sRNA Profiling Combined With Gene Function Analysis Reveals a Lack of Evidence for Cross-Kingdom RNAi in the Wheat - Zymoseptoria tritici Pathosystem

RNA interference: a promising biotechnological approach to combat plant pathogens, mechanism and future prospects

Plant pathogens and other biological pests represent significant obstacles to crop Protection worldwide. Even though there are many effective conventional methods for controlling plant diseases, new methods that are also effective, environmentally safe, and cost-effective are required. While plant breeding has traditionally been used to manipulate the plant genome to develop resistant cultivars for controlling plant diseases, the emergence of genetic engineering has introduced a completely new approach to render plants resistant to bacteria, nematodes, fungi, and viruses. The RNA interference (RNAi) approach has recently emerged as a potentially useful tool for mitigating the inherent risks associated with the development of conventional transgenics. These risks include the use of specific transgenes, gene control sequences, or marker genes. Utilizing RNAi to silence certain genes is a promising solution to this dilemma as disease-resistant transgenic plants can be generated within a legislative structure. Recent investigations have shown that using target double stranded RNAs via an effective vector system can produce significant silencing effects. Both dsRNA-containing crop sprays and transgenic plants carrying RNAi vectors have proven effective in controlling plant diseases that threaten commercially significant crop species. This article discusses the methods and applications of the most recent RNAi technology for reducing plant diseases to ensure sustainable agricultural yields.

Biotechnology and Genomic Approaches to Mitigating Disease Impacts on Forest Health

Outbreaks of insects and diseases are part of the natural disturbance regime of all forests. However, introduced pathogens have had outsized impacts on many dominant forest tree species over the past century. Mitigating these impacts and restoring these species are dilemmas of the modern era. Here, we review the ecological and economic impact of introduced pathogens, focusing on examples in North America. We then synthesize the successes and challenges of past biotechnological approaches and discuss the integration of genomics and biotechnology to help mitigate the effects of past and future pathogen invasions. These questions are considered in the context of the transgenic American chestnut, which is the most comprehensive example to date of how biotechnological tools have been used to address the impacts of introduced pathogens on naïve forest ecosystems.

The expression landscape and pangenome of long non-coding RNA in the fungal wheat pathogen Zymoseptoria tritici

Long non-coding RNAs (lncRNAs) are regulatory molecules interacting in a wide array of biological processes. lncRNAs in fungal pathogens can be responsive to stress and play roles in regulating growth and nutrient acquisition. Recent evidence suggests that lncRNAs may also play roles in virulence, such as regulating pathogenicity-associated enzymes and on-host reproductive cycles. Despite the importance of lncRNAs, only a few model fungi have well-documented inventories of lncRNA. In this study, we apply a recent computational pipeline to predict high-confidence lncRNA candidates in Zymoseptoria tritici, an important global pathogen of wheat impacting global food production. We analyse genomic features of lncRNAs and the most likely associated processes through analyses of expression over a host infection cycle. We find that lncRNAs are frequently expressed during early infection, before the switch to necrotrophic growth. They are mostly located in facultative heterochromatic regions, which are known to contain many genes associated with pathogenicity. Furthermore, we find that lncRNAs are frequently co-expressed with genes that may be involved in responding to host defence signals, such as oxidative stress. Finally, we assess pangenome features of lncRNAs using four additional reference-quality genomes. We find evidence that the repertoire of expressed lncRNAs varies substantially between individuals, even though lncRNA loci tend to be shared at the genomic level. Overall, this study provides a repertoire and putative functions of lncRNAs in Z. tritici enabling future molecular genetics and functional analyses in an important pathogen.

Comparison of the impact of two key fungal signalling pathways on Zymoseptoria tritici infection reveals divergent contribution to invasive growth through distinct regulation of infection-associated genes

The lifecycle of Zymoseptoria tritici requires a carefully regulated asymptomatic phase within the wheat leaf following penetration of the mesophyll via stomata. Here we compare the roles in this process of two key fungal signalling pathways, mutants of which were identified through forward genetics due to their avirulence on wheat. Whole-genome resequencing of avirulent Z. tritici T-DNA transformants identified disruptive mutations in ZtBCK1 from the kinase cascade of the cell wall integrity (CWI) pathway, and the adenylate cyclase gene ZtCYR1. Targeted deletion of these genes abolished the pathogenicity of the fungus and led to similar in vitro phenotypes to those associated with disruption of putative downstream kinases, both supporting previous studies and confirming the importance of these pathways in virulence. RNA sequencing was used to investigate the effect of ZtBCK1 and ZtCYR1 deletion on gene expression in both the pathogen and host during infection. ZtBCK1 was found to be required for the adaptation to the host environment, controlling expression of infection-associated secreted proteins, including known virulence factors. Meanwhile, ZtCYR1 is implicated in controlling the switch to necrotrophy, regulating expression of effectors associated with this transition. This represents the first study to compare the influence of CWI and cAMP signalling on in planta transcription of a fungal plant pathogen, providing insights into their differential regulation of candidate effectors during invasive growth.

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.

Exogenous dsRNA trigger RNAi in Venturia inaequalis resulting in down regulation of target genes and growth reduction

BACKGROUND

Venturia inaequalis is an apple scab causing fungal pathogen. It is a highly contagious and destructive pathogen which rapidly spreads infection in the surrounding orchards if not managed. The management and control of disease require multiple fungicides to be sprayed at different development stages of the apple. Persistent applications of fungicides also raises environmental concerns. Here, we demonstrate the potential of using spray induced gene silencing (SIGS) by developing target specific gene constructs for the synthesis of corresponding double-stranded RNA (dsRNA).

