Immunochemical detection of ds-RNA in healthy and virus-infected plants and specific detection of viral ds-RNA by hybridization to labelled complementary DNA

A 28-day oral toxicity evaluation of small interfering RNAs and a long double-stranded RNA targeting vacuolar ATPase in mice

New biotechnology-derived crop traits have been developed utilizing the natural process of RNA interference (RNAi). However, plant-produced double stranded RNAs (dsRNAs) are not known to present a hazard to mammals because numerous biological barriers limit uptake and potential for activity. To evaluate this experimentally, dsRNA sequences matching the mouse vATPase gene (an established target for control of corn rootworms) were evaluated in a 28-day toxicity study with mice. Test groups were orally gavaged with escalating doses of either a pool of four 21-mer vATPase small interfering RNAs (siRNAs) or a 218-base pair vATPase dsRNA. There were no treatment-related effects on body weight, food consumption, clinical observations, clinical chemistry, hematology, gross pathology, or histopathology endpoints. The highest dose levels tested were considered to be the no observed adverse effect levels (NOAELs) for the 21-mer siRNAs (48 mg/kg/day) and the 218 bp dsRNA (64 mg/kg/day). As an additional exploratory endpoint, vATPase gene expression, was evaluated in selected gastrointestinal tract and systemic tissues. The results of this assay did not indicate treatment-related suppression of vATPase. The results of this study indicate that orally ingested dsRNAs, even those targeting a gene in the test species, do not produce adverse health effects in mammals.

Safety assessment of food and feed from biotechnology-derived crops employing RNA-mediated gene regulation to achieve desired traits: a scientific review

Gene expression can be modulated in plants to produce desired traits through agricultural biotechnology. Currently, biotechnology-derived crops are compared to their conventional counterparts, with safety assessments conducted on the genetic modification and the intended and unintended differences. This review proposes that this comparative safety assessment paradigm is appropriate for plants modified to express mediators of RNA-mediated gene regulation, including RNA interference (RNAi), a gene suppression mechanism that naturally occurs in plants and animals. The molecular mediators of RNAi, including long double-stranded RNAs (dsRNA), small interfering RNAs (siRNA), and microRNAs (miRNA), occur naturally in foods; therefore, there is an extensive history of safe consumption. Systemic exposure following consumption of plants containing dsRNAs that mediate RNAi is limited in higher organisms by extensive degradation of ingested nucleic acids and by biological barriers to uptake and efficacy of exogenous nucleic acids. A number of mammalian RNAi studies support the concept that a large margin of safety will exist for any small fraction of RNAs that might be absorbed following consumption of foods from biotechnology-derived plants that employ RNA-mediated gene regulation. Food and feed derived from these crops utilizing RNA-based mechanisms is therefore expected to be as safe as food and feed derived through conventional plant breeding.

Double-stranded ribonucleic acid from carnation cryptic virus

The nucleic acid of carnation cryptic virus has been identified as double-stranded RNA on the basis of (i) susceptibility to RNase in 0.1 x SSC (SSC = 0.15 M NaCl-0.015 M sodium citrate buffer, pH 7.0) and resistance to RNase in 2 x SSC and to DNase, (ii) reaction with an antiserum containing antibodies to dsRNA, and (iii) electron microscopic appearance in nondenaturing media similar to that of the dsRNA of maize rough dwarf virus. Upon electrophoresis in 5% polyacrylamide gels carnation cryptic virus RNA separated into three major segments plus a fourth minor component, and had a total molecular weight of about 4 x 10(6).

Detection of virus infection in plants and differentiation between coexisting viruses by monoclonal antibodies to double-stranded RNA

Monoclonal antibodies to double-stranded RNA (dsRNA) are described for use as a universal diagnostic tool to detect infection in plants by RNA viruses. Crude nucleic acid extracts from plants infected with one of 25 different viruses were examined by sandwich-ELISA and immunoblotting. In comparison to the corresponding controls elevated dsRNA concentrations were found in 21 infected samples by ELISA; virus-specific dsRNA bands from 18 viruses were detected by immunoblotting. Using this method the identification of infecting virus is potentially possible on the basis of the electrophoretic banding pattern of the dsRNA, which in turn depends on the number, molecular weight and/or thermodynamic stability of the dsRNA species present in the extract. Immunoblot analyses in combination with temperature-gradient gel electrophoresis were used to demonstrate that the four individual genomic dsRNAs of the coexisting beet cryptic viruses BCV1 and BCV2 can be distinguished from one another and from other dsRNAs present in the extracts. It is shown that the thermal denaturation profiles and the Tm-values of the main structural transitions of BCV genomic dsRNAs are essentially the same in viruses from sugar beet as well as from wild Beta maritima. The reliability of dsRNA-immunoblotting for detecting virus infection in plants is discussed. Its use is especially recommended for the detection and characterization of cryptic viruses.

