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Differences in Intensity and Specificity of Hypersensitive Response Induction in Nicotiana spp. by INF1, INF2A, and INF2B of Phytophthora infestans

March 2005 , Volume 18 , Number  3
Pages  183 - 193

Edgar Huitema , 1 Vivianne G. A. A. Vleeshouwers , 2 Cahit Cakir , 1 Sophien Kamoun , 1 , 2 and Francine Govers 2

1Department of Plant Pathology, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster, U.S.A.; 2Laboratory of Phytopathology, Wageningen University, The Netherlands

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Accepted 11 November 2004.

Elicitins form a family of structurally related proteins that induce the hypersensitive response (HR) in plants, particularly Nicotiana spp. The elicitin family is composed of several classes. Most species of the plant-pathogenic oomycete genus Phytophthor produce the well-characterized 10-kDa canonical elicitins (class I), such as INF1 of the potato and tomato pathogen Phytophthora infestans. Two genes, inf2A and inf2B, encoding a distinct class (class III) of elicitinlike proteins, also occur in P. infestans. Unlike secreted class I elicitins, class III elicitins are thought to be cell-surface- anchored polypeptides. Molecular characterization of the inf2 genes indicated that they are widespread in Phytophthora spp. and occur as a small gene family. In addition, Southern blot and Northern blot hybridizations using gene-specific probes showed that inf2A and inf2B genes and transcripts can be detected in 17 different P. infestans isolates. Functional secreted expression in plant cells of the elicitin domain of the inf1 and inf2 genes was conducted using a binary Potato virus X (PVX) vector (agroinfection) and Agrobacterium tumefaciens transient transformation assays (agroinfiltration), and resulted in HR-like necrotic symptoms and induction of defense response genes in tobacco. However, comparative analyses of elicitor activity of INF1, INF2A, and INF2B revealed significant differences in intensity, specificity, and consistency of HR induction. Whereas INF1 induced the HR in Nicotiana benthamiana, INF2A induced weak symptoms and INF2B induced no symptoms on this plant. Nonetheless, similar to INF1, HR induction by INF2A in N. benthamiana required the ubiquitin ligase-associated protein SGT1. Overall, these results suggest that variation in the resistance of Nicotiana spp. to P. infestans is shadowed by variation in the response to INF elicitins. The ability of tobacco, but not N. benthamiana, to respond to INF2B could explain differences in resistance to P. infestans observed for these two species.

© 2005 The American Phytopathological Society