METHODS AND RESULTS

The exogenous application of dsRNAs was found to reduce mycelial growth and spore formation of V. inaequalis on culture plates. Four genes of V. inaequalis viz. CIN1, CE5, VICE12 and VICE16 which get upregulated during infection, were selected as targets for the development of gene construct expressing the corresponding dsRNA. The effect of exogenously supplied in vitro synthesized dsRNA on V. inaequalis was assessed in culture bioassay experiments with respect to growth, and spore formation. The expression level of the target genes in treated and control fungus was evaluated using quantitative PCR. Fungus treated with VICE12 targeted dsRNA showed maximum reduction in colony size (~ 55%), conidia formation (~ 93%) and expression level of the corresponding gene (2.2 fold), which was followed by CIN1-dsRNA. VICE16-dsRNA treatment was least effective with 32% reduction in growth, the non-significant effect of conidial spore formation and 1.13 fold down regulation of corresponding target gene expression level.

CONCLUSION

The result of this investigation validates the hypothesis that RNAi is evoked in V. inaequalis by exogenously supplied dsRNA and spray induced gene silencing (SIGS) based solutions may reduce burden of fungicide usage on apple crop against apple scab disease in future.

RNA-Based Control of Fungal Pathogens in Plants

Our duty to conserve global natural ecosystems is increasingly in conflict with our need to feed an expanding population. The use of conventional pesticides not only damages the environment and vulnerable biodiversity but can also still fail to prevent crop losses of 20-40% due to pests and pathogens. There is a growing call for more ecologically sustainable pathogen control measures. RNA-based biopesticides offer an eco-friendly alternative to the use of conventional fungicides for crop protection. The genetic modification (GM) of crops remains controversial in many countries, though expression of transgenes inducing pathogen-specific RNA interference (RNAi) has been proven effective against many agronomically important fungal pathogens. The topical application of pathogen-specific RNAi-inducing sprays is a more responsive, GM-free approach to conventional RNAi transgene-based crop protection. The specific targeting of essential pathogen genes, the development of RNAi-nanoparticle carrier spray formulations, and the possible structural modifications to the RNA molecules themselves are crucial to the success of this novel technology. Here, we outline the current understanding of gene silencing pathways in plants and fungi and summarize the pioneering and recent work exploring RNA-based biopesticides for crop protection against fungal pathogens, with a focus on spray-induced gene silencing (SIGS). Further, we discuss factors that could affect the success of RNA-based control strategies, including RNA uptake, stability, amplification, and movement within and between the plant host and pathogen, as well as the cost and design of RNA pesticides.

Recent transposable element bursts are associated with the proximity to genes in a fungal plant pathogen

The activity of transposable elements (TEs) contributes significantly to pathogen genome evolution. TEs often destabilize genome integrity but may also confer adaptive variation in pathogenicity or resistance traits. De-repression of epigenetically silenced TEs often initiates bursts of transposition activity that may be counteracted by purifying selection and genome defenses. However, how these forces interact to determine the expansion routes of TEs within a pathogen species remains largely unknown. Here, we analyzed a set of 19 telomere-to-telomere genomes of the fungal wheat pathogen Zymoseptoria tritici. Phylogenetic reconstruction and ancestral state estimates of individual TE families revealed that TEs have undergone distinct activation and repression periods resulting in highly uneven copy numbers between genomes of the same species. Most TEs are clustered in gene poor niches, indicating strong purifying selection against insertions near coding sequences, or as a consequence of insertion site preferences. TE families with high copy numbers have low sequence divergence and strong signatures of defense mechanisms (i.e., RIP). In contrast, small non-autonomous TEs (i.e., MITEs) are less impacted by defense mechanisms and are often located in close proximity to genes. Individual TE families have experienced multiple distinct burst events that generated many nearly identical copies. We found that a Copia element burst was initiated from recent copies inserted substantially closer to genes compared to older copies. Overall, TE bursts tended to initiate from copies in GC-rich niches that escaped inactivation by genomic defenses. Our work shows how specific genomic environments features provide triggers for TE proliferation in pathogen genomes.

Molecular characterization reveals no functional evidence for naturally occurring cross-kingdom RNA interference in the early stages of Botrytis cinerea-tomato interaction

Plant immune responses are triggered during the interaction with pathogens. The fungus Botrytis cinerea has previously been reported to use small RNAs (sRNAs) as effector molecules capable of interfering with the host immune response. Conversely, a host plant produces sRNAs that may interfere with the infection mechanism of an intruder. We used high-throughput sequencing to identify sRNAs produced by B. cinerea and Solanum lycopersicum (tomato) during early phases of interaction and to examine the expression of their predicted mRNA targets in the other organism. A total of 7042 B. cinerea sRNAs were predicted to target 3185 mRNAs in tomato. Of the predicted tomato target genes, 163 were indeed transcriptionally down-regulated during the early phase of infection. Several experiments were performed to study a causal relation between the production of B. cinerea sRNAs and the down-regulation of predicted target genes in tomato. We generated B. cinerea mutants in which a transposon region was deleted that is the source of c.10% of the fungal sRNAs. Furthermore, mutants were generated in which both Dicer-like genes (Bcdcl1 and Bcdcl2) were deleted and these displayed a >99% reduction of transposon-derived sRNA production. Neither of these mutants was significantly reduced in virulence on any plant species tested. Our results reveal no evidence for any detectable role of B. cinerea sRNAs in the virulence of the fungus.

