For more information, see Virologist Alma Laney’s detailed article on How virus resistance works in GMOs.
Plant viruses can be serious pathogens in crops as they can cause anywhere from minor losses to total crop failure. Viruses can be transmitted to crops in a number of ways ranging from contaminated tools to seed and pollen infections. But the primary mechanism of virus transmission is by arthropod vectors1,2 such as mites and insects. Because of the speed of infection and the devastation that crops can suffer, farmers focus on preventing the introduction of these viruses, which can slow or reduce its spread. However, one of the best strategies is to engineer resistance to the viruses themselves.
How does it work?
Genetically engineered plant virus resistance induces two different forms of resistance and one of the methods can use two different approaches.
- Early on, virologists transformed plants with the complete virus coat protein gene, which forms the shell of the virus to protect the genetic material. It was found in the case of Tobacco mosaic virus (TMV), that over-expression of the coat protein gene led to virus resistance because the excess coat protein interfered with the ability of the virus to complete its lifecycle and move systemically in the plant3.
- Later research uncovered that the the RNA silencing (RNAi) can induce resistance to plant viruses. In plants, the RNAi pathway serves as a type of immune system to target pathogens4, including viruses. There are multiple ways that the RNAi pathway can be triggered:
- dsRNA: In plants and animals, most RNA is single-stranded. However, the majority of the described plant viruses have RNA genomes and one of the byproducts of viral replication is double-stranded RNA (dsRNA)5. This dsRNA is very stable and triggers the RNAi pathway. Some genetically engineered crops that utilized the complete coat protein also triggered the RNAi pathway, and it was found that in some cases the secondary structure of the RNA produced regions with dsRNA that could trigger RNA silencing in plants6.
- Short hairpins: The discovery that short dsRNA segments could trigger RNAi led to the use of constructs that generate a hairpin cassette that forms into dsRNA7. Early uses of virus-derived transgenic resistance used entire genes; however, with advances in our understanding of the RNAi pathway, many researchers have adopted the hairpin cassette method. This allows for the targeting of multiple viruses and/or viral genes8, which minimizes the chances of the targeted virus developing resistance.
What crops are modified?
- Papaya resistant to Papaya ringspot potyvirus (PRSV): This product saved the papaya industry in Hawaii as PRSV makes fruit unmarketable and eventually kills infected trees9. The virus spreads quickly and attempts to introgress resistance to PRSV from wild relatives failed for decades, leaving papaya growers with the only strategy of moving their operations to another island. Each time they moved, there was a short reprieve, but the virus eventually made it to that area. By the time PRSV made it to the last papaya growing area in Puna, Hawaii, the transgenic was ready and the industry was saved.
- Summer squash: There are two different transgenic events for virus resistance in summer squash:
- Crops in development: There are multiple crops in development, the most notable are Cassava with resistance to Cassava mosaic disease (CMD)12 and Cassava with resistance to Cassava brown streak disease (CBSD)13, to their potential impact to food security in several African nations.
Virus-derived transgenic resistance holds great promise in sparing growers and consumers the costs of losses due to virus infection. Furthermore, this technology has saved at least one crop, papaya grown in Hawaii, and holds the potential to grant those in developing nations food security by preventing losses in staple crops. Some of the other benefits of this approach to controlling plant viruses is that it reduces sprays that were used to control the arthropod vectors, while not altering how the crops are grown. One of the main challenges is that resistance to one strain of virus may not give strong resistance to other strains, so the evolution of new virus strains must be closely monitored.
1: Leitner et al., 2015. Arthropod Vectors and Disease Transmission: Translational Aspects. PLoS Neglected Tropical Pathogens 9(11): e0004107. DOI: 10.1371/journal.pntd.0004107
2: Whitfield et al., 2015. Insect vector-mediated transmission of plant viruses. Virology
Volumes 479–480: 278–289 DOI: 10.1016/j.virol.2015.03.026
3: Beachy, 1999. Coat-protein-mediated resistance to tobacco mosaic virus: discovery mechanisms and exploitation. Philos Trans R Soc Lond B Biol Sci 354:659-664. DOI: 10.1098/rstb.1999.0418
4: Obbard et al., 2009. The evolution of RNAi as a defence against viruses and transposable elements. Philos Trans R Soc Lond B Biol Sci 364(1513): 99–115. DOI: 10.1098/rstb.2008.0168
5: Weber et al., 2006. Double-Stranded RNA Is Produced by Positive-Strand RNA Viruses and DNA Viruses but Not in Detectable Amounts by Negative-Strand RNA Viruses. Journal of Virology. 80(10): 5059–5064. DOI: 10.1128/JVI.80.10.5059-5064.2006
6: Lindbo and Falk, 2017. The Impact of “Coat Protein-Mediated Virus Resistance” in Applied Plant Pathology and Basic Research. Phytopathology 107(6): 624-634 DOI: 10.1094/PHYTO-12-16-0442-RVW
7: Jia et al., 2007. A strategy for constructing and verifying short hairpin RNA expression vectors. J RNAi Gene Silencing 3(1): 248–253. PMCID: PMC2737214
8: Lambeth et al., 2010. A direct comparison of strategies for combinatorial RNA interference. BMC Molecular Biology 11:77. DOI: 10.1186/1471-2199-11-77
9: Gonsalves et al., 2004. Transgenic Virus Resistant Papaya: From Hope to Reality for Controlling Papaya Ringspot Virus in Hawaii. APSnet Features. Online. DOI: 10.1094/APSnetFeature-2004-0704
10: Fuchs and Gonsalves, 1995. Resistance of Transgenic Hybrid Squash ZW-20 Expressing the Coat Protein Genes of Zucchini Yellow Mosaic Virus and Watermelon Mosaic Virus 2 to Mixed Infections by Both Potyviruses. Nature Biotechnology 13: 1466 – 1473 DOI: 10.1038/nbt1295-1466
11: Tricoll et al., 1995. Field Evaluation of Transgenic Squash Containing Single or Multiple Virus Coat Protein Gene Constructs for Resistance to Cucumber Mosaic Virus, Watermelon Mosaic Virus 2, and Zucchini Yellow Mosaic Virus. Nature Biotechnology 13: 1458 – 1465 DOI: 10.1038/nbt1295-1458
12: Chellappan et al., 2004. Broad Spectrum Resistance to ssDNA Viruses Associated with Transgene-Induced Gene Silencing in Cassava. Plant Molecular Biology 56: 601-611 DOI: 10.1007/s11103-004-0147-9
13: Ogwok et al., 2012. Transgenic RNA interference (RNAi)-derived field resistance to cassava brown streak disease. Molecular Plant Pathology 13: 1019-1031 DOI: 10.1111/j.1364-3703.2012.00812.x
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