Plant resistance to fungal infection induced by nontoxic pokeweed antiviral protein mutants

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Abstract
Pokeweed antiviral protein (PAP), a 29-kD protein isolated from Phytolacca americana inhibits translation by catalytically removing a specific adenine residue from the large rRNA of the 60S subunit of eukaryotic 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 enzymatic activity, and another mutant, delta C25PAP, had a nonsense mutation near the C-terminus (W237stop) that deleted 25 C-terminal amino acids. 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 overexpressed in transgenic plants expressing PAP and the nontoxic PAP mutants. Although PR-proteins are constitutively expressed, no increase in salicylic acid levels was detected. Homozygous progeny of transgenic plants expressing either PAP or the nontoxic PAP mutants displayed resistance to the fungal pathogen Rhizoctonia solani. These results show that expression of PAP or the nontoxic PAP mutants activates multiple plant defense pathways independently of salicylic acid and confers resistance to fungal infection. The C-terminal 25 amino acids of PAP, which are required for toxicity in vivo, are not critical for resistance to viral or fungal infection, indicating that toxicity of PAP can be separated from pathogen resistance.