Evidence that RNA silencing-mediated resistance to beet necrotic yellow vein virus is less effective in roots than in leaves

A vicinal oxygen chelate protein facilitates viral infection by triggering the unfolded protein response in Nicotiana benthamiana

Vicinal oxygen chelate (VOC) proteins are members of an enzyme superfamily with dioxygenase or non-dioxygenase activities. However, the biological functions of VOC proteins in plants are poorly understood. Here, we show that a VOC in Nicotiana benthamiana (NbVOC1) facilitates viral infection. NbVOC1 was significantly induced by infection by beet necrotic yellow vein virus (BNYVV). Transient overexpression of NbVOC1 or its homolog from Beta vulgaris (BvVOC1) enhanced BNYVV infection in N. benthamiana, which required the nuclear localization of VOC1. Consistent with this result, overexpressing NbVOC1 facilitated BNYVV infection, whereas, knockdown and knockout of NbVOC1 inhibited BNYVV infection in transgenic N. benthamiana plants. NbVOC1 interacts with the basic leucine zipper transcription factors bZIP17/28, which enhances their self-interaction and DNA binding to the promoters of unfolded protein response (UPR)-related genes. We propose that bZIP17/28 directly binds to the NbVOC1 promoter and induces its transcription, forming a positive feedback loop to induce the UPR and facilitating BNYVV infection. Collectively, our results demonstrate that NbVOC1 positively regulates the UPR that enhances viral infection in plants.

Assessment of co-infection with BNYVV and BSCTV on resistance against Rhizomania disease in transgenic sugar beet plants

Sugar beet is an economically important crop and one of the major sources of sucrose around the world. Beet necrotic yellow vein virus (BNYVV) and Beet severe curly top virus (BSCTV) are two widespread viruses in sugar beet that cause severe damage to its performance. Previously, we have successfully produced resistance to BNYVV based on RNA silencing in sugar beet by introducing constructs carrying the viral coat-protein-encoding DNA sequence, CP21, in sense and anti-sense orientations. Yet, the RNA silencing-mediated resistance to a specific virus could be affected by other ones as a part of synergistic interactions. In this study, we assayed the specificity of the induced resistance against BNYVV in two sets of transgenic events, S3 and S6 carrying 5'-UTR with or without CP21-coding sequences, respectively. These events were subjected to viral challenges with either BNYVV, an Iranian isolate of BSCTV (BSCTV-Ir) or both. All the plants inoculated with just BSCTV-Ir displayed curly-leaf symptoms. However, partial resistance was evident in S3 events as shown by mild symptoms and reduced PCR amplification of the BSCTV-Ir coat protein encoding sequence. Based on the presented data, resistance to BNYVV was stable in almost all the transgenic plants co-infected with BSCTV-Ir, except for one event, S3-229. In general, it seems that the co-infection does not affect the resistance to BNYVV in transgenic plants. These findings demonstrated that the introduced RNA silencing-mediated resistance against BNYVV in transgenic sugar beets is specific and is not suppressed after co-infection with a heterologous virus.

Recent trends and advances of RNA interference (RNAi) to improve agricultural crops and enhance their resilience to biotic and abiotic stresses

Over the last two decades, significant advances have been made using genetic engineering technology to modify genes from various exotic origins and introduce them into plants to induce favorable traits. RNA interference (RNAi) was discovered earlier as a natural process for controlling the expression of genes across all higher species. It aims to enhance precision and accuracy in pest/pathogen resistance, quality improvement, and manipulating the architecture of plants. However, it existed as a widely used technique recently. RNAi technologies could well be used to down-regulate any genes' expression without disrupting the expression of other genes. The use of RNA interference to silence genes in various organisms has become the preferred method for studying gene functions. The establishment of new approaches and applications for enhancing desirable characters is essential in crops by gene suppression and the refinement of knowledge of endogenous RNAi mechanisms in plants. RNAi technology in recent years has become an important and choicest method for controlling insects, pests, pathogens, and abiotic stresses like drought, salinity, and temperature. Although there are certain drawbacks in efficiency of this technology such as gene candidate selection, stability of trigger molecule, choice of target species and crops. Nevertheless, from past decade several target genes has been identified in numerous crops for their improvement towards biotic and abiotic stresses. The current review is aimed to emphasize the research done on crops under biotic and abiotic stress using RNAi technology. The review also highlights the gene regulatory pathways/gene silencing, RNA interference, RNAi knockdown, RNAi induced biotic and abiotic resistance and advancements in the understanding of RNAi technology and the functionality of various components of the RNAi machinery in crops for their improvement.

Coat protein of Chinese wheat mosaic virus upregulates and interacts with cytosolic glyceraldehyde-3-phosphate dehydrogenase, a negative regulator of plant autophagy, to promote virus infection

Autophagy is an intracellular degradation mechanism involved in antiviral defense, but the strategies employed by plant viruses to counteract autophagy-related defense remain unknown for the majority of the viruses. Herein, we describe how the Chinese wheat mosaic virus (CWMV, genus Furovirus) interferes with autophagy and enhances its infection in Nicotiana benthamiana. Yeast two-hybrid screening and in vivo/in vitro assays revealed that the 19 kDa coat protein (CP19K) of CWMV interacts with cytosolic glyceraldehyde-3-phosphate dehydrogenases (GAPCs), negative regulators of autophagy, which bind autophagy-related protein 3 (ATG3), a key factor in autophagy. CP19K also directly interacts with ATG3, possibly leading to the formation of a CP19K-GAPC-ATG3 complex. CP19K-GAPC interaction appeared to intensify CP19K-ATG3 binding. Moreover, CP19K expression upregulated GAPC gene transcripts and reduced autophagic activities. Accordingly, the silencing of GAPC genes in transgenic N. benthamiana reduced CWMV accumulation, whereas CP19K overexpression enhanced it. Overall, our results suggest that CWMV CP19K interferes with autophagy through the promotion and utilization of the GAPC role as a negative regulator of autophagy.

RNA Interference and CRISPR/Cas Gene Editing for Crop Improvement: Paradigm Shift towards Sustainable Agriculture

With the rapid population growth, there is an urgent need for innovative crop improvement approaches to meet the increasing demand for food. Classical crop improvement approaches involve, however, a backbreaking process that cannot equipoise with increasing crop demand. RNA-based approaches i.e., RNAi-mediated gene regulation and the site-specific nuclease-based CRISPR/Cas9 system for gene editing has made advances in the efficient targeted modification in many crops for the higher yield and resistance to diseases and different stresses. In functional genomics, RNA interference (RNAi) is a propitious gene regulatory approach that plays a significant role in crop improvement by permitting the downregulation of gene expression by small molecules of interfering RNA without affecting the expression of other genes. Gene editing technologies viz. the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein (CRISPR/Cas) have appeared prominently as a powerful tool for precise targeted modification of nearly all crops' genome sequences to generate variation and accelerate breeding efforts. In this regard, the review highlights the diverse roles and applications of RNAi and CRISPR/Cas9 system as powerful technologies to improve agronomically important plants to enhance crop yields and increase tolerance to environmental stress (biotic or abiotic). Ultimately, these technologies can prove to be important in view of global food security and sustainable agriculture.

RNA silencing of South African cassava mosaic virus in transgenic cassava expressing AC1/AC4 hp- RNA induces tolerance

Cassava mosaic disease (CMD), caused by geminiviruses, is a major hurdle to cassava production. Due to the heterozygous nature of cassava, breeding for virus resistance is difficult, but cassava has been shown to be a good candidate for genetic engineering using RNA interference (RNAi). T This study reports on the ability of a transgene-derived RNA hairpin, homologous to an overlapping region of the SACMV replication associated protein and putative virus suppressor of silencing protein (AC1/AC4), to confer tolerance in the CMD-susceptible model cassava cultivar 60444. Three of the fourteen transgenic lines expressing SACMV AC1/AC4 hairpin-derived siRNAs showed decreased symptoms and viral loads compared to untransformed control plants. Expression of SACMV AC1/AC4 homologous siRNAs showed that this tolerance is most likely associated with post-transcriptional gene silencing of the virus. This is the first report of targeting the overlapping AC1 and AC4 genes of SACMV conferring CMD tolerance in cassava.

