Virus Resistance,Coat Protein Gene,Defective Viral Genome,Antisense RNA Approach,Expression of Disease,Tomato Mosaic Virus,Viral Genome

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	Virus Resistance - Several approaches have been used to engineer plants for virus resistance, which are as follows: (1) coat protein gene, (2) cDNA of satellite RNA, (3) defective viral genome, (4) antisense RNA approach, and (5) ribozyme mediated protection. Of these strategies, use of coat protein gene has been the most successful. Transgenic plants having virus coat protein gene linked to a strong promoter have been produced m many crop plants, tobacco, tomato, alfalfa, sugarbeet, potato, etc.
The first transgenic plant of this type was tobacco produced in 1986; it contained the coat protein gene of tobacco mosaic virus (TMV) strain U I. When these plants were inoculated with TMV U I, symptoms either failed to develop or were considerably delayed. Further, there was a much less accumulation of virus than in the control plants in both inoculated and systemically infected leaves. In addition, these plants showed delayed expression of disease symptoms when inoculated with the related tomato mosaic virus (ToMV) and with tobacco mild green mosaic virus (TMGMV). It appears to be a common feature that expression of a virus coat protein gene not only confers resistance to the concerned virus but also gives a measure of resistance to related viruses.
The effectiveness of coat protein (CP) gene in conferring virus resistance can be affected by both the amount of coat protein produced in transgenic plants and by the concentration of virus inoculum. Most likely the resistance generated by CP is due to the blocking of the process of uncoating of virus particles, which is necessary for viral genome replication as well as expression. However, other effects seem to be involved in producing coat protein mediated virus resistance; one such mechanism appears to be the prevention or delay of systemic spread of the viruses. But at least in some cases, the resistance mechanism does not involve the coat protein itself since CP genes even in antisense orientation produce resistance to the virus.