Chinese wheat mosaic virus-derived vsiRNA-20 can regulate virus infection in wheat through inhibition of vacuolar- (H+ )-PPase induced cell death

Interplay between tobacco curly shoot virus vsiRNA24 and triosephosphate isomerase: implications for Nicotiana benthamiana viral defense

Virus-derived small interfering RNAs (vsiRNAs) play an important role in viral infection by regulating the expression of host genes. At present, research on the regulation of plant primary metabolic pathways by vsiRNAs is very limited. TvsiRNA24 derived from tobacco curly shoot virus (TbCSV) was amplified by reverse transcription polymerase chain reaction, and its target gene NbTPI (triosephosphate isomerase) was verified using reverse transcription quantitative polymerase chain reaction and GFP fluorescence observation. The effect of the interaction between TvsiRNA24 and NbTPI on TbCSV infection was analyzed by virus mediated, genetic transformation, western blotting, and quantitative detection. The expression of TvsiRNA24 retards the growth of Nicotiana benthamiana and enhances TbCSV accumulation within N. benthamiana. The overexpression of NbTPI attenuates the accumulation of TbCSV, and the silencing of NbTPI leads to the growth retardation of N. benthamiana and intensifies symptoms post-TbCSV infection. Moreover, the expression of some genes related to photosynthesis, primary metabolism and immune response is regulated by NbTPI. Our results unveil the specific role of TvsiRNA24-NbTPI in the pathogenicity of TbCSV, resulting in hindrance to plant growth and facilitation of viral infection. The identification of this regulatory pathway provides valuable insights that can be utilized to devise novel antiviral approaches targeting the reduction of viral pathogenicity.

Profiling of small RNAs derived from tomato brown rugose fruit virus in infected Solanum lycopersicum plants by deep sequencing

Tomato brown rugose fruit virus (ToBRFV) is an emerging, rapidly spreading virus belonging to the genus Tobamovirus which seriously decreases tomato yields. RNA silencing is an evolutionarily conserved antiviral mechanism. In virus-infected plants, virus-derived small interfering RNAs (vsiRNAs) are one of the key components involved in the RNA silencing-based antiviral activity in plants. The main function of vsiRNAs is to target and degrade viral RNA. Studies have found that some vsiRNAs can also target host transcripts, further regulating host responses and symptoms and promoting viral survival and spread. In this study, the vsiRNA profiles of ToBRFV-infected tomato plants were obtained by deep sequencing. VsiRNAs were mainly 21 and 22 nucleotides in length and had a U-bias at the 5' end. The single-nucleotide resolution profile shows that vsiRNAs exhibit a heterogeneous continuous distribution in the ToBRFV genomic RNA, with hotspot regions on the antisense strand located at the 5' end of the RdRP and in the coding regions of MP and CP. The presence of vsiRNAs was confirmed in tomato plants infected with ToBRFV through RT-PCR, and GO and KEGG enrichment analyses were performed on the predicted vsiRNA target genes (with an expectation value less than or equal to 2.5). Seven potential target genes were selected for qRT-PCR analysis, confirming that their transcript accumulation significantly decreased in the leaves of tomato plants infected with ToBRFV. These genes may play an important role in the process of viral infection in tomatoes. Our results suggest a role for vsiRNAs in the ToBRFV-tomato interaction.

The dynamic TaRACK1B-TaSGT1-TaHSP90 complex modulates NLR-protein-mediated antiviral immunity in wheat

Nucleotide-binding leucine-rich repeat (NLR) proteins contribute widely to plant immunity by regulating defense mechanisms through the elicitation of a hypersensitive response (HR). Here, we find that TaRACK1B (the receptor for activated C-kinase 1B) regulates wheat immune response against Chinese wheat mosaic virus (CWMV) infection. TaRACK1B recruits TaSGT1 and TaHSP90 to form the TaRACK1B-TaSGT1-TaHSP90 complex. This complex is essential for maintaining NLR proteins' stability (TaRGA5-like and TaRGH1A-like) in order to control HR activation and inhibit viral infection. However, the cysteine-rich protein encoded by CWMV can disrupt TaRACK1B-TaSGT1-TaHSP90 complex formation, leading to the reduction of NLR-protein stability and suppression of HR activation, thus promoting CWMV infection. Interestingly, the 7K protein of wheat yellow mosaic virus also interferes with this antiviral immunity. Our findings show a shared viral counter-defense strategy whereby two soil-borne viruses may disrupt the TaRACK1B-TaSGT1-TaHSP90 complex, suppressing NLR-protein-mediated broad-spectrum antiviral immunity and promoting viral infection in wheat.

Combating wheat yellow mosaic virus through microbial interactions and hormone pathway modulations

BACKGROUND

The rhizosphere microbiome is critical for promoting plant growth and mitigating soil-borne pathogens. However, its role in fighting soil-borne virus-induced diseases, such as wheat yellow mosaic virus (WYMV) transmitted by Polymyxa graminis zoospores, remains largely underexplored. In this study, we hypothesized that during viral infections, plant microbiomes engage in critical interactions with plants, with key microbes playing vital roles in maintaining plant health. Our research aimed to identify microbial taxa that not only suppress the disease but also boost wheat yield by using a blend of techniques, including field surveys, yield assessments, high-throughput sequencing of plant and soil microbiomes, microbial isolation, hydroponic experiments, and transcriptome sequencing.

RESULTS

We found that, compared with roots and leaves, the rhizosphere microbiome showed a better performance in predicting wheat yield and the prevalence of P. graminis and WYMV across the three WYMV-impacted regions in China. Using machine learning, we found that healthy rhizospheres were marked with potentially beneficial microorganisms, such as Sphingomonas and Allorhizobium-Neorhizobium-Parararhizobium-Rhizobium, whereas diseased rhizospheres were associated with a higher prevalence of potential pathogens, such as Bipolaris and Fusicolla. Structural equation modeling showed that these biomarkers both directly and indirectly impacted wheat yield by modulating the rhizosphere microbiome and P. graminis abundance. Upon re-introduction of two key healthy rhizosphere biomarkers, Sphingomonas azotifigens and Rhizobium deserti, into the rhizosphere, wheat growth and health were enhanced. This was attributed to the up-regulation of auxin and cytokinin signaling pathways and the regulation of jasmonic acid and salicylic acid pathways during infections.

CONCLUSIONS

Overall, our study revealed the critical role of the rhizosphere microbiome in combating soil-borne viral diseases, with specific rhizosphere microbes playing key roles in this process. Video Abstract.

