Are elicitins cryptograms in plant-Oomycete communications?

Stimulation of plant natural defenses is an important challenge in phytopro tection prospects. In that context, elicitins, which are small proteins sec reted by Phytophthora and Pythium species, have been shown to induce a hype rsensitive-like reaction in tobacco plants. Moreover, these plants become r esistant to their pathogens, and thus this interaction constitutes an excel lent model to investigate the signaling pathways leading to plant resistanc e. However, most plants are not reactive to elicitins, although they posses s the functional signaling pathways involved in tobacco responses to elicit in. The understanding of factors involved in this reactivity is needed to d evelop agronomic applications. In this review, it is proposed that elicitin s could interact with regulating cell wall proteins before they reach the p lasma membrane. Consequently, the plant reactivity or nonreactivity status could result from the equilibrium reached during this interaction. The poss ibility of overexpressing the elicitins directly from genomic DNA in Pichia pastoris allows site-directed mutagenesis experiments and structure/functi on studies. The recent discovery of the sterol carrier activity of elicitin s brings a new insight on their molecular activity. This constitutes a cruc ial property, since the formation of a sterol-elicitin complex is required to trigger the biological responses of tobacco cells and plants. Only the e licitins loaded with a sterol are able to bind to their plasmalemma recepto r, which is assumed to be an allosteric calcium channel. Moreover, Phytopht hora and Pythium do not synthesize the sterols required for their growth an d their fructification, and elicitins may act as shuttles trapping the ster ols from the host plants. Sequence analysis of elicitin genes from several Phytophthora species sheds unexpected light on the phylogenetic relationshi ps among the genus, and suggests that the expression of elicitins is under tight regulatory control. Finally, general involvement of these lipid trans fer proteins in the biology of Pythiaceae, and in plant defense responses, is discussed. A possible scheme for the coevolution between Phytophthora an d tobacco plants is approached.