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Cloning and Characterization of a cDNA Encoding an Elicitor of Phytophthora parasitica var. nicotianae That Shows Cellulose-Binding and Lectin-Like Activities

Phytophthora parasitica var. nicotianae produces a 34-kDa glycoprotein elicitor (CBEL) that is localized in the cell wall. A cDNA encoding the protein moiety of this elicitor was cloned and characterized. The deduced amino acid sequence consisted of two direct repeats of a cysteine-rich domain, joined by a Thr/Pro-rich region. Although having no general homology with published sequences, the positions of the cysteine residues in the two repeats show a conserved pattern, similar to that of the cellulose-binding domain of fungal glycanases. CBEL did not possess hydrolytic activity on a variety of glycans, but bound to fibrous cellulose and plant cell walls. In addition, it exerted a lectin-like hemagglutinating activity. Infiltration of tobacco leaves (cultivar 46--8) with this molecule elicited necrosis and defense gene expression at 150 nM. Elicitor pre-treatment of this tobacco cultivar resulted in protection against subsequent inoculation with an otherwise virulent race of P. parasitica var. nicotianae. All these biological activities were exerted within a low concentration range. This is the first report that a fungal elicitor exhibits cellulose-binding and lectin-like activities. The possible implications of such a multifunctional elicitor in plant-microbe interactions are discussed.