Genetically engineered protection against viruses in transgenic plants.

Transgenic plants carrying nucleotide sequences derived from plant viruses can exhibit increased resistance to viral disease. Many viral sequences confer some level of either resistance to infection or suppression of disease symptoms (tolerance). These include segments of viral genomes encoding capsid or coat proteins, sequences encoding proteins that 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 virus, spread of the infection throughout the plant, and symptom development. More than one of these processes can be impaired by a single transgene derived from a single viral gene. The level of protection ranges from very low to high, while the breadth of protection ranges from very narrow, where protection is only observed against closely related strains 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 examples. In addition, although the use of a particular segment of the viral genome confers resistance in one virus-host system, analogous sequences from a different virus in another host may be ineffective.