RESISTANCE OF NICOTIANA-BENTHAMIANA TO PHYTOPHTHORA-INFESTANS IS MEDIATED BY THE RECOGNITION OF THE ELICITOR PROTEIN INF1

Phytophthora infestans, the agent of potato and tomato late blight dis ease, produces a 10-kD extracellular protein, INF1 elicitin. INF1 indu ces a hypersensitive response in a restricted number of plants, partic ularly those of the genus Nicotiana. In virulence assays with differen t P. infestans isolates, five Nicotiana species displayed resistance r esponses. In all of the interactions, after inoculation with P. infest ans zoospores, penetration of an epidermal cell was observed, followed by localized necrosis typical of a hypersensitive response. To determ ine whether INF1 functions as an avirulence factor in these interactio ns, we adopted a gene-silencing strategy to inhibit INF1 production. S everal transformants deficient in inf1 mRNA and INF1 protein were obta ined. These strains remained pathogenic on host plants. However, in co ntrast to the wild-type and control transformant strains, INF1-deficie nt strains induced disease lesions when inoculated on N. benthamiana. These results demonstrate that the elicitin INF1 functions as an aviru lence factor in the interaction between N. benthamiana and P. infestan s.