ENOD40, A GENE EXPRESSED DURING NODULE ORGANOGENESIS, CODES FOR A NONTRANSLATABLE RNA INVOLVED IN PLANT-GROWTH

Rhizobium meliloti can interact symbiotically with Medicago plants, th ereby inducing root nodules. However, certain Medicago plants can form nodules spontaneously, in the absence of rhizobia. A differential scr eening was performed using spontaneous nodule versus root cDNAs from M edicago sativa ssp. varia. Transcripts of a differentially expressed c lone, Msenod40, were detected in all differentiating cells of nodule p rimordia and spontaneous nodules, but were absent in fully differentia ted cells. Msenod40 showed homology to a soybean early nodulin gene, G menod40, although no significant open reading frame (ORF) or coding ca pacity was found in the Medicago sequence. Furthermore, in the sequenc es of cDNAs and a genomic clone (Mtenod40) isolated from Medicago trun catula, a species containing a unique copy of this gene, no ORFs were found either. In vitro translation of purified Mtenod40 transcripts di d not reveal any protein product. Evaluation of the RNA secondary stru cture indicated that both Msenod40 and Gmenod40 transcripts showed a h igh degree of stability, a property shared with known non-coding RNAs. The Mtenod40 RNA was localized in the cytoplasm of cells in the nodul e primordium. Infection with Agrobacterium tumefaciens strains bearing antisense constructs of Mtenod40 arrested callus growth of Medicago e xplants, while overexpressing Mtenod40 embryos developed into teratoma s. These data suggest that the enod40 genes might have a role in plant development, acting as 'riboregulators', a novel class of untranslate d RNAs associated with growth control and differentiation.