Nod factors modulate the concentration of cytosolic free calcium differently in growing and non-growing root hairs of Medicago sativa L.

Using Ca2+-selective microelectrodes, the concentration of free calcium ([C a2+]) in the cytosol has been measured in root hair cells of Medicago sativ a L. in the presence of nodulation (Nod) factors. Growing root hairs of M. sativa displayed a steep apical [Ca2+] gradient, i.e. 604-967 nM in the tip compared with 95-235 nM in the basal region. When tested within the first 5 to 10 mu m of the tip, addition of NodRm-IV(C16:2,S) decreased the cytoso lic [Ca2+], whereas an increase was observed when tested behind the tip. Ov erall, this led to a partial dissipation of the [Ca2+] gradient. The Ca2+ r esponse was specific: it was equally well observed in the presence of NodRm -IV(Ac,C16:2,S), reduced with NodRm-IV(C16:0,S), but not with chitotetraose , the nonactive glucosamine backbone. In contrast to growing root hairs, no ngrowing root hairs without a tip-to-base cytosolic [Ca2+] gradient respond ed to NodRm-IV(C16:2,S) with an increase in cytosolic [Ca2+] at the tip as well as at the root hair base. We suggest that the response to Nod factors depends on the stage of development of the root hairs, and that changes in cytosolic [Ca2+] may play different roles in Nod-factor signaling: changes of cytosolic [Ca2+] in the apical part of the root hair may be related to r oot hair deformation, while the increase in [Ca2+] behind the tip may be es sential for the amplification of the Nod signal, for its propagation and tr ansduction to trigger downstream events.