by a fungal mutualist. Therefore, analysing the role of RNAi across a broader range of plant-microbe interactions has identified previously unknown functions for AGO proteins. For some pathogen interactions, however, all tested mutants exhibited wild type-like infection phenotypes, suggesting that the roles of AGO and DCL proteins in these interactions may be more complex to elucidate.
Bidirectional communication between pathogenic microbes and their plant hosts via small (s)RNA-mediated cross-kingdom RNA interference (ckRNAi) is one key element for successful host colonisation. However, whether mutualistic fungi from the Serendipitaceae family, known for their extremely broad host range, employ sRNAs to colonize plant roots is still under discussion. To address this question, we developed a pipeline to validate the accumulation, translocation, and activity of fungal sRNAs in post-transcriptional silencing of Arabidopsis thaliana genes. Using stem-loop PCR, we detected the expression of a specific set of Serendipita indica (Si)sRNAs, targeting host genes involved in cell wall organization, hormonal signalling regulation, immunity, and gene regulation. To confirm the gene silencing activity of these sRNA in plant cells, SisRNAs were transiently expressed in protoplasts. Stem-loop PCR proved the expression of sRNAs, while qPCR validated post-transcriptional gene silencing of their predicted target genes. Furthermore, ARGONAUTE 1 immunoprecipitation (AtAGO1-IP) revealed the loading of fungal SisRNAs into the plant RNAi machinery, suggesting the translocation of SisRNA from the fungus into root cells. In conclusion, this study provides a blueprint for rapid selection and analysis of sRNA effectors in plant-microbe interactions and further suggests cross-kingdom communication in the Sebacinoid symbiosis. Highlight Small RNAs of the beneficial fungus Serendipita indica are translocated and silence Arabidopsis genes at the onset of the interaction, revealing cross-kingdom communication in sebacinoid symbiosis.
Cross-kingdom RNA interference (ckRNAi), the bidirectional communication between microbes and their host counterparts, is a key element in the outcome of host colonization. Whether mutualistic fungi of the Serendipitaceae family with their broad host range use small RNA effectors (sRNAs) to colonize plant roots is still under debate. To investigate if ckRNAi is a factor in the symbiosis of Serendipita indica (Si) with Arabidopsis thaliana (Ath), we established a pipeline to validate expression, translocation and post-transcriptional gene silencing of host genes by Si-derived sRNAs (SisRNAs). First, we confirmed the expression of SisRNAs both in axenic fungal culture and during Ath root colonization using stem-loop PCR. Then, to verify the translocation of putative SisRNA effectors, an ARGONAUTE 1 immuno-purification assay (AtAGO1-IP) was employed, detecting fungal SisRNAs being loaded into the plant RNAi machinery in Si-colonised roots. Subsequently, SisRNAs and artificial sRNAs (amiRNAs), were transiently expressed in Ath protoplasts to test their gene silencing activity. Stem-loop PCR confirmed expression of sRNA effectors and qPCR validated post-transcriptional gene silencing of their predicted target genes involved in cell wall organization, hormonal signalling regulation, plant immunity and gene expression. Moreover, 5’-RLM-RACE analysis revealed amiRNA-mediated canonical cleavage in Arabidopsis targets. In conclusion, this study provides a blueprint for rapid selection and analysis of sRNA effectors in plant-microbe interactions in general and suggests cross-kingdom communication in the Sebacinoid symbiosis.
Circular RNAs (circRNAs) are single-stranded molecules that have attracted increasing attention in recent years due to their covalently closed structure and their diverse functional roles in mammalian cells, where they are involved in the regulation of gene expression and protein function. Increasing evidence suggests that circRNAs have similar functions in plants, where they play a role in plant development, resistance to biotic stress, and abiotic stress tolerance. Here, we investigated the agronomically relevant question of whether synthetic designer circRNAs can be used to modulate in a sequence-specific manner gene expression in plants. We show that treatment of GFP-expressing Arabidopsis protoplasts with designer 50 nt GFP antisense circRNA (circRNAGFP) reduces the cellular accumulation of the reporter protein in a sequence-specific and dose-dependent manner. This inhibitory activity of circRNAGFP was not abolished in various Arabidopsis ago and dcl mutants with defective RNAi pathways. Moreover, and in contrast to other types of RNA such as double-stranded (ds)RNA, circRNAs did not induce a PTI response in plant leaves. We discuss the possibility that circRNA may be applied to regulate endogenous plant genes and thus may have future potential as a novel bioherbicide.
In filamentous fungi, gene silencing by RNA interference (RNAi) shapes many biological processes, including pathogenicity. Recently, fungal small RNAs (sRNAs) have been shown to act as effectors that disrupt gene activity in interacting plant hosts, thereby undermining their defence responses. We show here that the devastating mycotoxin-producing ascomycete Fusarium graminearum (Fg) utilizes DICER-like (DCL)-dependent sRNAs to target defence genes in two Poaceae hosts, barley (Hordeum vulgare Hv) and Brachypodium distachyon (Bd). We identified 104 Fg-sRNAs with sequence homology to host genes that were repressed during interactions of Fg and Hv, while they accumulated in plants infected by the DCL double knock-out (dKO) mutant PH1-dcl1/2. The strength of target gene expression correlated with the abundance of the corresponding Fg-sRNA. Specifically, the abundance of three tRNA-derived fragments (tRFs) targeting immunity-related Ethylene overproducer 1-like 1 (HvEOL1) and three Poaceae orthologues of Arabidopsis thaliana BRI1-associated receptor kinase 1 (HvBAK1, HvSERK2 and BdSERK2) was dependent on fungal DCL. Additionally, RNA-ligase-mediated Rapid Amplification of cDNA Ends (RLM-RACE) identified infection-specific degradation products for the three barley gene transcripts, consistent with the possibility that tRFs contribute to fungal virulence via targeted gene silencing. Significance Statement Fusarium graminearum is one of the most devastating fungal pathogens in cereals, while understanding the mechanisms of fungal pathogenesis is a prerequisite for developing efficient and environmentally friendly crop protection strategies. We show exploratory data suggesting that fungal small RNAs play a critical role in Fusarium virulence by suppressing plant immunity.
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