Potato Virus Y Infection Alters Small RNA Metabolism and Immune Response in Tomato

Citrus psorosis virus 24K protein inhibits the processing of miRNA precursors by interacting with components of the biogenesis machinery

Sweet orange (Citrus sinensis) is one of the most important fruit crops worldwide. Virus infections in this crop can interfere with cellular processes, causing dramatic economic losses. By performing RT-qPCR analyses, we demonstrated that citrus psorosis virus (CPsV)-infected orange plants exhibited higher levels of unprocessed microRNA (miRNA) precursors than healthy plants. This result correlated with the reported reduction of mature miRNAs species. The protein 24K, the CPsV suppressor of RNA silencing (VSR), interacts with miRNA precursors in vivo. Thus, this protein becomes a candidate responsible for the increased accumulation of unprocessed miRNAs. We analyzed 24K RNA-binding and protein-protein interaction domains and described patterns of its subcellular localization. We also showed that 24K colocalizes within nuclear D-bodies with the miRNA biogenesis proteins DICER-LIKE 1 (DCL1), HYPONASTIC LEAVES 1 (HYL1), and SERRATE (SE). According to the results of bimolecular fluorescence complementation and co-immunoprecipitation assays, the 24K protein interacts with HYL1 and SE. Thus, 24K may inhibit miRNA processing in CPsV-infected citrus plants by direct interaction with the miRNA processing complex. This work contributes to the understanding of how a virus can alter the regulatory mechanisms of the host, particularly miRNA biogenesis and function.IMPORTANCESweet oranges can suffer from disease symptoms induced by virus infections, thus resulting in drastic economic losses. In sweet orange plants, CPsV alters the accumulation of some precursors from the regulatory molecules called miRNAs. This alteration leads to a decreased level of mature miRNA species. This misregulation may be due to a direct association of one of the viral proteins (24K) with miRNA precursors. On the other hand, 24K may act with components of the cell miRNA processing machinery through a series of predicted RNA-binding and protein-protein interaction domains.

Profiling of Groundnut bud necrosis orthotospovirus-responsive microRNA and their targets in tomato based on deep sequencing

Tomato, an extensively cultivated vegetable crop produces miRNAs in response to infection with Groundnut bud necrosis orthotospovirus, a viral pathogen causing significant economic losses. High-throughput miRNA sequencing was performed on tomato leaves inoculated with GBNV and mock-inoculated leaves as controls. Analysis revealed 73 known miRNAs belonging to 24 miRNA families, with variable expression levels. Interestingly, 39 miRNAs were upregulated, and 34 were downregulated in response to GBNV infection. Stem-loop quantitative reverse transcription PCR validated the differential expression of selected miRNAs. Additionally, 30 miRNA encoded proteins were identified to be involved in disease resistance and susceptibility. The miRNA-target interactions were found to play significant roles in cellular and metabolic activities, as well as modulating signaling pathways during the plant-virus interaction. The findings shed light on the intricate regulatory network of miRNAs in tomato response to viral infection and may contribute to developing strategies for improving crop protection against viral diseases.

Recovery from virus infection: plant's armory in action

MAIN CONCLUSION

This review focuses on different factors involved in promoting symptom recovery in plants post-virus infection such as epigenetics, transcriptional reprogramming, phytohormones with an emphasis on RNA silencing as well as role of abiotic factors such as temperature on symptom recovery. Plants utilize several different strategies to defend themselves in the battle against invading viruses. Most of the viral proteins interact with plant proteins and interfere with molecular dynamics in a cell which eventually results in symptom development. This initial symptom development is countered by the plant utilizing various factors including the plant's adaptive immunity to develop a virus tolerant state. Infected plants can specifically target and impede the transcription of viral genes as well as degrade the viral transcripts to restrict their proliferation by the production of small-interfering RNA (siRNA) generated from the viral nucleic acid, known as virus-derived siRNA (vsiRNA). To further escalate the degradation of viral nucleic acid, secondary siRNAs are generated. The production of virus-activated siRNA (vasiRNA) from the host genome causes differential regulation of the host transcriptome which plays a major role in establishing a virus tolerant state within the infected plant. The systemic action of vsiRNAs, vasiRNA, and secondary siRNAs with the help of defense hormones like salicylic acid can curb viral proliferation, and thus the newly emerged leaves develop fewer symptoms, maintaining a state of tolerance.

