1.8.44
RESISTANCE OF NICOTIANA TO PHYTOPHTHORA INFESTANS IS MEDIATED BY THE RECOGNITION OF THE ELICITOR PROTEIN INF 1

S KAMOUN1,3, P VAN WEST2,3 and F GOVERS3

1Department of Plant Pathology, Ohio State University, Ohio Agricultural Research and Development Center, USA; 2Department of Molecular and Cell Biology, University of Aberdeen, Scotland; 3Department of Phytopathology, Wageningen Agricultural University, The Netherlands

Background and objectives
Most Phytophthora and Pythium species produce 10-kDa extracellular protein elicitors, generally termed elicitins. Elicitins induce a hypersensitive response in a restricted number of plants, particularly in the genus Nicotiana within the Solanaceae family. Elicitins are thought to act as avirulence factors that restrict the host range of the pathogen by triggering plant defence responses [1, 2]. Phytophthora infestans, the causal agent of the potato late blight disease, produces an elicitin named INF 1. A cDNA clone encoding INFL was isolated and characterized [3]. In virulence assays with different P. infestans isolates, five INFL-responsive Nicotiana species displayed resistant responses. In all interactions, following inoculation with P. infestans zoospores, cyst germination and penetration of a Nicotiana epidermal cell was observed. This was followed by a localized necrotic hypersensitive response that varied in timing and severity between different Nicotiana spp. The objective of this study was to determine whether INF 1 functions as an avirulence factor that governs resistance in interactions between P. infestans and Nicotiana.

Results and conclusions
To determine whether INF 1 functions as an avirulence factor in interactions between P. infestans and Nicotiana, we engineered P. infestans mutants deficient in INFL production. Since P. infestans is a diploid, and homologous recombination of introduced DNA was never detected, we adopted a gene-silencing strategy to inhibit INF1 production. P. infestans was transformed with a construct carrying the oomycete promoter ham34 fused to the inf1-coding sequence in antisense orientation. Several transformants totally deficient in inf1 MRNA and INF 1 protein were obtained. The silenced state of the inf1 gene was shown to be mitotically stable under various conditions in vitro and in planta, suggesting that the mutant strains can be used in functional analyses. In virulence assays, INF 1 mutants remained pathogenic on the host plants potato and tomato. However, in contrast to wild-type and control transformant strains, INF 1 mutants induced disease lesions and extensive sporulation when inoculated on Nicotiana benthamiana. These results demonstrate that the recognition of INF 1 elicitin is a major component of the resistance of N. benthamiana to P. infestans and that INF 1 functions as an avirulence factor in this interaction.

References
1. Kamoun S, Young M, Glascock C, Tyler BM, 1993. Molecular Plant-Microbe Interactions 6, 15-25. 2. Yu L, 1995. Proceedings of the National Academy of Sciences, USA 92, 4088-4094.
3. Kamoun S, van West P, de Jong AJ et al., 1997. Molecular Plant-Microbe Interactions 10, 13-20.