Summary Nod factor [NodRm‐IV(Ac,S)], isolated from the bacterium Rhizobium meliloti, induces a well‐known depolarization in Medicago sativa (cv Sitel) root hairs. Analysis of this membrane response using the discontinuous single‐electrode voltage‐clamp technique (dSEVC) shows that anion channel, K+ channel and H+‐ATPase pump currents are involved in young growing root hairs. The early Nod‐factor‐induced depolarization is due to increase of the inward ion current and inhibition of the H+ pump. It involved an instantaneous inward anion current (IIAC) and/or a time‐dependent inward K+ current (IRKC). These two ion currents are then down‐regulated while the H+ pump is stimulated, allowing long‐term rectification of the membrane potential (Em). Our results support the idea that the regulation of inward current plays a primary role in the Nod‐factor‐induced electrical response, the nature of the ions carried by these currents depending on the activated anion and/or K+ channels at the plasma membrane.

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The purpose of our work was to investigate the functioning of K+ channels in protoplasts of laticifers of Hevea brasiliensis Muell. Arg., anastomosed into a network devoid of large central vacuoles, after tapping stress. Physiological functions such as proton pump activity and uptake of sucrose (a rubber precursor) were maintained, when the voltage‐clamp method was used in vivo to record the whole‐cell K+ current during the stress response. A time‐dependent inward current was induced in 50 mM KCl and rapidly inactivated (about 100 ms). The activation potential of this inward K+ channel was not closely dependent on Ek. This would be coherent with the ‘valve model’ of Schroeder and Fang (1991, Proc. Natl. Acad. Sci. USA 88: 11583–11587) involving the activation of a H+‐pump accounting for the K+ uptake observed in laticiferous cells under stress. The activation half‐time of outward currents was clearly voltage dependent: from about 350 to 60 ms for 125 and 155 mV, respectively. Time‐dependent outward current sensitivity to 5 mM BaCl2 or CaCl2 or to 5 μM Erythrosin B showed that the K+ channels could be Ca2+‐dependent. Because of the positive values of the activation potential of the outward current, the possibility opens that an action potential exists, these cells being specialized for stress response.

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