Articles
BIOTECHNOLOGY AND RESISTANCE
Several disease resistance genes have been cloned and sequenced, and possibilities exist for their introduction into new genetic backgrounds in transgenic plants, under suitable control to ensure appropriate expression.
Other resistance genes have not yet been isolated, yet closely linked marker genes or gene products have been identified; assays for the marker genes can be used to study inheritance of the resistance gene, and introgression of the gene into breeding lines.
Means of following introgression of marker genes include isozyme analysis, restriction fragment length polymorphisms (RFLPs), random amplified polymorphic DNAs (RAPDs), and amplified restriction fragment length polymorphisms (AFLPs).
Pathogen-derived resistance has now been demonstrated with many virus groups, and a variety of viral genes, to interfere with the viral life cycle.
The actual mechanisms of resistance are not yet clear.
No single gene or gene product has been universally effective; nor is there necessarily a direct relationship between the level of gene expression and the degree of resistance.
The greatest degree of resistance is typically restricted to closely related viral isolates, a situation not likely to have long-term utility in field situations.
Lower levels of resistance to a broader spectrum of virus isolates may be more valuable, with the combination of different mechanisms to increase the overall level of resistance and reduce the appearance of resistance-breaking strains.
Other means of introducing resistance to viruses or other pathogens include expression of satellite RNAs, defective interfering RNAs, ribozymes, specific protease inhibitors, or antibodies to either structural or non-structural proteins, in order to interfere with viral processes.
Broad-spectrum virus resistance may be possible through the expression of anti-viral proteins, nucleases specific for double-stranded RNA, the mammalian interferon-induced antiviral gene 2–5 A synthase, antibodies specific for double-stranded RNA, or inhibitors of RNA-dependent RNA polymerase.