Development of Beet necrotic yellow vein virus-based vectors for multiple-gene expression and guide RNA delivery in plant genome editing

Many plant viruses with monopartite or bipartite genomes have been developed as efficient expression vectors of foreign recombinant proteins. Nonetheless, due to lack of multiple insertion sites in these plant viruses, it is still a big challenge to simultaneously express multiple foreign proteins in single cells. The genome of Beet necrotic yellow vein virus (BNYVV) offers an attractive system for expression of multiple foreign proteins owning to a multipartite genome composed of five positive-stranded RNAs. Here, we have established a BNYVV full-length infectious cDNA clone under the control of the Cauliflower mosaic virus 35S promoter. We further developed a set of BNYVV-based vectors that permit efficient expression of four recombinant proteins, including some large proteins with lengths up to 880 amino acids in the model plant Nicotiana benthamiana and native host sugar beet plants. These vectors can be used to investigate the subcellular co-localization of multiple proteins in leaf, root and stem tissues of systemically infected plants. Moreover, the BNYVV-based vectors were used to deliver NbPDS guide RNAs for genome editing in transgenic plants expressing Cas9, which induced a photobleached phenotype in systemically infected leaves. Collectively, the BNYVV-based vectors will facilitate genomic research and expression of multiple proteins, in sugar beet and related crop plants.

Symptomatic plant viroid infections in phytopathogenic fungi

Viroids are pathogenic agents that have a small, circular noncoding RNA genome. They have been found only in plant species; therefore, their infectivity and pathogenicity in other organisms remain largely unexplored. In this study, we investigate whether plant viroids can replicate and induce symptoms in filamentous fungi. Seven plant viroids representing viroid groups that replicate in either the nucleus or chloroplast of plant cells were inoculated to three plant pathogenic fungi, Cryphonectria parasitica, Valsa mali, and Fusarium graminearum By transfection of fungal spheroplasts with viroid RNA transcripts, each of the three, hop stunt viroid (HSVd), iresine 1 viroid, and avocado sunblotch viroid, can stably replicate in at least one of those fungi. The viroids are horizontally transmitted through hyphal anastomosis and vertically through conidia. HSVd infection severely debilitates the growth of V. mali but not that of the other two fungi, while in F. graminearum and C. parasitica, with deletion of dicer-like genes, the primary components of the RNA-silencing pathway, HSVd accumulation increases. We further demonstrate that HSVd can be bidirectionally transferred between F. graminearum and plants during infection. The viroids also efficiently infect fungi and induce disease symptoms when the viroid RNAs are exogenously applied to the fungal mycelia. These findings enhance our understanding of viroid replication, host range, and pathogenicity, and of their potential spread to other organisms in nature.

Identification of a Novel Hypovirulence-Inducing Hypovirus From Alternaria alternata

Mycoviruses are wide spread throughout almost all groups of fungi but only a small number of mycoviruses can attenuate the growth and virulence of their fungal hosts. Alternaria alternata is an ascomycete fungus that causes leaf spot diseases on various crop plants. In this study, we identified a novel ssRNA mycovirus infecting an A. alternata f. sp. mali strain isolated from an apple orchard in China. Sequence analyses revealed that this virus is related to hypoviruses, in particular to Wuhan insect virus 14, an unclassified hypovirus identified from insect meta-transcriptomics, as well as other hypoviruses belonging to the genus Hypovirus, and therefore this virus is designed as Alternaria alternata hypovirus 1 (AaHV1). The genome of AaHV1 contains a single large open-reading frame encoding a putative polyprotein (∼479 kDa) with a cysteine proteinase-like and replication-associated domains. Curing AaHV1 from the fungal host strain indicated that the virus is responsible for the slow growth and reduced virulence of the host. AaHV1 defective RNA (D-RNA) with internal deletions emerging during fungal subcultures but the presence of D-RNA does not affect AaHV1 accumulation and pathogenicities. Moreover, AaHV1 could replicate and confer hypovirulence in Botryosphaeria dothidea, a fungal pathogen of apple white rot disease. This finding could facilitate better understanding of A. alternata pathogenicity and is relevant for development of biocontrol methods of fungal diseases.

Barley stripe mosaic virus infection requires PKA-mediated phosphorylation of γb for suppression of both RNA silencing and the host cell death response

The Barley stripe mosaic virus (BSMV) γb protein is a viral suppressor of RNA silencing (VSR) and symptom determinant. However, it is unclear how post-translational modification affects the different functions of γb. Here, we demonstrate that γb is phosphorylated at Ser-96 by a PKA-like kinase in vivo and in vitro. Mutant viruses containing a nonphosphorylatable substitution (BSMVS96A or BSMVS96R ) exhibited reduced viral accumulation in Nicotiana benthamiana due to transient induction of the cell death response that constrained the virus to necrotic areas. By contrast, a BSMVS96D mutant virus that mimics γb phosphorylation spread similarly to the wild-type virus. Furthermore, the S96A mutant had reduced local and systemic γb VSR activity due to having compromised its binding activity to 21-bp dsRNA. However, overexpression of other VSRs in trans or in cis failed to rescue the necrosis induced by BSMVS96A , demonstrating that suppression of cell death by γb phosphorylation is functionally distinct from its RNA silencing suppressor activities. These results provide new insights into the function of γb phosphorylation in regulating RNA silencing and the BSMV-induced host cell death response, and contribute to our understanding of how the virus optimizes the balance between viral replication and virus survival in the host plants during virus infection.

The resistance of sour orange to Citrus tristeza virus is mediated by both the salicylic acid and RNA silencing defence pathways

Citrus tristeza virus (CTV) induces in the field the decline and death of citrus varieties grafted on sour orange (SO) rootstock, which has forced the use of alternative decline-tolerant rootstocks in affected countries, despite the highly desirable agronomic features of the SO rootstock. Declining citrus plants display phloem necrosis below the bud union. In addition, SO is minimally susceptible to CTV compared with other citrus varieties, suggesting partial resistance of SO to CTV. Here, by silencing different citrus genes with a Citrus leaf blotch virus-based vector, we have examined the implication of the RNA silencing and salicylic acid (SA) defence pathways in the resistance of SO to CTV. Silencing of the genes RDR1, NPR1 and DCL2/DCL4, associated with these defence pathways, enhanced virus spread and accumulation in SO plants in comparison with non-silenced controls, whereas silencing of the genes NPR3/NPR4, associated with the hypersensitive response, produced a slight decrease in CTV accumulation and reduced stunting of SO grafted on CTV-infected rough lemon plants. We also found that the CTV RNA silencing suppressors p20 and p23 also suppress the SA signalling defence, with the suppressor activity being higher in the most virulent isolates.

Differential Characteristics of Viral siRNAs between Leaves and Roots of Wheat Plants Naturally Infected with Wheat Yellow Mosaic Virus, a Soil-Borne Virus

RNA silencing is an important innate antiviral defense in plants. Soil-borne plant viruses naturally infect roots via soil-inhabiting vectors, but it is unclear how antiviral RNA silencing responds to virus infection in this particular tissue. In this study, viral small interfering RNA (siRNA) profiles from leaves and roots of wheat plants naturally infected with a soil-borne virus, wheat yellow mosaic virus (WYMV, genus Bymovirus), were analyzed by deep sequencing. WYMV siRNAs were much more abundant in roots than leaves, which was positively correlated with the accumulation of viral RNA. WYMV siRNAs in leaves and roots were predominantly 21- and 22-nt long and equally derived from the positive- and negative-strands of the viral genome. WYMV siRNAs from leaves and roots differed in distribution pattern along the viral genome. Interestingly, compared to siRNAs from leaves (and most other reports), those from roots obviously had a lower A/U bias at the 5'-terminal nucleotide. Moreover, the expression of Dicer-like genes upon WYMV infection were differently regulated between leaves and roots. Our data suggest that RNA silencing in roots may operate differently than in leaves against soil-borne virus invasion.

