Efficient expression of foreign proteins in roots from tobravirus vectors

Expression of the Nicotiana benthamiana Retrozyme 1 (NbRZ1) Genomic Locus

Retrozymes are a class of non-autonomous plant retrotransposons that have long terminal repeats (LTRs) containing hammerhead ribozymes (HHRs) that facilitate the circularization of the retrozyme RNA. The LTR of Nicotiana benthamiana retrozyme 1 (NbRZ1) has been shown to contain a promoter that directs transcription of this retroelement. In this study, we identified the transcription start site of the promoter contained in the LTR of NbRZ1 and mapped the promoter region essential for its transcriptional activity. Using transgenic Arabidopsis thaliana plants carrying the GUS gene under the control of the NbRZ1 LTR, the NbRZ1 transcript was demonstrated to potentially encode a protein targeted for proteasomal degradation in the plant cell. Overexpression of this protein in plants using a viral expression vector was found to cause severe necrosis. The data presented suggest a tight regulation of the expression of the NbRZ1-encoded polypeptide in plants and its potential functional importance, although further research is needed to determine whether circular and/or linear retrozyme RNA forms can be translated in plants.

Viral delivery of recombinases activates heritable genetic switches in plants

Viral vectors provide an increasingly versatile platform for transformation-free reagent delivery to plants. RNA viral vectors can be used to induce gene silencing, overexpress proteins, or introduce gene editing reagents; however, they are often constrained by carrying capacity or restricted tropism in germline cells. Site-specific recombinases that catalyze precise genetic rearrangements are powerful tools for genome engineering that vary in size and, potentially, efficacy in plants. In this work, we show that viral vectors based on tobacco rattle virus (TRV) deliver and stably express four recombinases ranging in size from ∼0.6 to ∼1.5 kb and achieve simultaneous marker removal and reporter activation through targeted excision in transgenic Nicotiana benthamiana lines. TRV vectors with Cre, FLP, CinH, and Integrase13 efficiently mediated recombination in infected somatic tissue and led to heritable modifications at high frequency. An excision-activated Ruby reporter enabled simple and high-resolution tracing of infected cell lineages without the need for molecular genotyping. Together, our experiments broaden the scope of viral recombinase delivery and offer insights into infection dynamics that may be useful in developing future viral vectors.

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

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

Defense Responses Induced by Viral Movement Protein and Its Nuclear Localization Modulate Virus Cell-to-Cell Transport

Movement proteins (MPs) encoded by plant viruses are essential for cell-to-cell transport of viral genomes through plasmodesmata. The genome of hibiscus green spot virus contains a module of two MP genes termed 'binary movement block' (BMB), encoding the proteins BMB1 and BMB2. Here, BMB1 is shown to induce a defense response in Nicotiana benthamiana plants that inhibits BMB-dependent virus transport. This response is characterized by the accumulation of reactive oxygen species, callose deposition in the cell wall, and upregulation of 9-LOX expression. However, the BMB1-induced response is inhibited by coexpression with BMB2. Furthermore, BMB1 is found to localize to subnuclear structures, in particular to Cajal bodies, in addition to the cytoplasm. As shown in experiments with a BMB1 mutant, the localization of BMB1 to nuclear substructures enhances BMB-dependent virus transport. Thus, the virus transport mediated by BMB proteins is modulated by (i) a BMB1-induced defense response that inhibits transport, (ii) suppression of the BMB1-induced response by BMB2, and (iii) the nuclear localization of BMB1 that promotes virus transport. Collectively, the data presented demonstrate multiple levels of interactions between viral pathogens and their plant hosts during virus cell-to-cell transport.

Establishment and application of a root wounding-immersion method for efficient virus-induced gene silencing in plants

In the post-genomic era, virus-induced gene silencing (VIGS) has played an important role in research on reverse genetics in plants. Commonly used Agrobacterium-mediated VIGS inoculation methods include stem scratching, leaf infiltration, use of agrodrench, and air-brush spraying. In this study, we developed a root wounding-immersion method in which 1/3 of the plant root (length) was cut and immersed in a tobacco rattle virus (TRV)1:TRV2 mixed solution for 30 min. We optimized the procedure in Nicotiana benthamiana and successfully silenced N. benthamiana, tomato (Solanum lycopersicum), pepper (Capsicum annuum L.), eggplant (Solanum melongena), and Arabidopsis thaliana phytoene desaturase (PDS), and we observed the movement of green fluorescent protein (GFP) from the roots to the stem and leaves. The silencing rate of PDS in N. benthamiana and tomato was 95-100%. In addition, we successfully silenced two disease-resistance genes, SITL5 and SITL6, to decrease disease resistance in tomatoes (CLN2037E). The root wounding-immersion method can be used to inoculate large batches of plants in a short time and with high efficiency, and fresh bacterial infusions can be reused several times. The most important aspect of the root wounding-immersion method is its application to plant species susceptible to root inoculation, as well as its ability to inoculate seedlings from early growth stages. This method offers a means to conduct large-scale functional genome screening in plants.

Efficient generation of recombinant eggplant mottled dwarf virus and expression of foreign proteins in solanaceous hosts

Reverse genetics systems have only been successfully developed for a few plant rhabdoviruses. Additional systems are needed for molecular virology studies of these diverse viruses and development of viral vectors for biotechnological applications. Eggplant mottled dwarf virus (EMDV) is responsible for significant agricultural losses in various crops throughout the Mediterranean region and the Middle East. In this study, we report efficient recovery of infectious EMDV from cloned DNAs and engineering of EMDV-based vectors for the expression of foreign proteins in tobacco, eggplant, pepper, and potato plants. Furthermore, we show that the EMDV-based vectors are capable of simultaneously expressing multiple foreign proteins. The developed EMDV reverse genetics system offers a versatile tool for studying virus pathology and plant-virus interactions and for expressing foreign proteins in a range of solanaceous crops.

Development of Stable Infectious cDNA Clones of Tomato Black Ring Virus Tagged with Green Fluorescent Protein

Tomato black ring virus (TBRV) is a member of the Nepovirus genus in the Secoviridae family, which infects a wide range of important crop species worldwide. In this work, we constructed four cDNA infectious clones of the TBRV tagged with the green fluorescent protein (TBRV-GFP), which varied in (i) the length of the sequences flanking the GFP insert, (ii) the position of the GFP insert within the RNA2 polyprotein, and (iii) the addition of a self-cutting 2A protein. The presence of the GFP coding sequence in infected plants was verified by RT-PCR, while the infectivity and stability of the constructs were verified by mechanical inoculation of the host plants. The systemic spread of TBRV-GFP within plants was observed under UV light at a macroscopic level, monitoring GFP-derived fluorescence in leaves, and at a microscopic level using confocal microscopy. The obtained clones are a valuable tool for future studies of TBRV-host interactions, virus biology, and the long-term monitoring of its distribution in infected plants.

