Molecular mechanism of RNA silencing suppression mediated by p19 protein of tombusviruses

A tomato mottle mosaic virus-based vector system for gene function studies in tomato

Plant virus-based vectors provide a convenient tool for rapid protein expression and gene function studies. However, there is little research on viral vectors capable of systemically expressing proteins in tomatoes. Here, we substituted the coat protein (CP) gene with GFP gene in a previously constructed tomato mottle mosaic virus (ToMMV; genus Tobamovirus) infectious clone pCBToMMV to produce transiently expressing vectors, which could express GFP in the infiltrated leaves of Nicotiana benthamiana plants. We then inserted the sub-genomic promoter and CP gene from another tomamovirus tomato mosaic virus downstream of GFP gene to form a vector capable of systemically expressing GFP in both N. benthamiana and Solanum lycopersicum plants. The ToMMV-based vector also expressed the MYB-related transcription factor Rosea1, inducing anthocyanin biosynthesis in the systemic leaves of both N. benthamiana and S. lycopersicum plants. Additionally, expressing the bialaphos resistance gene using the ToMMV vector conferred resistance to the herbicide glufosinate-ammonium in plants. Furthermore, the ToMMV vector successfully expressed a 68 kDa β-glucuronidase in systemic leaves and roots of N. benthamiana and S. lycopersicum plants. This ToMMV-based vector provides a simple and efficient tool for gene function studies in tomatoes.

Shared and unique mechanisms of RNAi-mediated antiviral immunity in C. elegans

Small interfering RNAs (siRNAs), generated by Dicer proteins, play a pivotal role in antiviral immunity in eukaryotes. Dicer proteins also produce microRNAs (miRNAs), a class of endogenous small non-coding RNAs that regulate essential cellular functions through post-transcriptional mechanisms. In plants and insects, multiple Dicer proteins are produced and deployed to separately manage the biogenesis of antiviral siRNAs and miRNAs. This separation ensures that viral infections, especially the production of viral RNAi suppressors, do not severely compromise host growth or development. In contrast, nematode worms, such as Caenorhabditis elegans, rely on a single Dicer protein to produce both types of small RNAs. Probably as a strategy to mitigate the potential disruption of miRNA production by viral infections, nematodes have evolved distinct strategies for generating primary and secondary siRNAs for antiviral defense. This review explores the shared and unique features of siRNA-mediated antiviral immunity in Caenorhabditis elegans, shedding light on the specialized adaptations that enable robust antiviral defenses without compromising miRNA-mediated function.

A phytoplasma effector suppresses insect melanization immune response to promote pathogen persistent transmission

Insect melanization triggered by the conversion of prophenoloxidase to active phenoloxidase via serine proteases (SPs) is an important immediate immune response. However, how phytoplasmas evade this immune response to promote their propagation in insect vectors remains unknown. Here, we demonstrate that infection of leafhopper vectors with rice orange leaf phytoplasma (ROLP) activates the mild melanization response in hemolymph. ROLP-encoded effector protein SRP1 is highly expressed in leafhopper hemolymph, where it competitively binds to SP2, thereby inhibiting SP2-mediated cleavage of prophenoloxidase into active phenoloxidase. Consequently, microinjection of SRP1 effectively suppresses the melanization response and enhances ROLP propagation. The histidine residue at position 23 of SRP1 is essential for SRP1-SP2 interaction, and the mutation of this position abolishes its ability to inhibit such SP2-meidated cleavage, ultimately promoting melanization response and inhibiting ROLP propagation. Our findings provide insights into how phytoplasmas antagonize insect melanization response to facilitate their persistent transmission.

Transient Expression of Zika NS2B Protein Antigen in Nicotiana benthamiana and Use for Arboviruses Diagnosis

Zika (ZIKV) and Dengue (DENV) viruses are clinically significant due to their severe neurological and hemorrhagic complications. Rapid diagnostics often rely on nonstructural proteins to generate specific antibodies. This study aimed to produce IgG antibodies from the recombinant ZIKV protein and plant-expressed NS2B protein for arbovirus detection in serum and urine samples. The NS2B protein was expressed in Nicotiana benthamiana and purified chromatographically. Validation of recombinant NS2B as an antigen in indirect immunoassays demonstrated 95% sensitivity and 100% specificity in IgM/IgG ELISA tests, enabling effective detection of ZIKV and DENV. Notably, r-ZIKV-NS2B IgG identified positive ZIKV and DENV cases in urine but failed to detect negatives, suggesting limitations in specificity for urine diagnostics. Using urine as a diagnostic medium offers a less invasive and more practical approach, broadening the test applicability. This study utilized patient-derived positive urine samples and healthy samples spiked with an exogenous virus. Findings highlight the potential of the ZIKV-NS2B protein as a robust antigen for arbovirus diagnosis and demonstrate the viability of plant-based systems for antigen production, advancing diagnostics for neglected tropical diseases.

Pupylation-based proximity labeling reveals regulatory factors in cellulose biosynthesis in Arabidopsis

Knowledge about how and where proteins interact provides a pillar for cell biology. Protein proximity-labeling has emerged as an important tool to detect protein interactions. Biotin-related proximity labeling approaches are by far the most commonly used but may have labeling-related drawbacks. Here, we use pupylation-based proximity labeling (PUP-IT) as a tool for protein interaction detection in plants. We show that PUP-IT readily confirmed protein interactions for several known protein complexes across different types of plant hosts and that the approach increased detection of specific interactions as compared to biotin-based proximity labeling systems. To further demonstrate the power of PUP-IT, we used the system to identify protein interactions of the protein complex that underpin cellulose synthesis in plants. Apart from known complex components, we identified the ARF-GEF BEN1 (BFA-VISUALIZED ENDOCYTIC TRAFFICKING DEFECTIVE1). We show that BEN1 contributes to cellulose synthesis by regulating both clathrin-dependent and -independent endocytosis of the cellulose synthesis protein complex from the plasma membrane. Our results highlight PUP-IT as a powerful proximity labeling system to identify protein interactions in plant cells.

Virus-like Particles Produced in Plants: A Promising Platform for Recombinant Vaccine Development

The capsid proteins of many viruses are capable of spontaneous self-assembly into virus-like particles (VLPs), which do not contain the viral genome and are therefore not infectious. VLPs are structurally similar to their parent viruses and are therefore effectively recognized by the immune system and can induce strong humoral and cellular immune responses. The structural features of VLPs make them an attractive platform for the development of potential vaccines and diagnostic tools. Chimeric VLPs can be obtained by attaching foreign peptides to capsid proteins. Chimeric VLPs present multiple copies of the antigen on their surface, thereby increasing the effectiveness of the immune response. Recombinant VLPs can be produced in different expression systems. Plants are promising biofactories for the production of recombinant proteins, including VLPs. The main advantages of plant expression systems are the overall low cost and safety of plant-produced products due to the absence of pathogens common to plants and animals. This review provides an overview of the VLP platform as an approach to developing plant-produced vaccines, focusing on the use of transient expression systems.

Engineered plants provide a photosynthetic platform for the production of diverse human milk oligosaccharides

Human milk oligosaccharides (HMOs) are a diverse class of carbohydrates which support the health and development of infants. The vast health benefits of HMOs have made them a commercial target for microbial production; however, producing the approximately 200 structurally diverse HMOs at scale has proved difficult. Here we produce a diversity of HMOs by leveraging the robust carbohydrate anabolism of plants. This diversity includes high-value and complex HMOs, such as lacto-N-fucopentaose I. HMOs produced in transgenic plants provided strong bifidogenic properties, indicating their ability to serve as a prebiotic supplement with potential applications in adult and infant health. Technoeconomic analyses demonstrate that producing HMOs in plants provides a path to the large-scale production of specific HMOs at lower prices than microbial production platforms. Our work demonstrates the promise in leveraging plants for the low-cost and sustainable production of HMOs.

