Transgenic potato plants, cultivar Désirée, were produced that contained the coat protein gene of potato leafroll luteovirus (PLRV). The transformed potato plants expressed the PLRV coat protein (CP) RNA sequences but accumulation of coat protein in transgenic tissues could not be detected. Upon inoculation with PLRV, the PLRV CP RNA expressing potato plants showed a reduced rate of virus multiplication.

Plant viral genomes are relatively small and in the past decade many have been characterized at the molecular level. This has prompted research into the development of virus resistance based on interference with the viral multiplication cycle by teh introduction of viral sequences into the plant genome. Several strategies have been tested. The most successful one so far involves the constitutive expression of the coat protein gene of the virus against which resistance is desired. In this review we describe progress made in engineering virus resistance into potato, an important agricultural crop. To this end the molecular structure of the potato viruses X and Y and leafroll is discussed as well as the introduction of resistance against potato virus X into potato. In addition, we address the question of preservation of cultivar-specific characteristics, an important prerequisite for commercial application. Finally, recent investigations for alternative forms of virus resistance are described against the background of the results of coat protein-mediated protection.

To engineer resistance against potato virus X (PVX), the viral coat protein (CP) gene has been introduced into two potato cultivars. Stable expression of the gene in transgenic clones throughout the growing season has been obtained and resulted in considerably increased virus resistance. With varying frequencies depending on the original cultivar used, true-to-type PVX resistant transgenic clones have been obtained. Since deviant light sprout characteristics were invariably associated with aberrations in plant phenotype, they can be used in procedures to early screen for deviations. Furthermore, it has been possible to unequivocally discriminate between the original untransformed and independent transgenic cultivars. Although no relation has been found between the presence, if any, of the CP of potato virus Y (PVY) or potato leafroll virus (PLRV) in CP gene transgenic potato, appreciable levels of resistance to these viruses has been obtained. This suggests that the mechanism by which a viral CP gene in the potato genome evokes resistance, differs amongst various viruses.

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