Delivery of progeny virus from the infectious clone of cucumber green mottle mosaic virus and quantification of the viral load in different host plants

Cucumber green mottle mosaic virus (CGMMV, genus Tobamovirus) is a widely occurring tobamovirus in cucurbits. The genome of CGMMV has been used previously for the expression of foreign genes in the plant. High throughput delivery and high viral titer are important requirements of foreign protein expression in plant through virus genome-based vector, in this study, Agrobacterium containing infectious construct of CGMMV was infiltrated through syringe, vacuum and high-speed spray to N. benthamiana, cucumber and bottle gourd leaves. The success rate of systemic infection of CGMMV agro-construct through all three methods was higher (80-100%) in N. benthamiana compared to the cucurbits (40-73.3%). To determine the high-throughput delivery of CGMMV in the plant system, four delivery methods viz. rubbing, syringe infiltration, vacuum infiltration and high-speed spray using the progeny virus derived through CGMMV agro-construct were compared in the three different plant species. Based on the rate of systemic infection and time required to perform delivery by different methods, vacuum infiltration was found most efficient for the high-throughput delivery of CGMMV. The quantification of CGMMV through qPCR revealed that CGMMV load varied considerably in leaf and fruit tissues depending with the time of infection. Immediately after expression of symptoms, a high load of CGMMV (~ 1 µg/100 mg of tissues) was noticed in young leaves of N. benthamiana and cucumber. In bottle gourd leaves, the CGMMV load was far low compared to N. benthamiana and cucumber plants. In the fruit tissues of cucumber and bottle gourd higher virus load was observed in mature fruit but not in immature fruit. The findings of the present study will serve as an important base line information to produce foreign protein through CGMMV genome-vector.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03630-y.

Multiple integrations of a sense transgene, including a tandem inverted repeat confer stable RNA-silencing mediated virus resistance under different abiotic and biotic conditions

In a previous study, tobacco plants, transformed with a sense construct of the 57K domain of the replicase gene of tobacco rattle virus (TRV), provided resistance against genetically distant isolates of the virus. In this work, 57K-specific siRNAs were detected with RT-qPCR solely in the resistant line verifying the RNA-silencing base of the resistance. The integration sites of the transgene into the plant genome were identified with inverse-PCR. Moreover, the resistance against TRV was practically unaffected by low temperature conditions and the presence of heterologous viruses. The mechanism of the resistance was further examined by a gene expression analysis that showed increased transcript levels of genes with a key-role in the RNA silencing pathway and the basal antiviral defence. This work provides a comprehensive characterization of the robust virus resistance obtained by a sense transgene and underlines the usefulness of transgenic plants obtained by such a strategy.

The Plant Viruses and Molecular Farming: How Beneficial They Might Be for Human and Animal Health?

Plant viruses have traditionally been studied as pathogens in the context of understanding the molecular and cellular mechanisms of a particular disease affecting crops. In recent years, viruses have emerged as a new alternative for producing biological nanomaterials and chimeric vaccines. Plant viruses were also used to generate highly efficient expression vectors, revolutionizing plant molecular farming (PMF). Several biological products, including recombinant vaccines, monoclonal antibodies, diagnostic reagents, and other pharmaceutical products produced in plants, have passed their clinical trials and are in their market implementation stage. PMF offers opportunities for fast, adaptive, and low-cost technology to meet ever-growing and critical global health needs. In this review, we summarized the advancements in the virus-like particles-based (VLPs-based) nanotechnologies and the role they played in the production of advanced vaccines, drugs, diagnostic bio-nanomaterials, and other bioactive cargos. We also highlighted various applications and advantages plant-produced vaccines have and their relevance for treating human and animal illnesses. Furthermore, we summarized the plant-based biologics that have passed through clinical trials, the unique challenges they faced, and the challenges they will face to qualify, become available, and succeed on the market.

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.

Ribozyme-processed guide RNA enhances virus-mediated plant genome editing

In virus-induced gene-editing system, subgenomic promoters have been used to express guide RNAs (gRNAs). However, the transcription initiation site of the subgenomic promoters remains elusive. Here, we examined the sequence of gRNAs expressed by subgenomic promoters and found the variable length of overhangs at 5'-end of gRNAs. The overhangs at 5'-end of gRNA decrease the cleavage activity of SpCas9. To overcome this problem, we inserted hammerhead ribozyme between the subgenomic promoter and gRNA and confirmed that gRNAs with a precise 5'-end increase the editing efficacy in wild tobacco. This system will be widely used for editing target genes in plants with high efficiency.

A GoldenBraid-Compatible Virus-Based Vector System for Transient Expression of Heterologous Proteins in Plants

We have developed a Potato virus X (PVX)-based vector system compatible with the GoldenBraid 2.0 (GB) cloning strategy to transiently express heterologous proteins or peptides in plants for biotechnological purposes. This vector system consists of three domestication vectors carrying three GB parts-the cauliflower mosaic virus (CaMV) 35S promoter with PVX upstream of the second subgenomic promoter of the PVX coat protein (PVX CP SGP), nopaline synthase (NOS) terminator with PVX downstream of the first PVX CP SGP and the gene of interest (GOI). The full-length PVX clone carrying the sequence encoding a green fluorescent protein (GFP) as GOI was incorporated into the binary GB vector in a one-step reaction of three GB parts using the four-nucleotide GB standard syntax. We investigated whether the obtained vector named GFP/pGBX enables systemic PVX infection and expression of GFP in Nicotiana benthamiana plants. We show that this GB-compatible vector system can be used for simple and efficient assembly of PVX-based expression constructs and that it meets the current need for interchange of standard biological parts used in different expression systems.

