In this article we will discuss about development of viral resistant plants:- 1. Why Engineer Resistance to Viruses in Plants? 2. Coat-Protein (CP) Mediated Resistance 2. Advantage of Virus Coat Proteins 3. Disadvantage of Virus Coat Proteins 4. Replicase Mediated Resistance 5. Rep Protein Mediated Resistance to Single Stranded DNA Viruses 6. Movement Protein Mediated Resistance and Other Details.
- Why Engineer Resistance to Viruses in Plants?
- Coat-Protein (CP) Mediated Resistance
- Advantages of Virus Coat Protein
- Disadvantage of Virus Coat Proteins
- Replicase Mediated Resistance
- Rep Protein Mediated Resistance to Single Stranded DNA Viruses
- Movement Protein Mediated Resistance
- RNA Mediated Resistance
- Generation of Resistance by using Plantibodies
1. Why Engineer Resistance to Viruses in Plants?
Plant viruses can cause severe damage to crops by substantially reducing vigour, yield, and product quality. The most commonly used control strategies for virus diseases are directed against vectors that spread viruses within and between fields.
By far, the most effective approach to control viruses relies on the use of resistant varieties of crop plants developed by plant breeding methods. Host-resistance genes have been extensively exploited by traditional breeding techniques for the development of virus-resistant plants.
However, host resistance has been identified for a few viruses only. Therefore, engineered viral resistance is attractive to complement conventional breeding approaches. Actually, the deployment of virus- resistant transgenic plants has become an important strategy to implement effective and sustainable control measures against major virus diseases.
2. Coat-Protein (CP) Mediated Resistance:
The Coat protein of TMV has been widely exploited for the development of virus resistant transgenic plants. Researchers has introduced TMV coat protein (CP) gene and have developed viral resistant transgenic plants.
The level of protection conferred by CP genes in transgenic plants varies from immunity to delay and attenuation of symptoms. In some cases protection is broad and effective against several strains of the virus from which the CP gene is derived.
Despite extensive studies, the molecular mechanisms that govern CP-mediated resistance (CPMR) are not fully understood.
However, several lines of evidence supports the hypothesis that CPMR against TMV is a consequence of interaction between the transgenic CP and the CP of the challenging virus:
1. Transgenic plants expressing CP showed high resistance to challenge by virions, but not to inoculation with RNA or partially stripped virions.
2. Transgenic plants expressing TMV CP showed greater levels of CPMR against closely related viruses than to more distantly related viruses.
3. Transgenic plants expressing mutant CPs affecting electrostatic interactions between the subunits showed modified CPMR according to their self-assembly capacity.
3. Advantage of Virus Coat Proteins:
The CP not provides resistant to one virus, from which it is derived, rather it confers resistant to all related viruses.
4. Disadvantage of Virus Coat Proteins:
It is only successful for the viruses with single stranded RNA genome.
5. Replicase Mediated Resistance:
Engineering virus resistance by using genes encoding viral RNA dependent RNA-polymerases (RdRps) was first reported for TMV by Golemboski in 1990. A notable inhibition of virus replication at the inoculation site in the whole plant and also at the single-cell level in tobacco transformed with a modified RdRpwas found.
Resistance appeared to be strain-specific, against infection initiated by both TMV virions and RNA. However, although the 54-kDa protein itself was never detected in transgenic tissue, the finding that a mutant encoding only 20% of the protein was ineffective suggested at that time. Replicase genes of other viruses, such as tobra virus, potex virus, poty virus, alfamo virus and cucumoviruses have also been described as resistance sources.
6. Rep Protein Mediated Resistance to Single Stranded DNA Viruses:
Unlike RNA viruses, the genomes of plant single-stranded DNA viruses do not encode polymerases. Instead, their replication requires interaction between a viral replication- associated protein (Rep) and host polymerases. Gemini viral Rep proteins have been widely exploited to generate resistance.
The Rep gene of African cassava mosaic virus (ACMV) inhibited virus replication in protoplasts and induced virus resistance in plants. A protein- mediated resistance was described with a truncated Tomato yellow leaf curl Sardinia virus (TYLCSV) Rep protein, that strongly inhibited virus replication in protoplasts and induced resistance when expressed at high levels.
7. Movement Protein Mediated Resistance:
Compared with CP-or replicase-mediated resistance strategies, the expression of dysfunctional or mutant movement proteins (MP) has been reported to confer broader resistance. Plants transgenic for the MP of TMV (p30), lacking three N-terminal amino acids or a temperature-sensitive version showed a delay in both symptom appearance and infection.
The dysfunctional MP (dMP) was thought to act as a dominant negative mutant, interfering with local and systemic movement of the challenging virus. Resistance was also effective against taxonomically distant viruses, as a TMV dMP transgene interfered with systemic spread of the tobravirus, Tobacco rattle virus, the caulimovirus, Peanut chlorotic, streak virus and the nepovirus.
8. RNA Mediated Resistance:
This post-transcriptional gene silencing (PTGS) process, occurring in plants but also in other eukaryotes, is also known as RNA interference (RNAi) or RNA silencing. In this strategy we introduce transgene, called as antisense gene, in a reverse orientation (from 3′ to 5′).
The mRNA produced by antisense gene is complementary to the RNA of the viruses. As a result of this there will be base pairing between the RNA produced by the antisense gene and the viral RNA. This double stranded RNA complex cannot be recognized by the ribosomes and hence the viral proteins are not manufactured. Due to this the virus fails to complete their life cycle on the plant cell and eventually fails to establish the disease.
9. Generation of Resistance by using Plantibodies:
A plantibody is an antibody produced by genetically modified crops. Antibodies are part of animal immune systems, and are produced in plants by transforming them with antibody genes from animals. This was first done in 1989, with a mouse antibody made by tobacco plants.
Although plants do not naturally make antibodies, plantibodies have been shown to function in the same way as normal antibodies. The expression of plantibodies in the transgenic plants have been found very effective in subsiding the action of plant viruses.