The fungal gene AVR9 and the oomycete gene inf1 confer avirulence to potato virus X on tobacco

The AVR9 peptide of the fungal pathogen Cladosporium fulvum and the INF1 pr otein of the oomycete pathogen Phytophthora infestans elicit the hypersensi tive response (HR) on Cf9 tomato or Cf-9 transgenic tobacco and on all cult ivars of tobacco, respectively. Expression of either the functional Avr9 or inf1 genes from engineered potato virus X (PVX) genomes resulted in locali zed HR lesions on tobacco plants responsive to the elicitors and inhibited spread of the recombinant virus. In contrast, PVX derivatives producing mut ant forms of AVR9 and INF1 with reduced elicitor activity caused systemic n ecrotic and/or mosaic symptoms, and were unable to inhibit PVX spread. Thes e results demonstrate that HR is a highly versatile defense mechanism activ e against unrelated pathogens irrespective of the HR-inducing agent, and th at resistance to recombinant PVX in tobacco is correlated with the strength of the transgene-encoded elicitor.