FATTY ACID DESATURASE4 enhances plant RNA virus replication and undergoes host vacuolar ATPase-mediated degradation

Emerging evidence indicates that fatty acid (FA) metabolic pathways regulate host immunity to vertebrate viruses. However, information on FA signaling in plant virus infection remains elusive. In this study, we demonstrate the importance of fatty acid desaturase (FAD), an enzyme that catalyzes the rate-limiting step in the conversion of saturated FAs into unsaturated FAs, during infection by a plant RNA virus. We previously found that the rare Kua-ubiquitin-conjugating enzyme (Kua-UEV1) fusion protein FAD4 from Nicotiana benthamiana (NbFAD4) was downregulated upon turnip mosaic virus (TuMV) infection. We now demonstrate that NbFAD4 is unstable and is degraded as TuMV infection progresses. NbFAD4 is required for TuMV replication, as it interacts with TuMV replication protein 6K2 and colocalizes with viral replication complexes. Moreover, NbFAD4 overexpression dampened the accumulation of immunity-related phytohormones and FA metabolites, and its catalytic activity appears to be crucial for TuMV infection. Finally, a yeast 2-hybrid library screen identified the vacuolar H+-ATPase component ATP6V0C as involved in NbFAD4 degradation and further suppression of TuMV infection. This study reveals the intricate role of FAD4 in plant virus infection, and sheds light on a new mechanism by which a V-ATPase is involved in plant antiviral defense.

The biogenesis, regulation and functions of transitive siRNA in plants

Small RNA (sRNA)-mediated RNA interference (RNAi) is a sequence-specific gene silencing mechanism that modulates gene expression in eukaryotes. As core molecules of RNAi, various sRNAs are encoded in the plant genome or derived from invading RNA molecules, and their biogenesis depends on distinct genetic pathways. Transitive small interfering RNAs (siRNAs), which are sRNAs produced from double-strand RNA (dsRNA) in a process that depends on RNA-dependent RNA polymerases (RDRs), can amplify and spread silencing signals to additional transcripts, thereby enabling a phenomenon termed "transitive RNAi". Members of this class of siRNAs function in various biological processes ranging from development to stress adaptation. In Arabidopsis thaliana, two RDRs participate in the generation of transitive siRNAs, acting cooperatively with various siRNA generation-related factors, such as the RNA-induced silencing complex (RISC) and aberrant RNAs. Transitive siRNAs are produced in diverse subcellular locations and structures under the control of various mechanisms, highlighting the intricacies of their biogenesis and functions. In this review, we discuss recent advances in understanding the molecular events of transitive siRNA biogenesis and its regulation, with a particular focus on factors involved in RDR recruitment. We aim to provide a comprehensive description of the generalized mechanism governing the biogenesis of transitive siRNAs. Additionally, we present an overview of the diverse biological functions of these siRNAs and raise some pressing questions in this area for further investigation.

A viral small interfering RNA-host plant mRNA pathway modulates virus-induced drought tolerance by enhancing autophagy

Virus-induced drought tolerance presents a fascinating facet of biotic-abiotic interaction in plants, yet its molecular intricacies remain unclear. Our study shows that cowpea mild mottle virus (CPMMV) infection enhances drought tolerance in common bean (Phaseolus vulgaris) plants through a virus-derived small interfering RNA (vsiRNA)-activated autophagy pathway. Specifically, a 21 nt vsiRNA originating from the CPMMV Triple Gene Block1 (TGB1) gene targeted the 5' untranslated region (UTR) of the host Teosinte branched 1, Cycloidea, Proliferating Cell Factor (TCP) transcription factor gene PvTCP2, independent of the known role of TGB1 as an RNA silencing suppressor. This targeting attenuated the expression of PvTCP2, which encodes a transcriptional repressor, and in turn upregulated the core autophagy-related gene (ATG) PvATG8c, leading to activated autophagy activity surpassing the level induced by drought or CPMMV infection alone. The downstream EARLY RESPONSIVE TO DEHYDRATION (ERD) effector PvERD15 is a homologue of Arabidopsis thaliana AtERD15, which positively regulates stomatal aperture. PvERD15 was degraded in PvATG8c-mediated autophagy. Therefore, we establish a TGB1-PvTCP2-PvATG8c-PvERD15 module as a trans-kingdom fine-tuning mechanism that contributes to virus-induced drought tolerance in plant-drought-virus interactions.

Identification of the Coexisting Virus-Derived siRNA in Maize and Rice Infected by Rice Black-Streaked Dwarf Virus

Rice black-streaked dwarf virus is transmitted by small brown planthoppers, which causes maize rough dwarf disease and rice black-streaked dwarf disease. This virus leads to slow growth or death of the host plants. During the coevolutionary arms race between viruses and plants, virus-derived small interfering RNAs (vsiRNAs) challenge the plant's defense response and inhibit host immunity through the RNA silencing system. However, it is currently unknown if rice black-streaked dwarf virus can produce the same siRNAs to mediate the RNA silencing in different infected species. In this study, four small RNA libraries and four degradome libraries were constructed by extracting total RNAs from the leaves of the maize (Zea mays) inbred line B73 and japonica rice (Oryza sativa) variety Nipponbare exposed to feeding by viruliferous and nonviruliferous small brown planthoppers. We analyzed the characteristics of small RNAs and explored virus-derived siRNAs in small RNA libraries through high-throughput sequencing. On analyzing the characteristics of small RNA, we noted that the size distributions of small RNAs were mainly 24 nt (19.74 to 62.00%), whereas those of vsiRNAs were mostly 21 nt (41.06 to 41.87%) and 22 nt (39.72 to 42.26%). The 5'-terminal nucleotides of vsiRNAs tended to be adenine or uracil. Exploring the distribution of vsiRNA hot spots on the viral genome segments revealed that the frequency of hotspots in B73 was higher than those in Nipponbare. Meanwhile, hotspots in the S9 and S10 virus genome segments were distributed similarly in both hosts. In addition, the target genes of small RNA were explored by degradome sequencing. Analyses of the regulatory pathway of these target genes unveiled that viral infection affected the ribosome-related target genes in maize and the target genes in the metabolism and biosynthesis pathways in rice. Here, 562 and 703 vsiRNAs were separately obtained in maize and rice and 73 vsiRNAs named as coexisting vsiRNAs (co-vsiRNAs) were detected in both hosts. Stem-loop PCR and real-time quantitative PCR confirmed that co-vsiRNA 3.1 and co-vsiRNA 3.5, derived from genome segment S3, simultaneously play a role in maize and rice and inhibited host gene expression. The study revealed that rice black-streaked dwarf virus can produce the same siRNAs in different species and provides a new direction for developing new antiviral strategies.