The mitochondrial and chloroplast dual targeting of a multifunctional plant viral protein modulates chloroplast-to-nucleus communication, RNA silencing suppressor activity, encapsidation, pathogenesis and tissue tropism

Plant defense against melon necrotic spot virus (MNSV) is triggered by the viral auxiliary replicase p29 that is targeted to mitochondrial membranes causing morphological alterations, oxidative burst and necrosis. Here we show that MNSV coat protein (CP) was also targeted to mitochondria and mitochondrial-derived replication complexes [viral replication factories or complex (VRC)], in close association with p29, in addition to chloroplasts. CP import resulted in the cleavage of the R/arm domain previously implicated in genome binding during encapsidation and RNA silencing suppression (RSS). We also show that CP organelle import inhibition enhanced RSS activity, CP accumulation and VRC biogenesis but resulted in inhibition of systemic spreading, indicating that MNSV whole-plant infection requires CP organelle import. We hypothesize that to alleviate the p29 impact on host physiology, MNSV could moderate its replication and p29 accumulation by regulating CP RSS activity through organelle targeting and, consequently, eluding early-triggered antiviral response. Cellular and molecular events also suggested that S/P domains, which correspond to processed CP in chloroplast stroma or mitochondrion matrix, could mitigate host response inhibiting p29-induced necrosis. S/P deletion mainly resulted in a precarious balance between defense and counter-defense responses, generating either cytopathic alterations and MNSV cell-to-cell movement restriction or some degree of local movement. In addition, local necrosis and defense responses were dampened when RSS activity but not S/P organelle targeting was affected. Based on a robust biochemical and cellular analysis, we established that the mitochondrial and chloroplast dual targeting of MNSV CP profoundly impacts the viral infection cycle.

The plasma membrane Cation binding protein 1 affects accumulation of Potato virus Y in pepper both at the systemic level and in protoplasts

The Plasma membrane Cation binding Protein 1 (PCaP1) has been shown to be important for the intra-cellular movement of two members of the Potyvirus genus in arabidopsis and tobacco plants. In this study, the orthologous PCaP1 gene of pepper (Capsicum annuum) was examined for its role in the accumulation of Potato virus Y, type member of the Potyvirus. Downregulation of C. annuum PCaP (CaPCaP) through tobacco rattle virus-induced gene silencing, resulted in lower accumulation of potato virus Y (PVY) in pepper plants. Using an improved pepper protoplast isolation protocol, we showed that knockdown of CaPCaP negatively affected PVY accumulation at the within-cell level in pepper in contrast with the turnip mosaic virus-arabidopsis pathosystem. Conversely, following overexpression of CaPCaP, the accumulation of PVY at the systemic level was increased. The results provide further knowledge on the role of PCaP in the potyvirus infection process and reveal differences of its action among different pathosystems.

Characterization of an infectious clone of pepper ringspot virus and its use as a viral vector

The genus Tobravirus comprises three species: Tobacco rattle virus, Pea early-browning virus and Pepper ringspot virus. The genomes of tobraviruses consist of two positive-sense single-stranded RNA segments (RNA1 and RNA2). Infectious clones of TRV are extensively used as virus-induced gene-silencing (VIGS) vectors for studies of virus-host interactions and functions of plant genes. Complete infectious clones of pepper ringspot virus (PepRSV), the only tobravirus present in Brazil, however, have not yet been reported. Infectious clones will help to identify unique features of PepRSV RNA2 and provide another option for development of VIGS vectors. We constructed infectious clones based on two PepRSV isolates, CAM (RNA1 and RNA2) and LAV (RNA2). The cDNA constructs for both homologous (RNA1 and RNA2 of the CAM isolate) and heterologous (RNA1/CAM and RNA2/LAV) combinations were infectious in Nicotiana benthamiana plants. VIGS vector constructs with green fluorescent protein or phytoene desaturase genes inserted in RNA2 silenced the target genes. The systemic translocation of the PepRSV RNA1 construct alone (nonmultiple infection) was also confirmed in an N. benthamiana plant. These results are similar to those reported for tobacco rattle virus.

Virus-induced gene silencing in the perennial woody Paeonia ostii

Tree peony is a perennial deciduous shrub with great ornamental and medicinal value. A limitation of its current functional genomic research is the lack of effective molecular genetic tools. Here, the first application of a Tobacco rattle virus (TRV)-based virus-induced gene silencing (VIGS) in the tree peony species Paeonia ostii is presented. Two different approaches, leaf syringe-infiltration and seedling vacuum-infiltration, were utilized for Agrobacterium-mediated inoculation. The vacuum-infiltration was shown to result in a more complete Agrobacterium penetration than syringe-infiltration, and thereby determined as an appropriate inoculation method. The silencing of reporter gene PoPDS encoding phytoene desaturase was achieved in TRV-PoPDS-infected triennial tree peony plantlets, with a typical photobleaching phenotype shown in uppermost newly-sprouted leaves. The endogenous PoPDS transcripts were remarkably down-regulated in VIGS photobleached leaves. Moreover, the green fluorescent protein (GFP) fluorescence was detected in leaves and roots of plants inoculated with TRV-GFP, suggesting the capability of TRV to silence genes in various tissues. Taken together, the data demonstrated that the TRV-based VIGS technique could be adapted for high-throughput functional characterization of genes in tree peony.

DCL-suppressed Nicotiana benthamiana plants: valuable tools in research and biotechnology

RNA silencing is a universal mechanism involved in development, epigenetic modifications and responses to biotic and abiotic stresses. The major components of this mechanism are Dicer-like (DCL), Argonaute (AGO) and RNA-dependent RNA polymerase (RDR) proteins. Understanding the role of each component is of great scientific and agronomic importance. Plants, including Nicotiana benthamiana, an important plant model, usually possess four DCL proteins, each of which has a specific role, namely being responsible for the production of an exclusive small RNA population. Here, we used RNA interference (RNAi) technology to target DCL proteins and produced single and combinatorial mutants for DCL. We analysed the phenotype for each DCL knockdown plant, together with the small RNA profile, by next-generation sequencing (NGS). We also investigated transgene expression, as well as viral infections, and were able to show that DCL suppression results in distinct developmental defects, changes in small RNA populations, increases in transgene expression and, finally, higher susceptibility in certain RNA viruses. Therefore, these plants are excellent tools for the following: (i) to study the role of DCL enzymes; (ii) to overexpress proteins of interest; and (iii) to understand the complex relationship between the plant silencing mechanism and biotic or abiotic stresses.