Do small RNAs unlock the below ground microbiome-plant interaction mystery?

Over the past few decades, regulatory RNAs, such as small RNAs (sRNAs), have received increasing attention in the context of host-microbe interactions due to their diverse roles in controlling various biological processes in eukaryotes. In addition, studies have identified an increasing number of sRNAs with novel functions across a wide range of bacteria. What is not well understood is why cells regulate gene expression through post-transcriptional mechanisms rather than at the initiation of transcription. The finding of a multitude of sRNAs and their identified associated targets has allowed further investigation into the role of sRNAs in mediating gene regulation. These foundational data allow for further development of hypotheses concerning how a precise control of gene activity is accomplished through the combination of transcriptional and post-transcriptional regulation. Recently, sRNAs have been reported to participate in interkingdom communication and signalling where sRNAs originating from one kingdom are able to target or control gene expression in another kingdom. For example, small RNAs of fungal pathogens that silence plant genes and vice-versa plant sRNAs that mediate bacterial gene expression. However, there is currently a lack of evidence regarding sRNA-based inter-kingdom signalling across more than two interacting organisms. A habitat that provides an excellent opportunity to investigate interconnectivity is the plant rhizosphere, a multifaceted ecosystem where plants and associated soil microbes are known to interact. In this paper, we discuss how the interconnectivity of bacteria, fungi, and plants within the rhizosphere may be mediated by bacterial sRNAs with a particular focus on disease suppressive and non-suppressive soils. We discuss the potential roles sRNAs may play in the below-ground world and identify potential areas of future research, particularly in reference to the regulation of plant immunity genes by bacterial and fungal communities in disease-suppressive and non-disease-suppressive soils.

Induction of distinct plant cell death programs by secreted proteins from the wheat pathogen Zymoseptoria tritici

Cell death processes in eukaryotes shape normal development and responses to the environment. For plant-microbe interactions, initiation of host cell death plays an important role in determining disease outcomes. Cell death pathways are frequently initiated following detection of pathogen-derived molecules which can lead to resistance or susceptibility to disease depending on pathogen lifestyle. We previously identified several small secreted proteins (SSPs) from the wheat-infecting fungus Zymoseptoria tritici that induce rapid cell death in Nicotiana benthamiana following Agrobacterium-mediated delivery and expression (agroinfiltration). Here we investigated whether the execution of host cells was mechanistically similar in response to different Z. tritici SSPs. Using RNA sequencing, we found that transient expression of four Z. tritici SSPs led to massive transcriptional reprogramming within 48 h of agroinfiltration. We observed that distinct host gene expression profiles were induced dependent on whether cell death occurs in a cell surface immune receptor-dependent or -independent manner. These gene expression profiles involved differential transcriptional networks mediated by WRKY, NAC and MYB transcription factors. In addition, differential expression of genes belonging to different classes of receptor-like proteins and receptor-like kinases was observed. These data suggest that different Z. tritici SSPs trigger differential transcriptional reprogramming in plant cells.

Concepts and considerations for enhancing RNAi efficiency in phytopathogenic fungi for RNAi-based crop protection using nanocarrier-mediated dsRNA delivery systems

Existing, emerging, and reemerging strains of phytopathogenic fungi pose a significant threat to agricultural productivity globally. This risk is further exacerbated by the lack of resistance source(s) in plants or a breakdown of resistance by pathogens through co-evolution. In recent years, attenuation of essential pathogen gene(s) via double-stranded (ds) RNA-mediated RNA interference (RNAi) in host plants, a phenomenon known as host-induced gene silencing, has gained significant attention as a way to combat pathogen attack. Yet, due to biosafety concerns regarding transgenics, country-specific GMO legislation has limited the practical application of desirable attributes in plants. The topical application of dsRNA/siRNA targeting essential fungal gene(s) through spray-induced gene silencing (SIGS) on host plants has opened up a transgene-free avenue for crop protection. However, several factors influence the outcome of RNAi, including but not limited to RNAi mechanism in plant/fungi, dsRNA/siRNA uptake efficiency, dsRNA/siRNA design parameters, dsRNA stability and delivery strategy, off-target effects, etc. This review emphasizes the significance of these factors and suggests appropriate measures to consider while designing in silico and in vitro experiments for successful RNAi in open-field conditions. We also highlight prospective nanoparticles as smart delivery vehicles for deploying RNAi molecules in plant systems for long-term crop protection and ecosystem compatibility. Lastly, we provide specific directions for future investigations that focus on blending nanotechnology and RNAi-based fungal control for practical applications.

The evolutionary and molecular features of the broad-host-range plant pathogen Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a pathogenic fungus that infects hundreds of plant species, including many of the world's most important crops. Key features of S. sclerotiorum include its extraordinary host range, preference for dicotyledonous plants, relatively slow evolution, and production of protein effectors that are active in multiple host species. Plant resistance to this pathogen is highly complex, typically involving numerous polymorphisms with infinitesimally small effects, which makes resistance breeding a major challenge. Due to its economic significance, S. sclerotiorum has been subjected to a large amount of molecular and evolutionary research. In this updated pathogen profile, we review the evolutionary and molecular features of S. sclerotiorum and discuss avenues for future research into this important species.