Isolate fitness and tissue-tropism determine superinfection success

The mechanism of cross-protection, the deliberate infection of plants with a "mild" virus isolate to protect against "severe" isolates, has long been a topic of debate. In our model system, Citrus tristeza virus (CTV), this appears to be genotype-specific superinfection-exclusion, suggesting a simple recipe for cross-protection. However, this concept failed in field trials, which led us to examine the process of superinfection-exclusion more closely. We found that exclusion relies on the relative fitness of the primary versus the challenge isolates, and the host infected, and that significant differences in superinfection success could occur between isolates that differ by as few as 3 nucleotides. Furthermore, we found that exclusion was not uniform throughout the plant, but was tissue-specific. These data suggest that cross-protection is not a simple like-for-like process but a complex interaction between the primary and challenge isolates and the host.

SAGA complex mediates the transcriptional up-regulation of antiviral RNA silencing

Pathogen recognition and transcriptional activation of defense-related genes are crucial steps in cellular defense responses. RNA silencing (RNAi) functions as an antiviral defense in eukaryotic organisms. Several RNAi-related genes are known to be transcriptionally up-regulated upon virus infection in some host organisms, but little is known about their induction mechanism. A phytopathogenic ascomycete, Cryphonectria parasitica (chestnut blight fungus), provides a particularly advantageous system to study RNAi activation, because its infection by certain RNA viruses induces the transcription of dicer-like 2 (dcl2) and argonaute-like 2 (agl2), two major RNAi players. To identify cellular factors governing activation of antiviral RNAi in C. parasitica, we developed a screening protocol entailing multiple transformations of the fungus with cDNA of a hypovirus mutant lacking the RNAi suppressor (CHV1-Δp69), a reporter construct with a GFP gene driven by the dcl2 promoter, and a random mutagenic construct. Screening for GFP-negative colonies allowed the identification of sgf73, a component of the SAGA (Spt-Ada-Gcn5 acetyltransferase) complex, a well-known transcriptional coactivator. Knockout of other SAGA components showed that the histone acetyltransferase module regulates transcriptional induction of dcl2 and agl2, whereas histone deubiquitinase mediates regulation of agl2 but not dcl2 Interestingly, full-scale induction of agl2 and dcl2 by CHV1-Δp69 required both DCL2 and AGL2, whereas that by another RNA virus, mycoreovirus 1, required only DCL2, uncovering additional roles for DCL2 and AGL2 in viral recognition and/or RNAi activation. Overall, these results provide insight into the mechanism of RNAi activation.

Comparison of Small RNA Profiles of Glycine max and Glycine soja at Early Developmental Stages

Small RNAs, including microRNAs (miRNAs) and phased small interfering RNAs (phasiRNAs; from PHAS loci), play key roles in plant development. Cultivated soybean, Glycine max, contributes a great deal to food production, but, compared to its wild kin, Glycine soja, it may lose some genetic information during domestication. In this work, we analyzed the sRNA profiles of different tissues in both cultivated (C08) and wild soybeans (W05) at three stages of development. A total of 443 known miRNAs and 15 novel miRNAs showed varying abundances between different samples, but the miRNA profiles were generally similar in both accessions. Based on a sliding window analysis workflow that we developed, 50 PHAS loci generating 55 21-nucleotide phasiRNAs were identified in C08, and 46 phasiRNAs from 41 PHAS loci were identified in W05. In germinated seedlings, phasiRNAs were more abundant in C08 than in W05. Disease resistant TIR-NB-LRR genes constitute a very large family of PHAS loci. PhasiRNAs were also generated from several loci that encode for NAC transcription factors, Dicer-like 2 (DCL2), Pentatricopeptide Repeat (PPR), and Auxin Signaling F-box 3 (AFB3) proteins. To investigate the possible involvement of miRNAs in initiating the PHAS-phasiRNA pathway, miRNA target predictions were performed and 17 C08 miRNAs and 15 W05 miRNAs were predicted to trigger phasiRNAs biogenesis. In summary, we provide a comprehensive description of the sRNA profiles of wild versus cultivated soybeans, and discuss the possible roles of sRNAs during soybean germination.

Different Virus-Derived siRNAs Profiles between Leaves and Fruits in Cucumber Green Mottle Mosaic Virus-Infected Lagenaria siceraria Plants

RNA silencing is an evolutionarily conserved antiviral mechanism, through which virus-derived small interfering RNAs (vsiRNAs) playing roles in host antiviral defense are produced in virus-infected plant. Deep sequencing technology has revolutionized the study on the interaction between virus and plant host through the analysis of vsiRNAs profile. However, comparison of vsiRNA profiles in different tissues from a same host plant has been rarely reported. In this study, the profiles of vsiRNAs from leaves and fruits of Lagenaria siceraria plants infected with Cucumber green mottle mosaic virus (CGMMV) were comprehensively characterized and compared. Many more vsiRNAs were present in infected leaves than in fruits. vsiRNAs from both leaves and fruits were mostly 21- and 22-nt in size as previously described in other virus-infected plants. Interestingly, vsiRNAs were predominantly produced from the viral positive strand RNAs in infected leaves, whereas in infected fruits they were derived equally from the positive and negative strands. Many leaf-specific positive vsiRNAs with lengths of 21-nt (2058) or 22-nt (3996) were identified but only six (21-nt) and one (22-nt) positive vsiRNAs were found to be specific to fruits. vsiRNAs hotspots were only present in the 5'-terminal and 3'-terminal of viral positive strand in fruits, while multiple hotspots were identified in leaves. Differences in GC content and 5'-terminal nucleotide of vsiRNAs were also observed in the two organs. To our knowledge, this provides the first high-resolution comparison of vsiRNA profiles between different tissues of the same host plant.

Interplays between Soil-Borne Plant Viruses and RNA Silencing-Mediated Antiviral Defense in Roots

Although the majority of plant viruses are transmitted by arthropod vectors and invade the host plants through the aerial parts, there is a considerable number of plant viruses that infect roots via soil-inhabiting vectors such as plasmodiophorids, chytrids, and nematodes. These soil-borne viruses belong to diverse families, and many of them cause serious diseases in major crop plants. Thus, roots are important organs for the life cycle of many viruses. Compared to shoots, roots have a distinct metabolism and particular physiological characteristics due to the differences in development, cell composition, gene expression patterns, and surrounding environmental conditions. RNA silencing is an important innate defense mechanism to combat virus infection in plants, but the specific information on the activities and molecular mechanism of RNA silencing-mediated viral defense in root tissue is still limited. In this review, we summarize and discuss the current knowledge regarding RNA silencing aspects of the interactions between soil-borne viruses and host plants. Overall, research evidence suggests that soil-borne viruses have evolved to adapt to the distinct mechanism of antiviral RNA silencing in roots.

Ins and Outs of Multipartite Positive-Strand RNA Plant Viruses: Packaging versus Systemic Spread

Viruses possessing a non-segmented genome require a specific recognition of their nucleic acid to ensure its protection in a capsid. A similar feature exists for viruses having a segmented genome, usually consisting of viral genomic segments joined together into one viral entity. While this appears as a rule for animal viruses, the majority of segmented plant viruses package their genomic segments individually. To ensure a productive infection, all viral particles and thereby all segments have to be present in the same cell. Progression of the virus within the plant requires as well a concerted genome preservation to avoid loss of function. In this review, we will discuss the "life aspects" of chosen phytoviruses and argue for the existence of RNA-RNA interactions that drive the preservation of viral genome integrity while the virus progresses in the plant.