Regiodivergent biosynthesis of bridged bicyclononanes

Medicinal compounds from plants include bicyclo[3.3.1]nonane derivatives, the majority of which are polycyclic polyprenylated acylphloroglucinols (PPAPs). Prototype molecules are hyperforin, the antidepressant constituent of St. John's wort, and garcinol, a potential anticancer compound. Their complex structures have inspired innovative chemical syntheses, however, their biosynthesis in plants is still enigmatic. PPAPs are divided into two subclasses, named type A and B. Here we identify both types in Hypericum sampsonii plants and isolate two enzymes that regiodivergently convert a common precursor to pivotal type A and B products. Molecular modelling and substrate docking studies reveal inverted substrate binding modes in the two active site cavities. We identify amino acids that stabilize these alternative binding scenarios and use reciprocal mutagenesis to interconvert the enzymatic activities. Our studies elucidate the unique biochemistry that yields type A and B bicyclo[3.3.1]nonane cores in plants, thereby providing key building blocks for biotechnological efforts to sustainably produce these complex compounds for preclinical development.

Reversible S-nitrosylation of bZIP67 by peroxiredoxin IIE activity and nitro-fatty acids regulates the plant lipid profile

Nitric oxide (NO) is a gasotransmitter required in a broad range of mechanisms controlling plant development and stress conditions. However, little is known about the specific role of this signaling molecule during lipid storage in the seeds. Here, we show that NO is accumulated in developing embryos and regulates the fatty acid profile through the stabilization of the basic/leucine zipper transcription factor bZIP67. NO and nitro-linolenic acid target and accumulate bZIP67 to induce the downstream expression of FAD3 desaturase, which is misregulated in a non-nitrosylable version of the protein. Moreover, the post-translational modification of bZIP67 is reversible by the trans-denitrosylation activity of peroxiredoxin IIE and defines a feedback mechanism for bZIP67 redox regulation. These findings provide a molecular framework to control the seed fatty acid profile caused by NO, and evidence of the in vivo functionality of nitro-fatty acids during plant developmental signaling.

The melon Fom-1-Prv resistance gene pair: Correlated spatial expression and interaction with a viral protein

The head-to-head oriented pair of melon resistance genes, Fom-1 and Prv, control resistance to Fusarium oxysporum races 0 and 2 and papaya ringspot virus (PRSV), respectively. They encode, via several RNA splice variants, TIR-NBS-LRR proteins, and Prv has a C-terminal extra domain with a second NBS homologous sequence. In other systems, paired R-proteins were shown to operate by "labor division," with one protein having an extra integrated domain that directly binds the pathogen's Avr factor, and the second protein executing the defense response. We report that the expression of the two genes in two pairs of near-isogenic lines was higher in the resistant isoline and inducible by F. oxysporum race 2 but not by PRSV. The intergenic DNA region separating the coding sequences of the two genes acted as a bi-directional promoter and drove GUS expression in transgenic melon roots and transgenic tobacco plants. Expression of both genes was strong in melon root tips, around the root vascular cylinder, and the phloem and xylem parenchyma of tobacco stems and petioles. The pattern of GUS expression suggests coordinated expression of the two genes. In agreement with the above model, Prv's extra domain was shown to interact with the cylindrical inclusion protein of PRSV both in yeast cells and in planta.

The plant siRNA landscape

Whereas micro (mi)RNAs are considered the clean, noble side of the small RNA world, small interfering (si)RNAs are often seen as a noisy set of molecules whose barbarian acronyms reflect a large diversity of often elusive origins and functions. Twenty-five years after their discovery in plants, however, new classes of siRNAs are still being identified, sometimes in discrete tissues or at particular developmental stages, making the plant siRNA world substantially more complex and subtle than originally anticipated. Focusing primarily on the model Arabidopsis, we review here the plant siRNA landscape, including transposable elements (TE)-derived siRNAs, a vast array of non-TE-derived endogenous siRNAs, as well as exogenous siRNAs produced in response to invading nucleic acids such as viruses or transgenes. We primarily emphasize the extraordinary sophistication and diversity of their biogenesis and, secondarily, the variety of their known or presumed functions, including via non-cell autonomous activities, in the sporophyte, gametophyte, and shortly after fertilization.

A tomato bushy stunt virus-based vector for simultaneous editing and sensing to survey the host antiviral RNA silencing machinery

A tomato bushy stunt virus (TBSV)-derived vector system was applied for the delivery of CRISPR/Cas9 gene editing materials, to facilitate rapid, transient assays of host-virus interactions involved in the RNA silencing pathway. Toward this, single guide RNAs designed to target key components of the virus-induced host RNA silencing pathway (AGO2, DCL2, HEN1) were inserted into TBSV-based GFP-expressing viral vectors TBSV-GFP (TG) and its P19 defective mutant TGΔP19. This produced rapid, efficient, and specific gene editing in planta. Targeting AGO2, DCL2, or HEN1 partially rescued the lack of GFP accumulation otherwise associated with TGΔP19. Since the rescue phenotypes are normally only observed in the presence of the P19 silencing suppressor, the results support that the DCL2, HEN1, and AGO2 proteins are involved in anti-TBSV RNA silencing. Additionally, we show that knockdown of the RNA silencing machinery increases cargo expression from a nonviral binary Cas9 vector. The TBSV-based gene editing technology described in this study can be adapted for transient heterologous expression, rapid gene function screens, and molecular interaction studies in many plant species considering the wide host range of TBSV. In summary, we demonstrate that a plant virus can be used to establish gene editing while simultaneously serving as an accumulation sensor for successful targeting of its homologous antiviral silencing machinery components.

The Split-Luciferase Complementation Assay to Detect and Quantify Protein-Protein Interactions in Planta

Protein-protein interactions play a critical role in plant viral infection and defense responses against pathogens. This protocol provides a detailed and reliable methodology for investigating protein-protein interactions using a luciferase-based complementation assay that includes easy luminescence-based normalization within a single plate. The protocol includes step-by-step procedures, reagent lists, and considerations for data interpretation, ensuring robust and reproducible results. By following this protocol, researchers can advance on understanding of the crucial role of protein-protein interactions in plant viral infection and defense responses to other pathogen attacks.

Beyond Loading: Functions of Plant ARGONAUTE Proteins

ARGONAUTE (AGO) proteins are key components of the RNA-induced silencing complex (RISC) that mediates gene silencing in eukaryotes. Small-RNA (sRNA) cargoes are selectively loaded into different members of the AGO protein family and then target complementary sequences to in-duce transcriptional repression, mRNA cleavage, or translation inhibition. Previous reviews have mainly focused on the traditional roles of AGOs in specific biological processes or on the molecular mechanisms of sRNA sorting. In this review, we summarize the biological significance of canonical sRNA loading, including the balance among distinct sRNA pathways, cross-regulation of different RISC activities during plant development and defense, and, especially, the emerging roles of AGOs in sRNA movement. We also discuss recent advances in novel non-canonical functions of plant AGOs. Perspectives for future functional studies of this evolutionarily conserved eukaryotic protein family will facilitate a more comprehensive understanding of the multi-faceted AGO proteins.