Small Talk: On the Possible Role of Trans-Kingdom Small RNAs during Plant-Virus-Vector Tritrophic Communication

Small RNAs (sRNAs) are the hallmark and main effectors of RNA silencing and therefore are involved in major biological processes in plants, such as regulation of gene expression, antiviral defense, and plant genome integrity. The mechanisms of sRNA amplification as well as their mobile nature and rapid generation suggest sRNAs as potential key modulators of intercellular and interspecies communication in plant-pathogen-pest interactions. Plant endogenous sRNAs can act in cis to regulate plant innate immunity against pathogens, or in trans to silence pathogens' messenger RNAs (mRNAs) and impair virulence. Likewise, pathogen-derived sRNAs can act in cis to regulate expression of their own genes and increase virulence towards a plant host, or in trans to silence plant mRNAs and interfere with host defense. In plant viral diseases, virus infection alters the composition and abundance of sRNAs in plant cells, not only by triggering and interfering with the plant RNA silencing antiviral response, which accumulates virus-derived small interfering RNAs (vsiRNAs), but also by modulating plant endogenous sRNAs. Here, we review the current knowledge on the nature and activity of virus-responsive sRNAs during virus-plant interactions and discuss their role in trans-kingdom modulation of virus vectors for the benefit of virus dissemination.

Characterization of Tomato Brown Rugose Fruit Virus (ToBRFV) Detected in Czech Republic

Tomato is the most consumed vegetable in the world. The tomato brown rugose fruit virus (ToBRFV) is an important destructive virus that damages tomatoes and peppers with significant economic impact. The detection and characterization of this important viral pathogen were evaluated at the molecular and morphological level. The viral isolate was purified and inoculated on tomato and pepper plants. Small RNAs were sequenced in both plants and the profiles were compared. The complete genome of the isolate was obtained, and microRNA (miRNA) profiles were unveiled by small RNA sequencing. Symptoms caused by the isolate were also described and the morphology of the isolate was observed by transmission electron microscopy. Our results contribute to further understanding of the role of miRNAs in ToBRFV pathogenesis, which may be crucial for understanding disease symptom development in tomatoes and peppers.

A Novel miRNA in Rice Associated with the Low Seed Setting Rate Symptom of Rice Stripe Virus

MicroRNAs play key regulatory roles in plant development. The changed pattern of miRNA expression is involved in the production of viral symptoms. Here, we showed that a small RNA, Seq119, a putative novel microRNA, is associated with the low seed setting rate, a viral symptom of rice stripe virus (RSV)-infected rice. The expression of Seq 119 was downregulated in RSV-infected rice. The overexpression of Seq119 in transgenic rice plants did not cause any obvious phenotypic changes in plant development. When the expression of Seq119 was suppressed in rice plants either by expressing a mimic target or by CRISPR/Cas editing, seed setting rates were extremely low, similar to the effects of RSV infection. The putative targets of Seq119 were then predicted. The overexpression of the target of Seq119 in rice caused a low seed setting rate, similar to that in Seq119-suppressed or edited rice plants. Consistently, the expression of the target was upregulated in Seq119-suppressed and edited rice plants. These results suggest that downregulated Seq119 is associated with the low seed setting rate symptom of the RSV in rice.

Heightened miR6024-NLR interactions facilitate necrotrophic pathogenesis in tomato