Insights into the Complexity and Functionality of Plant Virus Protein Phosphorylation

Phosphorylation, the most extensive and pleiotropic form of protein posttranslation modification, is central to cellular signal transduction. Throughout the extensive co-evolution of plant hosts and viruses, modifications to phosphorylation have served multiple purposes. Such modifications highlight the evolutionary trajectories of viruses and their hosts, with pivotal roles in regulation and refinement of host-virus interactions. In plant hosts, protein phosphorylation orchestrates immune responses, enhancing the activities of defense-related proteins such as kinases and transcription factors, thereby strengthening pathogen resistance in plants. Moreover, phosphorylation influences the interactions between host and viral proteins, altering viral spread and replication within host plants. In the context of plant viruses, protein phosphorylation controls key aspects of the infection cycle, including viral protein functionality and the interplay between viruses and host plant cells, leading to effects on viral accumulation and dissemination within plant tissues. Explorations of the nuances of protein phosphorylation in plant hosts and their interactions with viruses are particularly important. This review provides a systematic summary of the biological roles of the proteins of plant viruses carrying diverse genomes in regulating infection and host responses through changes in the phosphorylation status. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

TaWRKY50-TaSARK7 module-mediated cysteine-rich protein phosphorylation suppresses the programmed cell death response to Chinese wheat mosaic virus infection

WRKY transcription factors are widely involved in plant responses to biotic and abiotic stresses. However, there is currently a limited understanding of the regulation of viral infection by WRKY transcription factors in wheat (Triticum aestivum). The WRKY transcription factor TaWRKY50 in group IIb wheat exhibited a significant response to Chinese wheat mosaic virus infection. TaWRKY50 is localized in the nucleus and is an activating transcription factor. Interestingly, we found that silencing TaWRKY50 induces cell death following inoculation with CWMV. The protein kinase TaSAPK7 is specific to plants, whereas NbSRK is a closely related kinase with high homology to TaSAPK7. The transcriptional activities of both TaSAPK7 and NbSRK can be enhanced by TaWRKY50 binding to their promoters. CRP is an RNA silencing suppressor. Furthermore, TaWRKY50 may regulate CWMV infection by regulating the expression of TaSAPK7 and NbSRK to increase CRP phosphorylation and reduce the amount of programmed cell death (PCD).

The inorganic pyrophosphatases of microorganisms: a structural and functional review

Pyrophosphatases (PPases) are enzymes that catalyze the hydrolysis of pyrophosphate (PPi), a byproduct of the synthesis and degradation of diverse biomolecules. The accumulation of PPi in the cell can result in cell death. Although the substrate is the same, there are variations in the catalysis and features of these enzymes. Two enzyme forms have been identified in bacteria: cytoplasmic or soluble pyrophosphatases and membrane-bound pyrophosphatases, which play major roles in cell bioenergetics. In eukaryotic cells, cytoplasmic enzymes are the predominant form of PPases (c-PPases), while membrane enzymes (m-PPases) are found only in protists and plants. The study of bacterial cytoplasmic and membrane-bound pyrophosphatases has slowed in recent years. These enzymes are central to cell metabolism and physiology since phospholipid and nucleic acid synthesis release important amounts of PPi that must be removed to allow biosynthesis to continue. In this review, two aims were pursued: first, to provide insight into the structural features of PPases known to date and that are well characterized, and to provide examples of enzymes with novel features. Second, the scientific community should continue studying these enzymes because they have many biotechnological applications. Additionally, in this review, we provide evidence that there are m-PPases present in fungi; to date, no examples have been characterized. Therefore, the diversity of PPase enzymes is still a fruitful field of research. Additionally, we focused on the roles of H+/Na+ pumps and m-PPases in cell bioenergetics. Finally, we provide some examples of the applications of these enzymes in molecular biology and biotechnology, especially in plants. This review is valuable for professionals in the biochemistry field of protein structure-function relationships and experts in other fields, such as chemistry, nanotechnology, and plant sciences.

TaTHI2 interacts with Ca2+-dependent protein kinase TaCPK5 to suppress virus infection by regulating ROS accumulation

Thiamine (vitamin B1) biosynthesis involves key enzymes known as thiazole moieties (THI1/THI2), which have been shown to participate in plant responses to abiotic stress. However, the role of THI1/THI2 in plant immunity remains unclear. In this study, we cloned TaTHI2 from wheat and investigated its function in Chinese wheat mosaic virus (CWMV) infection. Overexpression of TaTHI2 (TaTHI2-OE) inhibited CWMV infection, while TaTHI2 silencing enhanced viral infection in wheat. Interestingly, the membrane-localized TaTHI2 protein was increased during CWMV infection. TaTHI2 also interacted with the Ca2+-dependent protein kinase 5 (TaCPK5), which is localized in the plasma membrane, and promoted reactive oxygen species (ROS) production by repressing TaCPK5-mediated activity of the catalase protein TaCAT1. CWMV CP disrupted the interaction between TaTHI2 and TaCAT1, reducing ROS accumulation and facilitating viral infection. Additionally, transgenic plants overexpressing TaTHI2 showed increased seed number per ear and 1000-kernel weight compared to control plants. Our findings reveal a novel function of TaTHI2 in plant immunity and suggest its potential as a valuable gene for balancing disease resistance and wheat yield. Furthermore, the disruption of the TaTHI2-mediated plant immune pathway by CWMV CP provides further evidence for the evolutionary arms race between plants and viruses.

Regulation of Disease-Resistance Genes against CWMV Infection by NbHAG1-Mediated H3K36ac

Post-translational modification of proteins plays a critical role in plant-pathogen interactions. Here, we demonstrate in Nicotiana benthamiana that knockout of NbHAG1 promotes Chinese wheat mosaic virus (CWMV) infection, whereas NbHAG1 overexpression inhibits infection. Transcriptome sequencing indicated that a series of disease resistance-related genes were up-regulated after overexpression of NbHAG1. In addition, cleavage under targets and tagmentation (Cut&Tag)-qPCR results demonstrated that NbHAG1 may activate the transcription of its downstream disease-resistance genes by facilitating the acetylation level of H3K36ac. Therefore, we suggest that NbHAG1 is an important positive regulator of resistance to CWMV infestation.