Small interfering RNA (siRNA)-based therapeutic applications against viruses: principles, potential, and challenges

RNA has emerged as a revolutionary and important tool in the battle against emerging infectious diseases, with roles extending beyond its applications in vaccines, in which it is used in the response to the COVID-19 pandemic. Since their development in the 1990s, RNA interference (RNAi) therapeutics have demonstrated potential in reducing the expression of disease-associated genes. Nucleic acid-based therapeutics, including RNAi therapies, that degrade viral genomes and rapidly adapt to viral mutations, have emerged as alternative treatments. RNAi is a robust technique frequently employed to selectively suppress gene expression in a sequence-specific manner. The swift adaptability of nucleic acid-based therapeutics such as RNAi therapies endows them with a significant advantage over other antiviral medications. For example, small interfering RNAs (siRNAs) are produced on the basis of sequence complementarity to target and degrade viral RNA, a novel approach to combat viral infections. The precision of siRNAs in targeting and degrading viral RNA has led to the development of siRNA-based treatments for diverse diseases. However, despite the promising therapeutic benefits of siRNAs, several problems, including impaired long-term protein expression, siRNA instability, off-target effects, immunological responses, and drug resistance, have been considerable obstacles to the use of siRNA-based antiviral therapies. This review provides an encompassing summary of the siRNA-based therapeutic approaches against viruses while also addressing the obstacles that need to be overcome for their effective application. Furthermore, we present potential solutions to mitigate major challenges.

Plant-based production of diverse human milk oligosaccharides

Human milk oligosaccharides (HMOs) are a diverse class of carbohydrates that aid in the health and development of infants. The vast health benefits of HMOs have made them a commercial target for microbial production; however, producing the ∼130 structurally diverse HMOs at scale has proven difficult. Here, we produce a vast diversity of HMOs by leveraging the robust carbohydrate anabolism of plants. This diversity includes high value HMOs, such as lacto-N-fucopentaose I, that have not yet been commercially produced using state-of-the-art microbial fermentative processes. HMOs produced in transgenic plants provided strong bifidogenic properties, indicating their ability to serve as a prebiotic supplement. Technoeconomic analyses demonstrate that producing HMOs in plants provides a path to the large-scale production of specific HMOs at lower prices than microbial production platforms. Our work demonstrates the promise in leveraging plants for the cheap and sustainable production of HMOs.

An integrase toolbox to record gene-expression during plant development

There are many open questions about the mechanisms that coordinate the dynamic, multicellular behaviors required for organogenesis. Synthetic circuits that can record in vivo signaling networks have been critical in elucidating animal development. Here, we report on the transfer of this technology to plants using orthogonal serine integrases to mediate site-specific and irreversible DNA recombination visualized by switching between fluorescent reporters. When combined with promoters expressed during lateral root initiation, integrases amplify reporter signal and permanently mark all descendants. In addition, we present a suite of methods to tune the threshold for integrase switching, including: RNA/protein degradation tags, a nuclear localization signal, and a split-intein system. These tools improve the robustness of integrase-mediated switching with different promoters and the stability of switching behavior over multiple generations. Although each promoter requires tuning for optimal performance, this integrase toolbox can be used to build history-dependent circuits to decode the order of expression during organogenesis in many contexts.

Recruitment of distinct UDP-glycosyltransferase families demonstrates dynamic evolution of chemical defense within Eucalyptus L'Hér

The economic and ecologically important genus Eucalyptus is rich in structurally diverse specialized metabolites. While some specialized metabolite classes are highly prevalent across the genus, the cyanogenic glucoside prunasin is only produced by c. 3% of species. To investigate the evolutionary mechanisms behind prunasin biosynthesis in Eucalyptus, we compared de novo assembled transcriptomes, together with online resources between cyanogenic and acyanogenic species. Identified genes were characterized in vivo and in vitro. Pathway characterization of cyanogenic Eucalyptus camphora and Eucalyptus yarraensis showed for the first time that the final glucosylation step from mandelonitrile to prunasin is catalyzed by a novel UDP-glucosyltransferase UGT87. This step is typically catalyzed by a member of the UGT85 family, including in Eucalyptus cladocalyx. The upstream conversion of phenylalanine to mandelonitrile is catalyzed by three cytochrome P450 (CYP) enzymes from the CYP79, CYP706, and CYP71 families, as previously shown. Analysis of acyanogenic Eucalyptus species revealed the loss of different ortholog prunasin biosynthetic genes. The recruitment of UGTs from different families for prunasin biosynthesis in Eucalyptus demonstrates important pathway heterogeneities and unprecedented dynamic pathway evolution of chemical defense within a single genus. Overall, this study provides relevant insights into the tremendous adaptability of these long-lived trees.

Critical points for the design and application of RNA silencing constructs for plant virus resistance

Control of plant virus diseases is a big challenge in agriculture as is resistance in plant lines to infection by viruses. Recent progress using advanced technologies has provided fast and durable alternatives. One of the most promising techniques against plant viruses that is cost-effective and environmentally safe is RNA silencing or RNA interference (RNAi), a technology that could be used alone or along with other control methods. To achieve the goals of fast and durable resistance, the expressed and target RNAs have been examined in many studies, with regard to the variability in silencing efficiency, which is regulated by various factors such as target sequences, target accessibility, RNA secondary structures, sequence variation in matching positions, and other intrinsic characteristics of various small RNAs. Developing a comprehensive and applicable toolbox for the prediction and construction of RNAi helps researchers to achieve the acceptable performance level of silencing elements. Although the attainment of complete prediction of RNAi robustness is not possible, as it also depends on the cellular genetic background and the nature of the target sequences, some important critical points have been discerned. Thus, the efficiency and robustness of RNA silencing against viruses can be improved by considering the various parameters of the target sequence and the construct design. In this review, we provide a comprehensive treatise regarding past, present and future prospective developments toward designing and applying RNAi constructs for resistance to plant viruses.

Development of tobacco rattle virus-based platform for dual heterologous gene expression and CRISPR/Cas reagent delivery

A large number of viral delivery systems have been developed for characterizing functional genes and producing heterologous recombinant proteins in plants, and but most of them are unable to co-express two fusion-free foreign proteins in the whole plant for extended periods of time. In this study, we modified tobacco rattle virus (TRV) as a TRVe dual delivery vector, using the strategy of gene substitution. The reconstructed TRVe had the capability to simultaneously produce two fusion-free foreign proteins at the whole level of Nicotiana benthamiana, and maintained the genetic stability for the insert of double foreign genes. Moreover, TRVe allowed systemic expression of two foreign proteins with the total lengths up to ∼900 aa residues. In addition, Cas12a protein and crRNA were delivered by the TRVe expression system for site-directed editing of genomic DNA in N. benthamiana 16c line constitutively expressing green fluorescent protein (GFP). Taker together, the TRV-based delivery system will be a simple and powerful means to rapidly co-express two non-fused foreign proteins at the whole level and facilitate functional genomics studies in plants.

SlS5H silencing reveals specific pathogen-triggered salicylic acid metabolism in tomato

BACKGROUND

Salicylic acid (SA) is a major plant hormone that mediates the defence pathway against pathogens. SA accumulates in highly variable amounts depending on the plant-pathogen system, and several enzyme activities participate in the restoration of its levels. Gentisic acid (GA) is the product of the 5-hydroxylation of SA, which is catalysed by S5H, an enzyme activity regarded as a major player in SA homeostasis. GA accumulates at high levels in tomato plants infected by Citrus Exocortis Viroid (CEVd), and to a lesser extend upon Pseudomonas syringae DC3000 pv. tomato (Pst) infection.

