Expression of multiple virus-derived resistance determinants in transgenicplants does not lead to additive resistance properties

Plants can be protected against infection by potyviruses by expressing diff erent portions of potyviral genomes as transgenes. This strategy has proven effective with several potyvirus gentes, including the Nla, Nlb,and coat p rotein coding regions. Given the effectiveness of separate potyvirus coding regions as determinants of resistance, we tested the hypothesis that combi nations of potyvirus coding regions would provide additively greater protec tion of plants against potyviruses. For this, we compared transgenic plant lines that expressed either the coat protein (CP) car the Nla+Nlb+coat prot ein (NNC) coding regions from tobacco vein mottling virus (TVMV). We found that plants that carry the NNC gene combination were invariably less resist ant to TVMV than were lines that contain a CP gene alone. Additionally, we found that NNC lines displayed virtually no resistance to tobacco etch viru s (TEV), in contrast to the CP lines. We conclude that combining more than one virus-derived resistance determinant in a single construct is detriment al to the production of virus-resistant plants.