Plant Genetic Control of Nodulation in Legumes

This article summarizes recent findings with our supernodulation (nts) and nonnodulation (nod) mutants of soybean. The nonnodulation mutant nod 139 (unable to curl root hairs and to develop subepidermal cell divisions) does not induce the autoregulation response that controls nodulation. In contrast, nonnodulation mutant nod49 (also unable to curl root hairs, but able to induce some cortical cell divisions) induces autoregulation as effectively as the wild type. Since we perceive autoregulation of nodulation to be an arrest of nodule development, we postulate that cell division associated events signal the shoot of the plant of their activity and that the shoot responds by restricting their further development. We discovered a second nodulation control mechanism, recognized by nodule excision, which acts independently of, and after the autoregulation mechanism, and is different in soybean and alfalfa. Using RFLP mapping we were able to position the nts locus on linkage group F. We are using chromosome walking strategies to clarify the relation between the genetic and physical distance between closely linked loci. Nodules formed in the absence of Rhizobium (Nar) share functional and structural properties with Rhizobium-induced nodules. Alfalfa plants with 100% Nar progeny were selected. Observations like the above suggest that the plant controls the entire nodulation ontogeny and that the bacterium has evolved to activate the process in restrictive genotypes.