RESULTS

We have studied the induction of tomato SlS5H gene by different pathogens, and its expression correlates with the accumulation of GA. Transient over-expression of SlS5H in Nicotiana benthamiana confirmed that SA is processed by SlS5H in vivo. SlS5H-silenced tomato plants were generated, displaying a smaller size and early senescence, together with hypersusceptibility to the necrotrophic fungus Botrytis cinerea. In contrast, these transgenic lines exhibited an increased defence response and resistance to both CEVd and Pst infections. Alternative SA processing appears to occur for each specific pathogenic interaction to cope with SA levels. In SlS5H-silenced plants infected with CEVd, glycosylated SA was the most discriminant metabolite found. Instead, in Pst-infected transgenic plants, SA appeared to be rerouted to other phenolics such as feruloyldopamine, feruloylquinic acid, feruloylgalactarate and 2-hydroxyglutarate.

CONCLUSION

Using SlS5H-silenced plants as a tool to unbalance SA levels, we have studied the re-routing of SA upon CEVd and Pst infections and found that, despite the common origin and role for SA in plant pathogenesis, there appear to be different pathogen-specific, alternate homeostasis pathways.

AC5 protein encoded by squash leaf curl China virus is an RNA silencing suppressor and a virulence determinant

Squash leaf curl China virus (SLCCNV) is a bipartite Begomovirus. The function of the protein AC5, which is encoded by SLCCNV, is unknown. Here, we confirmed that the 172-amino acids (aa) long AC5 protein of SLCCNV could suppress single-stranded RNA but not double-stranded RNA-induced post-transcriptional gene silencing (PTGS). Furthermore, we determined that the C-terminal domain (96–172 aa) of the AC5 protein was responsible for RNA silencing suppressor (RSS) activity via deletion mutant analysis. The AC5 protein can reverse GFP silencing and inhibit systemic silencing of GFP by interfering with the systemic spread of the GFP silencing signal. The SLCCNV AC5 protein was localized to both the nucleus and cytoplasm of Nicotiana benthamiana cells. Furthermore, deletion analysis showed that the putative nuclear localization signal (NLS, 102–155 aa) was crucial for the RNA silencing suppression activity of AC5. In addition, the AC5 protein elicited a hypersensitive response and enhanced potoao virus X (PVX) RNA accumulation in infected N. benthamiana plants. Using the infectious clones of the SLCCNV and SLCCNV-AC5 null mutants, mutational analysis confirmed that knockout of the AC5 gene abolished SLCCNV-induced leaf curl symptoms, showing SLCCNV AC5 is also a virulence determinant.

Barley GRIK1-SnRK1 kinases subvert a viral virulence protein to upregulate antiviral RNAi and inhibit infection

Viruses often usurp host machineries for their amplification, but it remains unclear if hosts may subvert virus proteins to regulate viral proliferation. Here, we show that the 17K protein, an important virulence factor conserved in barley yellow dwarf viruses (BYDVs) and related poleroviruses, is phosphorylated by host GRIK1‐SnRK1 kinases, with the phosphorylated 17K (P17K) capable of enhancing the abundance of virus‐derived small interfering RNAs (vsiRNAs) and thus antiviral RNAi. Furthermore, P17K interacts with barley small RNA‐degrading nuclease 1 (HvSDN1) and impedes HvSDN1‐catalyzed vsiRNA degradation. Additionally, P17K weakens the HvSDN1‐HvAGO1 interaction, thus hindering HvSDN1 from accessing and degrading HvAGO1‐carried vsiRNAs. Importantly, transgenic expression of 17K phosphomimetics (17K^5D), or genome editing of SDN1, generates stable resistance to BYDV through elevating vsiRNA abundance. These data validate a novel mechanism that enhances antiviral RNAi through host subversion of a viral virulence protein to inhibit SDN1‐catalyzed vsiRNA degradation and suggest new ways for engineering BYDV‐resistant crops.

Genetic effects of Red Lettuce Leaf genes on red coloration in leaf lettuce under artificial lighting conditions

Some cultivars of lettuce accumulate anthocyanins, which act as functional food ingredients. Leaf lettuce has been known to be erratic in exhibiting red color when grown under artificial light, and there is a need for cultivars that more stably exhibit red color in artificial light cultivation. In this study, we aimed to dissect the genetic architecture for red coloring in various leaf lettuce cultivars grown under artificial light. We investigated the genotype of Red Lettuce Leaf (RLL) genes in 133 leaf lettuce strains, some of which were obtained from publicly available resequencing data. By studying the allelic combination of RLL genes, we further analyzed the contribution of these genes to producing red coloring in leaf lettuce. From the quantification of phenolic compounds and corresponding transcriptome data, we revealed that gene expression level-dependent regulation of RLL1 (bHLH) and RLL2 (MYB) is the underlying mechanism conferring high anthocyanin accumulation in red leaf lettuce under artificial light cultivation. Our data suggest that different combinations of RLL genotypes cause quantitative differences in anthocyanin accumulation among cultivars, and some genotype combinations are more effective at producing red coloration even under artificial lighting.

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.

PIF-independent regulation of growth by an evening complex in the liverwort Marchantia polymorpha

Previous studies in the liverwort Marchantia polymorpha have shown that the putative evening complex (EC) genes LUX ARRHYTHMO (LUX) and ELF4-LIKE (EFL) have a function in the liverwort circadian clock. Here, we studied the growth phenotypes of MpLUX and MpEFL loss-of-function mutants, to establish if PHYTOCHROME-INTERACTING FACTOR (PIF) and auxin act downstream of the M. polymorpha EC in a growth-related pathway similar to the one described for the flowering plant Arabidopsis. We examined growth rates and cell properties of loss-of-function mutants, analyzed protein-protein interactions and performed gene expression studies using reporter genes. Obtained data indicate that an EC can form in M. polymorpha and that this EC regulates growth of the thallus. Altered auxin levels in Mplux mutants could explain some of the phenotypes related to an increased thallus surface area. However, because MpPIF is not regulated by the EC, and because Mppif mutants do not show reduced growth, the growth phenotype of EC-mutants is likely not mediated via MpPIF. In Arabidopsis, the circadian clock regulates elongation growth via PIF and auxin, but this is likely not an evolutionarily conserved growth mechanism in land plants. Previous inventories of orthologs to Arabidopsis clock genes in various plant lineages showed that there is high levels of structural differences between clocks of different plant lineages. Here, we conclude that there is also variation in the output pathways used by the different plant clocks to control growth and development.

Antiviral RNAi Mechanisms to Arboviruses in Mosquitoes: microRNA Profile of Aedes aegypti and Culex quinquefasciatus from Grenada, West Indies

Mosquito-borne arboviruses, such as dengue virus, West Nile virus, Zika virus and yellow fever virus, impose a tremendous cost on the health of populations around the world. As a result, much effort has gone into the study of the impact of these viruses on human infections. Comparatively less effort, however, has been made to study the way these viruses interact with mosquitoes themselves. As ingested arboviruses infect their midgut and subsequently other tissue, the mosquito mounts a multifaceted innate immune response. RNA interference, the central intracellular antiviral defense mechanism in mosquitoes and other invertebrates can be induced and modulated through outside triggers (small RNAs) and treatments (transgenesis or viral-vector delivery). Accordingly, modulation of this facet of the mosquito’s immune system would thereby suggest a practical strategy for vector control. However, this requires a detailed understanding of mosquitoes’ endogenous small RNAs and their effects on the mosquito and viral proliferation. This paper provides an up-to-date overview of the mosquito’s immune system along with novel data describing miRNA profiles for Aedes aegypti and Culex quinquefasiatus in Grenada, West Indies.

