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. Thi s pattern of nodule development presents features of indeterminate and determinate nodules in early and later stages, respectively. A S. ros trata 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 trans criptase PCR, enhanced SrENOD40-1 expression was observed in the adven titious root primordia between 4 and 8 h after inoculation with A. cau linodans. In situ hybridization showed that SrENOD40-1 transcripts, pr esent around the central vascular bundle of the uninfected root primor dia, were strongly enhanced upon induction of nodule development. De n ovo SrENOD40-1 expression was observed in the initiating and growing n odule primordia and around vascular bundles. When cell type specificat ion sets in, the expression became pronounced in cells derived from th e meristematic regions. In other parts of the plant, weak SrENOD40-1 e xpression was associated with vascular bundles and was observed in lea f and stipule primordia.