Transgenic resistance

Transgenic resistance to plant viruses is an important technology for control of plant virus infection, which has been demonstrated for many model systems, as well as for the most important plant viruses, in terms of the costs of crop losses to disease, and also for many other plant viruses infecting various fruits and vegetables. Different approaches have been used over the last 28 years to confer resistance, to ascertain whether particular genes or RNAs are more efficient at generating resistance, and to take advantage of advances in the biology of RNA interference to generate more efficient and environmentally safer, novel "resistance genes." The approaches used have been based on expression of various viral proteins (mostly capsid protein but also replicase proteins, movement proteins, and to a much lesser extent, other viral proteins), RNAs [sense RNAs (translatable or not), antisense RNAs, satellite RNAs, defective-interfering RNAs, hairpin RNAs, and artificial microRNAs], nonviral genes (nucleases, antiviral inhibitors, and plantibodies), and host-derived resistance genes (dominant resistance genes and recessive resistance genes), and various factors involved in host defense responses. This review examines the above range of approaches used, the viruses that were tested, and the host species that have been examined for resistance, in many cases describing differences in results that were obtained for various systems developed in the last 20 years. We hope this compilation of experiences will aid those who are seeking to use this technology to provide resistance in yet other crops, where nature has not provided such.

Small RNAs in plants: recent development and application for crop improvement

The phenomenon of RNA interference (RNAi) which involves sequence-specific gene regulation by small non-coding RNAs, i.e., small interfering RNA (siRNA) and microRNA (miRNA) has emerged as one of most powerful approaches for crop improvement. RNAi based on siRNA is one of the widely used tools of reverse genetics which aid in revealing gene functions in many species. This technology has been extensively applied to alter the gene expression in plants with an aim to achieve desirable traits. RNAi has been used for enhancing the crop yield and productivity by manipulating the gene involved in biomass, grain yield and enhanced shelf life of fruits and vegetables. It has also been applied for developing resistance against various biotic (bacteria, fungi, viruses, nematodes, insects) and abiotic stresses (drought, salinity, cold, etc.). Nutritional improvements of crops have also been achieved by enriching the crops with essential amino acids, fatty acids, antioxidants and other nutrients beneficial for human health or by reducing allergens or anti-nutrients. microRNAs are key regulators of important plant processes like growth, development, and response to various stresses. In spite of similarity in size (20-24 nt), miRNA differ from siRNA in precursor structures, pathway of biogenesis, and modes of action. This review also highlights the miRNA based genetic modification technology where various miRNAs/artificial miRNAs and their targets can be utilized for improving several desirable plant traits. microRNA based strategies are much efficient than siRNA-based RNAi strategies due to its specificity and less undesirable off target effects. As per the FDA guidelines, small RNA (sRNA) based transgenics are much safer for consumption than those over-expressing proteins. This review thereby summarizes the emerging advances and achievement in the field of sRNAs and its application for crop improvement.

Differential contributions of plant Dicer-like proteins to antiviral defences against potato virus X in leaves and roots

Members of the plant Dicer-like (DCL) protein family are the critical components of the RNA-silencing pathway that mediates innate antiviral defence. The distinct antiviral role of each individual DCL protein has been established with mostly based on observations of aerial parts of plants. Thus, although the roots are closely associated with the life cycle of many plant viruses, little is known about the antiviral activities of DCL proteins in roots. We observed that antiviral silencing strongly inhibits potato virus X (PVX) replication in roots of some susceptible Solanaceae species. Silencing of the DCL4 homolog in Nicotiana benthamiana partially elevated PVX replication levels in roots. In Arabidopsis thaliana, which was originally considered a non-host plant of PVX, high levels of PVX accumulation in inoculated leaves were achieved by inactivation of DCL4, while in the upper leaves and roots, it required the additional inactivation of DCL2. In transgenic A. thaliana carrying the PVX amplicon with a green fluorescent protein (GFP) gene insertion in the chromosome (AMP243 line), absence of DCL4 enabled high levels of PVX-GFP accumulation in various aerial organs but not in the roots, suggesting that DCL4 is critical for intracellular antiviral silencing in shoots but not in roots, where it can be functionally compensated by other DCL proteins. Together, the high level of functional redundancies among DCL proteins may contribute to the potent antiviral activities against PVX replication in roots.

Comparative analysis among the small RNA populations of source, sink and conductive tissues in two different plant-virus pathosystems

Background

In plants, RNA silencing plays a fundamental role as defence mechanism against viruses. During last years deep-sequencing technology has allowed to analyze the sRNA profile of a large variety of virus-infected tissues. Nevertheless, the majority of these studies have been restricted to a unique tissue and no comparative analysis between phloem and source/sink tissues has been conducted. In the present work, we compared the sRNA populations of source, sink and conductive (phloem) tissues in two different plant virus pathosystems. We chose two cucurbit species infected with two viruses very different in genome organization and replication strategy; Melon necrotic spot virus (MNSV) and Prunus necrotic ringspot virus (PNRSV).

Results

Our findings showed, in both systems, an increase of the 21-nt total sRNAs together with a decrease of those with a size of 24-nt in all the infected tissues, except for the phloem where the ratio of 21/24-nt sRNA species remained constant. Comparing the vsRNAs, both PNRSV- and MNSV-infected plants share the same vsRNA size distribution in all the analyzed tissues. Similar accumulation levels of sense and antisense vsRNAs were observed in both systems except for roots that showed a prevalence of (+) vsRNAs in both pathosystems. Additionally, the presence of overrepresented discrete sites along the viral genome, hot spots, were identified and validated by stem-loop RT-PCR. Despite that in PNRSV-infected plants the presence of vsRNAs was scarce both viruses modulated the host sRNA profile.

Conclusions

We compare for the first time the sRNA profile of four different tissues, including source, sink and conductive (phloem) tissues, in two plant-virus pathosystems. Our results indicate that antiviral silencing machinery in melon and cucumber acts mainly through DCL4. Upon infection, the total sRNA pattern in phloem remains unchanged in contrast to the rest of the analyzed tissues indicating a certain tissue-tropism to this polulation. Independently of the accumulation level of the vsRNAs both viruses were able to modulate the host sRNA pattern.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1327-5) contains supplementary material, which is available to authorized users.

Composite potato plants with transgenic roots on non-transgenic shoots: a model system for studying gene silencing in roots

Composite potato plants offer an extremely fast, effective and reliable system for studies on gene functions in roots using antisense or inverted-repeat but not sense constructs for gene inactivation. Composite plants, with transgenic roots on a non-transgenic shoot, can be obtained by shoot explant transformation with Agrobacterium rhizogenes. The aim of this study was to generate composite potato plants (Solanum tuberosum) to be used as a model system in future studies on root-pathogen interactions and gene silencing in the roots. The proportion of transgenic roots among the roots induced was high (80-100%) in the four potato cultivars tested (Albatros, Desirée, Sabina and Saturna). No wild-type adventitious roots were formed at mock inoculation site. All strains of A. rhizogenes tested induced phenotypically normal roots which, however, showed a reduced response to cytokinin as compared with non-transgenic roots. Nevertheless, both types of roots were infected to a similar high rate with the zoospores of Spongospora subterranea, a soilborne potato pathogen. The transgenic roots of composite potato plants expressed significantly higher amounts of β-glucuronidase (GUS) than the roots of a GUS-transgenic potato line event. Silencing of the uidA transgene (GUS) was tested by inducing roots on the GUS-transgenic cv. Albatros event with strains of A. rhizogenes over-expressing either the uidA sense or antisense transcripts, or inverted-repeat or hairpin uidA RNA. The three last mentioned constructs caused 2.5-4.0 fold reduction in the uidA mRNA expression. In contrast, over-expression of uidA resulted in over 3-fold increase in the uidA mRNA and GUS expression, indicating that sense-mediated silencing (co-suppression) was not functional in roots. The results suggest that composite plants offer a useful experimental system for potato research, which has gained little previous attention.

