Cross Protection,Citrus Tristeza Virus,Tomato Mosaic Virus,Plant Viruses

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	Cross Protection - It has been shown that if susceptible strain of a crop is inoculated with a mild strain of a virus, then the susceptible strain develops resistance against more virulent strains.This phenomenon is known as cross protection and has been used to reduce yield losses in crops like tomatoes against tomato mosaic virus (TMV), in potato against potato spindle tuber viroid and in citrus against citrus tristeza virus. In most cases of cross protection, the symptoms of infection are delayed, and even the replication of virus is suppressed, although eventually the severe strain may be able to overcome the protection.
There are various disadvantages of this practice of cross protection such as (i) possibility of mutation in inducing mild virus strain, (ii) possibility of synergism between inducing virus and another unrelated virus, (iii) possibility of unnecessary spread of mild virus causing threat for future yield losses and (iv) possibility of some yield losses due to mild strain also. These disadvantages can be overcome, if a single gene imparting this benefit is transferred and transgenic plants arc produced. Such transgenic plants have been produced in tobacco, tomato, and potato using a broad spectrum of plant viruses. Following three kinds of genes, have been used for this purpose.