Artificial nanovesicles for dsRNA delivery in spray-induced gene silencing for crop protection

Challenges and Opportunities Arising from Host-Botrytis cinerea Interactions to Outline Novel and Sustainable Control Strategies: The Key Role of RNA Interference

The necrotrophic plant pathogenic fungus Botrytis cinerea (Pers., 1794), the causative agent of gray mold disease, causes significant losses in agricultural production. Control of this fungal pathogen is quite difficult due to its wide host range and environmental persistence. Currently, the management of the disease is still mainly based on chemicals, which can have harmful effects not only on the environment and on human health but also because they favor the development of strains resistant to fungicides. The flexibility and plasticity of B. cinerea in challenging plant defense mechanisms and its ability to evolve strategies to escape chemicals require the development of new control strategies for successful disease management. In this review, some aspects of the host-pathogen interactions from which novel and sustainable control strategies could be developed (e.g., signaling pathways, molecules involved in plant immune mechanisms, hormones, post-transcriptional gene silencing) were analyzed. New biotechnological tools based on the use of RNA interference (RNAi) are emerging in the crop protection scenario as versatile, sustainable, effective, and environmentally friendly alternatives to the use of chemicals. RNAi-based fungicides are expected to be approved soon, although they will face several challenges before reaching the market.

Double-Stranded RNA Targeting White Mold Sclerotinia sclerotiorum Argonaute 2 for Disease Control via Spray-Induced Gene Silencing

Sclerotinia sclerotiorum, the causal agent of white mold infection, is a cosmopolitan fungal pathogen that causes major yield losses in many economically important crops. Spray-induced gene silencing has recently been shown to be a promising alternative method for controlling plant diseases. Based on our prior research, we focused on developing a spray-induced gene silencing approach to control white mold by silencing S. sclerotiorum argonaute 2 (SsAgo2), a crucial part of the fungal small RNA pathway. We compared the lesion size as a result of targeting each ∼500-bp segment of SsAgo2 from the 5' to the 3' end and found that targeting the PIWI/RNaseH domain of SsAgo2 is most effective. External application of double-stranded RNA (dsRNA)-suppressed white mold infection using either in vitro or in vivo transcripts was determined at the rate of 800 ng/0.2 cm2 area with a downregulation of SsAgo2 from infected leaf tissue confirmed by RT-qPCR. Furthermore, magnesium/iron-layered double hydroxide nanosheets loaded with in vitro- and in vivo-transcribed dsRNA segments significantly reduced the rate of S. sclerotiorum lesion expansion. In vivo-produced dsRNA targeting the PIWI/RNaseH domain of the SsAgo2 transcript showed increased efficacy in reducing the white mold symptoms of S. sclerotiorum when combined with layered double hydroxide nanosheets. This approach is promising to produce a large scale of dsRNA that can be deployed as an environmentally friendly fungicide to manage white mold infections in the field.

Trans-Species Mobility of RNA Interference between Plants and Associated Organisms

Trans-species RNA interference (RNAi) occurs naturally when small RNAs (sRNAs) silence genes in species different from their origin. This phenomenon has been observed between plants and various organisms including fungi, animals and other plant species. Understanding the mechanisms used in natural cases of trans-species RNAi, such as sRNA processing and movement, will enable more effective development of crop protection methods using host-induced gene silencing (HIGS). Recent progress has been made in understanding the mechanisms of cell-to-cell and long-distance movement of sRNAs within individual plants. This increased understanding of endogenous plant sRNA movement may be translatable to trans-species sRNA movement. Here, we review diverse cases of natural trans-species RNAi focusing on current theories regarding intercellular and long-distance sRNA movement. We also touch on trans-species sRNA evolution, highlighting its research potential and its role in improving the efficacy of HIGS.