miR6024 acts as a negative regulator of R genes, hence of Tomato plant immunity, and facilitates disease by the necrotrophic pathogen A. solani. Plant resistance genes or Nucleotide-binding leucine-rich repeat (NLR) genes, integral components of plant disease stress-signaling are targeted by variable groups of miRNAs. However, the significance of miRNA-mediated regulation of NLRs during a pathogen stress response, specifically for necrotrophic fungus, is poorly understood. A thorough examination of Tomato NLRs and miRNAs could map substantial interactions of which half the annotated NLRs were targets of Solanaceae-specific and conserved miRNAs, at the NB subdomain. The Solanaceae-specific miR6024 and its NLR targets analysed in different phytopathogenic stresses revealed differential and mutually antagonistic regulation. Interestingly, miR6024-targeted cleavage of a target NLR also triggered the generation of secondary phased siRNAs which could potentially amplify the defense signal. RNA-seq analysis of leaf tissues from miR6024 overexpressing Tomato plants evidenced a perturbation in the defense transcriptome with the transgenics showing unwarranted immune response-related genes' expression with or without infection with necrotrophic Alternaria solani, though no adverse effect could be observed in the growth and development of the transgenic plants. Transgenic plants exhibited constitutive downregulation of the target NLRs, aggravated disease phenotype with an enhanced lesion, greater ROS generation and hypersusceptibility to A. solani infection, thus establishing that miR6024 negatively impacts plant immune response during necrotrophic pathogenesis. Limited knowledge about the outcome of NLR-miRNA interaction during necrotrophic pathogenesis is a hindrance to the deployment of miRNAs in crop improvement programs. With the elucidation of the necrotrophic disease-synergistic role played by miR6024, it becomes a potent candidate for biotechnological manipulation for the rapid development of pathogen-tolerant solanaceous plants.

Transcriptome and Small RNA Profiling of Potato Virus Y Infected Potato Cultivars, Including Systemically Infected Russet Burbank

Potatoes are the world’s most produced non-grain crops and an important food source for billions of people. Potatoes are susceptible to numerous pathogens that reduce yield, including Potato virus Y (PVY). Genetic resistance to PVY is a sustainable way to limit yield and quality losses due to PVY infection. Potato cultivars vary in their susceptibility to PVY and include susceptible varieties such as Russet Burbank, and resistant varieties such as Payette Russet. Although the loci and genes associated with PVY-resistance have been identified, the genes and mechanisms involved in limiting PVY during the development of systemic infections have yet to be fully elucidated. To increase our understanding of PVY infection, potato antiviral responses, and resistance, we utilized RNA sequencing to characterize the transcriptomes of two potato cultivars. Since transcriptional responses associated with the extreme resistance response occur soon after PVY contact, we analyzed the transcriptome and small RNA profile of both the PVY-resistant Payette Russet cultivar and PVY-susceptible Russet Burbank cultivar 24 h post-inoculation. While hundreds of genes, including terpene synthase and protein kinase encoding genes, exhibited increased expression, the majority, including numerous genes involved in plant pathogen interactions, were downregulated. To gain greater understanding of the transcriptional changes that occur during the development of systemic PVY-infection, we analyzed Russet Burbank leaf samples one week and four weeks post-inoculation and identified similarities and differences, including higher expression of genes involved in chloroplast function, photosynthesis, and secondary metabolite production, and lower expression of defense response genes at those time points. Small RNA sequencing identified different populations of 21- and 24-nucleotide RNAs and revealed that the miRNA profiles in PVY-infected Russet Burbank plants were similar to those observed in other PVY-tolerant cultivars and that during systemic infection ~32% of the NLR-type disease resistance genes were targeted by 21-nt small RNAs. Analysis of alternative splicing in PVY-infected potato plants identified splice variants of several hundred genes, including isoforms that were more dominant in PVY-infected plants. The description of the PVYN-Wi-associated transcriptome and small RNA profiles of two potato cultivars described herein adds to the body of knowledge regarding differential outcomes of infection for specific PVY strain and host cultivar pairs, which will help further understanding of the mechanisms governing genetic resistance and/or virus-limiting responses in potato plants.

Identification of miRNAs Contributing to the Broad-Spectrum and Durable Blast Resistance in the Yunnan Local Rice Germplasm