Spraying of dsRNA molecules derived from Phytophthora infestans, along with nanoclay carriers as a proof of concept for developing novel protection strategy for potato late blight

BACKGROUND

Phytophthora infestans is a late blight-causing oomycetes pathogen. It rapidly evolves and adapts to the host background and new fungicide molecules within a few years of their release, most likely because of the predominance of transposable elements in its genome. Frequent applications of fungicides cause environmental concerns. Here, we developed target-specific RNA interference (RNAi)-based molecules, along with nanoclay carriers, that when sprayed on plants are capable of effectively reducing late blight infection.

RESULTS

Targeted the genes unique to sporulation, early satge infection and the metabolism pathway stages based on in an our own microarray data. We used nanoclay as a carrier for sorbitol dehydrogenase, heat shock protein 90, translation elongation factor 1-α, phospholipase-D like 3 and glycosylphosphatidylinositol-anchored acidic serine-threonine-rich HAM34-like protein double-stranded (ds)RNAs, which were assessed by culture bioassay, detached leaf assay and spray methods, and revealed a reduction in growth, sporulation and symptom expression. Plants sprayed with multigene targeted dsRNA-nanoclay showed enhanced disease resistance (4% disease severity) and less sporulation (<1 × 10³ ) compared with plants sprayed with dsRNA alone.

CONCLUSION

The use of nanoclay with multigene targeted dsRNA was assumed to be involved in effective delivery, protection and boosting the action of RNAi as a spray-induced gene silencing approach (SIGS). A significant reduction in growth, sporulation, disease severity and decreased gene expression authenticates the effects of SIGS on late blight progression. This study demonstrated as a proof of concept the dsRNA-nanoclay SIGS approach, which could be used as an alternative to chemical fungicides and transgenic approaches to develop an environmentally friendly novel plant protection strategy for late blight. © 2022 Society of Chemical Industry.

RNA Interference for Improving Disease Resistance in Plants and Its Relevance in This Clustered Regularly Interspaced Short Palindromic Repeats-Dominated Era in Terms of dsRNA-Based Biopesticides

RNA interference (RNAi) has been exploited by scientists worldwide to make a significant contribution in the arena of sustainable agriculture and integrated pest management. These strategies are of an imperative need to guarantee food security for the teeming millions globally. The already established deleterious effects of chemical pesticides on human and livestock health have led researchers to exploit RNAi as a potential agri-biotechnology tool to solve the burning issue of agricultural wastage caused by pests and pathogens. On the other hand, CRISPR/Cas9, the latest genome-editing tool, also has a notable potential in this domain of biotic stress resistance, and a constant endeavor by various laboratories is in progress for making pathogen-resistant plants using this technique. Considerable outcry regarding the ill effects of genetically modified (GM) crops on the environment paved the way for the research of RNAi-induced double-stranded RNAs (dsRNA) and their application to biotic stresses. Here, we mainly focus on the application of RNAi technology to improve disease resistance in plants and its relevance in today's CRISPR-dominated world in terms of exogenous application of dsRNAs. We also focused on the ongoing research, public awareness, and subsequent commercialization of dsRNA-based biocontrol products.

Exploring the Effectiveness and Durability of Trans-Kingdom Silencing of Fungal Genes in the Vascular Pathogen Verticillium dahliae

Host-induced gene silencing (HIGS) based on trans-kingdom RNA interference (RNAi) has been successfully exploited to engineer host resistance to pests and pathogens, including fungi and oomycetes. However, revealing the mechanisms underlying trans-kingdom RNAi between hosts and pathogens lags behind applications. The effectiveness and durability of trans-kingdom silencing of pathogenic genes are uncharacterized. In this study, using our transgenic 35S-VdH1i cotton plants in which dsVdH1-derived small RNAs (siVdH1) accumulated, small RNA sequencing analysis revealed that siVdH1s exclusively occur within the double-stranded (ds)VdH1 region, and no transitive siRNAs were produced beyond this region in recovered hyphae of Verticillium dahliae (V. dahliae). Accordingly, we found that VdH1 silencing was reduced over time in recovered hyphae cultured in vitro, inferring that once the fungus got rid of the 35S-VdH1i cotton plants would gradually regain their pathogenicity. To explore whether continually exporting dsRNAs/siRNAs from transgenic plants into recipient fungal cells guaranteed the effectiveness and stability of HIGS, we created GFP/RFP double-labeled V. dahliae and transgenic Arabidopsis expressing dsGFP (35S-GFPi plants). Confocal images visually demonstrate the efficient silencing of GFP in V. dahliae that colonized host vascular tissues. Taken together, our results demonstrate that HIGS effectively triggers long-lasting trans-kingdom RNAi during plant vasculature V. dahliae interactions, despite no amplification or transitivity of RNAi being noted in this soil-borne fungal pathogen.

