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The development of sustainable, environmentally-benign methods of crop protection is an important priority in agricultural research. A variety of insects attack crops, causing damage and reducing yields and crop quality. Insects cause crop loss directly through feeding on leaves, flowers, fruit or seed. A subset of insects damages crops indirectly, through transmission of plant viruses, resulting in reduced yield and crop quality. Breeding for disease resistance has been an important strategy for protection of crops against fungal, bacterial or viral diseases; however, resistances have not yet been identified or transferred for many major diseases. Although integrated pest management (IPM) strategies have been implemented with noted success, insect control has more often relied on the use of pesticides, leading to the evolution of pesticide-resistant insects and to increasing health and environmental concerns. The development of pest resistant plants is an attractive alternative strategy for the control of insects and the direct damage they cause. For a target pest that is also the vector of a plant pathogenic virus, the question arises as to whether an effective insect resistance could also serve as a component in an integrated control strategy for insect vectored viruses.

Cross-protection is a natural phenomenon whereby tolerance or resistance of a plant to one virus strain is induced by systemic infection with a second. Eighty years have passed since the phenomenon was first demonstrated by McKinney (1929), who observed that in tobacco plants systemically infected with a “light green strain” of Tobacco mosaic virus (TMV: Genus Tobamovirus ), the appearance of yellow symptoms after re inoculation with a TMV “yellow mosaic strain” was repressed. In contrast, a “mild dark green” strain did not repress these yellow symptoms upon challenge. Later Salaman (1933) demonstrated that an avirulent strain of Potato virus X (PVX: Genus Potexvirus ) provided protection against superinfection with a virulent strain of PVX in potato. Webb et al. (1952) showed that cross protection against the phloem-limited virus, Potato leafroll virus (PLRV: Genus Polerovirus ) could be achieved by infection with the aphid vector and not only by sap inoculation. The first demonstrations of virus-disease control by mild strains were done with Citrus tristeza virus (CTV: Genus Closterovirus ) ( Grant and Costa, 1951 ), and Cacao swollen shoot disease ( Posnette and Todd, 1955 ). For many years serological and cross-protection tests were used as routine methods to determine strain interrelationships in plant viruses ( Latorre and Flores, 1985 ). Apparently, cross-protection seemed to be a general phenomenon with viruses for which distinct strains could be found ( Fulton, 1986 ; Sherwood, 1987 ; Fraser, 1998 ).

After initial inoculation, most viruses spread in host plants via two mechanisms: local, cell-to-cell movement and systemic movement. Cell-to-cell movement occurs through intercellular connections, plasmodesmata (PD), between epidermal (EP) cells and mesophyll (MS) cells, or MS cells and MS cells. Systemic movement is more complex, comprising three distinct stages: viral entry into vascular system from MS cells in the inoculated leaf, long distance transport through the vasculature, and viral egress from the vascular tissues into MS cells within uninoculated, systemic organs. Generally, local movement is a relatively slow process (e.g., 5–15 µm/hr, see Gibbs, 1976 ), which, in some hosts, may be further restricted by limitations in the viral replication rate. On the other hand, long distance movement through the vascular system is rather rapid (e.g., 50–80 mm/hr, see Gibbs, 1976 ), occurring with the flow of photoassimilates and, in many if not all cases, not requiring viral replication ( Wintermantel et al. 1997 ; Susi et al. 1999 ). Studies to date show that these two processes are mediated by different sets of viral proteins, implying that cellular machineries, especially those for the PD transport that viruses utilize in their two modes of movement are quite different from each other.

Viruses are subcellular parasites that replicate within a host cell with no intervening membrane to insulate host and viral gene products from each other ( Hull, 2002 ). The highly intimate nature of this relationship suggests that the biochemical and physiological processes occurring in the various host cell types through which a virus must propagate will significantly affect the outcome of the infection. In plants, drastic alterations in, and redirection of, host metabolism have been observed in many studies of both incompatible and compatible host-virus interactions. However, is it safe to suggest that these changes in plant metabolism influence whether a plant is resistant or susceptible to the virus infection? The answer to this question is important for a number of reasons. Firstly, it will lead to a better general understanding of the plant-virus interaction. Secondly, it may reveal mechanisms underlying induced resistance phenomena. Finally, it may allow us to identify targets for novel, artificial methods of inducing resistance to plant viruses.

The European cultivated potato ( Solanum tuberosum ssp tuberosum ) is a self-compatible outbreeding tetraploid species (2n = 4x = 48) and ranks fourth after wheat, rice and maize in terms of importance to human nutrition. It was introduced into Europe in the late 16th century from the Andes of South America and later transported to the rest of the world. By the end of the 18th century, it had been adapted to long-day conditions through selection by its early cultivators for early-tubering and high yields. Potato is susceptible to a wide range of fungal, bacterial and virus diseases as well as various insect and nematode pests. As its importance as a staple food crop increased, so did problems associated with its clonal means of multiplication, notably caused by various virus diseases, described as a degeneration of seed tubers due to ‘the curl’ (reviewed by Salaman, 1926 ). In time it was realized that some of the viruses were transmitted by aphids. This led to the development of seed industries in many countries where highgrade virus-free seed tubers were produced in areas that are climatically and geographically suitable with regards to isolation from sources of infection and reduced numbers of virus vectors.

