5 research outputs found
Nod factors of <i>Rhizobium<i> are a key to the legume door
Symbiotic interactions between rhizobia and legumes are largely controlled by reciprocal signal exchange. Legume roots excrete flavonoids which induce rhizobial nodulation genes to synthesize and excrete lopoâoligosaccharide Nod factors. In turn, Nod factors provoke deformation of the root hairs and nodule primordium formation. Normally, rhizobia enter roots through infection threads in markedly curled root hairs. If Nod factors are responsible for symbiosisâspecific root hair deformation, they could also be the signal for entry of rhizobia into legume roots. We tested this hypothesis by adding, at inoculation, NodNGRâfactors to signalâproductionâdeficient mutants of the broadâhostârange Rhizobium sp. NGR234 and Bradyrhizobium japorticum strain USDA110. Between 10â»â· M and 10â»â¶ M NodNGR factors permitted these NodABC mutants to penetrate, nodulate and fix nitrogen on Vigna unguiculata and Glycine max, respectively. NodNGR factors also allowed Rhizobium fredii strain USDA257 to enter and fix nitrogen on Calopogonium caeruleum, a nonâhost. Detailed cytological investigations of V. unguiculata showed that the NodABC mutant UGR AnodABC, in the presence of NodNGR factors, entered roots in the same way as the wildâtype bacterium. Since infection threads were also present in the resulting nodules, we conclude that Nod factors are the signals that permit rhizobia to penetrate legume roots via infection threads