Cloning and characterization of cDNA encoding an elicitor of Phytophthora colocasiae

The rapid and effective activation of disease resistance responses is essential for plant defense against pathogen attack. These responses are initiated when pathogen-derived molecules (elicitors) are recognized by the host. A cDNA encoding elicitor, the major secreted extracellular glycoprotein of Phytophthora colocasiae, a pathogen of taro (Colocasia esculenta) plants, was isolated, sequenced and characterized. The expression of the corresponding elicitor gene during the disease cycle of P. colocasiae was analyzed. Elicitor was shown to be expressed in mycelium grown in culture media, whereas it was not expressed in sporangiospores and zoospores. In planta, during infection of taro, particularly during the biotrophic stage, expression of elicitor was down-regulated compared to in vitro. The highest levels of expression of elicitor were observed in in vitro grown mycelium and in late stages of infection when profuse sporulation and leaf necrosis occur. The elicitation of the suspension-cultured taro cells was effective in the induction of the enzyme activity of l-phenylalanine-ammonia lyase, peroxidase and lipoxygenase as well as the expression of defense-related endochitinase gene. All these biological activities were exerted within a low concentration range. The glycoprotein represents a powerful tool to investigate further the signals and their transduction pathways involved in induced disease resistance. It may also be useful to engineer broad disease protection in taro plant against Phytophthora leaf blight.