EXPRESSION OF POKEWEED ANTIVIRAL PROTEIN IN TRANSGENIC PLANTS INDUCESVIRUS-RESISTANCE IN GRAFTED WILD-TYPE PLANTS INDEPENDENTLY OF SALICYLIC-ACID ACCUMULATION AND PATHOGENESIS-RELATED PROTEIN-SYNTHESIS

Pokeweed antiviral protein (PAP), a 29-kD protein isolated from Phytol acca americana, inhibits translation by catalytically removing a speci fic adenine residue from the large rRNA of the 60S subunit of eukaryot ic ribosomes. Transgenic tobacco (Nicotiana tabacum) plants expressing PAP or a variant (PAP-v) were shown to be resistant to a broad spectr um of plant viruses. Expression of PAP-v in transgenic plants induces synthesis of pathogenesis-related proteins and a very weak (<2-fold) i ncrease in salicylic acid levels. Using reciprocal grafting experiment s, we demonstrate here that transgenic tobacco rootstocks expressing P AP-v induce resistance to tobacco mosaic virus infection in both N. ta bacum NN and nn scions. Increased resistance to potato virus X was als o observed in N. tabacum nn scions grafted on transgenic rootstocks. P AP expression was not detected in the wild-type scions or rootstocks t hat showed virus resistance, nor was there any increase in salicylic a cid levels or pathogenesis-related protein synthesis. Crafting experim ents with transgenic plants expressing an inactive PAP mutant demonstr ated that an intact active site of PAP is necessary for induction of v irus resistance in wild-type scions. These results indicate that enzym atic activity of PAP is responsible for generating a signal that rende rs wild-type scions resistant to virus infection in the absence of inc reased salicylic acid levels and pathogenesis-related protein synthesi s.