Amplification of cell signaling and disease resistance by an immunity receptor Ve1Ve2 heterocomplex in plants

Immunity cell-surface receptors Ve1 and Ve2 protect against fungi of the genus Verticillium causing early dying, a worldwide disease in many crops. Characterization of microbe-associated molecular pattern immunity receptors has advanced our understanding of disease resistance but signal amplification remains elusive. Here, we report that transgenic plants expressing Ve1 and Ve2 together, reduced pathogen titres by a further 90% compared to plants expressing only Ve1 or Ve2. Confocal and immunoprecipitation confirm that the two receptors associate to form heteromeric complexes in the absence of the ligand and positively regulate signaling. Bioassays show that the Ve1Ve2 complex activates race-specific amplified immunity to the pathogen through a rapid burst of reactive oxygen species (ROS). These results indicate a mechanism by which the composition of a cell-surface receptor heterocomplex may be optimized to increase immunity against devastating plant diseases.

Transgenic plants expressing both Ve1 and Ve2 conferred enhanced signaling and disease resistance in susceptible potato in a race-specific manner, a step forward in generating disease resistant plants against Verticillium.

The N-terminally truncated helper NLR NRG1C antagonizes immunity mediated by its full-length neighbors NRG1A and NRG1B

Both plants and animals utilize nucleotide-binding leucine-rich repeat immune receptors (NLRs) to perceive the presence of pathogen-derived molecules and induce immune responses. NLR genes are far more abundant and diverse in vascular plants than in animals. Truncated NLRs, which lack one or more of the canonical domains, are also commonly encoded in plant genomes. However, little is known about their functions, especially the N-terminally truncated ones. Here, we show that the Arabidopsis thaliana N-terminally truncated helper NLR (hNLR) gene N REQUIREMENT GENE1 (NRG1C) is highly induced upon pathogen infection and in autoimmune mutants. The immune response and cell death conferred by some Toll/interleukin-1 receptor-type NLRs (TNLs) were compromised in Arabidopsis NRG1C overexpression lines. Detailed genetic analysis revealed that NRG1C antagonizes the immunity mediated by its full-length neighbors NRG1A and NRG1B. Biochemical tests suggested that NRG1C might interfere with the EDS1-SAG101 complex, which functions in immunity signaling together with NRG1A/1B. Interestingly, Brassicaceae NRG1Cs are functionally exchangeable and that the Nicotiana benthamiana N-terminally truncated hNLR NRG2 also antagonizes NRG1 activity. Together, our study uncovers an unexpected negative role of N-terminally truncated hNLRs in immunity in different plant species.

Light-Engineering Technology for Enhancing Plant Disease Resistance

Insect vector-borne diseases are a major constraint to a wide variety of crops. Plants integrate environmental light and internal signalings to defend dual stresses both from the vector insects and vector-transmitted pathogens. In this review, we highlight a studies that demonstrate how light regulates plants deploying mechanisms against vector-borne diseases. Four major host defensive pathways involved in the host defense network against multiple biotic stresses are reviewed: innate immunity, phytohormone signaling, RNA interference, and protein degradation. The potential with light-engineering technology with light emitting diodes (LEDs) and genome engineering technology for fine-tuning crop defense and yield are also discussed.

Purification of His-Tagged Proteases from the Apoplast of Agroinfiltrated N. benthamiana

Protein expression in plants by agroinfiltration and subsequent purification is increasingly used for the biochemical characterization of plant proteins. In this chapter we describe the purification of secreted, His-tagged proteases from the apoplast of agroinfiltrated Nicotiana benthamiana using immobilized metal affinity chromatography (IMAC). We show quality checks for the purified protease and discuss potential problems and ways to circumvent them. As a proof of concept, we produce and purify tomato immune protease Pip1 and demonstrate that the protein is active after purification.

Bimolecular Fluorescence Complementation to Test for Protein-Protein Interactions and to Uncover Regulatory Mechanisms During Gametogenesis

Bimolecular fluorescence complementation (BiFC) assay is one of the sensitive techniques that allows to investigate direct protein-protein interactions (PPI) in vivo and visualize the subcellular localization of interacting proteins. It is based on splitting of a fluorescent protein into two nonfluorescent parts accordingly fused to two putative interacting partners. If interaction between studied proteins is possible, nonfluorescent parts come to close proximity resulting in reconstitution of the functional fluorescent protein and giving fluorescence under certain wavelength. BiFC analysis implies transient or stable expression of the proteins of interest and can be used as a method to test or validate the direct PPI in various biological pathways, including the regulation of gametogenesis, which is the main focus of this book. In our protocol we give detailed information for beginners about three main steps of BiFC analysis of centromeric protein interactions. These steps include (1) generation of appropriate expression clones with the help of Gateway cloning technology, (2) infiltration of Nicotiana benthamiana plants by Agrobacteria containing generated constructs, and (3) microscopic analysis of plants under fluorescence microscope. Also, we discuss appropriate negative controls that can be used for evaluation as well as recommendable vector systems, possible artifacts and measures to avoid artifactual interactions for BiFC assay.

Controlled RISC loading efficiency of miR168 defined by miRNA duplex structure adjusts ARGONAUTE1 homeostasis

Micro RNAs (miRNAs) are processed from precursor RNA molecules with precisely defined secondary stem-loop structures. ARGONAUTE1 (AGO1) is the main executor component of miRNA pathway and its expression is controlled via the auto-regulatory feedback loop activity of miR168 in plants. Previously we have shown that AGO1 loading of miR168 is strongly restricted leading to abundant cytoplasmic accumulation of AGO-unbound miR168. Here, we report, that intrinsic RNA secondary structure of MIR168a precursor not only defines the processing of miR168, but also precisely adjusts AGO1 loading efficiency determining the biologically active subset of miR168 pool. Our results show, that modification of miRNA duplex structure of MIR168a precursor fragment or expression from artificial precursors can alter the finely adjusted loading efficiency of miR168. In dcl1-9 mutant where, except for miR168, production of most miRNAs is severely reduced this mechanism ensures the elimination of unloaded AGO1 proteins via enhanced AGO1 loading of miR168. Based on this data, we propose a new competitive loading mechanism model for miR168 action: the miR168 surplus functions as a molecular buffer for controlled AGO1 loading continuously adjusting the amount of AGO1 protein in accordance with the changing size of the cellular miRNA pool.

The rice RNase P protein subunit Rpp30 confers broad-spectrum resistance to fungal and bacterial pathogens

RNase P functions either as a catalytic ribonucleoprotein (RNP) or as an RNA-free polypeptide to catalyse RNA processing, primarily tRNA 5' maturation. To the growing evidence of non-canonical roles for RNase P RNP subunits including regulation of chromatin structure and function, we add here a role for the rice RNase P Rpp30 in innate immunity. This protein (encoded by LOC_Os11g01074) was uncovered as the top hit in yeast two-hybrid assays performed with the rice histone deacetylase HDT701 as bait. We showed that HDT701 and OsRpp30 are localized to the rice nucleus, OsRpp30 expression increased post-infection by Pyricularia oryzae (syn. Magnaporthe oryzae), and OsRpp30 deacetylation coincided with HDT701 overexpression in vivo. Overexpression of OsRpp30 in transgenic rice increased expression of defence genes and generation of reactive oxygen species after pathogen-associated molecular pattern elicitor treatment, outcomes that culminated in resistance to a fungal (P. oryzae) and a bacterial (Xanthomonas oryzae pv. oryzae) pathogen. Knockout of OsRpp30 yielded the opposite phenotypes. Moreover, HA-tagged OsRpp30 co-purified with RNase P pre-tRNA cleavage activity. Interestingly, OsRpp30 is conserved in grass crops, including a near-identical C-terminal tail that is essential for HDT701 binding and defence regulation. Overall, our results suggest that OsRpp30 plays an important role in rice immune response to pathogens and provides a new approach to generate broad-spectrum disease-resistant rice cultivars.