MicroRNAs are 20-24 nucleotide non-coding RNAs and play important roles in plant-environment interactions. In recent years, many microRNAs (miRNAs) have been found to regulate rice immunity against rice blast fungus. However, there are limited studies about miRNAs that directly target resistance (R) genes to regulate rice immunity. In this study, by deep sequencing, small RNA libraries were constructed from four-leaf stage seedlings of the resistant variety Ziyu44 and susceptible variety Jiangnanxiangnuo (JNXN) upon Magnaporthe oryzae infection, we found that much more miRNAs were significantly differentially expressed in Ziyu44 than in JNXN. Among these miRNAs, we focused on miR9664, a newly identified rice miRNA in our sequencing, which was upregulated lightly in Ziyu44 and drastically in JNXN at 24-48 h post-inoculation (hpi). The transgenic plants overexpressing miR9664 (miR9664-oe) displayed reduced defense responses to M. oryzae, while those knocking down miR9664 (miR9664-m) displayed enhanced defense responses to M. oryzae. Most of the detected miR9664 predicted target genes were reduced in the miR9664-oe lines while increased in the miR9664-m lines. The cleavage site of LOC_Os08g07774 was confirmed by RLM-RACE. Meanwhile, after being inoculated with M. oryzae, the genes were expressed differently between Ziyu44 and JNXN. The results suggest that miR9664-mediated R gene turnover contributes to Ziyu44 broad-spectrum resistance to rice blast fungus. Taken together, our research identified a new rice miRNA that directly targets R genes to regulate rice immunity against rice blast fungus, adding significant information to the study of rice-M. oryzae interaction.

Genome-Wide Identification and Evolutionary Analysis of the SRO Gene Family in Tomato

SRO (SIMILAR TO RCD ONE) is a family of plant-specific small molecule proteins that play an important role in plant growth and development and environmental responses. However, SROs still lack systematic characterization in tomato. Based on bioinformatics methods, SRO family genes were identified and characterized from cultivated tomatoes and several wild tomatoes. qRT-PCR was used to study the expression of SRO gene in cultivated tomatoes. Phylogenetic and evolutionary analyses showed that SRO genes in angiosperms share a common ancestor and that the number of SRO family members changed as plants diverged and evolved. Cultivated tomato had six SRO members, five of which still shared some degree of identity with the ancestral SRO genes. Genetic structure and physicochemical properties showed that tomato SRO genes were highly conserved with chromosomal distribution. They could be divided into three groups based on exon-intron structure, and cultivated tomato contained only two of these subclades. A number of hormonal, light and abiotic stress-responsive cis-regulatory elements were identified from the promoter of the tomato SRO gene, and they also interacted with a variety of stress-responsive proteins and microRNAs. RNA-seq analysis showed that SRO genes were widely expressed in different tissues and developmental stages of tomato, with significant tissue-specific features. Expression analysis also showed that SRO genes respond significantly to high temperature and salt stress and mediate the tomato hormone regulatory network. These results provide a theoretical basis for further investigation of the functional expression of tomato SRO genes and provide potential genetic resources for tomato resistance breeding.

Integrated Analysis of microRNA and mRNA Transcriptome Reveals the Molecular Mechanism of Solanum lycopersicum Response to Bemisia tabaci and Tomato chlorosis virus

Tomato chlorosis virus (ToCV), is one of the most devastating cultivated tomato viruses, seriously threatened the growth of crops worldwide. As the vector of ToCV, the whitefly Bemisia tabaci Mediterranean (MED) is mainly responsible for the rapid spread of ToCV. The current understanding of tomato plant responses to this virus and B. tabaci is very limited. To understand the molecular mechanism of the interaction between tomato, ToCV and B. tabaci, we adopted a next-generation sequencing approach to decipher miRNAs and mRNAs that are differentially expressed under the infection of B. tabaci and ToCV in tomato plants. Our data revealed that 6199 mRNAs were significantly regulated, and the differentially expressed genes were most significantly associated with the plant-pathogen interaction, the MAPK signaling pathway, the glyoxylate, and the carbon fixation in photosynthetic organisms and photosynthesis related proteins. Concomitantly, 242 differentially expressed miRNAs were detected, including novel putative miRNAs. Sly-miR159, sly-miR9471b-3p, and sly-miR162 were the most expressed miRNAs in each sample compare to control group. Moreover, we compared the similarities and differences of gene expression in tomato plant caused by infection or co-infection of B. tabaci and ToCV. Taken together, the analysis reported in this article lays a solid foundation for further research on the interaction between tomato, ToCV and B. tabaci, and provide evidence for the identification of potential key genes that influences virus transmission in tomato plants.

