BIOLOGICAL-ACTIVITY OF RHIZOBIUM SP NGR234 NOD-FACTORS ON MACROPTILIUM-ATROPURPUREUM

The broad host range of Rhizobium sp. NGR234 is based mainly on its ab ility to secrete a family of lipooligosaccharide Nod factors. To monit or Nod-factor purification, we used the small seeded legume Macroptili um atropurpureum, which responds evenly and consistently to Nod factor s. At concentrations between congruent to 10(-11) M and 10(-9) M, this response fakes the form of deformation of the root hairs. Higher conc entrations (congruent to 10(-9) to 10(-7) M), provoked profound ''shep herd's crook'' type curling of the root hairs. Similar concentrations of Nod factors of Bradyrhizobium japonicum, Rhizobium leguminosarum, a nd R. meliloti also provoked marked curling of the root hairs, but the latter two species are unable to nodulate Macroptilium. On the other hand, plant hormones, hormone-like substances, inhibitors of hormone a ction, as well as substituents of Nod factors were without effect in t his bioassay. We thus conclude that only Nod factors are capable of in ducing shepherd's crook type curling of Macroptilium root hairs. Pertu rbations in the auxincytokinin balance induced ''pseudo'' nodulation o n M. atropurpureum, as did NodNGR factors at concentrations between 10 (-7) and 10(-6) M. Concomitant inoculation of Macroptilium with a NodA BC(-) mutant of NGR234 and sulfated NodNGR factors (NodNGR[S]) gave ri se to plants that slowly greened, showing that the NodNGR factors perm itted entry of the No-d- mutant into the roots.