Infection of Beet necrotic yellow vein virus with RNA4-encoded P31 specifically up-regulates pathogenesis-related protein 10 in Nicotiana benthamiana

BACKGROUND

Beet necrotic yellow vein virus (BNYVV) is the infectious agent of sugar beet rhizomania, which consists of four or five plus-sense RNAs. RNA4 of BNYVV is not essential for virus propagation in Nicotiana benthamiana but has a major effect on symptom expression. Early reports showed that RNA4-encoded P31 was associated with severe symptoms, such as curling and dwarfing, in N. benthamiana.

RESULTS

We discovered that the pathogenesis-related protein 10 (PR-10) gene can be up-regulated in BNYVV-infected N. benthamiana in the presence of RNA4 and that it had a close link with symptom development. Our frame-shift, deletion and substitution analysis showed that only the entire P31 could induce PR-10 up-regulation during BNYVV infection and that all the tryptophans and six cysteines (C174, C183, C186, C190, C197 and C199) in the cysteine-rich P31 had significant effects on PR-10 expression. However, P31 could not interact directly with PR-10 in yeast.

CONCLUSIONS

Our data demonstrated that only integrated P31 specifically induced PR-10 transcription, which coincided closely with the appearance of severe symptoms in BNYVV-infected N. benthamiana, although they could not interact directly with each other in yeast.

Durable field resistance to wheat yellow mosaic virus in transgenic wheat containing the antisense virus polymerase gene

Wheat yellow mosaic virus (WYMV) has spread rapidly and causes serious yield losses in the major wheat-growing areas in China. Because it is vectored by the fungus-like organism Polymyxa graminis that survives for long periods in soil, it is difficult to eliminate by conventional crop management or fungicides. There is also only limited resistance in commercial cultivars. In this research, fourteen independent transgenic events were obtained by co-transformation with the antisense NIb8 gene (the NIb replicase of WYMV) and a selectable gene bar. Four original transgenic lines (N12, N13, N14 and N15) and an offspring line (N12-1) showed high and durable resistance to WYMV in the field. Four resistant lines were shown to have segregated and only contain NIb8 (without bar) by PCR and herbicide resistance testing in the later generations. Line N12-1 showed broad-spectrum resistance to WYMV isolates from different sites in China. After growing in the infested soil, WYMV could not be detected by tissue printing and Western blot assays of transgenic wheat. The grain yield of transgenic wheat was about 10% greater than the wild-type susceptible control. Northern blot and small RNA deep sequencing analyses showed that there was no accumulation of small interfering RNAs targeting the NIb8 gene in transgenic wheat plants, suggesting that transgene RNA silencing, a common mechanism of virus-derived disease resistance, is not involved in the process of WYMV resistance. This durable and broad-spectrum resistance to WYMV in transgenic wheat will be useful for alleviating the damage caused by WYMV.

RNA interference: concept to reality in crop improvement

The phenomenon of RNA interference (RNAi) is involved in sequence-specific gene regulation driven by the introduction of dsRNA resulting in inhibition of translation or transcriptional repression. Since the discovery of RNAi and its regulatory potentials, it has become evident that RNAi has immense potential in opening a new vista for crop improvement. RNAi technology is precise, efficient, stable and better than antisense technology. It has been employed successfully to alter the gene expression in plants for better quality traits. The impact of RNAi to improve the crop plants has proved to be a novel approach in combating the biotic and abiotic stresses and the nutritional improvement in terms of bio-fortification and bio-elimination. It has been employed successfully to bring about modifications of several desired traits in different plants. These modifications include nutritional improvements, reduced content of food allergens and toxic compounds, enhanced defence against biotic and abiotic stresses, alteration in morphology, crafting male sterility, enhanced secondary metabolite synthesis and seedless plant varieties. However, crop plants developed by RNAi strategy may create biosafety risks. So, there is a need for risk assessment of GM crops in order to make RNAi a better tool to develop crops with biosafety measures. This article is an attempt to review the RNAi, its biochemistry, and the achievements attributed to the application of RNAi in crop improvement.

Deep sequencing-based transcriptome profiling reveals comprehensive insights into the responses of Nicotiana benthamiana to beet necrotic yellow vein virus infections containing or lacking RNA4

BACKGROUND

Beet necrotic yellow vein virus (BNYVV), encodes either four or five plus-sense single stranded RNAs and is the causal agent of sugar beet rhizomania disease, which is widely distributed in most regions of the world. BNYVV can also infect Nicotiana benthamiana systemically, and causes severe curling and stunting symptoms in the presence of RNA4 or mild symptoms in the absence of RNA4.

RESULTS

Confocal laser scanning microscopy (CLSM) analyses showed that the RNA4-encoded p31 protein fused to the red fluorescent protein (RFP) accumulated mainly in the nuclei of N. benthamiana epidermal cells. This suggested that severe RNA4-induced symptoms might result from p31-dependent modifications of the transcriptome. Therefore, we used next-generation sequencing technologies to analyze the transcriptome profile of N. benthamiana in response to infection with different isolates of BNYVV. Comparisons of the transcriptomes of mock, BN3 (RNAs 1+2+3), and BN34 (RNAs 1+2+3+4) infected plants identified 3,016 differentially expressed transcripts, which provided a list of candidate genes that potentially are elicited in response to virus infection. Our data indicate that modifications in the expression of genes involved in RNA silencing, ubiquitin-proteasome pathway, cellulose synthesis, and metabolism of the plant hormone gibberellin may contribute to the severe symptoms induced by RNA4 from BNYVV.

CONCLUSIONS

These results expand our understanding of the genetic architecture of N. benthamiana as well as provide valuable clues to identify genes potentially involved in resistance to BNYVV infection. Our global survey of gene expression changes in infected plants reveals new insights into the complicated molecular mechanisms underlying symptom development, and aids research into new strategies to protect crops against viruses.

Root-specific role for Nicotiana benthamiana RDR6 in the inhibition of Chinese wheat mosaic virus accumulation at higher temperatures

Some viruses only infect plants at cool temperatures but the molecular mechanism underlying this low-temperature dependence remains unclear. Chinese wheat mosaic virus (CWMV, genus Furovirus) was able to infect wheat and Nicotiana benthamiana plants at 16 but not at 24°C. When CWMV-infected plants were transferred to 24°C for 2 weeks, the newly emerged leaves and roots became virus free. Co-infection with Potato virus Y rescued CWMV accumulation in N. benthamiana plants after a temperature shift to 24°C. In transgenic N. benthamiana plants silenced for the N. benthamiana RNA-dependent RNA polymerase 6 (NbRDR6), CWMV was able to accumulate in roots but not in leaves after a temperature shift to 24°C. Deep sequencing of small RNAs showed that, at 16°C, abundant CWMV small interfering (si)RNAs accumulated in infected N. benthamiana plants. Silencing of NbRDR6 increased the abundance of CWMV siRNAs and the generation of siRNAs from hotspots in the CWMV genome. In contrast, when shifted to 24°C for 1 week, CWMV siRNAs were markedly fewer in roots of NbRDR6-silenced than in roots of wild-type plants but were similar in the leaves of those plants. Our results demonstrate the root-specific role of NbRDR6 in the inhibition of CWMV accumulation and biogenesis of CWMV siRNAs at higher temperatures.