Fol-milR1, a pathogenicity factor of Fusarium oxysporum, confers tomato wilt disease resistance by impairing host immune responses

Although it is well known that miRNAs play crucial roles in multiple biological processes, there is currently no evidence indicating that milRNAs from Fusarium oxysporum f. sp. lycopersici (Fol) interfere with tomato resistance during infection. Here, using sRNA-seq, we demonstrate that Fol-milR1, a trans-kingdom small RNA, is exported into tomato cells after infection. The knockout strain ∆Fol-milR1 displays attenuated pathogenicity to the susceptible tomato cultivar 'Moneymaker'. On the other hand, Fol-milR1 overexpression strains exhibit enhanced virulence against the resistant cultivar 'Motelle'. Several tomato mRNAs are predicted targets of Fol-milR1. Among these genes, Solyc06g007430 (encoding the CBL-interacting protein kinase, SlyFRG4) is regulated at the posttranscriptional level by Fol-milR1. Furthermore, SlyFRG4 loss-of-function alleles created using CRISPR/Cas9 in tomato ('Motelle') exhibit enhanced disease susceptibility to Fol, further supporting the idea that SlyFRG4 is essential for tomato wilt disease resistance. Notably, our results using immunoprecipitation with specific antiserum suggest that Fol-milR1 interferes with the host immunity machinery by binding to tomato ARGONAUTE 4a (SlyAGO4a). Furthermore, virus-induced gene silenced (VIGS) knock-down SlyAGO4a plants exhibit reduced susceptibility to Fol. Together, our findings support a model in which Fol-milR1 is an sRNA fungal effector that suppresses host immunity by silencing a disease resistance gene, thus providing a novel virulence strategy to achieve infection.

Developing specific leaf promoters tools for genetic use in transgenic plants towards food security

Significant yields enrichments are necessitated for meeting the rapid global growth population together with the expected demanding for food, particularly major crops. Photosynthesis improvement is an unexploited opportunity in research on improving crop yields. However, the lack of sufficient molecular promoters tools leads to the need to explore and analyze native leaf-specified promoters for manipulating photosynthesis activities in plants. Two B. distachyon promoters, sedoheptulose-1, 7-bisphosphatase (SBPase) and fructose-1, 6-bisphosphate aldolase (FBPA), were isolated and cloned into an expression vector upstream of the eYFP reporter gene. The results demonstrate that both promoters actively function in N. benthamiana leaves in both agro-transiently assays, successfully regulating expression specifically to leaf-tissues. Exploring these active promoters could potentially provide new well genetic tools for any transgene expression in plants or leaves to genetically manipulate photosynthesis for yield improvement.

Rice stripe virus: Exploring Molecular Weapons in the Arsenal of a Negative-Sense RNA Virus

Rice stripe disease caused by Rice stripe virus (RSV) is one of the most devastating plant viruses of rice and causes enormous losses in production. RSV is transmitted from plant to plant by the small brown planthopper (Laodelphax striatellus) in a circulative-propagative manner. The recent reemergence of this pathogen in East Asia since 2000 has made RSV one of the most studied plant viruses over the past two decades. Extensive studies of RSV have resulted in substantial advances regarding fundamental aspects of the virus infection. Here, we compile and analyze recent information on RSV with a special emphasis on the strategies that RSV has adopted to establish infections. These advances include RSV replication and movement in host plants and the small brown planthopper vector, innate immunity defenses against RSV infection, epidemiology, and recent advances in the management of rice stripe disease. Understanding these issues will facilitate the design of novel antiviral therapies for management and contribute to a more detailed understanding of negative-sense virus-host interactions at the molecular level.

Sequence and Gene Expression Analysis of Recently Identified NLP from Plasmopara viticola

Grapevine downy mildew, evoked by the obligate biotrophic oomycete Plasmopara viticola, is one of the most challenging diseases in viticulture. P. viticola establishes an infection by circumvention of plant immunity, which is achieved by the secretion of effector molecules. One family of potential effectors are the necrosis- and ethylene-inducing peptide 1 (Nep1)-like proteins (NLP). NLP are most abundant in plant pathogenic microorganisms and exist in cytotoxic and non-cyctotoxic forms. Cytotoxic NLP often act as virulence factors and are synthesized in necrotrophic or hemibiotrophic pathogens during the transition from biotrophic to necrotrophic growth. In addition to these cytotoxic NLP, many non-cytotoxic NLP have been identified; their function in biotrophic pathogens is still unknown. In 2020, eight different NLP coding genes were identified in P. viticola and named PvNLP1 to PvNLP8 (Plasmopara viticolaNLP 1–8). In the present study, PvNLP4 to PvNLP8 were characterized by using qPCR analysis and transient expression in the model plant Nicotiana benthamiana. Gene expression analysis showed high PvNLP expression during the early stages of infection. Necrosis-inducing activity of PvNLP was not observed in the nonhost N. benthamiana.

GTP binding by Arabidopsis extra-large G protein 2 is not essential for its functions

The extra-large guanosine-5'-triphosphate (GTP)-binding protein 2, XLG2, is an unconventional Gα subunit of the Arabidopsis (Arabidopsis thaliana) heterotrimeric GTP-binding protein complex with a major role in plant defense. In vitro biochemical analyses and molecular dynamic simulations show that affinity of XLG2 for GTP is two orders of magnitude lower than that of the conventional Gα, AtGPA1. Here we tested the physiological relevance of GTP binding by XLG2. We generated an XLG2(T476N) variant with abolished GTP binding, as confirmed by in vitro GTPγS binding assay. Yeast three-hybrid, bimolecular fluorescence complementation, and split firefly-luciferase complementation assays revealed that the nucleotide-depleted XLG2(T476N) retained wild-type XLG2-like interactions with the Gβγ dimer and defense-related receptor-like kinases. Both wild-type and nucleotide-depleted XLG2(T476N) restored the defense responses against Fusarium oxysporum and Pseudomonas syringae compromised in the xlg2 xlg3 double mutant. Additionally, XLG2(T476N) was fully functional restoring stomatal density, root growth, and sensitivity to NaCl, but failed to complement impaired germination and vernalization-induced flowering. We conclude that XLG2 is able to function in a GTP-independent manner and discuss its possible mechanisms of action.

Maize Plants Chimeric for an Autoactive Resistance Gene Display a Cell-Autonomous Hypersensitive Response but Non-Cell Autonomous Defense Signaling

The maize gene Rp1-D21 is a mutant form of the gene Rp1-D that confers resistance to common rust. Rp1-D21 triggers a spontaneous defense response that occurs in the absence of the pathogen and includes a programed cell death called the hypersensitive response (HR). Eleven plants heterozygous for Rp1-D21, in four different genetic backgrounds, were identified that had chimeric leaves with lesioned sectors showing HR abutting green nonlesioned sectors lacking HR. The Rp1-D21 sequence derived from each of the lesioned portions of leaves was unaltered from the expected sequence whereas the Rp1-D21 sequences from nine of the nonlesioned sectors displayed various mutations, and we were unable to amplify Rp1-D21 from the other two nonlesioned sectors. In every case, the borders between the sectors were sharp, with no transition zone, suggesting that HR and chlorosis associated with Rp1-D21 activity was cell autonomous. Expression of defense response marker genes was assessed in the lesioned and nonlesioned sectors as well as in near-isogenic plants lacking and carrying Rp1-D21. Defense gene expression was somewhat elevated in nonlesioned sectors abutting sectors carrying Rp1-D21 compared with near-isogenic plants lacking Rp1-D21. This suggests that, whereas the HR itself was cell autonomous, other aspects of the defense response initiated by Rp1-D21 were not.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

The serine/threonine/tyrosine kinase STY46 defends against hordeivirus infection by phosphorylating γb protein

