A type-I chalcone isomerase mRNA is highly expressed in the root nodules of Elaeagnus umbellata

We have used the hybridization-competition method to isolate EuNOD-CHI from a root nodule cDNA library of Elaeagnus umbellata. This cDNA clone encodes chalcone isomerase (CHI) for a protein of 256 amino-acid residues and a mature molecular mass of 28 kDa. Multiple sequence alignment and phylogenetic analysis have demonstrated that EuNOD-CHI can be classified as Type I. Moreover, northern hybridization shows that the EuNOD-CHI gene is highly expressed in root nodules, with levels increasing during nodule development. The highest level of expression is at 6 to 8 weeks after inoculation, decreasing thereafter. Genomic Southern hybridization also demonstrates that EuNOD-CHI has as many as two copies in the E. umbellata genome. Taken together with the previous results, we propose that the higher expression level of the EuNOD-CHI gene in root nodules is likely associated with this species' defense mechanism against infection by Frankia.