THE RELATIONSHIPS BETWEEN THE TOXICITY AND THE PRIMARY AND SECONDARY STRUCTURES OF ELICITINLIKE PROTEIN ELICITORS SECRETED BY THE PHYTOPATHOGENIC FUNGUS PYTHIUM VEXANS

Elicitins are toxic and signaling proteins secreted by Phytophthora spp. responsible for the incompatible reaction and systemic hypersensitive-like 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 alpha elicitinlike proteins were purified and sequenced. One of these novel elicitins (Vex(2)) exhibited a 100-residue sequence instead of 98 while the other (Vex(1)) had an N-glycosylation site, effectively glycosylated (equivalent of 16 hexose residues). In addition to the point mutations already observed in Phytophthora species, we found several novel amino acid changes. Furthermore, circular dichroism revealed some differences in their structure in solution compared with the Phytophthora elicitins that were correlated with specific point mutations. These sequences permitted the establishment of a phylogenic tree, suggesting that Pythium vexans is a species close to the Phytophthora 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.