Abstract
Symbiosis between legume plants and rhizobia causes the development of new organs, nodules, which function as an apparatus for symbiotic nitrogen fixation. In this study, the roles of auxin in nodule development in Lotus japonicus have been demonstrated using molecular genetic tools and auxin inhibitors. The expression of an auxin-reporter fused to GUS was analyzed in L. japonicus roots, which showed a strong signal in the central cylinder of the root, whereas upon rhizobium infection, the generation of GUS signal was observed at the dividing outer cortical cells during the first stage of nodule cell divisions. When nodules were developed to maturity, strong GUS staining was detected in vascular tissues of nodules, suggesting a distinct auxin involvement in the determinate nodule development. Numbers and the morphology of nodules were affected by several auxin inhibitors. The common phenotypical alteration by these auxin inhibitors was the strong inhibition in forming lenticels, which are normally developed on the nodule surface from the root outer cortex. These results indicate that auxin is required for the normal development of determinate nodules in a multidirectional manner.
