The Relationships Between theToxicity and the Primary and Secondary Structures of Elicitinlike Protein Elicitors Secreted by the Phytopathogenic Fungus Pythium vexans. Jean- Claude Huet. Laboratoire d'Etude des Proteines, Departement de Physiologie et Biochimie vegetales, INRA, Domaine de Vilvert, F-78352 Jouy-en-Josas Cedex, France. Jean-Pierre Le Caer(2), Claude Nespoulous(1), and Jean-Claude Pernollet(1) (1)Laboratoire d'Etude des Proteines, Departement de Physiologie et Biochimie vegetales, INRA, Domaine de Vilvert, F-78352 Jouy-en-Josas Cedex, France; (2)CNRS - Institut Alfred Fessard, 91198 Gif-sur-Yvette, France. MPMI 8:302-310. Accepted 16 December 1994. Copyright 1995 The American Phytopathological Society.

Elicitins are toxic and signaling proteins secreted by Phy-tophthora spp. responsible for the incompatible reaction and systemic hypersensilive-Iike necroses of diverse plant species leading to resistance against fungal or bacterial plant pathogens. Such proteins were observed in the culture filtrate of another species of the Oomycete genus, Pythium vexans. Two a elicitinlike proteins were purified and sequenced. One of these novel elicitins (Vex2) exhibited a 100-residue sequence instead of 98 while the other (Vex1) had an N-glycosylation site, effectively glycosylated (equivalent of 16 hexose residues). In addition to the point mutations already observed in Phylophthora species, we found several novel amino acid changes. Furthermore, circular dichroism revealed some differences in their structure in solution compared with the Phytophlhora elicitins that were correlated with specific point mutations. These sequences permitted the establishment of a phylo-genic tree, suggesting that Pythium vexans is a species close to the Phylophthora genus. The toxicity of the Pythium vexans elicitins to tobacco leaves was investigated and correlated with the occurrence of the carbohydrate moiety of one of the two isoforms, observed for the first time in an elicitin.