Development and application of an RNA nanostructure to induce transient RNAi in difficult transgenic plants

The development of RNA interference (RNAi) is crucial for studying plant gene function. Its use, is limited to a few plants with well-established transgenic techniques. Spray-induced gene silencing (SIGS) introduces exogenous double-stranded RNA (dsRNA) into plants by spraying, injection, or irrigation, triggering the RNAi pathway to instantly silence target genes. As is a transient RNAi technology that does not rely on transgenic methods, SIGS has significant potential for studying gene function in plants lacking advanced transgenic technology. In this study, to enhance their stability and delivery efficiency, siRNAs were used as structural motifs to construct RNA nanoparticles (NPs) of four shapes: triangle, square, pentagon, and hexagon. These NPs, when synthesized by Escherichia coli, showed that triangular and square shapes accumulated more efficiently than pentagon and hexagon shapes. Bioassays revealed that RNA squares had the highest RNAi efficiency, followed by RNA triangles, with GFP-dsRNA showing the lowest efficiency at 4 and 7 days post-spray. We further explored the use of RNA squares in inducing transient RNAi in plants that are difficult to transform genetically. The results indicated that Panax notoginseng-derived MYB2 (PnMYB2) and Camellia oleifera-derived GUT (CoGUT) were significantly suppressed in P. notoginseng and C. oleifera, respectively, following the application of PnMYB2- and CoGUT-specific RNA squares. These findings suggest that RNA squares are highly effective in SIGS and can be utilized for gene function research in plants.

Frontiers in plant RNA research in ICAR2023: from lab to innovative agriculture

The recent growth in global warming, soil contamination, and climate instability have widely disturbed ecosystems, and will have a significant negative impact on the growth of plants that produce grains, fruits and woody biomass. To conquer this difficult situation, we need to understand the molecular bias of plant environmental responses and promote development of new technologies for sustainable maintenance of crop production. Accumulated molecular biological data have highlighted the importance of RNA-based mechanisms for plant stress responses. Here, we report the most advanced plant RNA research presented in the 33rd International Conference on Arabidopsis Research (ICAR2023), held as a hybrid event on June 5-9, 2023 in Chiba, Japan, and focused on "Arabidopsis for Sustainable Development Goals". Six workshops/concurrent sessions in ICAR2023 targeted plant RNA biology, and many RNA-related topics could be found in other sessions. In this meeting report, we focus on the workshops/concurrent sessions targeting RNA biology, to share what is happening now at the forefront of plant RNA research.

Advancing tree genomics to future proof next generation orchard production

The challenges facing tree orchard production in the coming years will be largely driven by changes in the climate affecting the sustainability of farming practices in specific geographical regions. Identifying key traits that enable tree crops to modify their growth to varying environmental conditions and taking advantage of new crop improvement opportunities and technologies will ensure the tree crop industry remains viable and profitable into the future. In this review article we 1) outline climate and sustainability challenges relevant to horticultural tree crop industries, 2) describe key tree crop traits targeted for improvement in agroecosystem productivity and resilience to environmental change, and 3) discuss existing and emerging genomic technologies that provide opportunities for industries to future proof the next generation of orchards.

Improving RNA-based crop protection through nanotechnology and insights from cross-kingdom RNA trafficking

Spray-induced gene silencing (SIGS) is a powerful and eco-friendly method for crop protection. Based off the discovery of RNA uptake ability in many fungal pathogens, the application of exogenous RNAs targeting pathogen/pest genes results in gene silencing and infection inhibition. However, SIGS remains hindered by the rapid degradation of RNA in the environment. As extracellular vesicles are used by plants, animals, and microbes in nature to transport RNAs for cross-kingdom/species RNA interference between hosts and microbes/pests, nanovesicles and other nanoparticles have been used to prevent RNA degradation. Efforts examining the effect of nanoparticles on RNA stability and internalization have identified key attributes that can inform better nanocarrier designs for SIGS. Understanding sRNA biogenesis, cross-kingdom/species RNAi, and how plants and pathogens/pests naturally interact are paramount for the design of SIGS strategies. Here, we focus on nanotechnology advancements for the engineering of innovative RNA-based disease control strategies against eukaryotic pathogens and pests.

