RECEPTOR-MEDIATED PATHOGEN RECOGNITION AND SIGNAL TRANSDUCTION IN PLANT DEFENCE
D SCHEEL1, H HIRT2, T KROJ1, W LIGTERING2, D NENNSTIEL1, T NÜRNBERGER1, M TSCHÖPE1, H ZINECKER1 and U ZUR NIEDEN1
1Institut für Pflanzenbiochemie, Weinberg 3, D-06120 Halle (Saale), Germany; 2Institut für Mikrobiologie und Genetik, Dr-Bohr-Gasse 9, A-1030 Wien, Austria
Background and objectives
Results and conclusions
Early elements of the plant response are ion fluxes across the plasma membrane, activation of a MAP kinase and the production of reactive oxygen species, the oxidative burst, followed by defence gene activation and phytoalexin accumulation. Activation of the elicitor-responsive ion channels, the most rapid reaction, has been found to be necessary and sufficient for all other reactions of the plant cells [3, 4]. Inhibition of elicitor-stimulated production of reactive oxygen species with diphenylene iodonium, an inhibitor of the mammalian NADPH oxidase, blocks defence gene activation and phytoalexin accumulation without affecting ion fluxes, MAP kinase activation, cell viability and constitutive gene expression [4, 5]. Mimicking the oxidative burst in the absence of elicitor by addition of appropriate amounts of potassium superoxide to the medium of cultured parsley cells stimulates phytoalexin accumulation but not ion fluxes. In contrast, hydrogen peroxide either added directly to, or generated in the culture medium by glucose and glucose oxidase does not stimulate any defence response . Upon receptor-mediated activation, the MAP kinase is translocated to the nucleus where it might interact with transcription factors that induce expression of defence genes .
In summary, our results demonstrate a causal relationship between early and late elicitor reactions, establish a sequence of signalling events from receptor-mediated activation of ion channels through MAP kinase activation, the oxidative burst, and defence gene activation to phytoalexin production, and suggest that within this signal transduction chain the superoxide anion radical, rather than hydrogen peroxide, triggers defence gene activation and phytoalexin accumulation.