Spray-induced gene silencing for disease control is dependent on the efficiency of pathogen RNA uptake

Recent discoveries show that fungi can take up environmental RNA, which can then silence fungal genes through environmental RNA interference. This discovery prompted the development of Spray-Induced Gene Silencing (SIGS) for plant disease management. In this study, we aimed to determine the efficacy of SIGS across a variety of eukaryotic microbes. We first examined the efficiency of RNA uptake in multiple pathogenic and non-pathogenic fungi, and an oomycete pathogen. We observed efficient double-stranded RNA (dsRNA) uptake in the fungal plant pathogens Botrytis cinerea, Sclerotinia sclerotiorum, Rhizoctonia solani, Aspergillus niger and Verticillium dahliae, but no uptake in Colletotrichum gloeosporioides, and weak uptake in a beneficial fungus, Trichoderma virens. For the oomycete plant pathogen, Phytophthora infestans, RNA uptake was limited and varied across different cell types and developmental stages. Topical application of dsRNA targeting virulence-related genes in pathogens with high RNA uptake efficiency significantly inhibited plant disease symptoms, whereas the application of dsRNA in pathogens with low RNA uptake efficiency did not suppress infection. Our results have revealed that dsRNA uptake efficiencies vary across eukaryotic microbe species and cell types. The success of SIGS for plant disease management can largely be determined by the pathogen's RNA uptake efficiency.

Spray-induced gene silencing for disease control is dependent on the efficiency of pathogen RNA uptake

Recent discoveries show that fungi can take up environmental RNA, which can then silence fungal genes through environmental RNA interference. This discovery prompted the development of Spray-Induced Gene Silencing (SIGS) for plant disease management. In this study, we aimed to determine the efficacy of SIGS across a variety of eukaryotic microbes. We first examined the efficiency of RNA uptake in multiple pathogenic and non-pathogenic fungi, and an oomycete pathogen. We observed efficient double-stranded RNA (dsRNA) uptake in the fungal plant pathogens Botrytis cinerea, Sclerotinia sclerotiorum, Rhizoctonia solani, Aspergillus niger and Verticillium dahliae, but no uptake in Colletotrichum gloeosporioides, and weak uptake in a beneficial fungus, Trichoderma virens. For the oomycete plant pathogen, Phytophthora infestans, RNA uptake was limited and varied across different cell types and developmental stages. Topical application of dsRNA targeting virulence-related genes in pathogens with high RNA uptake efficiency significantly inhibited plant disease symptoms, whereas the application of dsRNA in pathogens with low RNA uptake efficiency did not suppress infection. Our results have revealed that dsRNA uptake efficiencies vary across eukaryotic microbe species and cell types. The success of SIGS for plant disease management can largely be determined by the pathogen's RNA uptake efficiency.

Small RNAs in Plant Immunity and Virulence of Filamentous Pathogens

Gene silencing guided by small RNAs governs a broad range of cellular processes in eukaryotes. Small RNAs are important components of plant immunity because they contribute to pathogen-triggered transcription reprogramming and directly target pathogen RNAs. Recent research suggests that silencing of pathogen genes by plant small RNAs occurs not only during viral infection but also in nonviral pathogens through a process termed host-induced gene silencing, which involves trans-species small RNA trafficking. Similarly, small RNAs are also produced by eukaryotic pathogens and regulate virulence. This review summarizes the small RNA pathways in both plants and filamentous pathogens, including fungi and oomycetes, and discusses their role in host-pathogen interactions. We highlight secondarysmall interfering RNAs of plants as regulators of immune receptor gene expression and executors of host-induced gene silencing in invading pathogens. The current status and prospects of trans-species gene silencing at the host-pathogen interface are discussed.

Ago1 Affects the Virulence of the Fungal Plant Pathogen Zymoseptoria tritici

In host-pathogen interactions RNA interference (RNAi) has emerged as a pivotal mechanism to modify both, the immune responses of the host as well as the pathogenicity and virulence of the pathogen. In addition, in some fungi RNAi is also known to affect chromosome biology via its effect on chromatin conformation. Previous studies reported no effect of the RNAi machinery on the virulence of the fungal plant pathogen Zymoseptoria tritici however the role of RNAi is still poorly understood in this species. Herein, we elucidate whether the RNAi machinery is conserved within the genus Zymoseptoria. Moreover, we conduct functional analyses of Argonaute and Dicer-like proteins and test if the RNAi machinery affects chromosome stability. We show that the RNAi machinery is conserved among closely related Zymoseptoria species while an exceptional pattern of allelic diversity was possibly caused by introgression. The deletion of Ago1 reduced the ability of the fungus to produce asexual propagules in planta in a quantitative matter. Chromosome stability of the accessory chromosome of Z. tritici was not prominently affected by the RNAi machinery. These results indicate, in contrast to previous finding, a role of the RNAi pathway during host infection, but not in the stability of accessory chromosomes in Z. tritici.

Requirements for fungal uptake of dsRNA and gene silencing in RNAi-based crop protection strategies

Growing evidence indicates that RNAi is an effective control strategy for agronomically important fungi. To implement RNAi-based crop protection strategies, dsRNA molecules are either sprayed on foliage or generated by genetically engineered plants. Here, we summarize current knowledge of the mechanisms governing dsRNA uptake and RNAi-mediated gene silencing in fungi, as well as the factors that influence these phenomena. Of primary importance is dsRNA design, as identifying an appropriate gene for silencing and determining which region of the gene to target are critical for maximizing efficiency. Strategies for enhancing dsRNA uptake, potentially by using formulations and/or carriers that prevent dsRNA degradation by (a)biotic factors and possibly facilitate translocation, also are a key consideration. Finally, determining whether the fungal pathogen of interest contains a functional RNAi machinery is a major consideration. Integrated experimental confirmation of these important factors is necessary for the successful development of crop protection strategies against fungal pathogens.

