Vargas, W.A., Djonović, S., Sukno, S.A., and Kenerley, C.M. 2008. JBC (Journal of Biological Chemestry). 283:19804-19815
Trichoderma virens secretes a small protein (Sm1) that induces local and systemic defenses in plants. This protein belongs to the ceratoplatanin protein family and is mainly present as a monomer in culture filtrates. However, Epl1 (a homologue of Sm1) secreted by Hypocrea atroviride is produced, with high levels of dimerization. Nonetheless, the molecular mechanisms involved in recognition and the signaling pathways involved in the induction of systemic resistance in plants are still unclear. In this report we demonstrate for the first time that the monomeric form of Epl1, likewise Sm1, induces defenses in maize plants. In addition, we demonstrate that Sm1 and Epl1 are mainly produced as monomer and a dimer, respectively, in the presence of maize seedlings. The results presented show that the ability to induce plant defenses reside only the monomeric form of both Sm1 and Epl1. Biochemical analyses indicate that monomeric Sm1 is produced as a glycoprotein, but the glycosyl moiety is missing from its dimeric form and Epl1 is produced as a non-glycosylated protein. Moreover, for Sm1 homologues in various fungal strains, there is a negative correlation between the presence of the glycosylation site and their ability to aggregate. We propose a subdivision in the ceratoplatanin protein family according to the presence of the glycosylation site and the ability of the proteins to aggregate. The data presented suggest that elicitor’s aggregation may control the Trichoderma-plant molecular dialogue and block the activation of induced systemic resistance in plants.