Characterization of rice black-streaked dwarf virus- and rice stripe virus-derived siRNAs in singly and doubly infected insect vector Laodelphax striatellus

Replication of RNA viruses in insect cells triggers an antiviral defense that is mediated by RNA interference (RNAi) which generates viral-derived small interfering RNAs (siRNAs). However, it is not known whether an antiviral RNAi response is also induced in insects by reoviruses, whose double-stranded RNA genome replication is thought to occur within core particles. Deep sequencing of small RNAs showed that when the small brown planthopper (Laodelphax striatellus) was infected by Rice black-streaked dwarf virus (RBSDV) (Reoviridae; Fijivirus), more viral-derived siRNAs accumulated than when the vector insect was infected by Rice stripe virus (RSV), a negative single-stranded RNA virus. RBSDV siRNAs were predominantly 21 and 22 nucleotides long and there were almost equal numbers of positive and negative sense. RBSDV siRNAs were frequently generated from hotspots in the 5′- and 3′-terminal regions of viral genome segments but these hotspots were not associated with any predicted RNA secondary structures. Under laboratory condition, L. striatellus can be infected simultaneously with RBSDV and RSV. Double infection enhanced the accumulation of particular genome segments but not viral coat protein of RBSDV and correlated with an increase in the abundance of siRNAs derived from RBSDV. The results of this study suggest that reovirus replication in its insect vector potentially induces an RNAi-mediated antiviral response.

Differential requirements for Tombusvirus coat protein and P19 in plants following leaf versus root inoculation

Traditional virus inoculation of plants involves mechanical rubbing of leaves, whereas in nature viruses like Tomato bushy stunt virus (TBSV) are often infected via the roots. A method was adapted to compare leaf versus root inoculation of Nicotiana benthamiana and tomato with transcripts of wild-type TBSV (wtTBSV), a capsid (Tcp) replacement construct expressing GFP (T-GFP), or mutants not expressing the silencing suppressor P19 (TBSVΔp19). In leaves, T-GFP remained restricted to the cells immediately adjacent to the site of inoculation, unless Tcp was expressed in trans from a Potato virus X vector; while T-GFP inoculation of roots gave green fluorescence in upper tissues in the absence of Tcp. Conversely, leaf inoculation with wtTBSV or TBSVΔp19 transcripts initiated systemic infections, while upon root inoculation this only occurred with wtTBSV, not with TBSVΔp19. Evidently the contribution of Tcp or P19 in establishing systemic infections depends on the point-of-entry of TBSV in the plants.

Identification of the amino acid residues and domains in the cysteine-rich protein of Chinese wheat mosaic virus that are important for RNA silencing suppression and subcellular localization

Cysteine-rich proteins (CRPs) encoded by some plant viruses in diverse genera function as RNA silencing suppressors. Within the N-terminal portion of CRPs encoded by furoviruses, there are six conserved cysteine residues and a Cys-Gly-X-X-His motif (Cys, cysteine; Gly, glycine; His, histidine; X, any amino acid residue) with unknown function. The central domains contain coiled-coil heptad amino acid repeats that usually mediate protein dimerization. Here, we present evidence that the conserved cysteine residues and Cys-Gly-X-X-His motif in the CRP of Chinese wheat mosaic virus (CWMV) are critical for protein stability and silencing suppression activity. Mutation of a leucine residue in the third coiled-coil heptad impaired CWMV CRP activity for suppression of local silencing, but not for the promotion of cell-to-cell movement of Potato virus X (PVX). In planta and in vitro analysis of wild-type and mutant proteins indicated that the ability of the CRP to self-interact was correlated with its suppression activity. Deletion of up to 40 amino acids at the C-terminus did not abolish suppression activity, but disrupted the association of CRP with endoplasmic reticulum (ER), and reduced its activity in the enhancement of PVX symptom severity. Interestingly, a short region in the C-terminal domain, predicted to form an amphipathic α-helical structure, was responsible for the association of CWMV CRP with ER. Overall, our results demonstrate that the N-terminal and central regions are the functional domains for suppression activity, whereas the C-terminal region primarily functions to target CWMV CRP to the ER.

The benyvirus RNA silencing suppressor is essential for long-distance movement, requires both zinc-finger and NoLS basic residues but not a nucleolar localization for its silencing-suppression activity

The RNA silencing-suppression properties of Beet necrotic yellow vein virus (BNYVV) and Beet soil-borne mosaic virus (BSBMV) cysteine-rich p14 proteins have been investigated. Suppression of RNA silencing activities were made evident using viral infection of silenced Nicotiana benthamiana 16C, N. benthamiana agroinfiltrated with green fluorescent protein (GFP), and GF-FG hairpin triggers supplemented with viral suppressor of RNA silencing (VSR) constructs or using complementation of a silencing-suppressor-defective BNYVV virus in Chenopodium quinoa. Northern blot analyses of small-interfering RNAs (siRNAs) in agroinfiltration tests revealed reduced amounts of siRNA, especially secondary siRNA, suggesting that benyvirus VSR act downstream of the siRNA production. Using confocal laser-scanning microscopy imaging of infected protoplasts expressing functional p14 protein fused to an enhanced GFP reporter, we showed that benyvirus p14 accumulated in the nucleolus and the cytoplasm independently of other viral factors. Site-directed mutagenesis showed the importance of the nucleolar localization signal embedded in a C4 zinc-finger domain in the VSR function and intrinsic stability of the p14 protein. Conversely, RNA silencing suppression appeared independent of the nucleolar localization of the protein, and a correlation between BNYVV VSR expression and long-distance movement was established.

High level resistance against rhizomania disease by simultaneously integrating two distinct defense mechanisms

With the aim of achieving durable resistance against rhizomania disease of sugar beet, the employment of different sources of resistance to Beet necrotic yellow vein virus was pursued. To this purpose, Nicotiana benthamiana transgenic plants that simultaneously produce dsRNA originating from a conserved region of the BNYVV replicase gene and the HrpZ(Psph) protein in a secreted form (SP/HrpZ(Psph)) were produced. The integration and expression of both transgenes as well as proper production of the harpin protein were verified in all primary transformants and selfed progeny (T1, T2). Transgenic resistance was assessed by BNYVV-challenge inoculation on T2 progeny by scoring disease symptoms and DAS-ELISA at 20 and 30 dpi. Transgenic lines possessing single transformation events for both transgenes as well as wild type plants were included in inoculation experiments. Transgenic plants were highly resistant to virus infection, whereas in some cases immunity was achieved. In all cases, the resistant phenotype of transgenic plants carrying both transgenes was superior in comparison with the ones carrying a single transgene. Collectively, our findings demonstrate, for a first time, that the combination of two entirely different resistance mechanisms provide high level resistance or even immunity against the virus. Such a novel approach is anticipated to prevent a rapid virus adaptation that could potentially lead to the emergence of isolates with resistance breaking properties.

The cysteine-rich proteins of beet necrotic yellow vein virus and tobacco rattle virus contribute to efficient suppression of silencing in roots

Many plant viruses encode proteins that suppress RNA silencing, but little is known about the activity of silencing suppressors in roots. This study examined differences in the silencing suppression activity of different viruses in leaves and roots of Nicotiana benthamiana plants. Infection by tobacco mosaic virus, potato virus Y and cucumber mosaic virus but not potato virus X (PVX) resulted in strong silencing suppression activity of a transgene in both leaves and roots, whereas infection by beet necrotic yellow vein virus (BNYVV) and tobacco rattle virus (TRV) showed transgene silencing suppression in roots but not in leaves. For most viruses tested, viral negative-strand RNA accumulated at a very low level in roots, compared with considerable levels of positive-strand genomic RNA. Co-inoculation of leaves with PVX and either BNYVV or TRV produced an increase in PVX negative-strand RNA and subgenomic RNA (sgRNA) accumulation in roots. The cysteine-rich proteins (CRPs) BNYVV p14 and TRV 16K showed weak silencing suppression activity in leaves. However, when either of these CRPs was expressed from a PVX vector, there was an enhancement of PVX negative-strand RNA and sgRNA accumulation in roots compared with PVX alone. Such enhancement of PVX sgRNAs was also observed by expression of CRPs of other viruses and the well-known suppressors HC-Pro and p19 but not of the potato mop-top virus p8 CRP. These results indicate that BNYVV- and TRV-encoded CRPs suppress RNA silencing more efficiently in roots than in leaves.