Persistent Southern Tomato Virus (STV) Interacts with Cucumber Mosaic and/or Pepino Mosaic Virus in Mixed- Infections Modifying Plant Symptoms, Viral Titer and Small RNA Accumulation

Southern tomato virus (STV) is a persistent virus that was, at the beginning, associated with some tomato fruit disorders. Subsequent studies showed that the virus did not induce apparent symptoms in single infections. Accordingly, the reported symptoms could be induced by the interaction of STV with other viruses, which frequently infect tomato. Here, we studied the effect of STV in co- and triple-infections with Cucumber mosaic virus (CMV) and Pepino mosaic virus (PepMV). Our results showed complex interactions among these viruses. Co-infections leaded to a synergism between STV and CMV or PepMV: STV increased CMV titer and plant symptoms at early infection stages, whereas PepMV only exacerbated the plant symptoms. CMV and PepMV co-infection showed an antagonistic interaction with a strong decrease of CMV titer and a modification of the plant symptoms with respect to the single infections. However, the presence of STV in a triple-infection abolished this antagonism, restoring the CMV titer and plant symptoms. The siRNAs analysis showed a total of 78 miRNAs, with 47 corresponding to novel miRNAs in tomato, which were expressed differentially in the plants that were infected with these viruses with respect to the control mock-inoculated plants. These miRNAs were involved in the regulation of important functions and their number and expression level varied, depending on the virus combination. The number of vsiRNAs in STV single-infected tomato plants was very small, but STV vsiRNAs increased with the presence of CMV and PepMV. Additionally, the rates of CMV and PepMV vsiRNAs varied depending on the virus combination. The frequencies of vsiRNAs in the viral genomes were not uniform, but they were not influenced by other viruses.

Transcriptome analysis provides insights into the responses of sweet potato to sweet potato virus disease (SPVD)

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Transcriptome responses in sweet potato infected with SPCSV and/or SPFMV were studied.

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Numerous genes, miRNAs and phasiRNAs were responsive mainly to the dual infection.

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Salicylic acid-mediated pathways play important roles in antiviral defense responses.

Sweet potato (Ipomoea batatas) ranks among the most important crops in the world and provides nutritional and economic sustainability for subsistence farmers in sub-Saharan Africa. Its production is mainly constrained by sweet potato virus disease (SPVD) caused by the coinfection of two positive-sense single-stranded RNA viruses, sweet potato chlorotic stunt virus (SPCSV) and sweet potato feathery mottle virus (SPFMV). Current understanding of sweet potato responses to SPCSV and SPFMV at the molecular level remains very limited. In this study, we performed deep sequencing of both messenger RNA (mRNA) and small RNA (sRNA) populations in an SPVD-susceptible cultivar ‘Beauregard’ upon viral infection, to identify biological pathways that contribute to both general and specific host responses to these important viral pathogens. We found that pathways related to stress response and signaling were significantly affected by viral infection. sRNA components of these pathways were predominantly affected in late stages of the coinfection by SPCSV and SPFMV. We identified several novel microRNAs that were responsive to viral infection, some of which were predicted to target nucleotide-binding site leucine-rich repeat (NBS-LRR) disease resistance genes. The downregulation of the salicylic acid-mediated defense response pathway in particular seems to be a result of the viral infection process, and can in part explain the susceptible nature of the ‘Beauregard’ cultivar.

Secondary siRNAs in Plants: Biosynthesis, Various Functions, and Applications in Virology

The major components of RNA silencing include both transitive and systemic small RNAs, which are technically called secondary sRNAs. Double-stranded RNAs trigger systemic silencing pathways to negatively regulate gene expression. The secondary siRNAs generated as a result of transitive silencing also play a substantial role in gene silencing especially in antiviral defense. In this review, we first describe the discovery and pathways of transitivity with emphasis on RNA-dependent RNA polymerases followed by description on the short range and systemic spread of silencing. We also provide an in-depth view on the various size classes of secondary siRNAs and their different roles in RNA silencing including their categorization based on their biogenesis. The other regulatory roles of secondary siRNAs in transgene silencing, virus-induced gene silencing, transitivity, and trans-species transfer have also been detailed. The possible implications and applications of systemic silencing and the different gene silencing tools developed are also described. The details on mobility and roles of secondary siRNAs derived from viral genome in plant defense against the respective viruses are presented. This entails the description of other compatible plant-virus interactions and the corresponding small RNAs that determine recovery from disease symptoms, exclusion of viruses from shoot meristems, and natural resistance. The last section presents an overview on the usefulness of RNA silencing for management of viral infections in crop plants.

