5 NODULATION MUTANTS OF WHITE SWEETCLOVER (MELILOTUS-ALBA DESR) EXHIBIT DISTINCT PHENOTYPES BLOCKED AT ROOT HAIR CURLING, INFECTION THREAD DEVELOPMENT, AND NODULE ORGANOGENESIS

In an effort to obtain a developmental sequence of mutations in the Rh izobium-legume interaction within a single legume species, we have cha racterized the early events of nodule development in 10 nodulation mut ants of sweetclover, Melilotus alba Desr. cv U389, representing five g enetic loci. Both seed and root exudates from all of the sweetclover m utants induced expression of the nod genes of Rhizobium meliloti. Muta nts in three loci were blocked in the early stages of root hair curlin g. Of these, a mutant in the sym-3 locus exhibited root hair deformati ons in response to inoculation with R. meliloti but produced no nodule s or emerging nodule primordia, suggesting a blockage in the signal tr ansduction events leading to nodule organogenesis. In contrast, mutant s in both the sym-1 and sym-5 loci formed ineffective nodules in respo nse to inoculation but differed slightly in the type of root hair resp onse observed. None of these three early mutants formed infection thre ads. infection threads were observed in mutant sym-2 as well as in ine ffective nodules. Mutant sym-4 also formed infection threads but lacke d nodules. The phenotypes observed for mutants from these five loci su ggest that a secondary receptor or signal produced by the plant is req uired for nodule development.