Differential gene expression in an actinorhizal symbiosis: evidence for a nodule-specific cysteine proteinase

Nodules formed on the roots of actinorhizal plants as a consequence of nitrogen-fixing symbioses with the actinomycete Frankia appear to result from modification of the developmental pathway that leads to lateral root formation. Presently no information exists about factors that control this developmental switch or, until now, about genes that are differentially expressed as a result of an altered developmental pathway. Differential screening of an Alnus glutinosa nodule cDNA library revealed altered levels of gene expression in nodules as compared with roots and allowed isolation of host plant nodule-specific cDNA sequences. The deduced amino acid sequence of one full-length cDNA, AgNOD-CP1, represents a nodule-specific cysteine proteinase similar to cysteine proteinases of the papain superfamily. Residues critical to catalysis, active site, and disulfide bridges are conserved. Suggested roles for this enzyme are as a defense response to Frankia invasion, as a component of tissue remodeling in root and nodule tissues, as a cell cycle component, or as an element of protein turnover. Complexity of hybridization patterns revealed by Southern blot analysis suggests that the gene for AgNOD-CP1 is a member of a multigene family. Northern hybridization results indicate that this gene may have been recruited for a role specific to this symbiosis, a phenomenon observed in the Rhizobium-legume symbioses, perhaps common to many microbe-plant interactions.