Signals in Plant Defense Gene Activation

Abstract

Plants respond to pathogen attack by rapid activation of defense genes. In selected experimental systems, the transcription of these genes is also induced when cultured plant cells or protoplasts are treated with elicitors. We have purified an extracellular glycoprotein of 42 kDa from the culture filtrate of the soybean pathogen Phytophthora megasperma f. sp. glycinea (Pmg), which is a potent elicitor of phytoalexin accumulation and defense gene activation in parsley (1Petroselinum crispum) cells and protoplasts. Reversible binding of the 25I-labelled glycoprotein to microsomes and protoplasts indicates the presence of specific binding sites on the parsley plasma membrane. The transduction of the elicitor signal from the cell surface to the nucleus was found to involve a rapid and transient uptake of Ca2+, alkalization of the culture medium and effluxes of K+ and C1-. These ion fluxes probably result from opening of elicitor-responsive ion channels, as indicated by preliminary results from patch-clamp analysis of parsley protoplasts in the presence of pure glycoprotein elicitor. Omission of Ca2+ from the culture medium substantially reduced the elicitor-induced transcription rates of plant defense genes. Treatment of cultured parsley cells or protoplasts with amphotericin B resulted in ion fluxes and defense gene activation similar to those elicited by the crude Pmg elicitor. The callose elicitors chitosan and digitonin, however, induced ion fluxes of different intensities, activated only some defense genes, and did not stimulate the synthesis of phytoalexins at concentrations optimal for elicitation of callose formation.