The gene ENOD40 is expressed during early stages of legume nodule development. A homolog was isolated from tobacco, which, as does ENOD40 from legumes, encodes an oligopeptide of about 10 amino acids. In tobacco protoplasts, these peptides change the response to auxin at concentrations as low as 10(-12) to 10(-16)M. The peptides encoded by ENOD40 appear to act as plant growth regulators.

A pea cDNA clone representing the homologue of the soybean pGmENOD40-1 was isolated and characterized. At the nucleotide level both clones share 55% homology. Strikingly, the homology between the polypeptides derived from the pea and soybean ENOD40 cDNA sequences is only 14%. Despite this low homology Southern analyses revealed that the isolated pea cDNA clone represents the single pea ENOD40. In situ hybridizations showed that at early stages of nodule development and in mature nodules the expression pattern of pea ENOD40 is comparable to that of soybean ENOD40. Although ENOD40 show similar expression patterns in these two nodules, it is questionable whether the putative polypeptides have a similar function, since the homology is very low.

The early nodulin gene, PsENOD7, is expressed in pea root nodules induced by Rhizobium leguminosarum bv. viciae, but not in other plant organs. In situ hybridization showed that this gene is transcribed during nodule maturation in the infected cells of the proximal part of the prefixation zone II. At the transition of zone II into interzone II-III, the level of PsENOD7 mRNA drops markedly. PsENOD7 has no significant homology to other genes. RFLP mapping studies have shown that PsENOD7 is located in linkage group I between the leghaemoglobin genes and sym2.Rhizobium leguminosarum bv. viciae induces the formation of nitrogen fixing root nodules on the roots ofPisum sativum (pea). By mutagenesis and genetic studies several plant genes essential for normal nodule development have been identified and these genes have been named sym genes. In pea about 30 different sym genes have been described [7,18,19,26]. The sym genes are distributed randomly on the seven linkage groups of pea [19], but several sym genes, namely sym2, sym5, syml9 and nod3, are clustered on linkage group I, near the major leghaemoglobin (Lb) locus [32, 381.The different stages of legume nodule development are accompanied by the expression of plant genes, the so-called nodulin genes. These genes, that are only expressed during nodule development, have been divided into early and late nodulin genes; the early nodulin genes are expressed before the bacteria start to fix nitrogen, whereasThe nucleotide sequence data reported will appear in the EMBL, GenBank and DDBJ Nucleotide Sequence Databases under the accession number X93172.

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