Rice ENOD40: isolation and expression analysis in rice and transgenic soybean root nodules

The early nodulin ENOD40 has been proposed as playing a pivotal role in the organogenesis of legume root nodules. We have isolated the ENOD40 gene hom ologues ObENOD40 and OsENOD40 from the wild and cultivated rice genotypes O ryza brachyantha and Oryza sativa, respectively. Rice ENOD40s contain a seq uence at the 5' end (region I) for encoding an oligopeptide that is highly conserved in all legume ENOD40s. Furthermore, at the 3' end (region II), th e nucleotide sequence of rice ENOD40s exhibited a considerable homology to the corresponding region in legume ENOD40s. Among various organs of the ric e plant, expression of OsENOD40 was detected only in stems. In situ hybridi zation studies revealed that, within the stem, transcription of OsENOD40 is confined to parenchyma cells surrounding the protoxylem during the early s tages of development of lateral vascular bundles that conjoin an emerging l eaf. Expression pattern of OsENOD40 promoter-GUS fusion in nodules develope d on transgenic hairy roots of soybean was also found to be restricted to p eripheral cells of nodule vascular bundles, thus evidencing that rice ENOD4 0 promoter activity is essentially the same as that of soybean ENOD40. Take n together, these results strongly suggest that OsENOD40 and legume ENOD40s share common, if slot identical, functions in differentiation and/or funct ion of vascular bundles.