A papain-like cysteine protease-released small signal peptide confers wheat resistance to wheat yellow mosaic virus

Wheat yellow mosaic virus (WYMV), a soil-borne pathogen, poses a serious threat to global wheat production. Here, we identify a WYMV resistance gene, TaRD21A, that belongs to the papain-like cysteine protease family. Through genetic manipulation of TaRD21A expression, we establish its positive role in the regulation of wheat to WYMV resistance. Furthermore, our investigation shows that the TaRD21A-mediated plant antiviral response relies on the release of a small peptide catalyzed by TaRD21A protease activity. To counteract wheat resistance, WYMV-encoded nuclear inclusion protease-a (NIa) suppress TaRD21A activity to promote virus infection. In resistant cultivars, a natural variant of TaRD21A features a glycine-to-threonine substitution and this substitution enables the phosphorylation of threonine, thereby weakening the interaction between NIa and TaRD21A, reinforcing wheat resistance against WYMV. Our study not only unveils a WYMV resistance gene but also offers insights into the intricate mechanisms underpinning resistance against WYMV.

Genome-Wide Identification and Analysis of the ABCF Gene Family in Triticum aestivum

The ATP-binding cassette (ABC) superfamily of proteins is a group of evolutionarily conserved proteins. The ABCF subfamily is involved in ribosomal synthesis, antibiotic resistance, and transcriptional regulation. However, few studies have investigated the role of ABCF in wheat (Triticum aestivum) immunity. Here, we identified 18 TaABCFs and classified them into four categories based on their domain characteristics. Functional similarity between Arabidopsis and wheat ABCF genes was predicted using phylogenetic analysis. A comprehensive genome-wide analysis of gene structure, protein motifs, chromosomal location, and cis-acting elements was also performed. Tissue-specific analysis and expression profiling under temperature, hormonal, and viral stresses were performed using real-time quantitative reverse transcription polymerase chain reaction after randomly selecting one gene from each group. The results revealed that all TaABCF genes had the highest expression at 25 °C and responded to methyl jasmonate induction. Notably, TaABCF2 was highly expressed in all tissues except the roots, and silencing it significantly increased the accumulation of Chinese wheat mosaic virus or wheat yellow mosaic virus in wheat leaves. These results indicated that TaABCF may function in response to viral infection, laying the foundation for further studies on the mechanisms of this protein family in plant defence.

Genome-Wide Identification and Functional Analysis of NAP1 in Triticum aestivum

As a main molecular chaperone of histone H2A-H2B, nucleosome assembly protein 1 (NAP1) has been widely researched in many species. However, there is little research investigating the function of NAP1 in Triticum aestivum. To understand the capabilities of the family of NAP1 genes in wheat and the relationship between TaNAP1 genes and plant viruses, we performed comprehensive genome-wide analysis and quantitative real-time polymerase chain reaction (qRT-PCR) for testing expression profiling under hormonal and viral stresses. Our results showed that TaNAP1 was expressed at different levels in different tissues, with higher expression in tissues with high meristematic capacity, such as roots. Furthermore, the TaNAP1 family may participate in plant defense mechanisms. This study provides a systematic analysis of the NAP1 gene family in wheat and lays the foundation for further studies on the function of TaNAP1 in the response of wheat plants to viral infection.

vsiRNA18 derived from tobacco curly shoot virus can regulate virus infection in Nicotiana benthamiana

Virus-derived small interfering RNAs (vsiRNAs) play important roles in regulating host endogenous gene expression to promote virus infection and induce RNA silencing to suppress virus infection. However, to date, how vsiRNAs affect geminivirus infection in host plants has been less studied. In this study, we found that tobacco curly shoot virus (TbCSV)-derived vsiRNA18 (TvsiRNA18) can regulate TbCSV infection in Nicotiana benthamiana plants. The virus-mediated small RNA expression system and stable transformation technique were used to clarify the molecular role of TvsiRNA18 in TbCSV infection. The results indicate that TvsiRNA18 can aggravate disease symptoms in these plants and enhance viral DNA accumulation. ATP-dependent RNA helicase (ATP-dRH) was proven to be a target of TvsiRNA18, and down-regulation of ATP-dRH in plants was shown to induce virus-like leaf curling symptoms and increase TbCSV infection. These results suggest that TvsiRNA18 is an important regulator of TbCSV infection by suppressing ATP-dRH expression. This is the first report to demonstrate that TbCSV-derived vsiRNA can target host endogenous genes to affect symptom development, which helps to reveal the molecular mechanism of symptom occurrence after the virus infects the host.

Wheat yellow mosaic virus NIb targets TaVTC2 to elicit broad-spectrum pathogen resistance in wheat

GDP-L-galactose phosphorylase (VTC2) catalyses the conversion of GDP-L-galactose to L-galactose-1-P, a vital step of ascorbic acid (AsA) biosynthesis in plants. AsA is well known for its function in the amelioration of oxidative stress caused by most pathogen infection, but its function against viral infection remains unclear. Here, we have identified a VTC2 gene in wheat named as TaVTC2 and investigated its function in association with the wheat yellow mosaic virus (WYMV) infection. Our results showed that overexpression of TaVTC2 significantly increased viral accumulation, whereas knocking down TaVTC2 inhibited the viral infection in wheat, suggesting a positive regulation on viral infection by TaVTC2. Moreover, less AsA was produced in TaVTC2 knocking down plants (TaVTC2-RNAi) which due to the reduction in TaVTC2 expression and subsequently in TaVTC2 activity, resulting in a reactive oxygen species (ROS) burst in leaves. Furthermore, the enhanced WYMV resistance in TaVTC2-RNAi plants was diminished by exogenously applied AsA. We further demonstrated that WYMV NIb directly bound to TaVTC2 and inhibited TaVTC2 enzymatic activity in vitro. The effect of TaVTC2 on ROS scavenge was suppressed by NIb in a dosage-dependent manner, indicating the ROS scavenging was highly regulated by the interaction of TaVTC2 with NIb. Furthermore, TaVTC2 RNAi plants conferred broad-spectrum disease resistance. Therefore, the data indicate that TaVTC2 recruits WYMV NIb to down-regulate its own enzymatic activity, reducing AsA accumulation to elicit a burst of ROS which confers the resistance to WYMV infection. Thus, a new mechanism of the formation of plant innate immunity was proposed.

