Assessing the phylogeny of Frankia-actinorhizal plant nitrogen-fixing root nodule symbioses with Frankia 16S rRNA and glutamine synthetase gene sequences

Actinomycetes from the genus Frankia induce nitrogen-fixing root nodules on actinorhizal plants in the "core rosid" clade of eudicots. Reported here are nine partial Frankia 16S rRNA gene sequences including the first from host plants of the rosaceous genera Cercocarpus and Chamaebatia, 24 partial glutamine synthetase (GSI; glnA) sequences from Frankia in nodules of 17 of the 23 actinorhizal genera, and the partial glnA sequence of Acidothermus cellulolyticus. Phylogenetic analyses of combined Frankia 16S rDNA and glnA sequences indicate that infective strains belong to three major clades (I-III) and that Clade I strains consisting of unisolated symbionts from the Coriariaceae, Datiscaceae, Rosaceae, and Ceanothus of the Rhamnaceae are basal to the other clades. Clock-like mutation rates in glnA sequence alignments indicate that all three major Frankia clades diverged early during the emergence of eudicots in the Cretaceous period, and suggest that present-day symbioses are the result of an ancestral symbiosis that emerged before the divergence of extant actinorhizal plants.