A ribozyme gene and an antisense gene are equally effective in conferring resistance to tobacco mosaic virus on transgenic tobacco

Transgenic resistance

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

Unfolding of in planta activity of anti-rep ribozyme in presence of a RNA silencing suppressor

Antisense RNA ribozymes have intrinsic endonucleolytic activity to effect cleavage of the target RNA. However, this activity in vivo is often controlled by the dominance of antisense or other double-stranded RNA mechanism. In this work, we demonstrate the in planta activity of a hammerhead ribozyme designed to target rep-mRNA of a phytopathogen Mungbean Yellow Mosaic India virus (MYMIV) as an antiviral agent. We also found RNA-silencing is induced on introduction of catalytically active as well as inactive ribozymes. Using RNA-silencing suppressors (RSS), we demonstrate that the endonucleolytic activity of ribozymes is a true phenomenon, even while a mutated version may demonstrate a similar down-regulation of the target RNA. This helps to ease the confusion over the action mechanism of ribozymes in vivo.

The potential of genetically enhanced plants to address food insecurity

Food insecurity is one of the most important social issues faced today, with 840 million individuals enduring chronic hunger and three billion individuals suffering from nutrient deficiencies. Most of these individuals are poverty stricken and live in developing countries. Strategies to address food insecurity must aim to increase agricultural productivity in the developing world in order to tackle poverty, and must provide long-term improvements in crop yields to keep up with demand as the world's population grows. Genetically enhanced plants provide one route to sustainable higher yields, either by increasing the intrinsic yield capability of crop plants or by protecting them from biotic and abiotic constraints. The present paper discusses a range of transgenic approaches that could increase agricultural productivity if applied on a large scale, including the introduction of genes that confer resistance to pests and diseases, or tolerance of harsh environments, and genes that help to lift the intrinsic yield capacity by increasing metabolic flux towards storage carbohydrates, proteins and oils. The paper also explores how the nutritional value of plants can be improved by genetic engineering. Transgenic plants, as a component of integrated strategies to relieve poverty and deliver sustainable agriculture to subsistence farmers in developing countries, could have a significant impact on food security now and in the future.

Intervention of geminiviral replication in yeast by ribozyme mediated downregulation of its Rep protein

Geminiviruses pose serious threat to many economically important crops such as mungbean, tomato, cotton, etc. To devise a specific antiviral strategy at the viral DNA replication level, a hammerhead ribozyme was directed against the mRNA of the replication initiator protein (Rep). Rep is the most important viral protein for the DNA replication of the Mungbean yellow mosaic India virus (MYMIV), a member of the Geminiviridae family. The ribozyme showed approximately 33% cleavage activity on synthetic rep transcript within 1h under in vitro conditions, whereas the mutant ribozyme, designed to lack the catalytic activity but target the same site, showed no cleavage. The in vivo efficiency of ribozyme was evaluated in Saccharomyces cerevisiae as it can act as a surrogate host for replication of the MYMIV-DNA and lacks RNAi machinery. In the presence of the ribozyme, growth of the yeast cells that are dependent on geminiviral replication was inhibited by 30% and cellular generation time was increased by 2h. The RT-PCR analysis showed a maximum of about 50% reduction in the rep mRNA level in presence of the ribozyme compared to its noncatalytic mutant control. About 65% decrease in geminiviral DNA replication was observed due to the downregulation of replication initiator protein by the ribozyme. These results raise the possibility of engineering resistance to geminiviruses employing the ribozyme approach.

Development of an efficient cis-trans-cis ribozyme cassette to inactivate plant genes

Inactivation of a targeted gene is one of the main strategies used to understand their precise cellular role. In plants, apart from chemical or physical mutagenesis and random insertions of DNA elements followed by screening for a desired phenotype, the most common strategy to inhibit the expression of a given gene involves RNA silencing. This can be achieved either through antisense suppression, sense over-expression leading to co-suppression, or expression of double-stranded DNA constructs (dsRNA). The use of ribozymes to inhibit gene product accumulation has only been occasionally attempted, mainly because of the more complex genetic engineering procedure involved, although the specificity of ribozymes can be an important factor when targeting close members of a gene family. We report here the development of a new cis-acting ribozyme cassette for the production of RNAs with desired termini. Attention to many details has been brought in order to provide a powerful procedure for plant application. For example, ultrastable GNRA tetraloops were substituted for both loops II and III of cis-acting hammerhead sequences, thereby favouring folding into the catalytically active structure that results in the self-cleavage of all transcripts. We demonstrate the usefulness of this cassette by producing a ribozyme that cleaves in trans, originally embedded in the cis-acting self-cleaving cassette. The activity of the cis-trans-cis construct, was demonstrated both in vitro and in vivo, in transgenic plants with the specific cleavage of an mRNA encoding a 2-oxo-glutarate-dependant dioxygenase predominantly expressed in pistils tissues and in leaves, from the wild potato Solanum chacoense.

