The establishment of the nitrogen-fixing symbiosis between rhizobia and legumes re quires an ex change of sig nals be tween the two part ners. In re sponse to flavonoids ex creted by the host plant, rhizobia syn the size Nod fac tors (NFs) which elicit, at very low con cen tra tions and in a spe cific man ner, var i ous sym bi otic re sponses on the roots of the le gume hosts. NFs from sev eral rhizobial spe cies have been char ac ter ized. They all are lipo-chitooligosaccharides, con sist ing of a back bone of gen er ally four or five glucosamine residues N-acylated at the non-reducing end, and carrying various O-substituents. The N-acyl chain and the other substituents are important determi nants of the rhizobial host spec i fic ity. A num ber of nodulation genes which spec ify the syn the sis of NFs have been iden ti fied. All rhizobia, in spite of their di ver sity, pos sess conserved nodABC genes responsible for the synthesis of the N-acylated oligosaccharide core of NFs, which sug gests that these genes are of a monophyletic or i gin. Other genes, the host spe cificnod genes, spec ify the sub sti tu tions of NFs. The cen tral role of NFs and nod genes in the Rhizo bium-legume sym bi o sis sug gests that these factors could be used as mo lec u lar mark ers to study the evo lu tion of this sym bi o sis. We have stud ied a num ber of NFs which are N-acylated by α,β-unsaturated fatty acids. We found that the abil ity to syn the size such NFs does not cor re late with tax o- nomic po si tion of the rhizobia. How ever, all rhizobia that pro duce NFs such nodulate plants be long ing to re lated tribes of le gumes, the Trifolieae, Vicieae, and Galegeae, all of them be ing mem bers of the so-called galegoid group. This sug gests that the abil ity to recognize the NFs with α,β-unsaturated fatty acids is limited to this group of legumes, and thus might have ap peared only once in the course of le gume evo lu tion, in the galegoid phy lum.
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