RAPID ALKALINIZATION IN ALFALFA ROOT HAIRS IN RESPONSE TO RHIZOBIAL LIPOCHITOOLIGOSACCHARIDE SIGNALS

Rhizobial lipochitooligosaccharides (Nod factors) function as symbioti c signals that trigger root hair deformations and cortical cell divisi ons on the roots of leguminous plants in a host-specific manner. By us ing pH-sensitive microelectrodes, it is shown that alfalfa root hair c ells respond to Rhizobium meliloti Nod factors with a rapid intracellu lar alkalinization of 0.2-0.3 pH units. This alkalinization remained a s long as the Nod factor was present, but slowly reversed after remova l of the signal. The response was most sensitive to the sulfated tetra meric Nod factor, NodRm-IV(C16:2,S), which is morphogenic on the host plant alfalfa, suggesting a role in a signal transduction cascade. Non -sulfated Nod factor as well as chitooligosaccharides elicited a pH(c) change only at elevated concentrations. The increase of pH(c) in resp onse to sulfated Nod factor was concomitant with a depolarization of t he plasma membrane potential whereas the pH(c) change in response to n on-sulfated Nod factor occurred in the absence of membrane depolarizat ion. In addition, whereas a first dose of sulfated Nod factor inhibite d the subsequent response to a second dose of the same molecule, it di d not significantly repress the activity of non-sulfated Nod factor. T hese results indicate that sulfated and non-sulfated Nod factors act i ndependently and suggest the existence of two Nod signal perception sy stems, one transmitting the host-specific signal, the other representi ng an ancient reception system for a generic Nod factor structure.