Suppression of NS3 and MP is important for the stable inheritance of RNAi-mediated rice stripe virus (RSV) resistance obtained by targeting the fully complementary RSV-CP gene

Rice stripe virus (RSV) is a viral disease that seriously impacts rice production in East Asia, most notably in Korea, China, and Japan. Highly RSV-resistant transgenic japonica rice plants were generated using a dsRNAi construct designed to silence the entire sequence region of the RSV-CP gene. Transgenic rice plants were inoculated with a population of viruliferous insects, small brown planthoppers (SBPH), and their resistance was evaluated using ELISA and an infection rate assay. A correlation between the expression of the RSV-CP homologous small RNAs and the RSV resistance of the transgenic rice lines was discovered. These plants were also analyzed by comparing the expression pattern of invading viral genes, small RNA production and the stable transmission of the RSV resistance trait to the T3 generation. Furthermore, the agronomic trait was stably transmitted to the T4 generation of transgenic plants.

Genetic Transformation of Passionflower and Evaluation of R1 and R2 Generations for Resistance to Cowpea aphid borne mosaic virus

We report on the production and evaluation of passionflower transgenic lines for resistance to Cowpea aphid borne mosaic virus (CABMV). Genetic transformation was done using Agrobacterium tumefaciens and transgene integration was confirmed by Southern blot analyses, resulting in nine transgenic lines for 'IAC 275' and three for 'IAC 277'. Transgenic lines were clonally propagated and evaluated for resistance to CABMV. After the third inoculation, under higher inoculum pressure, only propagated plants of the transgenic line T16 remained asymptomatic, indicating a high resistance to infection with CABMV. This transgenic line was self-pollinated and the R₁ generation was evaluated together with the R₁ generation of another resistant transgenic line (T2) identified previously. Plants were inoculated with CABMV by means of viruliferous Myzus nicotianae. All 524 T2R₁ plants became infected, whereas 13 of 279 T16R₁ remained asymptomatic after four successive inoculations. A T16R₂ generation was obtained and plants were inoculated with CABMV mechanically or by aphids. After successive inoculations, 118 of 258 plants were symptomless, suggesting that the resistance to CABMV was maintained in the plant genome as the homozygous condition was achieved. Five selected resistant T16R₂ plants which contained the capsid protein gene are being crossed for further analyses.

Detection and characterization of spontaneous internal deletion mutants of Beet Necrotic yellow vein virus RNA3 from systemic host Nicotiana benthamiana

BACKGROUND

Beet Necrotic Yellow Vein virus (BNYVV) is a member of the genus Benyvirus causing a worldwide sugar beet disease rhizomania. BNYVV contains four or five plus-sense single stranded RNAs. In altered selective conditions, multipartite RNA viruses of plant are prone to undergoing internal deletions, thus turning into Defective RNAs (D RNAs). Although several D RNAs have been reported in BNYVV infection, the spontaneous internal deletion mutants responsible for severe symptom in systemic host Nicotiana benthamiana (N. benthamiana) are not described so far.

RESULTS

Systemic host N. benthamiana was inoculated by Chinese BNYVV isolates. RT-PCR and Northern blot showed that the D RNAs forms of BNYVV RNA3 were present in the systemic infection of the N. benthamiana. Three distinct D-RNA3s, named as D-RNA 3α, D-RNA 3β and D-RNA 3γ, were made into infectious clones. When inoculated on the N. benthamiana, the in vitro transcripts of D forms exhibited more stable than that of wild-type RNA3 in systemic movement. Among the detected mutant, the p25 protein frame-shift mutant (D-RNA3α) induced obvious necrotic lesions on Tetragonia.expansa (T. expansa) and pronounced systemic symptom on the N. benthamiana. The D-RNA3α was further mutated artificially to pre-terminate the downstream N protein, leading to the abolishment of the pathogenicity, indicating the N protein was responsible for the necrotic symptom.

CONCLUSION

Our studies demonstrated the internal deletion mutants of BNYVV-RNA3 were spontaneously generated in the systemic infection on N. benthamiana. The internal deletions didn't affect the efficient replication of D-RNA3s, instead by improving the stability and pathogenicity of RNA3 in the systemic host N. benthamiana. Besides, our results also suggested the downstream N protein of RNA3, but not the upstream p25 protein, may play an important role in the systemic infection on N. benthamiana.

Role of RNA interference in plant improvement

Research to alter crops for their better performance involving modern technology is underway in numerous plants, and achievements in transgenic plants are impacting crop improvements in unparalleled ways. Striking progress has been made using genetic engineering technology over the past two decades in manipulating genes from diverse and exotic sources, and inserting them into crop plants for inducing desirable characteristics. RNA interference (RNAi) has recently been identified as a natural mechanism for regulation of gene expression in all higher organisms from plants to humans and promises greater accuracy and precision to plant improvement. The expression of any gene can be down-regulated in a highly explicit manner exclusive of affecting the expression of any other gene by using RNAi technologies. Additional research in this field has been focused on a number of other areas including microRNAs, hairpin RNA, and promoter methylation. Manipulating new RNAi pathways, which generate small RNA molecules to amend gene expression in crops, can produce new quality traits and having better potentiality of protection against abiotic and biotic stresses. Nutritional improvement, change in morphology, or enhanced secondary metabolite synthesis are some of the other advantages of RNAi technology. In addition to its roles in regulating gene expression, RNAi is also used as a natural defense mechanism against molecular parasites such as jumping genes and viral genetic elements that affect genome stability. Even though much advancement has been made on the field of RNAi over the preceding few years, the full prospective of RNAi for crop improvement remains to be fully realized. The intricacy of RNAi pathway, the molecular machineries, and how it relates to plant development are still to be explained.

The hrpZ gene of Pseudomonas syringae pv. phaseolicola enhances resistance to rhizomania disease in transgenic Nicotiana benthamiana and sugar beet

To explore possible sources of transgenic resistance to the rhizomania-causing Beet necrotic yellow vein virus (BNYVV), Nicotiana benthamiana plants were constructed to express the harpin of Pseudomonas syringae pv. phaseolicola (HrpZ(Psph)). The HrpZ protein was expressed as an N-terminal fusion to the PR1 signal peptide (SP/HrpZ) to direct harpin accumulation to the plant apoplast. Transgene integration was verified by mPCR in all primary transformants (T0), while immunoblot analysis confirmed that the protein HrpZ(Psph) was produced and the signal peptide was properly processed. Neither T0 plants nor selfed progeny (T1) showed macroscopically visible necrosis or any other macroscopic phenotypes. However, plants expressing the SP/HrpZ(Psph) showed increased vigor and grew faster in comparison with non-transgenic control plants. Transgenic resistance was assessed after challenge inoculation with BNYVV on T1 progeny by scoring of disease symptoms and by DAS-ELISA at 20 and 30 dpi. Transgenic and control lines showed significant differences in terms of the number of plants that became infected, the timing of infection and the disease symptoms displayed. Plants expressing the SP/HrpZ(Psph) developed localized leaf necrosis in the infection area and had enhanced resistance upon challenge with BNYVV. In order to evaluate the SP/HrpZ-based resistance in the sugar beet host, A. rhizogenes-mediated root transformation was exploited as a transgene expression platform. Upon BNYVV inoculation, transgenic sugar beet hairy roots showed high level of BNYVV resistance. In contrast, the aerial non-transgenic parts of the same seedlings had virus titers that were comparable to those of the seedlings that were untransformed or transformed with wild type R1000 cells. These findings indicate that the transgenically expressed SP/HrpZ protein results in enhanced rhizomania resistance both in a model plant and sugar beet, the natural host of BNYVV. Possible molecular mechanisms underlying the enhanced resistance and plant growth phenotypes observed in SP/HrpZ transgenic plants are discussed.

