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Cation Fluxes Cause Plasma Membrane Depolarization Involved in β-Glucan Elicitor-Signaling in Soybean Roots

September 2005 , Volume 18 , Number  9
Pages  983 - 990

Axel Mithöfer , 1 Jürgen Ebel , 2 and Hubert H. Felle 3

1Max-Planck-Institut für Chemische ökologie, Bioorganische Chemie, Hans-Knöll-Str. 8, D-07745 Jena, Germany; 2Department Biologie I der Ludwig-Maximilians-Universität, Botanik, Menzinger Str. 67, D-80638 München, Germany; 3Botanisches Institut I, Universität Gießen, Senckenbergstraße 17, D-35390 Gießen, Germany

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Accepted 9 May 2005.

Inducible and specific ion fluxes on plasma membranes represent very early events during elicitation of plant cells. The hierarchy of such ion fluxes involved is still unknown. The effect of Phytophthora sojae-derived β-glucan elicitors on the plasma membrane potential as well as on surface K+, Ca2+, and H+ fluxes has been investigated on soybean roots using ion-selective microelectrodes. β-Glucans with different degrees of polymerization transiently depolarized the plasma membrane. The elicitor concentration necessary for half-maximal depolarization closely resembled the corresponding binding affinities of soybean root membranes toward the respective β-glucans. Upon repeated elicitor treatment, the root cells responded partially refractory, suggesting a complex responsiveness of the system. Within the root hair space, characteristic decreasing K+- and Ca2+-free concentrations were induced by the elicitors, probably causing depolarization through the influx of positive charges. Whereas K+ fluxes were inverted after passing the K+ equilibrium (Nernst-) potential, Ca2+ influx continued. No anion fluxes sufficient to account for charge compensation were observed under the same experimental conditions. K+ and Ca2+ fluxes as well as depolarization were inhibited by 100 μM or less of the Ca2+ antagonist La3+. Contrasting other systems, in soybean the main cause for elicitor-induced plasma membrane depolarization is the activation of cation instead of anion fluxes.

Additional keywords: β-Glucan receptor, Glycine max.

© 2005 The American Phytopathological Society