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Transgenic plants for viral disease resistance
- More than 300 plant viruses have been reported which can cause diseases in crops like retarded growth, lowered yields, and even complete crop failure. - Plants have natural defenses against viruses like hypersensitive response and extreme resistance which causes infected cells to die in order to prevent spread of the virus. - Transgenic techniques can also be used to develop virus-resistant plants, such as inserting virus coat protein, replicase, or movement protein genes, using antisense RNA or RNA interference strategies. - While transgenic virus-resistant crops could control plant virus diseases, there are some potential safety issues to consider like increased virulence of viruses, recombination between viral sequences, and effects on non-target organisms
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Transgenic plants for viral disease resistance
- 2. • Up tillnow, more than have been reported which include (Beachy, 1997). • Virus infections of crops may result in retarded cell division (hypoplasia), excessive cell division (hyperplasia) cell death (necrosis). • The overall effects of virus infections are growth retardation, lowered product yield and sometimes complete crop failure. • This fact makes the use of transgenic resistant plants as the only useful and potential virus control measures.
- 3. safe agricultural practicesto control/reduce viral infections to plants • i. Use of seeds that are virus – free. • ii. Control insects that spread plant viruses. • iii. Control weeds that serve as alternate hosts for viruses. • iv. Use cultivars that possess virus resistance.
- 5. Natural Defense ofPlants against Viruses • As the virus enters the cell, the plant responds either by hypersensitive response (HR) or extreme resistance (ER), commence to die (Goldbach et al. 2003). • naturally produced secondary metabolites which are toxic to virus • resistance genes (R genes). R genes are activated by recognizing the specific avirulence genes (avr) of the infecting virus . This induces either the HR or ER response, leading to the ultimate death of the cells surrounding the site of infection.
- 7. Cross Protection /PlantVaccination • by inoculating plant(employed on tomato, papaya and citrus ) with milder strain of the of target virus . • drawbacks 1) degree of virulence of each viral strain vary from crop to crop 2) milder strain of the virus that provides protection to one crop may cause serious diseases on varieties growing nearby.
- 8. Pathogen Derived Resistance: •insertion of resistant genes that are derived from the pathogen (virus) into the host plant. • Coat Protein Mediated Resistance • Replicase-Mediated Resistance • Movement Protein Mediated Resistance
- 10. • Advantages ofvirus coat proteins: The coat protein gene from one virus sometimes provides resistance (cross protection) to some other viruses, which may be unrelated e.g. TMV of tobacco plant provides resistance to potato virus X, alfalfa mosaic virus and cucumber mosaic virus. • Limitation of virus coat proteins: The virus coat protein-mediated protection is successful for viruses with single-stranded RNA genomes. However, this approach is of not much use for viruses with genomes containing double- stranded RNA and single- stranded DNA.
- 11. Replicase-Mediated Resistance • Plantviruses encode specific Replicase proteins enable virus replication in host cell by co-interaction of virus encoded replicase protein and host protein factors. • Plants transformed with a modified RNA dependent RNA polymerase gene conferred resistance which was strain specific. • the protein encoded by the transgene interferes with the function of the viral replicase, either by binding to viral proteins or host factors that regulate virus replication and its subsequent gene expression. • Replicase transgene trigger two possible mechanisms; 1. one targeting replication of challenging virus in the cell 2. limit the systemic spread of the virus by interacting with movement proteins. Eg:Presence of 54 kDa gene in transgenic tobacco
- 12. Movement Protein MediatedResistance • Plant viruses encode special movement proteins (MPs) enable them to spread the infection between adjacent cells as well as systemically . • This movement involves plasmodesmata and the channels that traverse plant cell walls and thus provide a systemic movement of virus between cells and tissues. • A plant having transgenic expression resistance through competition for plasmodesmatal binding sites between the mutant MP and functional MP of the challenging virus. • Eg :32 kD movement protein in transgenic tobacco • The advantage with movement protein strategy is that it is applicable to single-stranded DNA viruses also.
- 13. Homology derived resistance •Antisense RNA Strategies • RNA Silencing
- 14. Antisense RNA Strategies •antisense RNA (complementary to part of the viral genome protecting plants from systemic virus infection . • Antisense RNAs refer to small untranslatable RNA molecules that pair with a target RNA sequence on homology basis and thereby exert a negative control on interaction of target RNA with other nucleic acids or protein factors . • Further, RNase H is the endonuclease responsible for digestion of duplex RNA, thus blocking translation of target mRNA. • more useful for DNA viruses • Eg In fact, tomato golden mosaic virus (TGMV) replicase coding sequence was cloned in antisense orientation and introduced into tobacco plants. The transgenic tobacco plants expressed antisense RNA of TGMV replicase. These plants were resistant to TGMV infection.
- 15. RNA INTERFERENCE Key proteinsinvolved in RNA interference or silencing are • Dicer like proteins (DCL) • Argonaute proteins (AGO) • RNA dependent RNA polymerases (RdRp) Pathways observed in RNA silencing • Cytoplasmic siRNA silencing • The silencing of endogenous mRNAs by miRNAs • Pathway associated with DNA methylation and the suppression of transcription
- 17. • There isa good coordination and interaction between plant viruses and insect vectors for the spread of viruses from one plant to another. Certain viral-encoded transmission proteins do this job effectively. It is possible to produce mutated transmission proteins and block the spread of viruses. Thus, the spread of insect-transmitted viruses can be prevented by engineering crops to express a defective virus-transmission protein. • Plant viral satellite RNAs are small RNA molecules that multiply in the host cells with the help of specific helper viruses. These satellite RNAs are encapsulated together with the respective helper viruses. In general, the presence of satellite RNAs reduces the severity of the viral disease and the symptoms, and thus reduces the effect of the virus. • Transgenic plants containing satellite sequences have been developed to provide resistance to virus diseases. One example is given here. When cucumber mosaic virus (CMV) infects pepper plants, severe symptoms appear. These symptoms can be minimized with higher plant yield when CMV is co-inoculated with a satellite RNA. • Satellite RNA approach is not widely used due to several limitations: i.Some of the satellite RNA may increase the severity of disease symptoms in some plants. ii. Satellite RNAs mutate very rapidly which may sometimes result in a highly virulent agent. iii. Re-combinations between satellite RNAs have been detected. This may lead to serious consequences. Transmission proteins Satellite RNAs
- 18. Ribozymes • Ribozymes aresmall RNA molecules which promote the catalytic cleavage of RNA. • For providing virus resistance, ribozymes in the form of antisense RNAs capable of cleaving the target viral (sense) RNAs have been developed. • The ribozyme (antisense RNA) binds to a small sequence of viral RNA and splits
- 21. Virus-derived transgenic productsin the market
- 26. POTENTIAL SAFETY ISSUESASSOCIATED WITH VIRUS- RESISTANT TRANSGENIC PLANTS