Protein phosphorylation is a common post-translational modification that frequently occurs during plant-virus interaction. Host protein kinases often regulate virus infectivity and pathogenicity by phosphorylating viral proteins. The Barley stripe mosaic virus (BSMV) γb protein plays versatile roles in virus infection and the coevolutionary arms race between plant defense and viral counter-defense. Here, we identified that the autophosphorylated cytosolic serine/threonine/tyrosine (STY) protein kinase 46 of Nicotiana benthamiana (NbSTY46) phosphorylates and directly interacts with the basic motif domain (aa 19-47) of γb in vitro and in vivo. Overexpression of wild-type NbSTY46, either transiently or transgenically, suppresses BSMV replication and ameliorates viral symptoms, whereas silencing of NbSTY46 leads to increased viral replication and exacerbated symptom. Moreover, the antiviral role of NbSTY46 requires its kinase activity, as the NbSTY46T436A mutant, lacking kinase activity, not only loses the ability to phosphorylate and interact with γb but also fails to impair BSMV infection when expressed in plants. NbSTY46 could also inhibit the replication of Lychnis ringspot virus, another chloroplast-replicating hordeivirus. In summary, we report a function of the cytosolic kinase STY46 in defending against plant viral infection by phosphorylating a viral protein in addition to its basal function in plant growth, development, and abiotic stress responses.

Transgenic Kalanchoë blossfeldiana, Containing Individual rol Genes and Open Reading Frames Under 35S Promoter, Exhibit Compact Habit, Reduced Plant Growth, and Altered Ethylene Tolerance in Flowers

Reduced growth habit is a desirable trait for ornamental potted plants and can successfully be obtained through Rhizobium rhizogenes transformation in a stable and heritable manner. Additionally, it can also be obtained by transformation with Agrobacterium tumefaciens harboring specific genes from R. rhizogenes. The bacterial T-DNA harbors four root oncogenic loci (rol) genes and 14 less known open reading frames (ORFs). The four rol genes, i.e., rolA, rolB, rolC, and rolD, are conceived as the common denominator for the compact phenotype and the other less characterized ORFs seem auxiliary but present a potential breeding target for less aberrant and/or more tailored phenotypes. In this study, Kalanchoë blossfeldiana 'Molly' was transformed with individual rol genes and selected ORFs in 35S overexpressing cassettes to comprehensively characterize growth traits, gene copy and expression, and ethylene tolerance of the flowers. An association of reduced growth habit, e.g. height and diameter, was observed for rolB2 and ORF14-2 when a transgene single copy and high gene expression were detected. Chlorophyll content was reduced in overexpressing lines compared to wild type (WT), except for one ΔORF13a (a truncated ORF13a, where SPXX DNA-binding motif is absent). The flower number severely decreased in the overexpressing lines compared to WT. The anthesis timing showed that WT opened the first flower at 68.9 ± 0.9 days and the overexpressing lines showed similar or up to 24 days delay in flowering. In general, a single or low relative gene copy insertion was correlated to higher gene expression, ca. 3 to 5-fold, in rolB and ΔORF13a lines, while in ORF14 such relation was not directly linked. The increased gene expression observed in rolB2 and ΔORF13a-2 contributed to reducing plant growth and a more compact habit. Tolerance of detached flowers to 0.5 μl L-1 ethylene was markedly higher for ORF14 with 66% less flower closure at day 3 compared to WT. The subcellular localization of rolC and ΔORF13a was investigated by transient expression in Nicotiana benthamiana and confocal images showed that rolC and ΔORF13a are soluble and localize in the cytoplasm being able to enter the nucleus.

RNA2-encoded VP37 protein of Broad bean wilt virus 1 is a determinant of pathogenicity, host susceptibility, and a suppressor of post-transcriptional gene silencing

Broad bean wilt virus 1 (BBWV-1, genus Fabavirus, family Secoviridae) is a bipartite, single-stranded positive-sense RNA virus infecting many horticultural and ornamental crops worldwide. RNA1 encodes proteins involved in viral replication whereas RNA2 encodes two coat proteins (the large and small coat proteins) and two putative movement proteins (MPs) of different sizes with overlapping C-terminal regions. In this work, we determined the role played by the small putative BBWV-1 MP (VP37) on virus pathogenicity, host specificity, and suppression of post-transcriptional gene silencing (PTGS). We engineered a BBWV-1 35S-driven full-length cDNA infectious clone corresponding to BBWV-1 RNA1 and RNA2 (pBBWV1-Wt) and generated a mutant knocking out VP37 (pBBWV1-G492C). Agroinfiltration assays showed that pBBWV1-Wt, as the original BBWV-1 isolate, infected broad bean, tomato, pepper, and Nicotiana benthamiana, whereas pBBWV1-G492C did not infect pepper and tomato systemically. Also, pBBWV1-G492C induced milder symptoms in broad bean and N. benthamiana than pBBWV1-Wt. Differential retrotranscription and amplification of the (+) and (-) strands showed that pBBWV1-G492C replicated in the agroinfiltrated leaves of pepper but not in tomato. All this suggests that VP37 is a determinant of pathogenicity and host specificity. Transient expression of VP37 through a potato virus X (PVX) vector enhanced PVX symptoms and induced systemic necrosis associated with programmed cell death in N. benthamiana plants. Finally, VP37 was identified as a viral suppressor of RNA silencing by transient expression in N. benthamiana 16c plants and movement complementation of a viral construct based on turnip crinkle virus (pTCV-GFP).

Characterization of a DCL2-Insensitive Tomato Bushy Stunt Virus Isolate Infecting Arabidopsis thaliana

Tomato bushy stunt virus (TBSV), the type member of the genus Tombusvirus in the family Tombusviridae is one of the best studied plant viruses. The TBSV natural and experimental host range covers a wide spectrum of plants including agricultural crops, ornamentals, vegetables and Nicotiana benthamiana. However, Arabidopsis thaliana, the well-established model organism in plant biology, genetics and plant-microbe interactions is absent from the list of known TBSV host plant species. Most of our recent knowledge of the virus life cycle has emanated from studies in Saccharomyces cerevisiae, a surrogate host for TBSV that lacks crucial plant antiviral mechanisms such as RNA interference (RNAi). Here, we identified and characterized a TBSV isolate able to infect Arabidopsis with high efficiency. We demonstrated by confocal and 3D electron microscopy that in Arabidopsis TBSV-BS3Ng replicates in association with clustered peroxisomes in which numerous spherules are induced. A dsRNA-centered immunoprecipitation analysis allowed the identification of TBSV-associated host components including DRB2 and DRB4, which perfectly localized to replication sites, and NFD2 that accumulated in larger viral factories in which peroxisomes cluster. By challenging knock-out mutants for key RNAi factors, we showed that TBSV-BS3Ng undergoes a non-canonical RNAi defensive reaction. In fact, unlike other RNA viruses described, no 22nt TBSV-derived small RNA are detected in the absence of DCL4, indicating that this virus is DCL2-insensitive. The new Arabidopsis-TBSV-BS3Ng pathosystem should provide a valuable new model for dissecting plant-virus interactions in complement to Saccharomyces cerevisiae.