Monoclonal antibodies to double-stranded RNA as probes of RNA structure in crude nucleic acid extracts

We describe four monoclonal antibodies (MAB) which specifically recognize double-stranded RNA (dsRNA) together with their use in new methods for detecting and characterizing dsRNA in unfractionated nucleic acid extracts. The specificity of the antibodies was analyzed using a panel of 27 different synthetic and naturally occurring nucleic acids. All four antibodies reacted in a highly specific manner with long dsRNA helices, irrespective of their sequence; no binding to single-stranded RNA homopolymers or to DNA or RNA-DNA hybrids was observed. The apparent affinity of the antibodies to short (less than or equal to 11 bp) RNA helices was very low in all test systems used: only background levels of binding were obtained on single-stranded RNA species which contain double-helical secondary structures (e.g. rRNA, tRNA, viroid RNA). A sandwich ELISA and a dsRNA-immunoblotting procedure have been established which allow detection and characterization of dsRNA by MAB even in the presence of a large excess of other nucleic acids. In combination with temperature-gradient gelelectrophoresis (TGGE) not only the molecular weights but also the highly characteristic Tm-values of conformational transitions of individual dsRNA species could be determined by immunoblotting. An example of the general use of these methods for the detection of plant virus infections is demonstrated with groundnut rosette virus (GRV) dsRNAs. We were able to estimate the dsRNA content of infected leaves, identify the dsRNA species present in crude extracts and to determine the Tm- values of GRV dsRNA-3.

Detection of double-stranded RNA by ELISA and dot immunobinding assay using an antiserum to synthetic polynucleotides

An antiserum against polyinosinic-polycytidylic acid (In-Cn) was used to detect double-stranded RNA (dsRNA) by several serological techniques. DsRNA was readily detected by indirect ELISA (ELISA-I) and dot immunobinding assay (DIA). Addition of the antigen to poly-L-lysine-precoated plates and blocking with uncreamed milk powder allowed detection levels of 100 pg.ml-1 In-Cn by ELISA-I. Concentrations as low as 1 ng.ml-1 were detected by DIA using polyvinyliden difluoride (PVDF) membranes. Detection capacity with nitrocellulose membranes was 1000 times lower than with PVDF. ELISA-I and DIA enabled detection of dsRNA in enriched fractions from cucumber mosaic virus (CMV)- and citrus tristeza virus (CTV)-infected plants and from virus-infected Penicillium chrysogenum mycelium. These techniques showed similar or higher sensitivity for detection of dsRNA than separation by polyacrylamide gel electrophoresis and silver staining.

Mechanism of synthesis of turnip yellow mosaic virus coat protein subgenomic RNA in vivo

Turnip yellow mosaic virus (TYMV) possesses a monopartite single-stranded (+) sense RNA genome in which the coat protein (cp) gene is 3' proximal and is expressed in vivo via a subgenomic RNA. Evidence is presented here that this subgenomic RNA is synthesized in vivo by internal initiation of replication on (-) RNA strands of genomic length. The double-stranded RNAs (dsRNAs) from TYMV-infected plants have been isolated, purified, and characterized. Under native conditions, no dsRNAs (replicative intermediates and/or replicative forms) of subgenomic length corresponding to subgenomic cp RNA can be detected by ethidium bromide staining of RNA-sizing gels or by Northern blot hybridization using RNA probes. The presence of nascent subgenomic cp (+) RNA strands on the dsRNA of genomic length has been demonstrated using two different approaches: (1) Northern blot hybridization using (-) RNA probes under denaturing conditions and (2) characterization of the 5' ends of nascent (+) RNA strands upon labeling by vaccinia virus nucleoside-2'-methyltransferase.

Kinetics of velvet tobacco mottle virus satellite RNA synthesis and encapsidation

Synthesis of circular (RNA 2) and linear (RNA 3) molecules of velvet tobacco mottle virus (VTMoV) satellite RNA (sat RNA) has been studied by incubating strips of tissues excised from systemically infected Nicotiana clevelandii in solutions of [14C]uridine. After a short lag, RNA and virus synthesis proceeded at a constant rate for at least 24 hr, during which time most of the synthesis was directed to the production of RNAs 2 and 3. The kinetics of [14C]uridine incorporation into the sat RNA molecules after increasing times of incubation and during pulses of [14C]uridine followed by chase incubation with excess [12C]uridine suggest that RNA 3 is a percursor of RNA 2. However, not all the RNA 3 synthesized was shown to end up as RNA 2, even after 72 hr of incubation. Several lines of evidence are presented supporting the conclusion that VTMoV-infected cells contain large pools of unencapsidated sat RNA. It is suggested that the sat RNA may have a greater affinity for the VTMoV replicase than the helper viral RNA which results in copious production of the sat RNA.

Cellular double-stranded RNA in Phaseolus vulgaris

High molecular weight double-stranded (ds) RNAs have been detected in apparently virus-free French (common) bean Phaseolus vulgaris cv. Black Turtle Soup (BTS). Several other bean cultivars were free of detectable high molecular weight dsRNAs. The dsRNAs have been partially characterized and have homology to the BTS genome as well as to the genomes of other bean cultivars. The T m of hybrids formed between BTS DNA and denatured dsRNA have been estimated.

The application of spot hybridization to the detection of DNA and RNA viruses in plant tissues

A solid-phase nucleic acid hybridization technique for the detection of DNA and RNA viruses in plant tissues is described. The method involves spotting crude samples onto nitrocellulose and using 12P-labelled DNA hybridization probes. The limit of sensitivity is 5-20 pg virus/spot or approximately 5 micrograms/g leaf tissue. The method is quantitative for DNA viruses in crude homogenates, but not for RNA viruses. The amount of cauliflower mosaic virus in infected leaves and protoplasts was estimated. The amplitude of spot hybridization to screening plant material from glasshouses and field is discussed.