RESISTANCE IN PLANTS TRANSFORMED WITH THE P1 OR P3 GENE OF TOBACCO VEIN MOTTLING POTYVIRUS

Tobacco plants were transformed with genes encoding the tobacco vein m ottling potyvirus (TVMV) P1 or P3 protein. When compared with vector-t ransformed or P1 transgenic lines, seedlings of P3 transgenic lines (e xcept a low expressor line) developed poorly, suggesting a detrimental effect of P3 on the plant. All P1 and P3 transgenic lines were protec ted against the homologous TVMV strain and showed variable proportions of two resistance phenotypes: asymptomatic plants or symptomatic plan ts that recovered from infection, The resistance was effective against a high inoculum dose but had a narrow spectrum. The heterologous stra in TVMV-S was able to overcome resistance in most P1 lines but did not break the resistance of most P3 lines, No line showed resistance to a nother potyvirus (potato virus Y) or to potato virus X. These features and the low levels of transgene expression in resistant plants sugges t that protection in P1 and P3 lines is RNA-mediated. In contrast with most reports on virus-activated gene silencing, some P3 lines with th e predominant 'recovery' phenotype showed silencing of the transgene t hat was activated at a certain developmental stage of the plant indepe ndently of virus infection.