Cloning of a cDNA of the Dimer-Forming Nod Factor Hydrolase from Medicago: Root Specific Expression and Presence in Different Legumes

Abstract

Nod factors act as potent mitogens on the roots of leguminous host plants and their rapid inactivation might be important (Staehelin et al. 1995). In the rhizosphere of Medicago sativa, the R. meliloti Nod factors are degraded by a specific plant hydrolase releasing exclusively lipo-disaccharides from Nod factors. Therefore it was named dimer-forming enzyme (DFE). The hydrolase is stimulated up to 6-fold by Nod factors suggesting the presence of an autocontrol circuit (Staehelin et al. 1995). DFE activity was demonstrated to act specifically on Nod factors, while it was much less active on chitin oligomers and inactive on chitin (Staehelin et al. 1997). The enzyme was purified to homogeneity, peptide sequences were obtained and cDNA and genomic clones were isolated. Data bank searches showed homology to glycosylhydrolases but no similarity to chitinases, confirming our biochemical data. Northern analysis revealed that the level of dfe mRNA is highest in nodules and roots and barely detectable in leaves. This tissue-specific expression of dfe correlated with DFE activity which was detectable only in roots and nodules of alfalfa, but not in the aerial parts. By contrast, Nod factor treatment did not increase the amount of mRNA in roots, indicating that the previously reported increase in DFE activity upon Nod factor treatment occurs at posttranscriptional level. Southern analysis revealed the presence of several bands of different intensity suggesting the presence of different dfe alleles and additional dfe-related sequences.