NOD SIGNAL-INDUCED PLASMA-MEMBRANE POTENTIAL CHANGES IN ALFALFA ROOT HAIRS ARE DIFFERENTIALLY SENSITIVE TO STRUCTURAL MODIFICATIONS OF THE LIPOCHITOOLIGOSACCHARIDE

Lipochitooligosaccharide Nod signals are important determinants of hos t specificity in the Rhizobium-legume symbiosis. The most rapid respon se of plant cells to the R. meliloti Nod signal NodRm-IV(C16:2,S) repo rted so far is the depolarization of the plasma membrane potential in alfalfa root hairs. In order to investigate whether this response may be part of a specific signal transduction cascade involved in the nodu lation process, its specificity was studied with respect to host-speci fic modifications of the lipochitooligosaccharide. Five different Nod factors displaying different degrees of activity in inducing root hair deformation or cortical cell divisions on alfalfa were tested. The ab ility of the Nod factors to elicit plasma membrane depolarization corr elated well with their activity in the bioassays. Removal of the sulfa te group (NodRm-IV(C16:2)] led to inactivation of the Nod factor. An i ncrease in the length of the chitooligosaccharide backbone (NodRm-V(C1 6:2,S)) or saturation of the acyl chain (NodRm-IV(C16:0,S)) resulted i n severely reduced activity. In contrast, the O-acetyl group at the no n-reducing terminus in NodRm-IV(Ac,C16:2,S), which confers substantial ly higher activity in long-term bioassays, did not enhance plasma memb rane depolarization significantly in comparison with the non-O-acetyla ted factor. Thus, the rapid plasma membrane response is differentially sensitive to various structural motifs of the lipochitooligosaccharid e. These data suggest that the different substituents modifying the ba sic Nod factor structure may have distinct functions, not all of them contributing to the interaction with a putative receptor in root hair cells. However, the overall specificity of the membrane depolarization for the cognate Nod factors raises the possibility that it is involve d in a Nod signal transduction pathway.