Electrophysiologie des Membranes

 Root hairs are a primary site for nutrient absorption and for initiation of signaling processes linked to variations of the  root environment: plant-microbe interactions or abiotic changes. In many of these cases, the earliest detectable response is the modification of plasma membrane transports, detected through alteration of the electrical membrane potential. In spite of this, root hairs have not been extensively used in electrophysiological research so far. Problems with cell shape and current coupling are often prohibitive for microelectrode voltage clamp on intact root hairs. In the present study, these difficulties have been overcome and the ion channel currents are described for young root hairs from alfalfa seedlings (Medicago sativa cv Sitel). Electrophysiological and pharmacological studies indicated an inward rectifying K+ time-dependent current. This current was sensitive to tetraethylammonium and Cs+ (10 mM each). Two others currents never shown in root hairs were described: an outward rectifying time-dependent K+ current, inhibited by tetraethylammonium and Cs+ (10 mM each) allowing K+ efflux under strong depolarizations and an instantaneous inward current identified as an anion current, inhibited by 4-acetamido-4'-isothiocyanatostilbene-2,2', disulfonic acid and anthracene-9-carboxylic acid (100 µM each). These results should contribute to the understanding of root hair development and of signaling processes in M. sativa root hairs.

Nod-factor [NodRm-IV(Ac,S)], isolated from the bacterium Rhizobium meliloti, induces a well known depolarization in Medicago sativa (cv Sitel) root hairs (Kurkdjian Plant Physiol. 107: 783-790, 1995). The 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 the increase of the inward ion current and to the 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 the 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 ions carried by these currents depending on the activated anion and/or K+ channels at the plasma membrane.