The Heterologous Expression of the p22 RNA Silencing Suppressor of the Crinivirus Tomato Chlorosis Virus from Tobacco Rattle Virus and Potato Virus X Enhances Disease Severity but Does Not Complement Suppressor-Defective Mutant Viruses

To counteract host antiviral RNA silencing, plant viruses express suppressor proteins that function as pathogenicity enhancers. The genome of the Tomato chlorosis virus (ToCV) (genus Crinivirus, family Closteroviridae) encodes an RNA silencing suppressor, the protein p22, that has been described as having one of the longest lasting local suppressor activities when assayed in Nicotiana benthamiana. Since suppression of RNA silencing and the ability to enhance disease severity are closely associated, we analyzed the effect of expressing p22 in heterologous viral contexts. Thus, we studied the effect of the expression of ToCV p22 from viral vectors Tobacco rattle virus (TRV) and Potato virus X (PVX), and from attenuated suppressor mutants in N. benthamiana plants. Our results show that although an exacerbation of disease symptoms leading to plant death was observed in the heterologous expression of ToCV p22 from both viruses, only in the case of TRV did increased viral accumulation occur. The heterologous expression of ToCV p22 could not complement suppressor-defective mutant viruses.

Virus-Based MicroRNA Silencing and Overexpressing in Common Wheat (Triticum aestivum L.)

MicroRNAs (miRNAs) are a class of endogenous small non-coding RNAs that arise from large RNA precursors with a stem-loop structure and play important roles in plant development and responses to environmental stress. Although a hundred and nineteen wheat miRNAs have been identified and registered in the miRBase (Release 21.0, June, 2014; http://www.mirbase.org), the functional characterization of these miRNAs in wheat growth and development is lagging due to lack of effective techniques to investigate endogenous miRNA functions in wheat. Here we report barley stripe mosaic virus(BSMV)-based miRNA overexpression and silence systems that can be applied to study miRNA functions in wheat. By utilizing the BSMV system, we successfully knocked down endogenous miR156 and miR166 levels and over-expressed endogenous miR156 and artificial miRNA against phytoene desaturase gene PDS (amiR-PDS) in wheat. amiR-PDS expression caused a great reduction in endogenous mRNA abundance of PDS gene in wheat plant, leading to leaf obviously photobleaching. miR156 silencing led to a great increase in the mRNA level of its target gene SPL2, resulting in a leaf-curl phenotype in wheat seedlings. In contrast, overexpression of miR156 led to a significant reduction in the mRNA level of SPL2 in wheat seedlings, resulting in serious delay of the vegetative phase transitions as well as booting and flowering in wheat. These confirmed that miR156 regulates wheat development and booting time through SPL genes. In summary, the BSMV-based miRNA overexpression and silence systems have extraordinary potential not only for functional study of protein-encoding genes but also for miRNA genes in wheat.

Transient Expression Systems in Plants: Potentialities and Constraints

Plants have been used from old to extract and isolate by different means the products of interest that they store. In recent years new techniques have emerged that allow the use of plants as factories to overexpress transiently and often efficiently, specific genes of interest, either endogenous or foreign, in their native form or modified. These techniques allow and facilitate the targeted purification of gene products for research and commercial purposes without resorting to lengthy, time-consuming and sometimes challenging plant stable transformations, while avoiding some of their associated regulatory constraints. In this chapter we describe the main strategies available for the transient expression of gene sequences and their encoded products in plants. We discuss biological issues affecting transient expression, including resistance responses elicited by the plant against sequences that it recognizes naturally as foreign, and ways to neutralize them. We also discuss the relative advantages of each expression strategy as well as their inherent drawbacks and technical limitations, and how to partially prevent or overcome them, whenever possible.

When plant virology met Agrobacterium: the rise of the deconstructed clones

In the early days of molecular farming, Agrobacterium-mediated stable genetic transformation and the use of plant virus-based vectors were considered separate and competing technologies with complementary strengths and weaknesses. The demonstration that 'agroinfection' was the most efficient way of delivering virus-based vectors to their target plants blurred the distinction between the two technologies and permitted the development of 'deconstructed' vectors based on a number of plant viruses. The tobamoviruses, potexviruses, tobraviruses, geminiviruses and comoviruses have all been shown to be particularly well suited to the development of such vectors in dicotyledonous plants, while the development of equivalent vectors for use in monocotyledonous plants has lagged behind. Deconstructed viral vectors have proved extremely effective at the rapid, high-level production of a number of pharmaceutical proteins, some of which are currently undergoing clinical evaluation.

Different roles for RNA silencing and RNA processing components in virus recovery and virus-induced gene silencing in plants

A major antiviral mechanism in plants is mediated by RNA silencing, which relies on the cleavage of viral dsRNA into virus-derived small interfering RNAs (vsiRNAs) by DICER-like enzymes. Members of the Argonaute (AGO) family of endonucleases then use these vsiRNA as guides to target viral RNA. This can result in a phenomenon known as recovery, whereby the plant silences viral gene expression and recovers from viral symptoms. Endogenous mRNAs can also be targeted by vsiRNAs in a phenomenon known as virus-induced gene silencing (VIGS). Although related to other RNA silencing mechanisms, it has not been established if recovery and VIGS are mediated by the same molecular mechanisms. We used tobacco rattle virus (TRV) carrying a fragment of the phytoene desaturase (PDS) gene (TRV-PDS) or expressing green fluorescent protein (TRV-GFP) as readouts for VIGS and recovery, respectively, in Arabidopsis ago mutants. Our results demonstrated roles for AGO2 and AGO4 in susceptibility to TRV, whereas VIGS of endogenous genes appeared to be largely mediated by AGO1. However, recovery appeared to be mediated by different components, as all the aforementioned mutants were able to recover from TRV-GFP inoculation. TRV RNAs from recovered plants associated less with ribosomes, suggesting that recovery involves translational repression of viral transcripts. Translationally repressed RNAs often accumulate in RNA processing bodies (PBs), where they are eventually processed by decapping enzymes. Consistent with this, we found that viral recovery induced increased PB formation and that a decapping mutant (DCP2) showed increased VIGS and virus RNA accumulation, indicating an important role for PBs in eliminating viral RNA.

Virus-induced gene silencing for comparative functional studies in Gladiolus hybridus

Functional analysis of genes in gladiolus has previously been impractical due to the lack of an efficient stable genetic transformation method. However, virus-induced gene silencing (VIGS) is effective in some plants which are difficult to transform through other methods. Although the Tobacco rattle virus (TRV)-based VIGS system has been developed and used for verifying gene functions in diverse plants, an appropriate TRV-VIGS approach for gladiolus has not been established yet. In this report we describe the first use of the TRV-VIGS system for gene silencing in gladiolus. Vacuum infiltration of cormels and young plants with the GhPDS-VIGS vector effectively down-regulated the PHYTOENE DESATURASE ortholog GhPDS gene and also resulted in various degrees of photobleaching in Gladiolus hybridus. The reduction in GhPDS expression was tested after TRV-based vector infection using real-time RT-PCR. In addition, the progress of TRV infection was detected by fluorescence visualization using a pTRV2: CP-GFP vector. In conclusion, the TRV-mediated VIGS described here will be an effective gene function analysis mechanism in gladiolus.

