Development of Transgenics for Virus Resistance
By: Manisha Mangal and Arpita Srivastav | Corresponding author: manishamangal@rediffmail.com

Advancements in transgenic technology has offered new means of incorporating traits for virus resistance into existing desirable crops. There are two approaches for developing transgenics for virus resistance depending on the source of the genes used. The genes can be either from the pathogenic virus itself or from any other source.

Pathogen derived resistance:

Virus resistant transgenics have been developed in many crops by using genes for viral coat proteins, replicases, movement proteins, satellite RNA and defective interfering viral genomic components.

Coat protein mediated resistance (CPMR):

Genes for viral coat protein have been used to transform a number of crops and these transformed crops were found to show a high level of resistance in comparison to untransformed plants. The resistance in most of the cases has been manifested as delayed appearance of symptoms and reduced titre of virus in the infected transgenic plants. CPMR has been shown to affect disassembly and long distance transport of virus. In some cases, it has been shown that transgenically expressed CP sub units recoat the nascent disassembled viral RNA thereby decreasing the pool of available viral RNA for translation.

Replicase protein mediated resistance (RPMR):

RPMR against a virus in transgenic plant was first shown in tobacco against TMV in plants containing the 54 K Da putative Rep gene. In earlier studies it was considered to be RNA mediated homology dependent resistance. However there are exceptions to this observation. In one study, the TMV replicase transgene was modified by the unintended insertion of a bacterial transposable element during propagation in bacterial host, however, plants expressing this construct were found to be resistant to even those tobamoviruses which were only slightly similar to TMV, indicating that gene silencing mechanisms were not involved. In yet another study, constructs designed to express the replicase caused suppression of TMV accumulation, however, when the construct was expressed without an initiation codon there was no suppression of TMV, suggesting the involvement of protein instead of RNA mediated homology dependent resistance. In view of these contradictory findings, the nature of the mechanism leading to resistance remains an open question in this case.

Movement protein mediated resistance (MPMR):

Movement proteins are essential for cell to cell movement of plant viruses as they modify the gating function of plasmodesmata. This phenomenon has been utilized to engineer virus resistance in plants by producing transgenic plants with genes for modified movement proteins. Resistance is based on the competition between wild type virus encoded movement proteins and the preformed dysfunctional movement proteins to bind to plasmodesmatal sites.

Satellite RNA:

Some strains of virus possess satellite RNA which acts as a parasite of the virus and depend on the host or helper virus for its replication, movement within plant, encapsidation and transmission. The presence of satellite RNA depresses symptoms as well as accumulation of helper virus in the host. This phenomenon has been utilized to engineer virus resistance in plants by introducing partial copies of satellite RNA for a particular virus in the desired plants.

Defective interfering viral nucleic acid:

In a number of virus infected tissues truncated genomic components are often detected which interfere with the replication of the genomic components of the virus. These species of DNA are called defective interfering DNA (DI-DNA) and plants engineered with DI-DNA have been found to show increased resistance to viruses.

Plant Derived resistance:

Use of R genes:

A number of plant genes imparting resistance to viruses have been identified which offer potential for generation of virus resistant crops.

Ribosomal inactivating proteins (RIP):

Several plants contain antiviral proteins which inhibit translocation step of translation by catalytically removing adenine base from 28 S r-RNA. These proteins are called Ribosomal inactivating proteins and have been isolated from a number of plant species and used to develop transgenics with broad range of antiviral activities.

Protease inhibitors:

Many viruses need cystein protease activity to process their own poly proteins for their replication and propagation. Plants possessing cystein protease inhibitor resist these kinds of viruses and this strategy has been utilized to engineer resistance for these viruses eg. Tobacco lines expressing rice cystein protease Inhibitor show resistance to tobacco etch virus (TEV) and potato virus Y (PVY).

Antiviral Plantabodies:

Specific viral antibodies called plantabodies are expressed in plants to make them virus resistant. In this technique, a panel of monoclonal antibodies is raised against the target virus and the gene for most reactive antibody of this panel is cloned and expressed in desired plant.

Utilization of genes involved in Systemic acquired resistance (SAR) pathway :

Systemic acquired resistance is induced in the infected plants following infection by a pathogen, and is characterized by the activation of several genes, accumulation of salicylic acid (SA), enhanced expression of pathogenesis related proteins, increase in reactive oxygen species, activation of phenyl propanoid pathway etc. SA is an important component of SAR and is involved in the degradation of H2O2 degrading enzyme catalase 1. Plants engineered with antisense catalase 1 showed reduction in catalase 1 activity, high level of SA and increased resistance to viruses. Similarly when plants were transformed with genes which resulted in increased accumulation of SA (by conversion of Chorismate to SA), they showed higher resistance to viruses.

Post transcriptional gene silencing (PTGS):

Post transcriptional gene silencing is a specific RNA degradation mechanism of any organism that takes care of unwanted, excess, aberrant or foreign RNA intracellularly in a homology dependent manner. This activity could be present constitutively to help normal development and can also be induced in response to cellular defence against pathogens especially viruses.

Introduction of transgenes constitutively expressing part of the genome of a virus can lead to resistance of the plant to infection by this virus. Resistant plants are immune to secondary infection by the same virus or by another recombinant virus carrying part of the genome of the first virus, which indicates that plants have a 'memory' of the first virus which is based on the presence of a silencing signal.

About Author / Additional Info:
Senior Scientist (Biotechnology) at IARI-New Delhi.