2.1. Expression of the virus coat protein (CP) gene

This technique is most common. In this CP-mediated resistance (CP-MR) is developed, based on the well known process of cross protection. It is protected against super infection by a severe strain of related virus. This method has been commonly used in agriculture to confer protection against severe virus infection. However, the technique has following disadvantages:

•  Due to synergistic interaction, infection of cross protected plants with a second unrelated virus may cause a severe disease,

•  The suspicious virus strain might mutate to a more severe form, leading to extensive crop losses,

•  Protecting virus strain may cause a small but significant decrease in yields, and

•  In cross protection, the protecting virus must be applied each growing season.

Most of these problems can be offset by genetic engineering of CP-MR in plants. CP-MR produced a c-DNA encoding the capsid protein (CP) sequences of TMV, ligated it to a strong transcriptional promoter (CaMV 35S promoter) and transport sequences to provide constitutive expression of the gene throughout the transgenic plant, and flanked on the 3' end by poly A signal from the nopaline synthase gene. This chimeric gene was introduced into a disarmed plasmid of A. tumefaciens and the modified bacterium was used.

 

2.2. Expression of satellite RNAs

Some viruses have specific feature to contain, in addition to their genomic RNA, a small RNA molecule known as satellite RNA (S-RNA). The S-RNAs require the company of a specific 'helper' virus (closely related virus) for their replication. S-RNA does not have sequence to encode CP. They are encapsulated in the coat protein of their helper virus or satellite vi-ruses which encode their own coat protein. Due to ability to modify disease symptoms S-RNAs now have point of attention in genetic engineering. Most of the S-RNAs decrease the severity of viral infection, presumably through interference with viral replication. By this method, tomato, a number of pepper varieties, cucumber, eggplant, cabbage and tobacco plants against CMV have been protected. The first time S-RNA induced attenuation of viral symptoms involved the introduction of cDNA copies of CMV S-RNA into the genome of tobacco plants.

 

2.3. Use of antisense viral RNA

Here using the antisense RNA, which is a single stranded RNA molecule complementary to the mRNA (sense RNA), transcribed by a given gene, is another approach suggested for introducing viral resistance in plants. The sense RNA carries codons to translate to a specified sequence of amino acids. The antisense RNA, on the other hand, does not contain the functional protein sequences. When both sense and antisense RNA are present together in cytoplasm they anneal to form a duplex RNA molecule which cannot be translated. Using this methodology, transgenic plants expressing 3' region of antisense RNA, including CP gene of TMV or CMV. RNAs were produced which have property to protect against infection with respective viruses or viral RNA.