PLANT-RESISTANCE TO FUNGAL INFECTION-INDUCED BY NONTOXIC POKEWEED ANTIVIRAL PROTEIN MUTANTS

Pokeweed antiviral protein (PAP), a 29-kD protein isolated from Phytol acca americana inhibits translation by catalytically removing a specif ic adenine residue from the large rRNA of the 60S subunit of eukaryoti c ribosomes. Transgenic plants expressing PAP are resistant to a broad spectrum of plant viruses. Nontoxic PAP mutants have been isolated by random mutagenesis and selection in yeast. One of these mutants, PAP- X, had a point mutation at the active-site (E176V) that abolished enzy matic activity, and another mutant, Delta C25PAP, had a nonsense mutat ion near the C-terminus (W237stop) that deleted 25 C-terminal amino ac ids. Unlike the wild-type PAP, expression of neither mutant was toxic to transgenic plants. We show that both class I (basic) and class II ( acidic) isoforms of pathogenesis-related (PR) proteins are overexpress ed in transgenic plants expressing PAP and the nontoxic PAP mutants. A lthough PR-proteins are constitutively expressed, no increase in salic ylic acid levels was detected. Homozygous progeny of transgenic plants expressing either PAP or the nontoxic PAP mutants displayed resistanc e to the fungal pathogen Rhizoctonia solani. These results show that e xpression of PAP or the nontoxic PAP mutants activates multiple plant defense pathways independently of salicylic acid and confers resistanc e to fungal infection. The C-terminal 25 amino acids of PAP, which are required for toxicity in vivo, are not critical for resistance to vir al or fungal infection, indicating that toxicity of PAP can be separat ed from pathogen resistance.