NOD FACTORS OF RHIZOBIUM ARE A KEY TO THE LEGUME DOOR

Symbiotic interactions between rhizobia and legumes are largely contro lled by reciprocal signal exchange. Legume roots excrete flavonoids wh ich induce rhizobial nodulation genes to synthesize and excrete lipo-o ligosaccharide Nod factors. In turn, Nod factors provoke deformation o f the root hairs and nodule primordium formation. Normally, rhizobia e nter roots through infection threads in markedly curled root hairs. If Nod factors are responsible for symbiosis-specific root hair deformat ion, they could also be the signal for entry of rhizobia into legume r oots. We tested this hypothesis by adding, at inoculation, NodNGR-fact ors to signal-production-deficient mutants of the broad-host-range Rhi zobium sp. NGR234 and Bradyrhizobium japonicum strain USDA110. Between 10(-7) M and 10(-6) M NodNGR factors permitted these NodABC(-) mutant s to penetrate, nodulate and fix nitrogen on Vigna unguiculata and Gly cine max, respectively. NodNGR factors also allowed Rhizobium fredii s train USDA257 to enter and fix nitrogen on Calopogonium caeruleum, a n onhost. Detailed cytological investigations of V. unguiculata showed t hat the NodABC(-) mutant NGR Delta nodABC, in the presence of NodNGR f actors, entered roots in the same way as the wild-type bacterium. Sinc e infection threads were also present in the resulting nodules, we con clude that Nod factors are the signals that permit rhizobia to penetra te legume roots via infection threads.