THE AUXIN TRANSPORT INHIBITOR N-(1-NAPHTHYL)PHTHALAMIC ACID ELICITS PSEUDONODULES ON NONNODULATING MUTANTS OF WHITE SWEETCLOVER

The collection of symbiotic (sym) mutants of white sweetclover (Melilo tus alba Desr.) provides a developmental sequence of mutants blocked e arly in infection or nodule organogenesis. Mutant phenotypes include n on-nodulating mutants that exhibit root-hair deformations in response to Rhizobium meliloti, mutants that form ineffective nodules lacking i nfection threads, and mutants that form infection threads and ineffect ive nodules. Mutant alleles from both the sym-1 and the sym-3 loci exh ibited a non-nodulating phenotype in response to R. meliloti, although one allele in the sym-1 locus formed ineffective nodules at a low fre quency. Spot-inoculation experiments on a non-nodulating allele in the sym-3 locus indicated that this mutant lacked cortical cell divisions following inoculation with R. meliloti. The auxin transport inhibitor N-(1-naphthyl)phthalamic acid elicited development of pseudonodules a t a high frequency on all of the sweetclover sym mutants, including th e nonnodulating mutants, in which the early nodulin ENOD2 was expresse d. This suggests that N-(1-naphthyl)phthalamic acid activates cortical cell divisions by circumventing a secondary signal transduction event that is lacking in the non-nodulating sweetclover mutants. The sym-3 locus and possibly the sym-1 locus appear to be essential to early hos t plant responses essential to nodule organogenesis.