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 t he protein moiety of this elicitor was cloned and characterized. The d educed amino acid sequence consisted of two direct repeats of a cystei ne-rich domain, joined by a Thr/Pro-rich region. Although having no ge neral 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 po ssess hydrolytic activity on a variety of glycans, but bound to fibrou s cellulose and plant cell walls, In addition, it exerted a lectin-lik e hemagglutinating activity. Infiltration of tobacco leaves (cultivar 46-8) with this molecule elicited necrosis and defense gene expression at 150 nM. Elicitor pretreatment 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 repo rt that a fungal elicitor exhibits cellulose-binding and lectin-like a ctivities. The possible implications of such a multifunctional elicito r in plant-microbe interactions are discussed.