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.

Genome-wide identification and analysis of wheat LRR-RLK family genes following Chinese wheat mosaic virus infection

BACKGROUND

As the largest plant receptor-like protein kinase (RLK) superfamily, the 21 leucine-rich repeat receptor-like kinases (LRR-RLKs) family are involved in plant 22 growth, development, and stress responses. However, the functions of LRR-RLKs in 23 wheat immunity remain unknown.

RESULTS

In the current study, 929 LRR-RLKs were identified in Triticum aestivum 25 genome database using the BLAST and hidden Markov models (HMM) approach and 26 divided into 14 clades. Chromosomal localization and synteny analysis revealed that 27 TaLRR-RLKs were randomly distributed on all chromosomes with 921 collinear 28 events. Through the cis-acting elements analysis, we observed that TaLRR-RLKs 29 participated in hormone response, light response, development, metabolism, and 30 response to environmental stress. The transcript level of 14 random selected 31 TaLRR-RLKs from each subfamily was regulated by plant hormone treatment and 32 Chinese wheat mosaic virus (CWMV) infection. The function of TaLRR-RLKs in 33 wheat resistance to CWMV infection was further investigated by virus-induced gene 34 silencing assay. Additionally, the accumulation of MeJA response genes, as well as 35 CWMV RNA were not changed in the TaLRR-RLK silencing plants under MeJA 36 treatment.

CONCLUSIONS

Our results demonstrated that TaLRR-RLKs play an important role in 38 wheat resistance to viral infection via hormone signals and lay the groundwork for the 39 functional study of TaLRR-RLKs in wheat.

Rapid, Sensitive and Simultaneous Detection of Two Wheat RNA Viruses Using Reverse Transcription Recombinase Polymerase Amplification (RT-RPA)

In China, wheat yellow mosaic disease is mostly caused by wheat yellow mosaic virus (WYMV) and Chinese wheat mosaic virus (CWMV). If wheat is co-infected with these two viruses, it can cause severe yellow mosaic symptoms and yield losses. Early detection of viruses is crucial for preventing disease in the field. In this study, we optimized a sensitive, specific reverse transcription recombinase polymerase amplification (RT-RPA) detection method for two viruses, WYMV and CWMV. Two sets of primers were designed based on the capsid protein (CP)-encoding genes of the two viruses, and the reaction conditions were determined. The RT-RPA method, which amplified the target amplicon by a handheld reaction mixture for 20 min, was more sensitive than PCR-CP in the detection of WYMV. Finally, the RT-RPA method was performed on 110 randomly selected field samples, demonstrating its applicability to samples from different regions and specificity for co-infected samples. This study not only describes an improved method for detecting WYMV and CWMV using RT-RPA but also demonstrates the potential of this method, which could be applied under field conditions.

Phosphorylation of plant virus proteins: Analysis methods and biological functions

Phosphorylation is one of the most extensively investigated post-translational modifications that orchestrate a variety of cellular signal transduction processes. The phosphorylation of virus-encoded proteins plays an important regulatory role in the infection cycle of such viruses in plants. In recent years, molecular mechanisms underlying the phosphorylation of plant viral proteins have been widely studied. Based on recent publications, our study summarizes the phosphorylation analyses of plant viral proteins and categorizes their effects on biological functions according to the viral life cycle. This review provides a theoretical basis for elucidating the molecular mechanisms of viral infection. Furthermore, it deepens our understanding of the biological functions of phosphorylation in the interactions between plants and viruses.

Phosphorylated viral protein evades plant immunity through interfering the function of RNA-binding protein

Successful pathogen infection in plant depends on a proper interaction between the invading pathogen and its host. Post-translational modification (PTM) plays critical role(s) in plant-pathogen interaction. However, how PTM of viral protein regulates plant immunity remains poorly understood. Here, we found that S162 and S165 of Chinese wheat mosaic virus (CWMV) cysteine-rich protein (CRP) are phosphorylated by SAPK7 and play key roles in CWMV infection. Furthermore, the phosphorylation-mimic mutant of CRP (CRP^S162/165D) but not the non-phosphorylatable mutant of CRP (CRP^S162/165A) interacts with RNA-binding protein UBP1-associated protein 2C (TaUBA2C). Silencing of TaUBA2C expression in wheat plants enhanced CWMV infection. In contrast, overexpression of TaUBA2C in wheat plants inhibited CWMV infection. TaUBA2C inhibits CWMV infection through recruiting the pre-mRNA of TaNPR1, TaPR1 and TaRBOHD to induce cell death and H₂O₂ production. This effect can be supressed by CRP^S162/165D through changing TaUBA2C chromatin-bound status and attenuating it’s the RNA- or DNA-binding activities. Taken together, our findings provide new knowledge on how CRP phosphorylation affects CWMV infection as well as the arms race between virus and wheat plants.

Chinese wheat mosaic virus (CWMV) causes a damaging disease in cereal plants. However, CWMV interacts with host factors to facilitate virus infection is not clear yet. Here, we found that S162 and S165 of CWMV cysteine-rich protein (CRP) are phosphorylated by SAPK7 in vivo and in vitro. Mutational analyses have indicated that these two phosphorylation sites of CRP (CRP^S162/165D) promoting CWMV infection in plants, due to the supressed cell death and H₂O₂ production. Further investigations found the CRP^S162/165D can interact with TaUBA2C, while the non-phosphorylatable mutant of CRP (CRP^S162/165A) does not. Futhermore, we have determined that CRP^S162/165D and TaUBA2C interaction inhibited the formation of TaUBA2C speckles in nucleus to attenuate its RNA- and DNA-binding activity. We also showed that TaUBA2C recruit the pre-mRNA of TaNPR1, TaPR1 and TaRBOHD to up-regulated these genes expressions and then induce cell death and H₂O₂ production in plant. This effect can be supressed by the expression of CRP^S162/165D, in a dose-dependent manner. Taken together, our discovery may provide a new sight for the arms race between virus and its host plants.

eIF4A, a target of siRNA derived from rice stripe virus, negatively regulates antiviral autophagy by interacting with ATG5 in Nicotiana benthamiana

