T MÄKI-VALKAMA^1,2, A LEHTINEN², A SANTALA¹, K KOIVU¹, T PEHU¹, K LEHTO³, J VALKONEN⁴ and E PEHU¹

¹Department of Plant Production, PO Box 27, and ²Department of Plant Biology, PO Box 28, FIN-00014 Helsinki University, Finland; ³Department of Botany, Biocity, Tykistökatu 6, FIN-20521 Turku, Finland; ⁴Department of Plant Biology, Swedish University of Agricultural Sciences, Genetic Center, PO Box 7080, S-75007 Sweden

Background and objectives
Potyviruses form the largest and economically the most important group of plant viruses. Potato virus Y (PVY) is the type member of this virus group. It causes serious damage in potato (10-100% yield losses) and other solanaceous crop species worldwide. Disease severity depends on PVY strain, host tolerance, time of infection and environmental factors. In potato, PVY is transmitted to the new crop via seed tubers. Primary symptoms caused by PVY may be mild or hardly detectable, which causes problems particularly in potato seed production. In Finland, many of the commonly grown potato cultivars are susceptible to the ordinary strain of PVY (PVY-O) [1, 2]. Thus development of resistance to PVY in potato cultivars is of significant economic importance.

The objectives of this work were (i) to use PVY P1 gene to engineer resistance to PVY in potato; and (ii) to analyse the mechanism of the transgenic resistance thus achieved.

Results and conclusions
The P1 gene was cloned from the ordinary strain of PVY (PVY-O) and was transformed both in sense (S) [3] and in antisense (AS) orientation into the Finnish potato cv. Pito. Three S- and five AS-transgenic lines showed resistance to PVY-O infection following sap and graft inoculation. However, data showed that resistance was effective to PVY-O only, whereas no resistance to PVY tobacco vein necrosis strain (PVY-N), potato A potyvirus, or other potato viruses was observed.

In the transgenic, PVY-susceptible lines the P1 gene was integrated into the potato genome in one to three copies. In the PVY-resistant lines, one to five copies of the P1 insert were detected. However, the P1 mRNA expression levels were lower in the PVY-resistant lines than in the PVY-susceptible lines. Consistent with this result, P1 protein could be detected in the susceptible lines but not in the resistant lines. The overall transgene expression levels were low, but it could be shown that the expression levels in AS-lines were lower than those in the S-lines. These data indicate involvement of a gene-silencing mechanism [4], which is the subject of our current studies. We have also carried out a small-scale field evaluation of P1 transgenic plants under natural virus transmission pressure in 1997. Preliminary data suggest that at least some transgenic lines are resistant to PVY also in the field.

References
4. Baulcombe DC, 1996. Plant Molecular Biology 32, 79-88.