A fungal RNA-dependent RNA polymerase is a novel player in plant infection and cross-kingdom RNA interference

Small RNAs act as fungal pathogen effectors that silence host target genes to promote infection, a virulence mechanism termed cross-kingdom RNA interference (RNAi). The essential pathogen factors of cross-kingdom small RNA production are largely unknown. We here characterized the RNA-dependent RNA polymerase (RDR)1 in the fungal plant pathogen Botrytis cinerea that is required for pathogenicity and cross-kingdom RNAi. B. cinerea bcrdr1 knockout (ko) mutants exhibited reduced pathogenicity and loss of cross-kingdom small RNAs. We developed a "switch-on" GFP reporter to study cross-kingdom RNAi in real-time within the living plant tissue which highlighted that bcrdr1 ko mutants were compromised in cross-kingdom RNAi. Moreover, blocking seven pathogen cross-kingdom small RNAs by expressing a short-tandem target mimic RNA in transgenic Arabidopsis thaliana led to reduced infection levels of the fungal pathogen B. cinerea and the oomycete pathogen Hyaloperonospora arabidopsidis. These results demonstrate that cross-kingdom RNAi is significant to promote host infection and making pathogen small RNAs an effective target for crop protection.

Small RNAs: Promising Molecules to Tackle Climate Change Impacts in Coffee Production

Over the centuries, human society has evolved based on the ability to select and use more adapted species for food supply, which means making plant species tastier and more productive in particular environmental conditions. However, nowadays, this scenario is highly threatened by climate change, especially by the changes in temperature and greenhouse gasses that directly affect photosynthesis, which highlights the need for strategic studies aiming at crop breeding and guaranteeing food security. This is especially worrying for crops with complex phenology, genomes with low variability, and the ones that support a large production chain, such as Coffea sp. L. In this context, recent advances shed some light on the genome function and transcriptional control, revealing small RNAs (sRNAs) that are responsible for environmental cues and could provide variability through gene expression regulation. Basically, sRNAs are responsive to environmental changes and act on the transcriptional and post-transcriptional gene silencing pathways that regulate gene expression and, consequently, biological processes. Here, we first discuss the predicted impact of climate changes on coffee plants and coffee chain production and then the role of sRNAs in response to environmental changes, especially temperature, in different species, together with their potential as tools for genetic improvement. Very few studies in coffee explored the relationship between sRNAs and environmental cues; thus, this review contributes to understanding coffee development in the face of climate change and towards new strategies of crop breeding.

Extracellular RNAs released by plant-associated fungi: from fundamental mechanisms to biotechnological applications

Extracellular RNAs are an emerging research topic in fungal-plant interactions. Fungal plant pathogens and symbionts release small RNAs that enter host cells to manipulate plant physiology and immunity. This communication via extracellular RNAs between fungi and plants is bidirectional. On the one hand, plants release RNAs encapsulated inside extracellular vesicles as a defense response as well as for intercellular and inter-organismal communication. On the other hand, recent reports suggest that also full-length mRNAs are transported within fungal EVs into plants, and these fungal mRNAs might get translated inside host cells. In this review article, we summarize the current views and fundamental concepts of extracellular RNAs released by plant-associated fungi, and we discuss new strategies to apply extracellular RNAs in crop protection against fungal pathogens. KEY POINTS: • Extracellular RNAs are an emerging topic in plant-fungal communication. • Fungi utilize RNAs to manipulate host plants for colonization. • Extracellular RNAs can be engineered to protect plants against fungal pathogens.