Coilin, the signature protein of Cajal bodies, differentially modulates the interactions of plants with viruses in widely different taxa

Cajal bodies (CBs) are distinct nuclear bodies physically and functionally associated with the nucleolus. In addition to their traditional function in coordinating maturation of certain nuclear RNAs, CBs participate in cell cycle regulation, development, and regulation of stress responses. A key "signature" component of CBs is coilin, the scaffolding protein essential for CB formation and function. Using an RNA silencing (loss-of-function) approach, we describe here new phenomena whereby coilin also affects, directly or indirectly, a variety of interactions between host plants and viruses that have RNA or DNA genomes. Moreover, the effects of coilin on these interactions are manifested differently: coilin contributes to plant defense against tobacco rattle virus (tobravirus), tomato black ring virus (nepovirus), barley stripe mosaic virus (hordeivirus), and tomato golden mosaic virus (begomovirus). In contrast, with potato virus Y (potyvirus) and turnip vein clearing virus (tobamovirus), coilin serves to increase virus pathogenicity. These findings show that interactions with coilin (or CBs) may involve diverse mechanisms with different viruses and that these mechanisms act at different phases of virus infection. Thus, coilin (CBs) has novel, unexpected natural functions that may be recruited or subverted by plant viruses for their own needs or, in contrast, are involved in plant defense mechanisms that suppress host susceptibility to the viruses.

Modification of Tobacco rattle virus RNA1 to serve as a VIGS vector reveals that the 29K movement protein is an RNA silencing suppressor of the virus

Tobacco rattle virus (TRV) has a bipartite, positive-sense single-stranded RNA genome and is widely used for virus-induced gene silencing (VIGS) in plants. RNA1 of TRV that lacks the gene for the cysteine-rich 16K silencing-suppression protein infects plants systemically in the absence of RNA2. Here, we attempted to engineer RNA1 for use as a VIGS vector by inserting heterologous gene fragments to replace 16K. The RNA1 vector systemically silenced the phytoene desaturase (PDS) gene, although less efficiently than when the original VIGS vector system was used, which consists of wild-type RNA1 and engineered RNA2 carrying the heterologous gene. Infectious RNA1 mutants with a dysfunctional 16K suppressed silencing and enhanced transgene expression in green fluorescent protein-transgenic Nicotiana benthamiana following inoculation by agroinfiltration, unlike mutants that also lacked 29K, a movement protein (MP) gene. The 30K MP gene of Tobacco mosaic virus complemented in cis the movement defect but not the silencing suppression functions of TRV 29K. Silencing suppression by 29K occurred in the context of RNA1 replication but not in an agroinfiltration assay which tested 29K alone for suppression of sense-mediated silencing. Both 29K and 16K were needed to avoid necrotic symptoms in RNA1-infected N. benthamiana. The results shed new light on virulence factors of TRV.

Dynamic localization of two tobamovirus ORF6 proteins involves distinct organellar compartments

ORF6 is a small gene that overlaps the movement and coat protein genes of subgroup 1a tobamoviruses. The ORF6 protein of tomato mosaic virus (ToMV) strain L (L-ORF6), interacts in vitro with eukaryotic elongation factor 1α, and mutation of the ORF6 gene of tobacco mosaic virus (TMV) strain U1 (U1-ORF6) reduces the pathogenicity in vivo of TMV, whereas expression of this gene from two other viruses, tobacco rattle virus (TRV) and potato virus X (PVX), increases their pathogenicity. In this work, the in vivo properties of the L-ORF6 and U1-ORF6 proteins were compared to identify sequences that direct the proteins to different subcellular locations and also influence virus pathogenicity. Site-specific mutations in the ORF6 protein were made, hybrid ORF6 proteins were created in which the N-terminal and C-terminal parts were derived from the two proteins, and different subregions of the protein were examined, using expression either from a recombinant TRV vector or as a yellow fluorescent protein fusion from a binary plasmid in Agrobacterium tumefaciens. L-ORF6 caused mild necrotic symptoms in Nicotiana benthamiana when expressed from TRV, whereas U1-ORF6 caused severe symptoms including death of the plant apex. The difference in symptoms was associated with the C-terminal region of L-ORF6, which directed the protein to the endoplasmic reticulum (ER), whereas U1-ORF6 was directed initially to the nucleolus and later to the mitochondria. Positively charged residues at the N terminus allowed nucleolar entry of both U1-ORF6 and L-ORF6, but hydrophobic residues at the C terminus of L-ORF6 directed this protein to the ER.

RNA viral vectors for improved Agrobacterium-mediated transient expression of heterologous proteins in Nicotiana benthamiana cell suspensions and hairy roots

BACKGROUND

Plant cell suspensions and hairy root cultures represent scalable protein expression platforms. Low protein product titers have thus far limited the application of transient protein expression in these hosts. The objective of this work was to overcome this limitation by harnessing A. tumefaciens to deliver replicating and non-replicating RNA viral vectors in plant tissue co-cultures.

RESULTS

Replicating vectors derived from Potato virus X (PVX) and Tobacco rattle virus (TRV) were modified to contain the reporter gene β-glucuronidase (GUS) with a plant intron to prevent bacterial expression. In cell suspensions, a minimal PVX vector retaining only the viral RNA polymerase gene yielded 6.6-fold more GUS than an analogous full-length PVX vector. Transient co-expression of the minimal PVX vector with P19 of Tomato bushy stunt virus or HC-Pro of Tobacco etch virus to suppress post-transcriptional gene silencing increased GUS expression by 44 and 83%, respectively. A non-replicating vector containing a leader sequence from Cowpea mosaic virus (CPMV-HT) modified for enhanced translation led to 70% higher transient GUS expression than a control treatment. In hairy roots, a TRV vector capable of systemic movement increased GUS accumulation by 150-fold relative to the analogous PVX vector. Histochemical staining for GUS in TRV-infected hairy roots revealed the capacity for achieving even higher productivity per unit biomass.

CONCLUSIONS

For the first time, replicating PVX vectors and a non-replicating CPMV-HT vector were successfully applied toward transient heterologous protein expression in cell suspensions. A replicating TRV vector achieved transient GUS expression levels in hairy roots more than an order of magnitude higher than the highest level previously reported with a viral vector delivered by A. tumefaciens.

