Plant viruses reduce both the quantity and quality of crop yields by direct damage to plants, increasing sensitivity to adverse climatic conditions and to the direct pathogens. They cause trillions of Rupees of losses every year to crops worldwide, second only to the impact of fungal diseases (Waterworth and Hadidi, 1998). In several fruit crops virus diseases represent a particular problem, for example in grape with GCMV and GFLV, in Prunus spp. with Sharka and in some tropical species, such as papaya, with PRSV (Gonsalves, 1998). At present viral diseases are controlled in a number of ways including: planting virus-free plants, maintaining plant health, controlling plant pathogens which can be virus vectors and by cross protection (Alrefai and Korban, 1995). However, these techniques provide only limited protection from viral attack. Whilst, in the case of fungi, chemical defenses are available, such remedies are either not effective in the case of viruses or can make the impact of the virus even worse. The preventive use of resistant genotypes is thus essential (Khetarpal et al., 1998). Two types transgenic resistance are available:
1. Pathogen-derived resistance (PDR) (used most at present)
2. Resistance induced by sequences of alien DNA.
PDR is conferred to the plants by genes from the virus itself, cloned and transferred to the host genome (Sanford and Johnston, 1985). PDR is developed when the viral gene products or virus-related sequences in the plant genome interferes with the virus infection cycle. The mechanisms which confer PDR are not yet well understood, varying with the nature of the gene used (Carr and Zaitlin, 1993; Fitchen and Beachy, 1993; Baulcombe, 1994; Kaniewski and Lawson, 1998; Yie and Tien, 1998; Martelli et al., 1999; Smyth, 1999). Transgenic plants for the virus coat protein gene provide the most common strategy for gene transfer. The other strategies include antisense nucleic acids, satellitesequences, defective interfering molecules and non-structural genes (replicase, protease, and movement proteins), antibodies, and interferon-related proteins (Gadani et al., 1990; Baulcombe, 1994; Grumet, 1994; Kaniewski and Lawson, 1998; Wilson, 1993). Although a large number of crop plants has been successfully engineered using such strategies, for fruit crops only the coat protein strategy has been applied to confer PDR to potyvirus, nepovirus and closterovirus groups.
Studies demonstrate that this strategy is very promising, although in papaya Tennant et al. (1994) reported that CP-PRV was effective in protecting from some virus isolates but not from others. Studies by Singh et al. (1997) demonstrated that in tobacco, as a model plant, transgenic plants expressing a defective replicase gene of cucumber mosaic virus (CMV-FNY), acquired resistance to various banana isolates of CMV, suggesting this approach is worth further development. In most cases resistance has been successfully tested invivo or indirectly by testing the accumulation of coat protein by ELISA orWestern blot analysis or gus gene expression in the transgenic tissues. Examples of the resistance induced by sequences of alienDNA are not yet available but it should be possible to obtain them since in some species, such as Citrus spp., resistance to CTV is present in Poncirus trifoliata and is known to be controlled by a dominant gene at the Ctr locus. Developing transgenic fruits for virus resistance may lead to possible risks.
Trans capsidation, when nucleic acids of a virus are covered by the coat protein belonging to another virus expressed by the transgenic plant (Farinelli et al., 1992; Greene and Allison, 1994; Robinson etal., 1999; Buzkan et al., 2000). This problem is; however, already frequent in nature, with virusmultiple infections (Creamer and Falk,1990; Hobbs and McLaughlin, 1990; Bourdin and Lecoq, 1991; Buzkan et al., 2000);
Ø Recombination of nucleic acid expressed by the transgenic plants with nucleic acids of the virus occurring in transgenic plants, producing new more virulent viruses (Rybicki, 1994; Dolja et al., 1994; Miller et al., 1997; Aziz and Tepfer, 1999; Smith et al., 2000). This problem is also very common in nature and together with mutations, is responsible for much viral evolution (Roossinck, 1997). According to the studies of Miller et al., (1997), Jacquemond and Tepfer (1998), and other scientists, transgenic plants expressing viral sequences do not represent a source of risk greater than those already present in nature; Genetic depletion caused by abandoning susceptible varieties in favour of transgenic ones. This is a false problem since resistance can be conferred to susceptible varieties by biotechnologies;
Ø Compatible wild species which could become resistant following pollination with transgenic pollen produced by the transgenic crops. This is not usually a problem in areas where fruit/vegetable crops are cultivated because there are no wild relatives, except for the area of origin of the crop in question. Several fruit crops have been transformed with virus coat proteins; some of them showed resistance in field conditions, others have not been tested yet. An indirect strategy to fight viruses is to make plants resistant to their vectors. Yang et al. (2000) for example, have tried to make plants resistant to aphids, which are the vectors of grapefruit tristeza virus.
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