Accumulation of barley stripe mosaic virus is significantly reduced in transgenic wheat plants expressing a bacterial ribonuclease

An rnc70 gene encoding a mutant bacterial ribonuclease III (RNase III) was introduced into wheat (Triticum aestivum cv. Bobwhite) by microprojectile bombardment. T1, T2, and T3 plants regenerated from three transgenic callus lines were challenged with barley stripe mosaic virus. Plants expressing RNase III exhibited a high level of resistance to the virus infection. This resistance was evidenced by the absence of virus symptoms and reduced accumulation of virions in these plants. The result demonstrates that this pathogen-targeted resistance strategy can be effectively employed in conferring resistance to viral diseases of cereal crops.

Antibody-based resistance to plant pathogens

Plant diseases are a major threat to the world food supply, as up to 15% of production is lost to pathogens. In the past, disease control and the generation of resistant plant lines protected against viral, bacterial or fungal pathogens, was achieved using conventional breeding based on crossings, mutant screenings and backcrossing. Many approaches in this field have failed or the resistance obtained has been rapidly broken by the pathogens. Recent advances in molecular biotechnology have made it possible to obtain and to modify genes that are useful for generating disease resistant crops. Several strategies, including expression of pathogen-derived sequences or anti-pathogenic agents, have been developed to engineer improved pathogen resistance in transgenic plants. Antibody-based resistance is a novel strategy for generating transgenic plants resistant to pathogens. Decades ago it was shown that polyclonal and monoclonal antibodies can neutralize viruses, bacteria and selected fungi. This approach has been improved recently by the development of recombinant antibodies (rAbs). Crop resistance can be engineered by the expression of pathogen-specific antibodies, antibody fragments or antibody fusion proteins. The advantages of this approach are that rAbs can be engineered against almost any target molecule, and it has been demonstrated that expression of functional pathogen-specific rAbs in plants confers effective pathogen protection. The efficacy of antibody-based resistance was first shown for plant viruses and its application to other plant pathogens is becoming more established. However, successful use of antibodies to generate plant pathogen resistance relies on appropriate target selection, careful antibody design, efficient antibody expression, stability and targeting to appropriate cellular compartments.

Ribozyme-mediated resistance to rice dwarf virus and the transgene silencing in the progeny of transgenic rice plants

A hammerhead ribozyme (Rz) with long hybridizing arms targeting the mRNA of rice dwarf virus (RDV) segment 5 and a mutated nonfunctional ribozyme (mRz) were constructed. As predicted, Rz transcribed in vitro cleaved the target mRNA of RDV segment 5 into two fragments of 138 and 238 nucleotides in length. The Rz and mRz genes were each placed under the control of the CaMV 35S promoter and used to transform Japonica rice variety 'Tongling No. 1' via Agrobacterium tumefaciens. A total of 32 independent lines containing Rz or mRz was obtained as demonstrated by Southern blot analysis. Challenge inoculation with RDV viruliferous leafhoppers (Nephotettix cincticeps) showed that T1 plants containing the Rz transgene displayed high resistance or delayed and attenuated viral symptoms. In contrast, transgenic lines expressing mRz showed severe symptoms similar to the control plants transformed with the vector alone. These results suggest that Rz confers RDV resistance in transgenic rice. Genomic DNA PCR analysis confirmed that all of the examined T6 progeny plants contained the Rz transgene. However, accumulation of the Rz transcripts was detectable by RT-PCR only in the plants that were resistant to RDV. This suggested that loss of RDV resistance in progeny plants containing the Rz transgene may result from silencing of the Rz transgene.

Regulation of CAT protein by ribozyme and antisense mRNA in transgenic mice

Transgenic mouse lines were engineered to express stably antisense mRNA or antisense mRNA containing catalytic ribozyme (rbz) structures complementary to bacterial chloramphenicol acetyltransferase (CAT) gene transcripts. One transgenic line expressed antisense mRNA that specifically targeted full-length CAT coding sequences (ACAT). Another transgenic line expressed full-length antisense CAT mRNA which was modified by mutagensis to include four rbz cassettes (rbz-ACAT) in order to compare antisense versus antisense-rbz function in vivo. Preliminary data were also collected from a transgenic mouse line expressing antisense mRNA targeting 72% of the 5' region of CAT coding sequences (5' ACAT). All constructs contained similar control elements in their design. Promoter elements were derived from the bovine alpha s1-casein gene, while the small t intron and 3' control sequences were derived from SV40. The ability of these various constructs to down-regulate CAT protein levels was compared by analysis of CAT protein production in lactating double-hemizygous transgenic female mice. Every double-hemizygous mouse analysed expressed mRNA from the alpha s1-casein-CAT construct (Clarke et al., 1994) and equivalent levels of mRNA from one of the three antisense constructs. Transgenic mouse lines expressing both ACAT and CAT mRNA down-regulated CAT protein levels by 90% of that found in the CAT only transgenic population. Similarly, double-hemizygous transgenic lines expressing both rbz-ACAT and CAT mRNA regulated CAT protein levels by 87%. Preliminary data suggests that expression of mRNA from 5' ACAT/CAT double-hemizygote mice allowed approximately 67% down-regulation of normal CAT protein levels. We conclude that incorporation of multiple ribozymes within the full-length antisense CAT construct does not enhance the effectiveness of antisense mRNA in the down-regulation of CAT protein production in our system.

