Engineered Resistance to Plant Viruses

Viruses cause significant losses in most major food and fibre crops worldwide. A range of strategies can be used to control virus infection, including chemical treatments to kill virus vectors, identification and introduction of natural resistance genes from related species and use of diagnostics and indexing to ensure propagation of virus-free starting material (seeds, tubers, etc.). However, the major development has been the exploitation of pathogen-derived resistance, i.e. the use of virus-derived sequences expressed in transgenic plants to confer resistance to plant viruses. This approach is based on earlier observations that inoculation or infection of a plant initially with a mild strain of a virus confers protection against subsequent inoculation with a virulent strain of the same or a closely related virus.Pathogen-derived resistance thus involves transformation of plants with virus-derived sequences; host resistance appears to result from two different mechanisms: (i) protection thought to be mediated by expression of native or modified viral proteins (e.g. coat protein, replicase, defective replicase) and (ii) protection mediated at the transcriptional level (‘RNA-mediated resistance’) which requires transcription of RNA either from full or partial sequences derived from the target virus
(including genes for coat protein, replicase, defective replicase, protease, movement protein, etc.).
The molecular events which underlie pathogen-derived resistance are the subject of intensive research. The bases of RNA-mediated virus resistance and post-transcriptional gene silencing are probably similar and reflect fundamental activities in plant cells to detect, inactivate and eliminate foreign DNA or RNA. For example, endogenous plant genes inserted into viruses such as PVX can silence expression of the endogenous plant gene. It is probable that low molecular weight double-stranded RNA sequences homologous to the gene message to be silenced or degraded travel systemically through the plant from the site of induction, to ensure that viruses with homologous sequences are degraded when they arrive elsewhere in the plant. Understanding the basis of resistance is needed to ensure practical applications of transgenic virus resistance are stable and have the least environmental risks when deployed on a wide scale.
Most of the major crops have been transformed with genes from major viral pathogens based on the concept of pathogen-derived resistance.
For example, by expression of viral replicase derived sequences, host resistance has to be obtained to 13 genera of viruses representing 11 plant virus taxa. Major crops transformed for virus resistance include potato, tomato, canola, soybean, sugar beet, rice, barley, sugar cane, papaya, melons/ cucurbits, peanut, horticultural and tree species. Effective resistance against a wide range of viruses has been achieved, including PVX, PVY, PLRV, CMV, BYMV, PRSV, ACMV, CPMV, TYLCV, PPV,PMMV, TMV, PEBV, CymRSV, BYDV, RTV and BBTV.
A good practical application of this technology is that of effective protection of transgenic papaya (Carica papaya) against papaya ringspot virus (PRSV). In Hawaii, PRSV has devastated papaya production.
Resistance to PRSV (mediated via viral coat protein constructs) has held up under field conditions in Hawaii and these results suggest that long-term protection of perennial crops, such as papaya, will be possible using pathogen-derived resistance.