Nod factors integrate spontaneously in biomembranes and transfer rapidly between membranes and to root hairs, but transbilayer flip-flop does not occur

Three novel nodulation (Nod) factors were synthesized from chitotetraose and three structurally different fluorescent BODIPY-tagged fatty acids. With fluorescence spectroscopic and microscopictechniques, the following aspects were studied: whether these amphiphilic molecules insert in membranes, whether they transfer between different membranes, and whether they are able to transfer from a membrane to a legume root hair. Fluorescence correlation spectroscopy showed that fluorescent Nod factors are present as monomers in PBS buffer at a concentration of 10 nM, but that when either Triton X-100 micelles or dioleoylphosphatidylcholine (DOPC) vesicles are present, the Nod factors are associated withthese particles. With time-correlated single-photon counting fluorescence spectroscopy, it was shown that upon Nod factor insertion in the membrane, the rotation of the fluorescent acyl chain was markedly reduced. A fluorescence resonance energy transfer assay was used to study the transfer of Nod factors from onemembrane to the other, or from vesicles to root hairs. Nod factors transfer rapidly between membranes or from vesicles to root hair cell walls. However, they do not flip-flop between membrane leaflets. Theresults provide novel insights for the mode of secretion and transfer of Nod factors during the early steps of the Rhizobium-legume interaction.