Development of an efficient Tef-1α RNA hairpin structure to efficient management of Lasiodiplodia theobromae and Neofusicoccum parvum

Lasiodiplodia theobromae and Neofusicoccum parvum are serious worldwide-distributed plant pathogenic fungi with a wide host range in tropical and temperate climates. They cause fruit rot, canker, and dieback of twigs in various woody plants. Protection of pruning wounds using fungicides is the prevalent strategy for the management of the diseases caused by these fungi. Chemical control of plant diseases is not environmentally safe and the residues of fungicides are a threat to nature. Furthermore, genetic resources of resistance to plant diseases in woody plants are limited. The aim of this study was to investigate the efficiency of RNA silencing using an efficient hairpin structure based on Tef-1α gene for the management of L. theobromae and N. parvum. Hairpin structure of Tef-1α was cloned in pFGC5941 binary vector and the recombinant construct was named pFGC-TEF-d. Transient expression of pFGC-TEF-d using Agrobacterium LBA4404 in grapevine (Bidaneh Sefid cv.) and strawberry cultivars (Camarosa and Ventana) led to a reduction in disease progress of L. theobromae. The disease reduction in grapevine was estimated by 55% and in strawberries cultivars Camarosa and Ventana by 58% and 93%, respectively. Further analysis of transient expression of pFGC-TEF-d in strawberry (Camarosa) shown disease reduction using Neofusicoccum parvum. Here we introduce RNAi silencing using pFGC-TEF-d construct as an efficient strategy to the management of L. theobromae and N. parvum for the first time.

RNA Interference Strategies for Future Management of Plant Pathogenic Fungi: Prospects and Challenges

Plant pathogenic fungi are the largest group of disease-causing agents on crop plants and represent a persistent and significant threat to agriculture worldwide. Conventional approaches based on the use of pesticides raise social concern for the impact on the environment and human health and alternative control methods are urgently needed. The rapid improvement and extensive implementation of RNA interference (RNAi) technology for various model and non-model organisms has provided the initial framework to adapt this post-transcriptional gene silencing technology for the management of fungal pathogens. Recent studies showed that the exogenous application of double-stranded RNA (dsRNA) molecules on plants targeting fungal growth and virulence-related genes provided disease attenuation of pathogens like Botrytis cinerea, Sclerotinia sclerotiorum and Fusarium graminearum in different hosts. Such results highlight that the exogenous RNAi holds great potential for RNAi-mediated plant pathogenic fungal disease control. Production of dsRNA can be possible by using either in-vitro or in-vivo synthesis. In this review, we describe exogenous RNAi involved in plant pathogenic fungi and discuss dsRNA production, formulation, and RNAi delivery methods. Potential challenges that are faced while developing a RNAi strategy for fungal pathogens, such as off-target and epigenetic effects, with their possible solutions are also discussed.

Exogenous RNA as a Regulatory Signal during a Plant's Interaction with the Biotic Environment: An Evolutionary Perspective and Future Applications in Agriculture

Environmental RNAi (eRNAi) is a sequence-specific regulation of endogenous gene expression in a responsive organism by exogenous RNA. While exogenous RNA transfer between organisms of different kingdoms of life have been unambiguously identified in nature, our understanding of the biological significance of this phenomenon remains obscure, particularly within an evolutionary context. During the last decade multiple reports utilizing various mechanisms of natural eRNAi phenomena have been attempted to develop new agricultural traits and products including weed, disease and insect control. Although these attempts yielded mixed results, this concept remains extremely attractive for many agricultural applications. To better utilize eRNAi for practical applications, we would like to emphasize the necessity of understanding the biological significance of this phenomenon within an evolutionary context and learn from nature by developing advanced tools to identify and study new cases of exogeneous RNA transfer and eRNAi. In this opinion article we would like to look at the exogeneous RNA transfer from an evolutionary perspective, propose that new cases of exogeneous RNA transfer still remain to be identified in nature, and address a knowledge gap in understanding the biological function and significance of RNA transfer. We believe such approach may eventually result in a more successful use of this phenomenon for practical applications in agriculture.

Host-Induced Gene Silencing of a Sclerotinia sclerotiorum oxaloacetate acetylhydrolase Using Bean Pod Mottle Virus as a Vehicle Reduces Disease on Soybean

A lack of complete resistance in the current germplasm complicates the management of Sclerotinia stem rot (SSR) caused by Sclerotinia sclerotiorum in soybean. In this study, we used bean pod mottle virus (BPMV) as a vehicle to down-regulate expression of a key enzyme in the production of an important virulence factor in S. sclerotiorum, oxalic acid (OA). Specifically, we targeted a gene encoding oxaloacetate acetylhydrolase (Ssoah1), because Ssoah1 deletion mutants are OA deficient and non-pathogenic on soybean. We first established that S. sclerotiorum can uptake environmental RNAs by monitoring the translocation of Cy3-labeled double-stranded and small interfering RNA (ds/siRNAs) into fungal hyphae using fluorescent confocal microscopy. This translocation led to a significant decrease in Ssoah1 transcript levels in vitro. Inoculation of soybean plants with BPMV vectors targeting Ssoah1 (pBPMV-OA) also led to decreased expression of Ssoah1. Importantly, pBPMV-OA inoculated plants showed enhanced resistance to S. sclerotiorum compared to empty-vector control plants. Our combined results provide evidence supporting the use of HIGS and exogenous applications of ds/siRNAs targeting virulence factors such as OA as viable strategies for the control of SSR in soybean and as discovery tools that can be used to identify previously unknown virulence factors.