Molecular cloning and functional characterization of a putative sulfite oxidase (SO) ortholog from Nicotiana benthamiana

Sulfite oxidase (SO) catalyzes the oxidation of sulfite to sulfate and thus has important roles in diverse metabolic processes. However, systematic molecular and functional investigations on the putative SO from tobacco (Nicotiana benthamiana) have hitherto not been reported. In this work, a full-length cDNA encoding putative sulfite oxidase from N. benthamiana (NbSO) was isolated. The deduced NbSO protein shares high homology and typical structural features with other species SOs. Phylogenetic analysis indicates that NbSO cDNA clone encodes a tobacco SO isoform. Southern blot analysis suggests that NbSO is a single-copy gene in the N. benthamiana genome. The NbSO transcript levels were higher in aerial tissues and were up-regulated in N. benthamiana during sulfite stress. Reducing the SO expression levels through virus-induced gene silencing caused a substantial accumulation in sulfite content and less sulfate accumulation in N. benthamiana leaves when exposed to sulfite stress, and thus resulted in decreased tolerance to sulfite stress. Taken together, this study improves our understanding on the molecular and functional properties of plant SO and provides genetic evidence on the involvement of SO in sulfite detoxification in a sulfite-oxidizing manner in N. benthamiana plants.

STARTS--a stable root transformation system for rapid functional analyses of proteins of the monocot model plant barley

Large data sets are generated from plants by the various 'omics platforms. Currently, a limiting step in data analysis is the assessment of protein function and its translation into a biological context. The lack of robust high-throughput transformation systems for monocotyledonous plants, to which the vast majority of crop plants belong, is a major restriction and impedes exploitation of novel traits in agriculture. Here we present a stable root transformation system for barley, termed STARTS, that allows assessment of gene function in root tissues within 6 weeks. The system is based on the finding that a callus, produced on root induction medium from the scutellum of the immature embryo, is able to regenerate roots from single transformed cells by concomitant suppression of shoot development. Using Agrobacterium tumefaciens-mediated transfer of genes involved in root development and pathogenesis, we show that those calli regenerate large amounts of uniformly transformed roots for in situ functional analysis of newly expressed proteins.

Efficient production of human acidic fibroblast growth factor in pea (Pisum sativum L.) plants by agroinfection of germinated seeds

BACKGROUND

For efficient and large scale production of recombinant proteins in plants transient expression by agroinfection has a number of advantages over stable transformation. Simple manipulation, rapid analysis and high expression efficiency are possible. In pea, Pisum sativum, a Virus Induced Gene Silencing System using the pea early browning virus has been converted into an efficient agroinfection system by converting the two RNA genomes of the virus into binary expression vectors for Agrobacterium transformation.

RESULTS

By vacuum infiltration (0.08 Mpa, 1 min) of germinating pea seeds with 2-3 cm roots with Agrobacteria carrying the binary vectors, expression of the gene for Green Fluorescent Protein as marker and the gene for the human acidic fibroblast growth factor (aFGF) was obtained in 80% of the infiltrated developing seedlings. Maximal production of the recombinant proteins was achieved 12-15 days after infiltration.

CONCLUSIONS

Compared to the leaf injection method vacuum infiltration of germinated seeds is highly efficient allowing large scale production of plants transiently expressing recombinant proteins. The production cycle of plants for harvesting the recombinant protein was shortened from 30 days for leaf injection to 15 days by applying vacuum infiltration. The synthesized aFGF was purified by heparin-affinity chromatography and its mitogenic activity on NIH 3T3 cells confirmed to be similar to a commercial product.

Plasmodesmata: gateways to local and systemic virus infection

As channels that provide cell-to-cell connectivity, plasmodesmata are central to the local and systemic spread of viruses in plants. This review discusses the current state of knowledge of the structure and function of these channels and the ways in which viruses bring about functional changes that allow macromolecular trafficking to occur. Despite the passing of two decades since the first identification of a viral movement protein that mediates these changes, our understanding of the relevant molecular mechanisms remains in its infancy. However, viral movement proteins provide valuable tools for the modification of plasmodesmata and will continue to assist in the dissection of plasmodesmal properties in relation to their core roles in cell-to-cell communication.

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.

Tobacco rattle virus mediates gene silencing in a plant parasitic root-knot nematode

Root-knot nematodes (RKNs) are sedentary biotrophic parasites that induce the differentiation of root cells into feeding cells that provide the nematodes with the nutrients necessary for their development. The development of new control methods against RKNs relies greatly on the functional analysis of genes that are crucial for the development of the pathogen or the success of parasitism. In the absence of genetic transformation, RNA interference (RNAi) allows for phenotype analysis of nematode development and nematode establishment in its host after sequence-specific knock-down of the targeted genes. Strategies used to induce RNAi in RKNs are so far restricted to small-scale analyses. In the search for a new RNAi strategy amenable to large-scale screenings the possibility of using RNA viruses to produce the RNAi triggers in plants was tested. Tobacco rattle virus (TRV) was tested as a means to introduce double-stranded RNA (dsRNA) triggers into the feeding cells and to mediate RKN gene silencing. It was demonstrated that virus-inoculated plants can produce dsRNA and siRNA silencing triggers for delivery to the feeding nematodes. Interestingly, the knock-down of the targeted genes was observed in the progeny of the feeding nematodes, suggesting that continuous ingestion of dsRNA triggers could be used for the functional analysis of genes involved in early development. However, the heterogeneity in RNAi efficiency between TRV-inoculated plants appears as a limitation to the use of TRV-mediated silencing for the high-throughput functional analysis of the targeted nematode genes.

Use of a defective RNA of broad bean mottle bromovirus for stable gene expression in legumes

A viral vector that expressed foreign genes was engineered using a cDNA clone of broad bean mottle bromovirus (BBMV) defective interfering (DI) RNA. The expression vector required an active ORF to ensure efficient accumulation and replication in the host plants. After mechanical inoculation with BBMV RNAs, expression of the green fluorescent protein (GFP) reporter was driven by DI RNA constructs during consecutive passages through broad bean plants. Our data prove that BBMV DI RNAs, whose yield is similar to the virus genomic components, are useful as gene vectors after mechanical inoculation on legume plants.

Production of recombinant proteins in clonal root cultures using episomal expression vectors

We have developed a fully contained system for expressing recombinant proteins that is based on clonal root cultures and episomal expression vectors. Clonal root lines expressing green fluorescent protein (GFP) or human growth hormone were generated from Nicotiana benthamiana leaves infected with the tobacco mosaic virus-based vector 30B after exposure to Agrobacterium rhizogenes. These lines accumulated GFP at over 50 mg per kg fresh tissue, a level that is comparable with other plant production systems in early stage development. Accumulation of both hGH and GFP in the clonal root lines was sustained over a 3-year period, and in the absence of antibiotic selection. This technology shows promise for commercial production of vaccine antigens and therapeutic proteins in contained facilities.

