Viral-resistance in crop plants is engineered based on the premise that the host plants express genes that interfere with the essential functions of the virus thereby upsetting the balance of related components. Coat-protein mediated pathogen-derived resistance is the commonly used method to introduce resistance in crop plants to viruses. In this approach, plants are transformed with a specific virus coat protein gene which interfere with critical processes such as replication, post-transcriptional gene expression, virion coating and uncoating and intercellular transport (Beachy et al., 1990; Kaniewski and Lawson, 1998). Constitutive expression of the coat protein gene confers protection against infection with the virus from which the gene is derived and possibly against infection from other related viruses (Di et al., 1996).
Commercially available virus-resistant crops include papaya, summer squash and potato. Virus-resistant papaya and squash have been available since 1998. Virus-resistant trait was stacked with Bt to broaden the range of protection against pest populations in potato and was discussed in insect-resistant plants section.
Adoption of virus-resistant papaya has been rapid since its introduction (53% in 2000). It is expected that virus-resistant papaya will be planted on almost 90% of the acreage in the next few years. On the other hand, biotechnology-derived summer squash was planted on less than 10% of the total acreage in the United States in 2000. Lack of resistance to important pathogenic viruses coupled with the availability of virus-resistance trait in only few varieties is cited to be the reasons for low adoption.
Biotechnology-derived virus-resistant crops are particularly valuable as management options that limit viral infestations to prevent serious yield losses are limited. Since viral infestations cannot be controlled by chemical means, conventional way to manage viruses is to manage their transmission by controlling insects. Preventing the spread of virus by controlling insect vectors is not effective for two reasons: virus transmission through insects is almost instantaneous which render insecticide applications futile and secondary hosts that harbour the viruses do not exhibit symptoms. Another widely used management technique to control viruses is use of resistant varieties in crops such as squash. Natural resistance may not be available to combat viruses in crops such as papaya. However, both these methods are not completely effective in preventing viral infestations.
Papaya industry in the United States concentrated mainly in Hawaii was on the brink of extinction in 1990s due to the epidemic infestations of papaya ringspot virus (PRSV). PRSV is the most important disease of papayas. The PRSV is transmitted by aphids and cannot be eradicated as secondary hosts harbour the virus without exhibiting any symptoms. Hawaiian farmers had no choice other than destroying the infested plants to contain the disease.
Viruses that limit summer squash production in the United States are zucchini mosaic virus (ZMV), watermelon mosaic virus 2 (WMV), cucumber mosaic virus (CMV) and papaya ringspot virus (PRSV). All these viruses are transmitted by aphids and affect a range of plants making it difficult and impossible to prevent virus infestations. Foliar applications of highly refined petroleum oil are widely used to serve as a barrier between aphid and the plant to prevent virus transmission. However, frequent applications are needed to ensure season-long protection.
Virus-resistant plants enable growers to reduce the use of pesticide by eliminating the need to spray insecticides to control the insects that transmit viral diseases, or herbicides to kill the weeds that harbour those insects. As a result, overall pesticide use and crop production costs have been reduced. An indirect benefit of virus-resistant crops is they do not serve as reservoirs for viruses unlike their conventional counterparts. As a result, further spread of virus to susceptible plants by vectors is prevented.
Virus-resistant papaya is an exemplary example that demonstrated the promise biotechnology holds. It literally saved an industry that could disappear. A recent survey by USDA suggested that papaya yields increased by 33% in 2000 compared to 1998, which is a direct consequence of using PSRV-resistant plants (USDA-NASS, 2001b).
Biotechnology-derived virus protection in squash translated to increased number of harvests and increased yield per harvest. Evidence suggests that virus-resistant squash produces greater marketable yields of high quality fruit, particularly in production areas where high virus incidence limits the growing season both in terms of number of plantings and number of harvests per planting (Fuchs et al., 1998; Schultheis and Walters, 1998). However, virus-resistant squash has not reduced insecticide use as chemical applications that control aphids also control white flies and will be made to biotechnology-derived squash also.