dsRNA Uptake in Plant Pests and Pathogens: Insights into RNAi-Based Insect and Fungal Control Technology

Efforts to develop more environmentally friendly alternatives to traditional broad-spectrum pesticides in agriculture have recently turned to RNA interference (RNAi) technology. With the built-in, sequence-specific knockdown of gene targets following delivery of double-stranded RNA (dsRNA), RNAi offers the promise of controlling pests and pathogens without adversely affecting non-target species. Significant advances in the efficacy of this technology have been observed in a wide range of species, including many insect pests and fungal pathogens. Two different dsRNA application methods are being developed. First, host induced gene silencing (HIGS) harnesses dsRNA production through the thoughtful and precise engineering of transgenic plants and second, spray induced gene silencing (SIGS) that uses surface applications of a topically applied dsRNA molecule. Regardless of the dsRNA delivery method, one aspect that is critical to the success of RNAi is the ability of the target organism to internalize the dsRNA and take advantage of the host RNAi cellular machinery. The efficiency of dsRNA uptake mechanisms varies across species, and in some uptake is negligible, rendering them effectively resistant to this new generation of control technologies. If RNAi-based methods of control are to be used widely, it is critically important to understand the mechanisms underpinning dsRNA uptake. Understanding dsRNA uptake mechanisms will also provide insight into the design and formulation of dsRNAs for improved delivery and provide clues into the development of potential host resistance to these technologies.

Clathrin mediated endocytosis is involved in the uptake of exogenous double-stranded RNA in the white mold phytopathogen Sclerotinia sclerotiorum

RNA interference (RNAi) technologies have recently been developed to control a growing number of agronomically significant fungal phytopathogens, including the white mold pathogen, Sclerotinia sclerotiorum. Exposure of this fungus to exogenous double-stranded RNA (dsRNA) results in potent RNAi-mediated knockdown of target genes' transcripts, but it is unclear how the dsRNA can enter the fungal cells. In nematodes, specialized dsRNA transport proteins such as SID-1 facilitate dsRNA uptake, but for many other eukaryotes in which the dsRNA uptake mechanisms have been examined, endocytosis appears to mediate the uptake process. In this study, using live cell imaging, transgenic fungal cultures and endocytic inhibitors, we determined that the uptake mechanism in S. sclerotiorum occurs through clathrin-mediated endocytosis. RNAi-mediated knockdown of several clathrin-mediated endocytic genes' transcripts confirmed the involvement of this cellular uptake process in facilitating RNAi in this fungus. Understanding the mode of dsRNA entry into the fungus will prove useful in designing and optimizing future dsRNA-based control methods and in anticipating possible mechanisms by which phytopathogens may develop resistance to this novel category of fungicides.

Oomycete small RNAs bind to the plant RNA-induced silencing complex for virulence

The exchange of small RNAs (sRNAs) between hosts and pathogens can lead to gene silencing in the recipient organism, a mechanism termed cross-kingdom RNAi (ck-RNAi). While fungal sRNAs promoting virulence are established, the significance of ck-RNAi in distinct plant pathogens is not clear. Here, we describe that sRNAs of the pathogen Hyaloperonospora arabidopsidis, which represents the kingdom of oomycetes and is phylogenetically distant from fungi, employ the host plant’s Argonaute (AGO)/RNA-induced silencing complex for virulence. To demonstrate H. arabidopsidis sRNA (HpasRNA) functionality in ck-RNAi, we designed a novel CRISPR endoribonuclease Csy4/GUS reporter that enabled in situ visualization of HpasRNA-induced target suppression in Arabidopsis. The significant role of HpasRNAs together with AtAGO1 in virulence was revealed in plant atago1 mutants and by transgenic Arabidopsis expressing a short-tandem-target-mimic to block HpasRNAs, that both exhibited enhanced resistance. HpasRNA-targeted plant genes contributed to host immunity, as Arabidopsis gene knockout mutants displayed quantitatively enhanced susceptibility.

RNA-based biocontrol compounds: current status and perspectives to reach the market

Facing current climate challenges and drastically reduced chemical options for plant protection, the exploitation of RNA interference (RNAi) as an agricultural biotechnology tool has unveiled possible new solutions to the global problems of agricultural losses caused by pests and other biotic and abiotic stresses. While the use of RNAi as a tool in agriculture is still limited to a few transgenic crops, and only adopted in restricted parts of the world, scientists and industry are already seeking innovations in leveraging and exploiting the potential of RNAi in the form of RNA-based biocontrol compounds for external applications. Here, we highlight the expanding research and development pipeline, commercial landscape and regulatory environment surrounding the pursuit of RNA-based biocontrol compounds with improved environmental profiles. The commitments of well-established agrochemical companies to invest in research endeavours and the role of start-up companies are crucial for the successful development of practical applications for these compounds. Additionally, the availability of standardized guidelines to tackle regulatory ambiguities surrounding RNA-based biocontrol compounds will help to facilitate the entire commercialization process. Finally, communication to create awareness and public acceptance will be key to the deployment of these compounds. © 2019 Society of Chemical Industry.