Reference gene selection for miRNA and mRNA normalization in potato in response to potato virus Y

This was the first report on evaluating candidate reference genes for quantifying the expression profiles of both coding (e.g., mRNA) and non-coding (e.g., miRNA) genes in potato response to potato virus Y (PVY) inoculation. The reverse transcription-quantitative real-time polymerase chain reaction (RT-qPCR) method was employed to quantify the expression profiles of eight selected candidate reference genes; their expression stability was analyzed by four statistical algorithms, i.e., geNorm, BestKeeper, NormFinder and RefFinder. The most stable reference genes were sEF1a, sTUBb and seIF5 with a high stability. The least stable ones were sPP2A, sSUI1 and sGAPDH. The same reference gene allows for normalization of both miRNA and mRNA levels from a single RNA sample using cDNAs synthesized in a single RT reaction, in which a stem-loop primer was used for miRNAs and the oligo (dT) for mRNAs.

Novel targets for engineering Physostegia chlorotic mottle and tomato brown rugose fruit virus-resistant tomatoes: in silico prediction of tomato microRNA targets

Background

Physostegia chlorotic mottle virus (PhCMoV; genus: Alphanucleorhabdovirus, family: Rhabdoviridae) and tomato brown rugose fruit virus (ToBRFV; genus: Tobamovirus, family: Virgaviridae) are newly emerging plant viruses that have a dramatic effect on tomato production. Among various known virus-control strategies, RNAi-mediated defence has shown the potential to protect plants against various pathogens including viral infections. Micro(mi)RNAs play a major role in RNAi-mediated defence.

Methods

Using in silico analyses, we investigated the possibility of tomato-encoded miRNAs (TomiRNA) to target PhCMoV and ToBRFV genomes using five different algorithms, i.e., miRanda, RNAhybrid, RNA22, Tapirhybrid and psRNATarget.

Results

The results revealed that 14 loci on PhCMoV and 10 loci on ToBRFV can be targeted by the TomiRNAs based on the prediction of at least three algorithms. Interestingly, one TomiRNA, miR6026, can target open reading frames from both viruses, i.e., the phosphoprotein encoding gene of PhCMoV, and the two replicase components of ToBRFV. There are currently no commercially available PhCMoV- or ToBRFV-resistant tomato varieties, therefore the predicted data provide useful information for the development of PhCMoV- and ToBFRV-resistant tomato plants.

Special Issue: "The Complexity of the Potyviral Interaction Network"

Many potyvirus species are among the most economically-significant plant viruses as they cause substantial yield losses to crop plants globally [...].

Roles of small RNAs in the establishment of tolerant interaction between plants and viruses

In a tolerant plant-virus interaction, viral multiplication is sustained without substantial effects on plant growth or reproduction. Such interactions are, in natural environments, frequent and sometimes even beneficial for both interactors. Here we compiled evidence showing that small RNAs modulate plant immune responses and growth, hence adjusting its physiology to enable a tolerant interaction. Importantly, the role of small RNAs in tolerant interactions resembles that required for establishment of a mutualistic symbiosis. Tolerance can become a sustainable strategy for breeding for virus resistance as selection pressure for emergence of more aggressive strains is low. Understanding the processes underlying establishment of tolerance is, therefore, important for the development of future crops.

Plant Molecular Responses to Potato Virus Y: A Continuum of Outcomes from Sensitivity and Tolerance to Resistance

Potato virus Y (PVY) is the most economically important virus affecting potato production. PVY manipulates the plant cell machinery in order to successfully complete the infecting cycle. On the other side, the plant activates a sophisticated multilayer immune defense response to combat viral infection. The balance between these mechanisms, depending on the plant genotype and environment, results in a specific outcome that can be resistance, sensitivity, or tolerance. In this review, we summarize and compare the current knowledge on molecular events, leading to different phenotypic outcomes in response to PVY and try to link them with the known molecular mechanisms.