The evolutionary history of Beet necrotic yellow vein virus deduced from genetic variation, geographical origin and spread, and the breaking of host resistance

Beet necrotic yellow vein virus (BNYVV) is an economically important pathogen of sugar beet and has been found worldwide, probably as the result of recent worldwide spread. The BNYVV genome consists of four or five RNA components. Here, we report analysis of sequence variation in the RNA3-p25, RNA4-p31, RNA2-CP, and RNA5-p26 genes of 73 worldwide isolates. The RNA3-p25 gene encodes virulence and avirulence factors. These four sets of gene sequences each fell into two to four groups, of which the three groups of p25 formed eight subgroups with different geographical distributions. Each of these subgroup isolates (strains) could have arisen from four original BNYVV population and their mixed infections. The genetic diversity for BNYVV was relatively small. Selection pressure varied greatly depending on the BNYVV gene and geographical location. Isolates of the Italy strain, in which p25 was subject to the strongest positive selection, were able to overcome the Rz1-host resistance gene to differing degrees, whereas other geographically limited strains could not. Resistance-breaking variants were generated by p25 amino acid changes at positions 67 and 68. Our studies suggest that BNYVV originally evolved in East Asia and has recently become a pathogen of cultivated sugar beet followed by the emergence of new resistance-breaking variants.

Tobraviruses--plant pathogens and tools for biotechnology

The tobraviruses, Tobacco rattle virus (TRV), Pea early-browning virus (PEBV) and Pepper ringspot virus (PepRSV), are positive-strand RNA viruses with rod-shaped virus particles that are transmitted between plants by trichodorid nematodes. As a group, these viruses infect many plant species, with TRV having the widest host range. Recent studies have begun to dissect the interaction of TRV with potato, currently the most commercially important crop disease caused by any of the tobraviruses. As well as being successful plant pathogens, these viruses have become widely used as vectors for expression in plants of nonviral proteins or, more frequently, as initiators of virus-induced gene silencing (VIGS). Precisely why tobraviruses should be so effective as VIGS vectors is not known; however, molecular studies of the mode of action of the tobravirus silencing suppressor protein are shedding some light on this process.

BNYVV-derived dsRNA confers resistance to rhizomania disease of sugar beet as evidenced by a novel transgenic hairy root approach

Agrobacterium rhizogenes-transformed sugar beet hairy roots, expressing dsRNA from the Beet necrotic yellow vein virus replicase gene, were used as a novel approach to assess the efficacy of three intron-hairpin constructs at conferring resistance to rhizomania disease. Genetically engineered roots were similar in morphology to wild type roots but were characterized by a profound abundancy, rapid growth rate and, in some cases, plagiotropic development. Upon challenge inoculation, seedlings showed a considerable delay in symptom development compared to untransformed or vector-transformed seedlings, expressing dsRNA from an unrelated source. The transgenic root system of almost all seedlings contained no or very low virus titer while the non-transformed aerial parts of the same plants were found infected, leading to the conclusion that the hairy roots studied were effectively protected against the virus. This readily applicable novel method forms a plausible approach to preliminarily evaluate transgenic rhizomania resistance before proceeding in transformation and whole plant regeneration of sugar beet, a tedious and time consuming process for such a recalcitrant crop species.

Gene silencing induced by hairpin or inverted repeated sense transgenes varies among promoters and cell types

*In transgenic calli and different tissues of Arabidopsis thaliana plants, the in trans silencing capacity of a 35S-beta-glucuronidase (GUS) hairpin RNA construct was investigated on a target GUS gene, under the control of the 35S, a WRKY or several cell cycle-specific promoters. *GUS histochemical staining patterns were analyzed in all tissues of the parental lines and supertransformants harboring the hairpin construct. Quantitative GUS activity measurements determined GUS suppression by a 35S-GUS hairpin or inverted repeated GUS transgenes in leaves and calli. *In some supertransformants, GUS-based staining disappeared in all tissues, including calli. In most supertransformants, however, a significant reduction was found in mature roots and leaves, but residual GUS activity was observed in the root tips, young leaves and calli. In leaves of most hairpin RNA supertransformants, the GUS activity was reduced by c. 1000-fold or more, but, in derived calli, generally by less than 200-fold. The silencing efficiency of inverted repeated sense transgenes was similar to that of a hairpin RNA construct in leaves, but weaker in calli. *These results imply that the tissue type, nature of the silencing inducer locus and the differential expression of the targeted gene codetermine the silencing efficiency.

A transformation booster sequence (TBS) from Petunia hybrida functions as an enhancer-blocking insulator in Arabidopsis thaliana

Several matrix-attachment regions (MARs) from animals have been shown to block interactions between an enhancer and promoter when situated between the two. Since a similar function for plant MARs has not been discerned, we tested the Zea mays ADH1 5' MAR, Nicotiana tabacum Rb7 3' MAR and a transformation booster sequence (TBS) MAR from Petunia hybrida for their ability to impede enhancer-promoter interactions in Arabidopsis thaliana. Stable transgenic lines containing vectors in which one of the three MAR elements or a 4 kb control sequence were interposed between the cauliflower mosaic virus 35S enhancer and a flower-specific AGAMOUS second intron-derived promoter (AGIP)::beta-glucuronidase (GUS) fusion were assayed for GUS expression in vegetative tissues. We demonstrate that the TBS MAR element, but not the ADH1 or Rb7 MARs, is able to block interactions between the 35S enhancer and AGIP without compromising the function of either with elements from which they are not insulated.

A simple and sensitive high-throughput GFP screening in woody and herbaceous plants

Green fluorescent protein (GFP) has been used widely as a powerful bioluminescent reporter, but its visualization by existing methods in tissues or whole plants and its utilization for high-throughput screening remains challenging in many species. Here, we report a fluorescence image analyzer-based method for GFP detection and its utility for high-throughput screening of transformed plants. Of three detection methods tested, the Typhoon fluorescence scanner was able to detect GFP fluorescence in all Arabidopsis thaliana tissues and apple leaves, while regular fluorescence microscopy detected it only in Arabidopsis flowers and siliques but barely in the leaves of either Arabidopsis or apple. The hand-held UV illumination method failed in all tissues of both species. Additionally, the Typhoon imager was able to detect GFP fluorescence in both green and non-green tissues of Arabidopsis seedlings as well as in imbibed seeds, qualifying it as a high-throughput screening tool, which was further demonstrated by screening the seedlings of primary transformed T(0) seeds. Of the 30,000 germinating Arabidopsis seedlings screened, at least 69 GFP-positive lines were identified, accounting for an approximately 0.23% transformation efficiency. About 14,000 seedlings grown in 16 Petri plates could be screened within an hour, making the screening process significantly more efficient and robust than any other existing high-throughput screening method for transgenic plants.

Progress towards the understanding and control of sugar beet rhizomania disease

Rhizomania is a soil-borne disease that occurs throughout the major sugar beet growing regions of the world, causing severe yield losses in the absence of effective control measures. It is caused by Beet necrotic yellow vein virus (BNYVV), which is transmitted by the obligate root-infecting parasite Polymyxa betae. BNYVV has a multipartite RNA genome with all natural isolates containing four RNA species, although some isolates have a fifth RNA. The larger RNA1 and RNA2 contain the housekeeping genes of the virus and are always required for infection, whereas the smaller RNAs are involved in pathogenicity and vector transmission. RNA5-containing isolates are restricted to Asia and some parts of Europe, and these isolates tend to be more aggressive. With no acceptable pesticides available to restrict the vector, the control of rhizomania is now achieved almost exclusively through the use of resistant cultivars. A single dominant resistance gene, Rz1, has been used to manage the disease worldwide in recent years, although this gene confers only partial resistance. More recently, new variants of BNYVV have evolved (both with and without RNA5) that are able to cause significant yield penalties on resistant cultivars. These isolates are not yet widespread, but their appearance has resulted in accelerated searches for new sources of resistance to both the virus and the vector. Combined virus and vector resistance, achieved either by conventional or transgenic breeding, offers the sugar beet industry a new approach in its continuing struggle against rhizomania.