Transformation of Amorphadiene Synthase and Antisilencing P19 Genes into Artemisia annua L. and its Effect on Antimalarial Artemisinin Production

Purpose: The low content of artemisinin related to the biosynthetic pathway is influenced by the role of certain enzymes in the formation of artemisinin. The regulation of genes involved in artemisinin biosynthesis through genetic engineering is a choice to enhance the content. This research aims to transform ads and p19 gene as an antisilencing into Artemisia annua and to see their effects on artemisinin production. Methods: The presence of p19 and ads genes was confirmed through polymerase chain reaction (PCR) products and sequencing analysis. The plasmids, which contain ads and/or p19 genes, were transformed into Agrobacterium tumefaciens, and then inserted into leaves and hairy roots of A. annua by vacuum and syringe infiltration methods. The successful transformation was checked through the GUS histochemical test and the PCR analysis. Artemisinin levels were measured using HPLC. Results: The percentages of the blue area on leaves by using vacuum and syringe infiltration method and on hairy roots were up to 98, 92.55%, and 99.00% respectively. The ads-p19 sample contained a higher level of artemisinin (0.18%) compared to other samples. Transformed hairy root with co-transformation of ads-p19 contained 0.095% artemisinin, where no artemisinin was found in the control hairy root. The transformation of ads and p19 genes into A. annua plant has been successfully done and could enhance the artemisinin content on the transformed leaves with ads-p19 up to 2.57 folds compared to the untransformed leaves, while for p19, cotransformed and ads were up to 2.25, 1.29, and 1.14 folds respectively. Conclusion: Antisilencing p19 gene could enhance the transformation efficiency of ads and artemisinin level in A. annua.

Movement and differential consumption of short interfering RNA duplexes underlie mobile RNA interference

In RNA interference (RNAi), the RNase III Dicer processes long double-stranded RNA (dsRNA) into short interfering RNA (siRNA), which, when loaded into ARGONAUTE (AGO) family proteins, execute gene silencing¹. Remarkably, RNAi can act non-cell autonomously^2,3: it is graft transmissible^4-7, and plasmodesmata-associated proteins modulate its cell-to-cell spread^8,9. Nonetheless, the molecular mechanisms involved remain ill defined, probably reflecting a disparity of experimental settings. Among other caveats, these almost invariably cause artificially enhanced movement via transitivity, whereby primary RNAi-target transcripts are converted into further dsRNA sources of secondary siRNA^5,10,11. Whether siRNA mobility naturally requires transitivity and whether it entails the same or distinct signals for cell-to-cell versus long-distance movement remains unclear, as does the identity of the mobile signalling molecules themselves. Movement of long single-stranded RNA, dsRNA, free/AGO-bound secondary siRNA or primary siRNA have all been advocated^12-15; however, an entity necessary and sufficient for all known manifestations of plant mobile RNAi remains to be ascertained. Here, we show that the same primary RNAi signal endows both vasculature-to-epidermis and long-distance silencing movement from three distinct RNAi sources. The mobile entities are AGO-free primary siRNA duplexes spreading length and sequence independently. However, their movement is accompanied by selective siRNA depletion reflecting the AGO repertoires of traversed cell types. Coupling movement with this AGO-mediated consumption process creates qualitatively distinct silencing territories, potentially enabling unlimited spatial gene regulation patterns well beyond those granted by mere gradients.

Agrobacterium tumefaciens: A Bacterium Primed for Synthetic Biology

Agrobacterium tumefaciens is an important tool in plant biotechnology due to its natural ability to transfer DNA into the genomes of host plants. Genetic manipulations of A. tumefaciens have yielded considerable advances in increasing transformational efficiency in a number of plant species and cultivars. Moreover, there is overwhelming evidence that modulating the expression of various mediators of A. tumefaciens virulence can lead to more successful plant transformation; thus, the application of synthetic biology to enable targeted engineering of the bacterium may enable new opportunities for advancing plant biotechnology. In this review, we highlight engineering targets in both A. tumefaciens and plant hosts that could be exploited more effectively through precision genetic control to generate high-quality transformation events in a wider range of host plants. We then further discuss the current state of A. tumefaciens and plant engineering with regard to plant transformation and describe how future work may incorporate a rigorous synthetic biology approach to tailor strains of A. tumefaciens used in plant transformation.

Superinfection by PHYVV Alters the Recovery Process in PepGMV-Infected Pepper Plants

Geminiviruses are important plant pathogens that affect crops around the world. In some geminivirus-host interactions, infected plants show recovery, a phenomenon characterized by symptom disappearance in newly emerging leaves. In pepper-Pepper golden mosaic virus (PepGMV) interaction, the host recovery process involves a silencing mechanism that includes both post-transcriptional (PTGS) and transcriptional (TGS) gene silencing pathways. Under field conditions, PepGMV is frequently found in mixed infections with Pepper huasteco yellow vein virus (PHYVV), another bipartite begomovirus. Mixed infected plants generally show a synergetic phenomenon and do not present recovery. Little is known about the molecular mechanism of this interaction. In the present study, we explored the effect of superinfection by PHYVV on a PepGMV-infected pepper plant showing recovery. Superinfection with PHYVV led to (a) the appearance of severe symptoms, (b) an increase of the levels of PepGMV DNA accumulation, (c) a decrease of the relative methylation levels of PepGMV DNA, and (d) an increase of chromatin activation marks present in viral minichromosomes. Finally, using heterologous expression and silencing suppression reporter systems, we found that PHYVV REn presents TGS silencing suppressor activity, whereas similar experiments suggest that Rep might be involved in suppressing PTGS.

Identification and Characterization of Nep1-Like Proteins From the Grapevine Downy Mildew Pathogen Plasmopara viticola

The obligate biotrophic oomycete Plasmopara viticola causes tremendous problems in viticulture by evoking grapevine downy mildew. P. viticola, like other plant pathogens, achieves infection by suppression of plant innate immunity by secretion of effector molecules into its host plant. An ever-expanding family of proteins with effector-like characteristics is formed by the "Necrosis and Ethylene inducing peptide 1 (Nep1)-like proteins" (NLPs). NLPs can be divided into two groups by their ability to induce necrosis. While cytotoxic NLPs may act as virulence factors for a necrotrophic or hemibiotrophic plant pathogen, the role of non-cytotoxic NLPs is so far unknown. In this study, we identified eight independent NLPs in P. viticola and selected three for functional analysis. While one was identified as a putative pseudo gene, two contain all so far described critical key elements for necrosis formation except for an N-terminal signal peptide. Further characterization revealed that none of the putative necrosis elicitors was able to actually induce necrosis, neither in several susceptible or resistant Vitis species nor in the dicot model plant Nicotiana benthamiana. This inability exists independently of the presence or absence of a signal peptide. However, any possible mechanism for the suppression of the ability to induce necrosis in planta was not detected. Interestingly, expression analysis of the presumed pseudo gene revealed remarkable differences between pure sporangia solution and sporangia in the presence of leaf material. To our knowledge, this is the first report of this kind of regulation that suggests an important function of so far nonfunctional "pseudo" NLP genes during the first hours of infection.

A Plant SMALL RNA-BINDING PROTEIN 1 Family Mediates Cell-to-Cell Trafficking of RNAi Signals

In plants, RNA interference (RNAi) plays a pivotal role in growth and development, and responses to environmental inputs, including pathogen attack. The intercellular and systemic trafficking of small interfering RNA (siRNA)/microRNA (miRNA) is a central component in this regulatory pathway. Currently, little is known with regards to the molecular agents involved in the movement of these si/miRNAs. To address this situation, we employed a biochemical approach to identify and characterize a conserved SMALL RNA-BINDING PROTEIN 1 (SRBP1) family that mediates non-cell-autonomous small RNA (sRNA) trafficking. In Arabidopsis, AtSRBP1 is a glycine-rich (GR) RNA-binding protein, also known as AtGRP7, which we show binds single-stranded siRNA. A viral vector, Zucchini yellow mosaic virus (ZYMV), was employed to functionally characterized the AtSRBP1-4 (AtGRP7/2/4/8) RNA recognition motif and GR domains. Cellular-based studies revealed the GR domain as being necessary and sufficient for SRBP1 cell-to-cell movement. Taken together, our findings provide a foundation for future research into the mechanism and function of mobile sRNA signaling agents in plants.