Silencing Dicer-Like Genes Reduces Virulence and sRNA Generation in Penicillium italicum, the Cause of Citrus Blue Mold

The Dicer protein is one of the most important components of RNAi machinery because it regulates the production of small RNAs (sRNAs) in eukaryotes. Here, Dicer1-like gene (Pit-DCL1) and Dicer2-like gene (Pit-DCL2) RNAi transformants were generated via pSilent-1 in Penicillium italicum (Pit), which is the causal agent of citrus blue mold. Neither transformant showed a change in mycelial growth or sporulation ability, but the pathogenicity of the Pit-DCL2 RNAi transformant to citrus fruits was severely impaired, compared to that of the Pit-DCL1 RNAi transformant and the wild type. We further developed a citrus wound-mediated RNAi approach with a double-stranded fragment of Pit-DCL2 generated in vitro, which achieved an efficiency in reducing Pi-Dcl2 expression and virulence that was similar to that of protoplast-mediated RNAi in P. italicum, suggesting that this approach is promising in the exogenous application of dsRNA to control pathogens on the surface of citrus fruits. In addition, sRNA sequencing revealed a total of 69.88 million potential sRNAs and 12 novel microRNA-like small RNAs (milRNAs), four of which have been predicated on target innate immunity or biotic stress-related genes in Valencia orange. These data suggest that both the Pit-DCL1 and Pit-DCL2 RNAi transformants severely disrupted the biogenesis of the potential milRNAs, which was further confirmed for some milRNAs by qRT-PCR or Northern blot analysis. These data suggest the sRNAs in P. italicum that may be involved in a molecular virulence mechanism termed cross-kingdom RNAi (ck-RNAi) by trafficking sRNA from P. italicum to citrus fruits.

Small RNA Bidirectional Crosstalk During the Interaction Between Wheat and Zymoseptoria tritici

Cross-kingdom RNA interference (RNAi) has been shown to play important roles during plant-pathogen interactions, and both plants and pathogens can use small RNAs (sRNAs) to silence genes in each other. This bidirectional cross-kingdom RNAi was still unexplored in the wheat-Zymoseptoria tritici pathosystem. Here, we performed a detailed analysis of the sRNA bidirectional crosstalk between wheat and Z. tritici. Using a combination of small RNA sequencing (sRNA-seq) and microRNA sequencing (mRNA-seq), we were able to identify known and novel sRNAs and study their expression and their action on putative targets in both wheat and Z. tritici. We predicted the target genes of all the sRNAs in either wheat or Z. tritici transcriptome and used degradome analysis to validate the cleavage of these gene transcripts. We could not find any clear evidence of a cross-kingdom RNAi acting by mRNA cleavage in this pathosystem. We also found that the fungal sRNA enrichment was lower in planta than during in vitro growth, probably due to the lower expression of the only Dicer gene of the fungus during plant infection. Our results support the recent finding that Z. tritici sRNAs cannot play important roles during wheat infection. However, we also found that the fungal infection induced wheat sRNAs regulating the expression of specific wheat genes, including auxin-related genes, as an immune response. These results indicate a role of sRNAs in the regulation of wheat defenses during Z. tritici infection. Our findings contribute to improve our understanding of the interactions between wheat and Z. tritici.

Exchange of Small Regulatory RNAs between Plants and Their Pests

Regulatory small RNAs are well known as antiviral agents, regulators of gene expression, and defenders of genome integrity in plants. Several studies over the last decade have also shown that some small RNAs are exchanged between plants and their pathogens and parasites. Naturally occurring trans-species small RNAs are used by host plants to silence mRNAs in pathogens. These gene-silencing events are thought to be detrimental to the pathogen and beneficial to the host. Conversely, trans-species small RNAs from pathogens and parasites are deployed to silence host mRNAs; these events are thought to be beneficial for the pests. The natural ability of plants to exchange small RNAs with invading eukaryotic organisms can be exploited to provide disease resistance. This review gives an overview of the current state of trans-species small RNA research in plants and discusses several outstanding questions for future research.

SIGS vs HIGS: a study on the efficacy of two dsRNA delivery strategies to silence Fusarium FgCYP51 genes in infected host and non-host plants

CYP3RNA, a double-stranded (ds)RNA designed to concomitantly target the two sterol 14α-demethylase genes FgCYP51A and FgCYP51B and the fungal virulence factor FgCYP51C, inhibits the growth of the ascomycete fungus Fusarium graminearum (Fg) in vitro and in planta. Here we compare two different methods (setups) of dsRNA delivery, viz. transgene expression (host-induced gene silencing, HIGS) and spray application (spray-induced gene silencing, SIGS), to assess the activity of CYP3RNA and novel dsRNA species designed to target one or two FgCYP51 genes. Using Arabidopsis and barley, we found that dsRNA designed to target two FgCYP51 genes inhibited fungal growth more efficiently than dsRNA targeting a single gene, although both dsRNA species reduced fungal infection. Either dsRNA delivery method reduced fungal growth stronger than anticipated from previous mutational knock-out (KO) strategies, where single gene KO had no significant effect on fungal viability. Consistent with the strong inhibitory effects of the dsRNAs on fungal development in both setups, we detected to a large extent dsRNA-mediated co-silencing of respective non-target FgCYP51 genes. Together, our data further support the valuation that dsRNA applications have an interesting potential for pesticide target validation and gene function studies, apart from their potential for crop protection.