The common bean ( Phaseolus vulgaris L.) is one of the most widely cultivated legumes in the world, occupying over 27 million hectares of tropical and temperate agricultural land in the Americas, Europe, Africa and Asia ( FAO, 2003 ). The genus Phaseolus is of American origin and comprises over 30 species ( Debouck, 1999 ). P. vulgaris is the most widely grown legume, occupying almost 90% of the area planted to Phaseolus species in the world. The centre of origin and domestication of common bean includes the Andean region of South America and Middle America, from Chile up to approximately the Tropic of Cancer in Mexico ( Singh, 2001 ). Genetic diversity in common bean is represented by large-seeded Andean, and small- and medium-seeded Middle American gene pools ( Evans, 1980 ). There are two major commercial classes of common bean: snap and dry beans. In the case of snap beans, the green pods are harvested, whereas for dry beans, the seed is extracted from mature pods. The dry bean is the preferred form of consumption, with over 70% of the total common bean production area corresponding to this commercial class. In general terms, the genetic base of common bean cultivars is narrow, because only a small proportion of wild common bean populations were domesticated ( Gepts et al. 1986 ).

Lettuce ( Lactuca sativa ) is a widely cultivated crop. Historically, the ancient Egyptians cultivated lettuce for its seed oil, which they believed had relaxing and aphrodisiac properties. Later, the Victorian English and others used its latex as a substitute for opium (“lactucarium”). Although stem lettuce is still cultivated in some Asian countries, lettuce is nowadays best known as a leafy vegetable and a raw ingredient in salads ( Ryder, 1999 ; Maisonneuve, 2003 ). Lettuce is a member of the family Asteraceae in the subfamily Cichorioideae and the tribe Lactuceae . The family Asteraceae also contains such crops as endive, chicory, artichoke, sunflower, safflower and many ornamental plants such as Chrysanthemum, Gazania, Osteospermum , etc. Lettuce shows a broad phenotypic diversity with several distinct horticultural types identified such as crisphead (or iceberg lettuce), romaine (cos lettuce), leaf lettuce, Batavia and butterhead lettuce ( Ryder, 1999 ; Maisonneuve, 2003 ). L. sativa is closely related to its common relative L. serriola L. (wild or prickly lettuce) and, more distantly, to two other wild species, L. saligna . and L. virosa . Lettuce is a naturally self-pollinating species so that the principal breeding strategies used with this species are pedigree breeding and back-crossing. Because it is possible to produce interspecific crosses between L. sativa and the three other species of the compatibility group ( L. serriola, L. saligna and L. virosa ), these have sometimes been used in lettuce breeding programs, in particular as sources of resistance to pathogens and pests.

Tobacco mosaic virus (TMV) and Tomato mosaic virus (ToMV) cause a serious disease in tomato, with systemic mosaic symptoms and losses in fruit yield and quality. Both viruses are closely related tobamoviruses, plus stranded RNA viruses with a rod like particle structure. The genomic structure of TMV and ToMV has been well characterized, as a positivesense single-stranded RNA genome that encodes at least four proteins ( Goelet et al. 1982 ; Ohno et al. 1984 ; Canto et al. 2004 ). The 130 kDa methyltransferase/helicase and the 180 kDa RNA dependent RNA polymerase are translated directly from the genomic RNA using the same first initiation codon, the latter is synthesised by the read-through of the amber termination codon of the 130 kDa protein gene. The movement protein (MP) and the coat protein (CP) are translated from their respective subgenomic mRNAs, which are synthesised during the replication cycle. Involvement of the 130 kDa and 180 kDa proteins in intracellular replication has been demonstrated by deletion or substitution mutants of each protein ( Ishikawa et al. 1986 ). It has also been shown that the MP is involved in cellto-cell transport ( Meshi et al. 1987 ), and that the CP is involved in long-distance movement ( Saito et al. 1990 ; Hilf and Dawson, 1993 ). In tomato, TMV infection is more or less a rare event because the virus is soon competed out in tomato populations by ToMV, which is more adapted to this host plant.

Turnip mosaic virus (TuMV) is a member of the Potyviridae , the largest group of plant viruses. It is a particularly interesting member of the family as it has a very broad host range including monocots and dicots and shows differing degrees of adaptation to different plant groups. In some respects e.g. interactions with plant resistance genes ( Walsh and Jenner, 2002 ), ecology in wild plants ( Raybould et al. 2003 ) and genetic diversity ( Tomimura et al. in press ) it is the best characterised potyvirus. As the potyvirus best adapted to the model, fully sequenced dicot plant Arabidopsis thaliana , it represents an excellent model with which to gain fundamental insights into plant — virus interactions.

The genus Oryza of the Family Gramineae comprises 18 tropical and subtropical species, of which two species are cultivated as rice: O. sativa and O. glaberrima . Rice provides the staple food of 60% of mankind, and is cultivated in all tropical and subtropical countries. Oryza sativa is thought to have been domesticated in China before 6500 BC, in India between 2000 and 1500 BC and even earlier than 5000 BC in northeast India ( Chauvet, 2004 ). Molecular markers strongly suggested that the asian rice has been domesticated twice independently to give rise to the so-called japonica and indica groups of varieties similar to subspecies in China and South-India, respectively ( Second, 1982 ). A third domestication took place in West Africa probably around 1500 BC from the wild relative O. brevigulata (syn. O. barthii ) to give the african cultivated rice species O. glaberrima which is isolated from O. sativa by reproductive barriers ( Oka, 1958 ; Second, 1982 ). Rice is a natural host for 20 viruses and an experimental host for 17. About 16 viruses may seriously affect rice yield ( Lapierre and Signoret, 2004 ). The distribution of each virus is generally restricted to only one of the continents in which rice is grown. Host plant resistance has been reported for several viruses. Intensive breeding programs have been carried out to obtain resistance to the main virus diseases: rice tungro viruses and Rice stripe virus in Asia, Rice hoja blanca virus in South-America, and Rice yellow mottle virus in Africa.