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Diversity of Frankia Strains in Root Nodules of Plants from the Families Elaeagnaceae and Rhamnaceae

ABSTRACT Partial 16S ribosomal DNAs (rDNAs) were PCR amplified and sequenced from Frankia strains living in root nodules of plants belonging to the families Elaeagnaceae and Rhamnaceae, includingColletia hystrix, Elaeagnus angustifolia, an unidentified Elaeagnus sp., Talguenea quinquenervia, and Trevoa trinervis. Nearly full-length 16S rDNAs were sequenced from strains ofFrankia living in nodules of Ceanothus americanus, C. hystrix, Coriaria arborea, and Trevoa trinervis. Partial sequences also were obtained from Frankia strains isolated and cultured from the nodules of C. hystrix, Discaria serratifolia, D. trinervis, Retanilla ephedra, T. quinquenervia, and T. trinervis (Rhamnaceae). Comparison of these sequences and other published sequences ofFrankia 16S rDNA reveals that the microsymbionts and isolated strains from the two plant families form a distinct phylogenetic clade, except for those from C. americanus. All sequences in the clade have a common 2-base deletion compared with other Frankia strains. Sequences from C. americanus nodules lack the deletion and cluster withFrankia strains infecting plants of the family Rosaceae. Published plant phylogenies (based on chloroplast rbcLsequences) group the members of the families Elaeagnaceae and Rhamnaceae together in the same clade. Thus, with the exception ofC. americanus, actinorhizal plants of these families and their Frankia microsymbionts share a common symbiotic origin.