Leguminous plants can interact with rhizobium for atmospheric nitrogen fixation, which is vitally important for the development of sustainable agro-ecological agriculture. The symbiotic interaction is initiated, in response to plant flavonoid secretion, by the rhizobial production of lipochitooligosaccharides, called Nod factors (NF), which, in turn, elicit a complex signaling in root hairs. An electrical signal characterized by a fast root hair membrane depolarization followed by a slower repolarization is the earliest detected event following the perception of NF. This was shown to successively involve rapid Ca2+ influx induction and inhibition of proton extrusion, activation of Cl- efflux then activation of K+ efflux. My thesis work aimed at characterizing Medicago truncatula root hair plasma membrane ion conductances potentially involved in the early Nod factor signaling. Patch-clamp analysis on M. truncatula root hair protoplasts generated a ‘repertoire’ of six root hair conductances with diverse electrical properties in terms of voltage dependency, kinetics, ion selectivity, pH sensitivity, etc., consisting in hyperpolarization and depolarization-activated conductances for both K+, Ca2+/cations and anions. Three of them, the hyperpolarization-activated Ca2+ conductance, the Shaker-like outwardly-rectifying K+ one and the slowly inactivating “type A” anionic one, were quickly responding by NF, suggesting an important role for these conductances in the early electrical NF signal. The electrophysiological analysis in both root hairs and guard cells of a knock-out mutant line displaying disruption of MtSKOR, the unique outward Shaker gene of M. truncatula genome, confirmed the genetic identity of the Shaker-like outwardly rectifying K+ conductance. The Mtskor mutant plants displayed reduced nodulation capacity and higher leaf water loss upon hydric stress, indicating a determinant role of MtSKOR for both symbiosis and stomatal function