How alfalfa root hairs discriminate between Nod factors and oligochitin elicitors

Using ion-selective microelectrodes, the problem of how signals coming from symbiotic partners or from potential microbial intruders are distinguished was investigated on root hairs of alfalfa (Medicago sativa,a). The Nod fac tor, NodRm-IV(C16:2,S), was used to trigger the symbiotic signal and (GlcNA c)(8) was selected from (GlcNAc)(4-8), to elicit defense-related reactions. To both compounds, root hairs responded with initial transient depolarizat ions and alkalinizations, which were followed by a hyperpolarization and ex ternal acidification in the presence of (GlcNAc)(8). We propose that alfalf a recognizes tetrameric Nod factors and N-acetylchitooligosaccharides (n = 4-8) with separate perception sites: (a) (GlcNAc)(4) and (GlcNAc)(6) reduce d the depolarization response to (GlcNAc)(8), but not to NodRm-IV(C16:2,6); and (b) depolarization and external alkalization were enhanced when NodRm- IV(C16:2,S) and (GlcNAc)(8) were added jointly without preincubation. We su ggest further that changes in cytosolic pH and Ca2+ are key events in the t ransduction, as well as in the discrimination of signals leading to symbiot ic responses or defense-related reactions. To (GlcNAc)(8), cells responded with a cytosolic acidification, and they responded to NodRm-IV(C16:2,S) wit h a sustained alkalinization. When both agents were added jointly, the cyto sol first alkalized and then acidified. (GlcNAc)(8) and NodRm-IV(C16:2,S) t ransiently increased cytosolic Ca2+ activity, whereby the response to (GlcN Ac)(8) exceeded the one to NodRm-IV(C16:2,S) by at least a factor of two.