Fungal small RNAs ride in extracellular vesicles to enter plant cells through clathrin-mediated endocytosis

Small RNAs (sRNAs) of the fungal pathogen Botrytis cinerea can enter plant cells and hijack host Argonaute protein 1 (AGO1) to silence host immunity genes. However, the mechanism by which these fungal sRNAs are secreted and enter host cells remains unclear. Here, we demonstrate that B. cinerea utilizes extracellular vesicles (EVs) to secrete Bc-sRNAs, which are then internalized by plant cells through clathrin-mediated endocytosis (CME). The B. cinerea tetraspanin protein, Punchless 1 (BcPLS1), serves as an EV biomarker and plays an essential role in fungal pathogenicity. We observe numerous Arabidopsis clathrin-coated vesicles (CCVs) around B. cinerea infection sites and the colocalization of B. cinerea EV marker BcPLS1 and Arabidopsis CLATHRIN LIGHT CHAIN 1, one of the core components of CCV. Meanwhile, BcPLS1 and the B. cinerea-secreted sRNAs are detected in purified CCVs after infection. Arabidopsis knockout mutants and inducible dominant-negative mutants of key components of the CME pathway exhibit increased resistance to B. cinerea infection. Furthermore, Bc-sRNA loading into Arabidopsis AGO1 and host target gene suppression are attenuated in those CME mutants. Together, our results demonstrate that fungi secrete sRNAs via EVs, which then enter host plant cells mainly through CME.

Fungal small RNAs ride in extracellular vesicles to enter plant cells through clathrin-mediated endocytosis

Small RNAs (sRNAs) of the fungal pathogen Botrytis cinerea can enter plant cells and hijack host Argonaute protein 1 (AGO1) to silence host immunity genes. However, the mechanism by which these fungal sRNAs are secreted and enter host cells remains unclear. Here, we demonstrate that B. cinerea utilizes extracellular vesicles (EVs) to secrete Bc-sRNAs, which are then internalized by plant cells through clathrin-mediated endocytosis (CME). The B. cinerea tetraspanin protein, Punchless 1 (BcPLS1), serves as an EV biomarker and plays an essential role in fungal pathogenicity. We observe numerous Arabidopsis clathrin-coated vesicles (CCVs) around B. cinerea infection sites and the colocalization of B. cinerea EV marker BcPLS1 and Arabidopsis CLATHRIN LIGHT CHAIN 1, one of the core components of CCV. Meanwhile, BcPLS1 and the B. cinerea-secreted sRNAs are detected in purified CCVs after infection. Arabidopsis knockout mutants and inducible dominant-negative mutants of key components of CME pathway exhibit increased resistance to B. cinerea infection. Furthermore, Bc-sRNA loading into Arabidopsis AGO1 and host target gene suppression are attenuated in those CME mutants. Together, our results demonstrate that fungi secrete sRNAs via EVs, which then enter host plant cells mainly through CME.

Extracellular vesicles: cross-organismal RNA trafficking in plants, microbes, and mammalian cells

Extracellular vesicles (EVs) are membrane-enclosed nanometer-scale particles that transport biological materials such as RNAs, proteins, and metabolites. EVs have been discovered in nearly all kingdoms of life as a form of cellular communication across different cells and between interacting organisms. EV research has primarily focused on EV-mediated intra-organismal transport in mammals, which has led to the characterization of a plethora of EV contents from diverse cell types with distinct and impactful physiological effects. In contrast, research into EV-mediated transport in plants has focused on inter-organismal interactions between plants and interacting microbes. However, the overall molecular content and functions of plant and microbial EVs remain largely unknown. Recent studies into the plant-pathogen interface have demonstrated that plants produce and secrete EVs that transport small RNAs into pathogen cells to silence virulence-related genes. Plant-interacting microbes such as bacteria and fungi also secrete EVs which transport proteins, metabolites, and potentially RNAs into plant cells to enhance their virulence. This review will focus on recent advances in EV-mediated communications in plant-pathogen interactions compared to the current state of knowledge of mammalian EV capabilities and highlight the role of EVs in cross-kingdom RNA interference.