Viral infection causes substantial crop lose every year. It reduces the rate of photosynthesis and growth vigor. The net result is the low yield and income. Viral infection does not take place in plants already infected with certain viruses. The principle is called cross protection. By using this principle plant genetic engineers have been developing transgenic virus resistant plants.

The transgenic plants produce viral coat proteins or viral nucleoprotein or satellite RNAs in their cells. Therefore viral infection does not take place. Growing such transgenic plants reduce the crop lose due to viral attack. Crop varieties raised by conventional breeding methods are resistant to only one virus and sometimes the resistance may often be reverted. Transgenic technology never causes such problems.

Transgenic Plant with Viral Coat Protein

Gene for viral coat protein have been introduced into plants in order to make them to produce the viral proteins. Transgenic plants having such viral coat proteins are prone to viral infections.e.g. TMV resistant tobacco and tomato.

Tobacco Mosaic Virus (TMV) is s single stranded RNA virus and its RNA is enclosed in a protein coat. The protein coat consists of 2130 identical sub-units called capsomers.
Each capsomer consists of 158 amino acids. The genes for the protein coat is expressed as a separate unit.

Some transgenic plants and their virus resistance are:

• Tomato is resistant to Yellow leaf curl virus (YLCV), Tomato mosaic virus (ToMV).
• Tobacco is resistant to Tobacco mosaic virus (TMV), Tobacco etch virus (TEV).
• Potato is resistant to Potato virus -X and-Y.
• Alfalfa is resistant to Alfalfa mosaic virus (AMV).
• Cucumber is resistant to Cucumber mosaic virus (CuMV).
• Rice is resistant to Rice stripe virus (RSV).
• Black gram and Green gram is resistant to Yellow mosaic virus (YMV).

Transgenic Plants with Viral Nucleoprotein

RNA of some plant viruses is found associated with certain proteins called nucleoproteins. The nucleoprotein constitutes the enzyme replicase and a protein that stabilizes its shape. The cDNA of nucleoprotein is constructed from the viral DNA and fused with a 35S promoter of CaMV. The DNA construct is introduced into plants through Ti plasmid of Agrobacterium. The transgenic plants are immune to the attack by pathogenic plant viruses.

Transgenic tobacco, tomato, lettuce, groundnut, pepper, Impatiens, Ageratm, Chrysanthemum, etc. have been developed to resist viruses by introducing nucleoprotein genes of tobacco rattle virus (TRV) and tobacco spotted wilt virus (TSWV).

Transgenic Plants with Viral SAT RNA

RNA of certain plant viruses contain some non coding sequences called satellite RNAs or SAT RNA. Gehrlach and his team in 1987 introduced a cDNA of satellite RNA of Tobacco ringspot virus (TRV) into tobacco through Ti plasmid. The transgenic tobacco has been resistant to tobacco ringspot virus. It produces viral SAT RNAs that attenuate the disease symptoms.
Some transgenic plants with virus resistance by having viral SAT are :

• Cucumber is resistant to Cucumber mosaic virus (CuMV).
• Tomato is resistant to Tomato aspermy virus (TAV).
• Tobacco is resistant to Tobacco ringspot virus (TRV).
• Papaya is resistant to Papaya ringspot virus (PRV).
• Alfalfa is resistant to Alfalfa mosaic virus (AMV).
• Rice is resistant to Rice tungro virus (RTV).
• Potato is resistant to Potato virus-X.

Transgenic Plants with Antisense RNA

The RNA produced by the cloned gene binds with target DNA and inhibits translation is called antisense DNA or antisense RNA producing DNA. It is used to raise virus resistant plants e.g. CuMv resistant tobacco.

The cucumber mosaic virus viral protein coat is encoded by RNA3. DNA that produce antisense RNA for RNA3 was linked with a 35S promoter and inserted into a Ti plasmid. The rDNA was introduced into tobacco calli through Agrobacterium tumefaciens. Tobacco regenerated from the calli are resistant to CuMV.

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