Autophagy is induced by viral infection and has antiviral functions in plants, but the underlying mechanism is poorly understood. We previously identified a viral small interfering RNA (vsiRNA) derived from rice stripe virus (RSV) RNA4 that contributes to the leaf-twisting and stunting symptoms caused by this virus by targeting the host eukaryotic translation initiation factor 4A (eIF4A) mRNA for silencing. In addition, autophagy plays antiviral roles by degrading RSV p3 protein, a suppressor of RNA silencing. Here, we demonstrate that eIF4A acts as a negative regulator of autophagy in Nicotiana benthamiana. Silencing of NbeIF4A activated autophagy and inhibited RSV infection by facilitating autophagic degradation of p3. Further analysis showed that NbeIF4A interacts with NbATG5 and interferes with its interaction with ATG12. Overexpression of NbeIF4A suppressed NbATG5-activated autophagy. Moreover, expression of vsiRNA-4A, which targets NbeIF4A mRNA for cleavage, induced autophagy by silencing NbeIF4A. Finally, we demonstrate that eIF4A from rice, the natural host of RSV, also interacts with OsATG5 and suppresses OsATG5-activated autophagy, pointing to the conserved function of eIF4A as a negative regulator of antiviral autophagy. Taken together, these results reveal that eIF4A negatively regulates antiviral autophagy by interacting with ATG5 and that its mRNA is recognized by a virus-derived siRNA, resulting in its silencing, which induces autophagy against viral infection.

Regulation of RNA Interference Pathways in the Insect Vector Laodelphax striatellus by Viral Proteins of Rice Stripe Virus

RNA interference (RNAi), especially the small interfering RNA (siRNA) and microRNA (miRNA) pathways, plays an important role in defending against viruses in plants and insects. However, how insect-transmitted phytoviruses regulate the RNAi-mediated antiviral response in vector insects has barely been uncovered. In this study, we explored the interaction between rice stripe virus (RSV) and the miRNA and siRNA pathways of the small brown planthopper, which is a vector insect. The transcript and protein levels of key genes in the two RNAi pathways did not change during the RSV infection process. When the expression of insect Ago1, Ago2, or Translin was silenced by the injection of double-stranded RNAs targeting these genes, viral replication was promoted with Ago2 silencing but inhibited with Translin silencing. Protein-protein binding assays showed that viral NS2 and RNA-dependent RNA polymerase interacted with insect Ago2 and Translin, respectively. When NS2 was knocked down, the transcript level of Ago2 increased and viral replication was inhibited. Therefore, viral NS2 behaved like an siRNA suppressor in vector insects. This protein-binding regulation of insect RNAi systems reflects a complicated and diverse coevolution of viruses with their vector insects.

Ferredoxin 1 is downregulated by the accumulation of abscisic acid in an ABI5-dependent manner to facilitate rice stripe virus infection in Nicotiana benthamiana and rice

Ferredoxin 1 (FD1) accepts and distributes electrons in the electron transfer chain of plants. Its expression is universally downregulated by viruses and its roles in plant immunity have been brought into focus over the past decade. However, the mechanism by which viruses regulate FD1 remains to be defined. In a previous report, we found that the expression of Nicotiana benthamiana FD1 (NbFD1) was downregulated following infection with potato virus X (PVX) and that NbFD1 regulates callose deposition at plasmodesmata to play a role in defense against PVX infection. We now report that NbFD1 is downregulated by rice stripe virus (RSV) infection and that silencing of NbFD1 also facilitates RSV infection, while viral infection was inhibited in a transgenic line overexpressing NbFD1, indicating that NbFD1 also functions in defense against RSV infection. Next, a RSV-derived small interfering RNA was identified that contributes to the downregulation of FD1 transcripts. Further analysis showed that the abscisic acid (ABA) which accumulates in RSV-infected plants also represses NbFD1 transcription. It does this by stimulating expression of ABA insensitive 5 (ABI5), which binds the ABA response element motifs in the NbFD1 promoter, resulting in negative regulation. Regulation of FD1 by ABA was also confirmed in RSV-infected plants of the natural host rice. The results therefore suggest a mechanism by which virus regulates chloroplast-related genes to suppress their defense roles.

A virus-derived siRNA activates plant immunity by interfering with ROS scavenging

Virus-derived small interference RNAs (vsiRNAs) not only suppress virus infection in plants via induction of RNA silencing but also enhance virus infection by regulating host defensive gene expression. However, the underlying mechanisms that control vsiRNA-mediated host immunity or susceptibility remain largely unknown. In this study, we generated several transgenic wheat lines using four artificial microRNA expression vectors carrying vsiRNAs from Wheat yellow mosaic virus (WYMV) RNA1. Laboratory and field tests showed that two transgenic wheat lines expressing amiRNA1 were highly resistant to WYMV infection. Further analyses showed that vsiRNA1 could modulate the expression of a wheat thioredoxin-like gene (TaAAED1), which encodes a negative regulator of reactive oxygen species (ROS) production in the chloroplast. The function of TaAAED1 in ROS scavenging could be suppressed by vsiRNA1 in a dose-dependent manner. Furthermore, transgenic expression of amiRNA1 in wheat resulted in broad-spectrum disease resistance to Chinese wheat mosaic virus, Barley stripe mosaic virus, and Puccinia striiformis f. sp. tritici infection, suggesting that vsiRNA1 is involved in wheat immunity via ROS signaling. Collectively, these findings reveal a previously unidentified mechanism underlying the arms race between viruses and plants.

Genome-wide identification and characterization of UBP gene family in wheat (Triticum aestivum L.)

Ubiquitination is essential for plant growth and development. Deubiquitination cooperates with ubiquitination to regulate the ubiquitination levels of target proteins. The ubiquitin-specific protease (UBP) family is the largest group of deubiquitinases (DUBs), which perform extensive and significant roles in eukaryotic organisms. However, the UBP genes in wheat (TaUBPs) are not identified, and the functions of TaUBPs are unknown. The present study identified 97 UBP genes in the whole genome of T. aestivum. These genes were divided into 15 groups and non-randomly distributed on chromosomes of T. aestivum. Analyses of evolutionary patterns revealed that TaUBPs mainly underwent purification selection. The studies of cis-acting regulatory elements indicated that they might be involved in response to hormones. Quantitative real-time PCR (qRT-PCR) results showed that TaUBPs were differentially expressed in different tissues. Besides, several TaUBPs were significantly up-regulated when plants were treated with salicylic acid (SA), implying that these DUBs may play a role in abiotic stress responses in plants and few TaUBPs displayed differential expression after viral infection. Furthermore, TaUBP1A.1 (TraesCS1A02G432600.1) silenced by virus-induced gene silencing (VIGS) facilitates Chinese wheat mosaic virus (CWMV) infection in wheat, indicating that TaUBP1A.1 may be involved in a defense mechanism against viruses. This study comprehensively analyzed the UBP gene family in wheat and provided a basis for further research of TaUBPs functions in wheat plant response to viral infection.