Distribution and pathway for phloem-dependent movement of Melon necrotic spot virus in melon plants

The translocation of Melon necrotic spot virus (MNSV) within tissues of inoculated and systemically infected Cucumis melo L. 'Galia' was studied by tissue-printing and in situ hybridization techniques. The results were compatible with the phloem vascular components being used to spread MNSV systemically by the same assimilate transport route that runs from source to sink organs. Virus RNAs were shown to move from the inoculated cotyledon toward the hypocotyl and root system via the external phloem, whereas the upward spread through the stem to the young tissues took place via the internal phloem. Virus infection was absent from non-inoculated source tissues as well as from both shoot and root apical meristems, but active sink tissues such as the young leaves and root system were highly infected. Finally, our results suggest that the MNSV invasion of roots is due to virus replication although a destination-selective process is probably necessary to explain the high levels of virus accumulation in roots. This efficient invasion of the root system is discussed in terms of natural transmission of MNSV by the soil-borne fungal vector.

Virus-induced gene silencing (VIGS) as a reverse genetic tool to study development of symbiotic root nodules

Virus-induced gene silencing (VIGS) can provide a shortcut to plants with altered expression of specific genes. Here, we report that VIGS of the Nodule inception gene (Nin) can alter the nodulation phenotype and Nin gene expression in Pisum sativum. PsNin was chosen as target because of the distinct non-nodulating phenotype of nin mutants in P. sativum, Lotus japonicus, and Medicago truncatula. The vector based on Pea early browning virus (PEBV) was engineered to carry one of three nonoverlapping fragments (PsNinA, PsNinB, and PsNinC) derived from the PsNin cDNA. Vector inoculation was mediated by agroinfiltration and, 2 weeks later, a Rhizobium leguminosarum bv. viceae culture was added in order to induce root nodulation. At this time point, it was estimated that systemic silencing was established because leaves of reference plants inoculated with PEBV carrying a fragment of Phytoene desaturase displayed photo bleaching. Three weeks after Rhizobium spp. application, plants inoculated with a control vector nodulated normally, whereas nodulation was almost eliminated in plants inoculated with a vector carrying PsNinA and PsNinC. For plants inoculated with a vector carrying PsNinB, nodulation was reduced by at least 45%. Down-regulation of PsNin transcripts in plants inoculated with vectors carrying PsNin cDNA fragments was confirmed and these plants displayed a relative increase in the root/shoot ratio, as expected if nitrogen fixation had been impaired.

Virus-induced gene silencing for functional analysis of selected genes

Virus-induced gene silencing (VIGS) is a technology that exploits an RNA-mediated antiviral defense mechanism and has been shown to be of great potential in plant reverse genetics. Circumvention of plant transformation, methodological simplicity, robustness, and speedy results makes VIGS an attractive alternative instrument in functional genomics, even in a high throughput fashion. The system is well established in Nicotiana benthamiana, and efforts are being made to improve VIGS in other species, including monocots. Here, we discuss the issues specific to the application of VIGS technology to determine gene function, which has revealed the roles of a variety of genes in disease resistance, abiotic stress, cellular signaling and secondary metabolite biosynthesis.

Tobacco rattle virus 29K movement protein is the elicitor of extreme and hypersensitive-like resistance in two cultivars of Solanum tuberosum

Leaf infection experiments were used to analyze the host responses of Solanum tuberosum cultivars known to be resistant or susceptible to natural, nematode-mediated infection of tubers and necrosis induction ("spraing") by Tobacco rattle virus (TRV) isolate PpK20 (TRV-PpK20). Extreme and hypersensitive-like resistance (ER and HR-like, respectively) as well as spreading veinal necrosis and systemic infection were observed. Agroinfection of leaves with a DsRed-expressing TRV cDNA clone revealed ER to function on the single-cell level, inhibiting virus replication and possessing the potential to initiate a cell death response. HR-like necrosis was characterized by initial virus replication and cell-to-cell movement before the onset of necrosis. Transient agroexpression and Potato virus X (PVX)-mediated expression assays demonstrated that the 29K-PpK20 movement protein (MP) can elicit ER and HR-like cell-death. A TRV isolate, PpO85M, known to overcome the resistance to spraing in plants that are resistant to TRV-PpK20 encoded a variant 29K protein which did not elicit HR in PpK20-HR plants. Our results show that the TRV MP is the elicitor of both ER and HR-like cell-death, that no other TRV-encoded proteins or RNA replication are required for its elicitor activity, and that the host reactions are likely to be controlled by single dominant resistance genes.

Delivery of macromolecules to plant parasitic nematodes using a tobacco rattle virus vector

Plant parasitic nematodes cause significant damage to crops on a worldwide scale. These nematodes are often soil dwelling but rely on plants for food and to sustain them during reproduction. Complex interactions occur between plants and nematodes during the nematode life cycle with plant roots developing specialized feeding structures through which nematodes withdraw nutrients. Here we describe a novel method for delivering macromolecules to feeding nematodes using a virus-based vector [tobacco rattle virus (TRV)]. We show that the parasitic nematode Heterodera schachtii will ingest fluorescent proteins transiently expressed in plant roots infected with a TRV construct carrying the appropriate protein sequence. A prerequisite for this delivery is the presence of replicating virus in root tips prior to the formation of nematode-induced syncytia. We show also that TRV vectors expressing nematode gene sequences can be used to induce RNAi in the feeding nematodes.

Infection, propagation, distribution and stability of plant virus in hairy root cultures

Nicotiana benthamiana hairy root cultures were infected with tobacco mosaic virus (TMV) and used for in vitro plant virus propagation. The roots were infected with TMV by addition of virus to the medium at the same time as root inoculation. Viral accumulation in the biomass was 7-11-fold greater when the initial infection was carried out in B5 medium rather than sodium phosphate buffer; virus accumulation also increased with increasing viral inoculum concentration. The amount of TMV accumulated in the biomass was similar when virus was retained in the medium for the duration of the cultures and when the inoculum virus was removed 23h after addition to the roots. In roots with established infections, the concentration of virus remained relatively constant and did not increase with further root growth. The distribution of virus within individual root mats harvested from shake flasks was not uniform; there was also significant variability in viral accumulation between replicate hairy root cultures. The picture that emerges from this work is that in vitro viral accumulation in hairy root cultures depends strongly on the viral inoculum concentration applied and the initial level of primary infection achieved, even though primary infection by external virus occurs mainly within only the first few hours of exposure to the biomass and is followed by substantial secondary infection by viral progeny within the root tissue.