A ribozyme targeted to cleave the polymerase gene sequences of different foot-and-mouth disease virus (FMDV) serotypes

Vaccinations against foot-and-mouth disease virus (FMDV) has dramatically reduced the number of disease outbreaks. Nevertheless, there are still many outbreaks in different regions around the world. In an effort to find new ways to control the disease, ribozymes able to cleave FMDV were designed and tested. In this work we tested the ability of FRZ4, a ribozyme targeted to the viral polymerase gene, to cleave polymerase sequences of several FMDV. Homology analysis was used to choose target sequences which consist of two conserved GUC which lie 15 bases apart and, their flanking sequences. These were the basis for the FRZ4 ribozyme gene sequence that contains two catalytic domains. We show that polymerase sequences from A, Asia 1, C and two different O1 Israeli isolates could be specifically cleaved by FRZ4. It is suggested that FRZ4 can cleave polymerase gene sequences from any FMDV serotype.

Antiviral ribozymes. New jobs for ancient molecules

Catalytic RNAs are a genetic property not only of some particular viroids or viruses, but also are more common naturally among eukaryotes and even prokaryotes than earlier expected. However, the major interest in ribozymes results from their potential for development of "tailor-made" cDNA constructions designed to be transcribed into catalytic RNAs that will recognize by hybridization and destroy by specific cleavage their cellular or viral RNA targets. The efficiency of an antiviral ribozyme is determined by both the accessibility and sequence conservation of the target region, as well as the design of the ribozyme: its type, size, and composition of flanking sequences; expression rates; and cellular compartment localization. Until now the most frequently selected viral target is the human immunodeficiency virus, where an up to a 10(4)-fold inhibition in its progeny production has been achieved. Although the first generation ribozymes focused on improvements in basic design and expression rates, more recently the efficiency of antiviral catalytic activity has been increased by employing polyribozymes and/or multitarget ribozymes, as well as special constructions to enhance the cellular co-compartmentation of the ribozyme with its viral RNA target.

Ribozyme-mediated high resistance against potato spindle tuber viroid in transgenic potatoes

A hammerhead ribozyme [R(-)] targeting the minus strand RNA of potato spindle tuber viroid (PSTVd) and a mutated nonfunctional ribozyme [mR(-)] were designed, cloned, and transcribed. As predicted, both monomer and dimer transcripts of the active R(-) ribozyme gene could cleave the PSTVd minus strand dimer RNA into three fragments of 77, 338, and 359 bases in vitro at 25 and 50 degrees C. The tandem dimer genes of R(-) and mR(-) were subcloned separately into the plant expression vector pROK2. Transgenic potato plants (cultivar Desirée) were generated by Agrobacterium tumefaciens-mediated transformation. Twenty-three of 34 independent transgenic plant lines expressing the active ribozyme R(-) resulted in having high levels of resistance to PSTVd, being free of PSTVd accumulation after challenge inoculation with PSTVd, but the remaining lines showed weaker levels of resistance to PSTVd with low levels of PSTVd accumulation. In contrast, 59 of 60 independent transgenic lines expressing the mutated ribozyme mR(-) were susceptible to PSTVd inoculation and had levels of PSTVd accumulation similar to that of the control plants transformed with the empty vector. The resistance against PSTVd replication was stably inherited to the vegetative progenies.

Comparison of the antiviral efficacy of ribozymes and antisense RNA directed against bovine leukemia virus rex/tax

Despite the theoretical attraction of ribozymes, which cleave their target RNA, as compared with antisense RNA, which only blocks translation, few studies have assessed the relative efficacy of ribozymes and antisense RNA directed against the identical target sequence. Previously, we described the efficacy of a hammerhead ribozyme targeted against rex/tax, a regulatory gene of bovine leukemia virus (BLV). In this study, we asked whether antisense RNA targeted against the same site would also be efficacious. BLV-infected bat lung cells were transfected with an antisense RNA-encoding plasmid under the control of sarcoma virus (RSV) promoter, and stable cell lines were selected. No inhibition of viral p24 expression was demonstrated in seven cell lines transfected with the antisense RNA targeted at the same site as the ribozyme or in three cell lines transfected with an antisense sequence targeted against the tax 5' initiation codon. Although in previous experiments antisense DNA oligonucleotides inhibited Tax expression in vitro, stably transfected plasmids encoding antisense RNA of the same sequence did not inhibit viral expression in BLV-infected cells in this study. These results suggest that in persistently infected cells producing high levels of BLV, the ribozyme is more effective than antisense RNA directed at rex/tax transcripts.