GENETICALLY-ENGINEERED PROTECTION AGAINST VIRUSES IN TRANSGENIC PLANTS

Transgenic plants carrying nucleotide sequences derived from plant vir uses can exhibit increased resistance to viral disease. Many viral seq uences confer some level of either resistance to infection or suppress ion of disease symptoms (tolerance). These include segments of viral g enomes encoding capsid or coat proteins, sequences encoding proteins t hat are or may be subunits of the viral replicase, sequences incapable of encoding proteins, entire genomes of defective interfering viruses and satellite viruses, and complete genomes of mild strains of virus. The transgene may act on initiation of infection, replication of viru s, spread of the infection throughout the plant, and symptom developme nt. More than one of these processes can be impaired by a single trans gene derived from a single viral gene. The level of protection ranges from very low to high, while the breadth of protection ranges from ver y narrow, where protection is only observed against closely related st rains of the virus from which the transgene was derived, to moderately broad, extending to other viruses. Data are insufficient to establish a molecular mechanism of resistance for most of the described example s. In addition, although the use of a particular segment of the viral genome confers resistance in one virus-host system, analogous sequence s from a different virus in another host may be ineffective.