VIRUS RESISTANCE

Viral infection reduces photosynthesis rate & growth region of crop plants, thereby lowering crop yields.

Viral infection does not occur in plants already infected with certain viruses.

This principle is called “cross protection”.

Cross protection is the ability of one virus to prevent or inhibit the effect of a second challenge virus.

If a susceptible strain of a crop is inoculated with a mild strain of a virus, then the susceptible crop strain develops resistance against more virulent virus strain.

This principle is used by genetic engineers to develop transgenic virus resistant plants.

The transgenic plants produced viral coat proteins or viral nucleiprotein (or) satellite RNA’s in their cells.

Therefore viral infection does not take place.

Crop varieties raised by conventional breeding method are resistant to only one virus and sometimes the resistance may be reverted. Transgenic technology tries to overcome this problem.

1) TRANSGENIC PLANTS WITH VIRAL COAT PROTEIN

Genes for viral coat proteins have been introduced into plants to make them produce the viral proteins.

Transgenic plants having such viral coat proteins are resistant to viral infection.

e.g. TMV resistant tobacco & tomato.

TMV is a single stranded RNA virus & the RNA is enclosed in a protein coat.

Powell – Abel et al (1986) first demonstrated that transgenic tobacco expressing tobacco mosaic virus (TMV) coat protein showed resistance.

Coat protein mediated resistance is correlated with the inhibition of virus replication at the initial point of infection.

This resistance takes the form of reduced numbers of infection sites on inoculated leaves, suggesting that an initial step in the virus life cycle has been disrupted.

It has been demonstrated that TMV cross protection may result from the coat protein of the protecting virus preventing uncoating of the challenge virus RNA.

Powell – Abel et al (1986) made cDNA of the TMV coat protein & introduced it into tobacco plant through disarmed Ti plasmid of Agrobaterium.

The transgenic tobacco plants produced TMV coat protein in their cells & were resistant to viral infection.

2) TRANSGENIC PLANTS WITH VIRAL NUCLEOPROTEIN

RNA of some plant viruses is found associated with certain proteins. These are necessary for replication replicase & a protein that stabilizes its shape.

Several of these nonstructural replicase proteins have been found to provide a high degree of resistance to virus infected when expressed in transgenic plants.

The cDNA of nucleoprotein is constructed & fused with a 35S promoter of CaMV & introduced into plants through disarmed Ti Plasmid of Agrobacteriumn.

Transgenic tobacco, tomato, lettuce, groundnut, pepper etc have been developed to resist viruses by introducing nucleoproteins gene of tobacco rattle virus (TRV) & tobacco spotted wilt virus (TSwV).

3) TRANSGENIC PLANTS WITH VIRAL SAT RNA

RNA of certain plant viruses contain some non – coding sequences called satellite RNA’s or SAT RNA.

SAT RNA’s are a class of small (approx 300 nucleotides), single stranded RNA molecule that are dependent upon a helper virus for replication & virion packaging to cause infection elsewhere.

SAT RNA depends on virus for its replication & transmission, even though its unrelated to viral genome.

A number of SAT RNA’s have been shown to modulate the replication & symptom of diseases of their helper viruses.

Satellite RNA’s that attenuate symptoms can be potentially used to reduce the disease severity of the helper virus.

Hence its use in transgenics to confer resistance in crops finds an important place.

Trin et al (1987) Trin & Causui (1991) demonstrated that the deliberate inoculation of a mild strain of CMV with a symptom attenuating satellite RNA successfully protected tobacco, pepper, tomato & cucumber plants from a virulent strain of CMV & reduced yield losses. This strategy is limited to those virus systems in which attenuating satellite RNA’s are found.

Gehrlach et al (1987) introduced a cDNA of a satellite RNA of tobacco ringspot virus (TRV), into tobacco through disarmed Ti plasmid.

The transgenic tobacco is resistant to tobacco ringspot virus.

It produces viral SAT RNA’s that attenuate the disease symptoms.

4) TRANSGENIC PLANTS WITH ANTISENSE RNA :

A strategy for the control of plant viruses is the transgenic expression of antisense segments of viral RNA’s.

“Anti – sense RNA“ is RNA complementary to mRNA.

Normal mRNA of a gene is said to be a “sense” because it carries the codons that are “read” during translation to produce specified sequence of amino acids.

The complement of mRNA “sense” strand will not contain a sequence of codons that can be translated to produce a functional protein.

Thus this complementary strand is antisense RNA.

The principle of this strategy is to bind viral RNA with complementary RNA sequences Expressed by the plant.

Potentially prevent accessibility accessibility of the viral RNA for replication or gene expression.

Thus, antisense constructs should be used to block the initial steps important in the establishment of viral infection.

This strategy was used to develop viruses resistant tobacco.

e.g. CuMV (cucumber mosaic virus)

In CuMV, viral protein is encoded by RNA3.

DNA that produce antisense RNA for RNA3 was linked with a 35S promoter & inserted into a disarmed Ti plasmid.

THE rDNA was introduced into tobacco calli, through A. tumifaciens.

Tobacco regenerated from the calli are resistant to CaMV.