Contribution of Small RNA Pathway to Interactions of Rice with Pathogens and Insect Pests

Small RNAs (sRNAs) are mainly classified into microRNAs (miRNAs) and small interfering RNAs (siRNAs) according to their origin. miRNAs originate from single-stranded RNA precursors, whereas siRNAs originate from double-stranded RNA precursors that are synthesized by RNA-dependent RNA polymerases. Both of single-stranded and double-stranded RNA precursors are processed into sRNAs by Dicer-like proteins. Then, the sRNAs are loaded into ARGONAUTE proteins, forming RNA-induced silencing complexes (RISCs). The RISCs repress the expression of target genes with sequences complementary to the sRNAs through the cleavage of transcripts, the inhibition of translation or DNA methylation. Here, we summarize the recent progress of sRNA pathway in the interactions of rice with various parasitic organisms, including fungi, viruses, bacteria, as well as insects. Besides, we also discuss the hormone signal in sRNA pathway, and the emerging roles of circular RNAs and long non-coding RNAs in rice immunity. Obviously, small RNA pathway may act as a part of rice innate immunity to coordinate with growth and development.

Construction and biological characterization of an infectious full-length cDNA clone of a Chinese isolate of Wheat yellow mosaic virus

Wheat yellow mosaic virus (family Potyviridae; genus Bymovirus), is an important soil-borne virus that causes serious economic losses in wheat. In this study, we constructed infectious cDNA clones of WYMV genomic RNAs under the control of 35S or SP6 promoter for versatile usage (agroinfiltration or in vitro RNA transcription). Our results showed that an Agrobacterium-mediated inoculation system enabled WYMV to infect the leaves of Nicotiana benthamiana without causing WYMV systemic infection. However, in vitro transcripts from infectious cDNA clones using the SP6 promoter promoted WYMV systemic infection of wheat plants, which was then developed for further assays. The optimal temperature for virus multiplication and systemic infection of wheat was 8 °C. Additionally, a synergistic effect between WYMV and Chinese wheat mosaic virus (CWMV) was also detected. This is the first report of the construction of a Chinese isolate of WYMV and should facilitate the investigation of viral pathogenesis.

Comparative proteomic analysis of Nicotiana benthamiana plants under Chinese wheat mosaic virus infection

BACKGROUND

Chinese wheat mosaic virus (CWMV) is a severe threat to winter wheat and is transmitted by Polymyxa graminis. The mechanisms of interactions between CWMV and plants are poorly understood. In this study, a comparative proteomics analysis based on nanoliquid chromatography mass spectrometry (MS)/MS was conducted to characterize proteomic changes in plants responding to CWMV infection.

RESULTS

In total, 2751 host proteins were identified, 1496 of which were quantified and 146 up-regulated and 244 down-regulated proteins were identified as differentially expressed proteins (DEPs). Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that DEPs were most strongly associated with photosynthesis antenna proteins, MAPK signaling plant and glyoxylate and dicarboxylate metabolism pathways. Subcellular localization analysis predicted that more than half of the DEPs were localized in the chloroplast, an organelle indispensable for abscisic acid (ABA) synthesis. Our results suggest that CWMV infection interrupts normal chloroplast functions and decreases ABA concentrations in Nicotiana benthamiana. Further analysis showed that the ABA pathway was suppressed during CWMV infection and that ABA treatment induced plant hosts defenses against CWMV.

CONCLUSIONS

We identified several candidate proteins expressed during CWMV infection, and the ABA pathway was strongly associated with responses to CWMV infection in N. benthamiana.

Virus and Viroid-Derived Small RNAs as Modulators of Host Gene Expression: Molecular Insights Into Pathogenesis

Virus-derived siRNAs (vsiRNAs) generated by the host RNA silencing mechanism are effectors of plant’s defense response and act by targeting the viral RNA and DNA in post-transcriptional gene silencing (PTGS) and transcriptional gene silencing (TGS) pathways, respectively. Contrarily, viral suppressors of RNA silencing (VSRs) compromise the host RNA silencing pathways and also cause disease-associated symptoms. In this backdrop, reports describing the modulation of plant gene(s) expression by vsiRNAs via sequence complementarity between viral small RNAs (sRNAs) and host mRNAs have emerged. In some cases, silencing of host mRNAs by vsiRNAs has been implicated to cause characteristic symptoms of the viral diseases. Similarly, viroid infection results in generation of sRNAs, originating from viroid genomic RNAs, that potentially target host mRNAs causing typical disease-associated symptoms. Pathogen-derived sRNAs have been demonstrated to have the propensity to target wide range of genes including host defense-related genes, genes involved in flowering and reproductive pathways. Recent evidence indicates that vsiRNAs inhibit host RNA silencing to promote viral infection by acting as decoy sRNAs. Nevertheless, it remains unclear if the silencing of host transcripts by viral genome-derived sRNAs are inadvertent effects due to fortuitous pairing between vsiRNA and host mRNA or the result of genuine counter-defense strategy employed by viruses to enhance its survival inside the plant cell. In this review, we analyze the instances of such cross reaction between pathogen-derived vsiRNAs and host mRNAs and discuss the molecular insights regarding the process of pathogenesis.

Genome-Wide Identification and Expression Analysis of the Histone Deacetylase Gene Family in Wheat (Triticum aestivum L.)

Histone acetylation is a dynamic modification process co-regulated by histone acetyltransferases (HATs) and histone deacetylases (HDACs). Although HDACs play vital roles in abiotic or biotic stress responses, their members in Triticumaestivum and their response to plant viruses remain unknown. Here, we identified and characterized 49 T. aestivumHDACs (TaHDACs) at the whole-genome level. Based on phylogenetic analyses, TaHDACs could be divided into 5 clades, and their protein spatial structure was integral and conserved. Chromosomal location and synteny analyses showed that TaHDACs were widely distributed on wheat chromosomes, and gene duplication has accelerated the TaHDAC gene family evolution. The cis-acting element analysis indicated that TaHDACs were involved in hormone response, light response, abiotic stress, growth, and development. Heatmaps analysis of RNA-sequencing data showed that TaHDAC genes were involved in biotic or abiotic stress response. Selected TaHDACs were differentially expressed in diverse tissues or under varying temperature conditions. All selected TaHDACs were significantly upregulated following infection with the barley stripe mosaic virus (BSMV), Chinese wheat mosaic virus (CWMV), and wheat yellow mosaic virus (WYMV), suggesting their involvement in response to viral infections. Furthermore, TaSRT1-silenced contributed to increasing wheat resistance against CWMV infection. In summary, these findings could help deepen the understanding of the structure and characteristics of the HDAC gene family in wheat and lay the foundation for exploring the function of TaHDACs in plants resistant to viral infections.

