Marianne J. Huisman

Nucleotide sequence and organization of potato leafroll virus genomic RNA

The nucleotide sequence of the genomic RNA of potato leafroll virus was determined and its genetic organization deduced. The RNA is 5882 nucleotides long and contains 6 open reading frames (ORFs) encoding proteins of 70, 70, 56, 28, 23 and 17 kDa. The putative genes for the coat protein (23 kDa) and the RNA‐dependent RNA polymerase (70 kDa) were identified by interviral amino acid sequence homologies. For expression of the different ORFS, translational frameshift and readthrough events are proposed.

Expression of the potato leafroll luteovirus coat protein gene in transgenic potato plants inhibits viral infection

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.

Engineering virus resistance in agricultural crops.

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 the 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.

Transgenic potato plants resistant to viruses

Traditional potato breeding is a laborious process in which, by intercrossing, valuable traits from different parental clones are combined in a progeny genotype. Depending on the availability of genes, molecular techniques can be used to add specific genes to existing cultivars that, although otherwise satisfactory, lack a few commercially important traits. For virus resistance the gene for the coat protein of a given virus transplanted into the genome of the plant renders the plant resistant to that virus. In conferring such resistance to potato varieties it proved to be possible to preserve their intrinsic properties.

Molecular breeding for virus resistant potato plants

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.

Transgenic Potato Cultivars Resistant to Potato Virus X

Extensive potato breeding programs over the years have yielded improved varieties with a whole set of proven valuable traits. The process of potato breeding, however, is laborious and time-consuming due to the tetraploid character of the potato genome. Techniques for the genetic engineering of plants present a new tool to improve potato via the introduction of traits presently missing in the existing cultivars. An essential element in the successful application of these new techniques is the preservation of the existing, desired traits of the cultivars. So far, no systematic studies on the actual performance of engineered plants have been performed. In this paper we report the engineering of resistance to potato virus X in the two commercial potato cultivars Bintje and Escort. Also, the results of two years of field trials testing the performance of these transgenic lines are discussed.