Plant tissue distribution and chemical inactivation of six carnation viruses

Carnation mottle virus (CarMV), Carnation etched ring virus (CERV), Carnation vein mottle virus (CVMV), Carnation ringspot virus (CRSV), Carnation Italian ringspot virus (CIRV) and Carnation latent virus (CLV) are the most important viruses affecting carnation crops. All except CERV are RNA viruses. Viral RNA or DNA accumulation on root, stem, leaf, sepal, petal, stamen, pistil and ovary tissues of infected carnation or Saponaria vaccaria plants was analysed by non-isotopic molecular hybridisation. High-titres of CarMV, CRSV, CIRV, and CLV accumulated in all plant tissues whereas CERV and CVMV were irregularly distributed over the plant. High-titres of all viruses accumulated in leaf, petal, stamen, pistil, and ovary tissues, so leaves or petals are a good tissue for routine diagnosis. Six chemicals were evaluated for inactivation of all carnation viruses in infected extracts. Commercial bleach at 7% (v/v) or NaOH at 0.5% (w/v) was found to inactivate all viruses after 60 s treatment in a systemic S. vaccaria bioassay.

Stability of recombinant plant viruses containing genes of unrelated plant viruses

The stability of hybrid plant viruses that might arise by recombination in transgenic plants was examined using hybrid viruses derived from the viral expression vectors potato virus X (PVX) and tobacco rattle virus (TRV). The potato virus Y (PVY) NIb and HCPro open reading frames (ORFs) were introduced into PVX to generate PVX-NIb and PVX-HCPro, while the PVY NIb ORF was introduced into a vector derived from TRV RNA2 to generate TRV-NIb. All three viruses were unstable and most of the progeny viruses had lost the inserted sequences between 2 and 4 weeks post-inoculation. There was some variation in the rate of loss of part or all of the inserted sequence and the number of plants containing the deleted viruses, depending on the sequence, the host (Nicotiana tabacum vs Nicotiana benthamiana) or the vector, although none of these factors was associated consistently with the preferential loss of the inserted sequences. PVX-NIb was unable to accumulate in NIb-transgenic tobacco resistant to infection by PVY and also showed loss of the NIb insert from PVX-NIb in some NIb-transgenic tobacco plants susceptible to infection by PVY. These data indicate that such hybrid viruses, formed in resistant transgenic plants from a transgene and an unrelated virus, would be at a selective disadvantage, first by being targeted by the resistance mechanism and second by not being competitive with the parental virus.

Propagation of plant viruses in hairy root cultures: a potential method for in vitro production of epitope vaccines and foreign proteins

Hairy roots were used as an in vitro culture system for the propagation of wild-type and transgenic plant viruses. Tobacco mosaic virus (TMV) was added to the liquid culture medium at the same time as root inoculation. Hairy root growth was unaffected by viral infection. Maximum concentrations of TMV in Nicotiana benthamiana hairy roots were 1-2 orders of magnitude greater than in suspended N. benthamiana cells and reached levels of 1-2 mg g(-1) dry weight or 20-28% total soluble protein. Virus accumulated in the roots initially with a constant doubling time of about 1.0 day; subsequent reductions in viral growth rate were correlated with a significant decline in infectivity relative to the inoculum virus. The morphological integrity of the viral particles was maintained during propagation in hairy roots. The contribution to the overall viral titer of passive association of virus with the biomass, for example, by surface adsorption, was negligible compared with active viral replication. N. benthamiana hairy roots were also infected with a TMV-based viral vector developed to express green fluorescent protein (GFP). This vector was about 260-fold less infectious than wild-type TMV and accumulated much more slowly in the roots. Maximum levels of TMV-GFP in the biomass were about 65-fold lower than for TMV. This work demonstrates that hairy root cultures are a feasible means for in vitro propagation of wild-type and transgenic plant viruses under conditions that allow a high degree of environmental containment and control.

Bioreactor technology: a novel industrial tool for high-tech production of bioactive molecules and biopharmaceuticals from plant roots

Plants are the richest source for different bioactive molecules. Because of the vast number of side effects associated with synthetic pharmaceuticals, medical biotechnologists turned to nature to provide new promising therapeutic molecules from plant biofactories. The large-scale availability of the disease- and pesticide-free raw material is, however, restricted in vivo. Many bioactive plant secondary metabolites are accumulated in roots. Engineered plants can also produce human therapeutic proteins. Vaccines and diagnostic monoclonal antibodies can be won from their roots, so that engineered plants hold immense potential for the biopharmaceutical industry. To obtain sufficient amounts of the plant bioactive molecules for application in human therapy, adventitious and hairy roots have to be cultured in in vitro systems. High-tech pilot-scale bioreactor technology for the establishment of a long-term adventitious root culture from biopharmaceutical plants has recently been established. In this review, I briefly discuss a technology for cultivating bioactive molecule-rich adventitious and hairy roots from plants using a high-tech bioreactor system, as well as the principles and application of genome-restructuring mechanisms for plant-based biopharmaceutical production from roots. High-tech bioreactor-derived bioactive phytomolecules and biopharmaceuticals hold the prospect of providing permanent remedies for improving human well-being.

Zygocactus virus X-based expression vectors and formation of rod-shaped virus-like particles in plants by the expressed coat proteins of Beet necrotic yellow vein virus and Soil-borne cereal mosaic virus

Expression vectors were constructed from 35S promoter-containing full-length cDNA clones of Zygocactus virus X (ZVX). The expression of foreign genes was driven by the ZVX coat protein (cp) subgenomic promoter. It was successful only when the variable region downstream of the conserved putative promoter region GSTTAAGTT(X(12-13))GAA was retained. Most of the ZVX cp gene, except for a short 3' part, was replaced by the corresponding sequence of the related Schlumbergera virus X (SVX) and its cp subgenomic promoter to enable encapsidation of the transcribed RNA by an SVX/ZVX hybrid cp. Vector-expressed cp of Beet necrotic yellow vein virus (BNYVV) assembled in Chenopodium quinoa, Tetragonia expansa and Beta vulgaris leaves into particles resembling true BNYVV particles. The virus produced from these constructs retained its ability to express BNYVV cp in local infections during successive passages on C. quinoa. This ability was lost, however, in the rarely occurring systemic infections.