Viruses also cause a considerable damage to crop plants and decrease the productivity significantly. There is no effective chemical treatment against the viruses.
Therefore, the plant breeders attempt to transfer naturally occurring virus resistant genes from one plant strain to another through breeding. However, resistant strains achieved by this method, often, revert back to the sensitive strains and moreover, resistance to one virus does not confer-resistance to the other.
Recently, genetic engineering of plants has circumvented these problems. Researchers have generated virus resistant transgenic plants. Such plants are immunized by a vaccine, usually consisting of a gene encoding for a viral coat protein. The transgene expresses a small amount of the viral coat protein.
The exact mechanism of protection is not known at this point of time. This approach has been used to confer tolerance to a number of different plant viruses including tobacco mosaic virus, cucumber mosaic virus, alfalfa mosaic virus, tobacco streak virus, tobacco rattle virus, potato virus X, potato virus Y and tobacco etch virus.
In another approach, the use of antisense RNA of the viral coat protein gene has been contemplated. The antisense RNA is complementary to the mRNA of the viral coat protein gene and both bind to each other by complementary base pairing, thereby inhibiting the translation of the coat protein mRNA.
Viral coat proteins cannot be synthesized and hence, the assembly of the viral particles. Theoretically, transgenic plants may be generated containing a gene transcribing an antisense RNA, complementary to the viral coat protein mRNA.
Many genes have been isolated from plants, which confer resistance to plants against a variety of pathogenic organisms. For example, a gene from A. thaliana and tomato confers resistance to the bacterial pathogen Pseudomonas syringe. Another gene from tomato confers resistant to the fungal pathogen Cladosporiumfidvum.
Thus, the application of biotechnology to crop plants has helped increase the productivity on two counts:
1. Generation of disease-resistant plants has decreased the incidence of infection by pathogenic organisims and hence the loss of crop yields due to diseases.
2. The generation of insect pest-resistant plants has decreased a considerable loss of food grains caused by insect pests.