Patterns of ENOD40 gene expression in stem-borne nodules of Sesbania rostrata

At the base of adventitious root primordia, located on the stem of the tropical legume Sesbania rostrata, nitrogen-fixing nodules are formed upon inoculation with the microsymbiont Azorhizobium caulinodans. This pattern of nodule development presents features of indeterminate and determinate nodules in early and later stages, respectively. A S. rostrata cDNA clone homologous to early nodulin ENOD40 genes was isolated from a cDNA library of developing stem nodules. SrENOD40-1 contained the conserved regions I and II of other ENOD40 genes. By reverse transcriptase PCR, enhanced SrENOD40-1 expression was observed in the adventitious root primordia between 4 and 8 h after inoculation with A. caulinodans. In situ hybridization showed that SrENOD40-1 transcripts, present around the central vascular bundle of the uninfected root primordia, were strongly enhanced upon induction of nodule development. De novo SrENOD40-1 expression was observed in the initiating and growing nodule primordia and around vascular bundles. When cell type specification sets in, the expression became pronounced in cells derived from the meristematic regions. In other parts of the plant, weak SrENOD40-1 expression was associated with vascular bundles and was observed in leaf and stipule primordia.