Interactions and links among the noncoding RNAs in plants under stresses

The complex interplay among sRNAs, lncRNAs and circRNAs has been implicated in plants under biotic and abiotic stresses. Here, we review current advances in our understanding of ncRNA interactions and links, which have considerable potential for improving the agronomic traits and the environmental adaptability of plants. Plants can respond to biotic or abiotic stresses. To cope with various conditions, numerous intricate molecular regulatory mechanisms have evolved in plants. Noncoding RNAs (ncRNAs) can be divided into small noncoding RNAs (sRNAs), long noncoding RNAs (lncRNAs) and circular RNAs (circRNAs). Emerging evidence has demonstrated that interplay among the ncRNAs acts as a novel layer in the regulatory mechanisms, which has attracted substantial interest. Links between sRNAs can affect plant immune responses and development in synergistic or antagonistic manners. Additionally, multiple interactions between lncRNAs and sRNAs are involved in crop breeding, disease resistance and high tolerance to environmental stresses. Here, we summarize current knowledge of the interactions and links among the ncRNAs in plant responses to stresses and the methods for identifying ncRNA interactions. Furthermore, challenges and prospects for further progress in elucidating ncRNA interactions and links are highlighted.

Barley yellow dwarf virus-GAV-derived vsiRNAs are involved in the production of wheat leaf yellowing symptoms by targeting chlorophyll synthase

BACKGROUND

Wheat yellow dwarf virus disease is infected by barley yellow dwarf virus (BYDV), which causes leaf yellowing and dwarfing symptoms in wheat, thereby posing a serious threat to China's food production. The infection of plant viruses can produce large numbers of vsiRNAs, which can target host transcripts and cause symptom development. However, few studies have been conducted to explore the role played by vsiRNAs in the interaction between BYDV-GAV and host wheat plants.

METHODS

In this study, small RNA sequencing was conducted to profile vsiRNAs in BYDV-GAV-infected wheat plants. The putative targets of vsiRNAs were predicted by the bioinformatics software psRNATarget. RT-qPCR and VIGS were employed to identify the function of selected target transcripts. To confirm the interaction between vsiRNA and the target, 5' RACE was performed to analyze the specific cleavage sites.

RESULTS

From the sequencing data, we obtained a total of 11,384 detected vsiRNAs. The length distribution of these vsiRNAs was mostly 21 and 22 nt, and an A/U bias was observed at the 5' terminus. We also observed that the production region of vsiRNAs had no strand polarity. The vsiRNAs were predicted to target 23,719 wheat transcripts. GO and KEGG enrichment analysis demonstrated that these targets were mostly involved in cell components, catalytic activity and plant-pathogen interactions. The results of RT-qPCR analysis showed that most chloroplast-related genes were downregulated in BYDV-GAV-infected wheat plants. Silencing of a chlorophyll synthase gene caused leaf yellowing that was similar to the symptoms exhibited by BYDV-GAV-inoculated wheat plants. A vsiRNA from an overlapping region of BYDV-GAV MP and CP was observed to target chlorophyll synthase for gene silencing. Next, 5' RACE validated that vsiRNA8856 could cleave the chlorophyll synthase transcript in a sequence-specific manner.

CONCLUSIONS

This report is the first to demonstrate that BYDV-GAV-derived vsiRNAs can target wheat transcripts for symptom development, and the results of this study help to elucidate the molecular mechanisms underlying leaf yellowing after viral infection.

Viral and subviral derived small RNAs as pathogenic determinants in plants and insects

The phenotypic manifestations of disease induced by viruses and subviral infectious entities are the result of complex molecular interactions between host and viral factors. The viral determinants of the diseased phenotype have traditionally been sought at the level of structural or non-structural proteins. However, the discovery of RNA silencing mechanisms has led to speculations that determinants of the diseased phenotype are caused by viral nucleic acid sequences in addition to proteins. RNA silencing is a gene regulation mechanism conserved within eukaryotic kingdoms (with the exception of some yeast species), and in plants and insects it also functions as an antiviral mechanism. Non-coding RNAs of viral origin, ranging in size from 21 to 24 nucleotides (viral small interfering RNAs, vsiRNAs) accumulate in virus-infected tissues and organs, in some cases to comparable levels as the entire complement of host-encoded small interfering RNAs. Upon incorporation into RNA-induced silencing complexes, vsiRNAs can mediate cleavage or induce translational inhibition of nucleic acid targets in a sequence-specific manner. This review focuses on recent findings that suggest an increased complexity of small RNA-based interactions between virus and host. We mainly address plant viruses, but where applicable discuss insect viruses as well. Prominence is given to studies that have indisputably demonstrated that vsiRNAs determine diseased phenotype by either carrying sequence determinants or, indirectly, by altering host-gene regulatory pathways. Results from these studies suggest biotechnological applications, which are also discussed.

Hijacking of host cellular components as proviral factors by plant-infecting viruses

Plant viruses are important pathogens that cause serious crop losses worldwide. They are obligate intracellular parasites that commandeer a wide array of proteins, as well as metabolic resources, from infected host cells. In the past two decades, our knowledge of plant-virus interactions at the molecular level has exploded, which provides insights into how plant-infecting viruses co-opt host cellular machineries to accomplish their infection. Here, we review recent advances in our understanding of how plant viruses divert cellular components from their original roles to proviral functions. One emerging theme is that plant viruses have versatile strategies that integrate a host factor that is normally engaged in plant defense against invading pathogens into a viral protein complex that facilitates viral infection. We also highlight viral manipulation of cellular key regulatory systems for successful virus infection: posttranslational protein modifications for fine control of viral and cellular protein dynamics; glycolysis and fermentation pathways to usurp host resources, and ion homeostasis to create a cellular environment that is beneficial for viral genome replication. A deeper understanding of viral-infection strategies will pave the way for the development of novel antiviral strategies.