DIVERSITY OF FRANKIA STRAINS IN ROOT-NODULES OF PLANTS FROM THE FAMILIES ELAEAGNACEAE AND RHAMNACEAE

Partial 16S ribosomal DNAs (rDNAs) were PCR amplified and sequenced fr om Frankia strains living in root nodules of plants belonging to the f amilies Elaeagnaceae and Rhamnaceae, including Colletia hystrix, Elaea gnus angustifolia, an unidentified Elaeagnus sp., Talguenea quinquener via, and Trevoa trinervis. Nearly full length 16S rDNAs were sequenced from strains of Frankia living in nodules of Ceanothus americanus, C. hystrix, Coriaria arborea, and Trevoa trinervis. Partial sequences al so were obtained from Frankia strains isolated and cultured from the n odules of C. hystrix, Discaria serratifolia, D. trinervis, Retanilla e phedra, T. quinquenervia, and T. trinervis (Rhamnaceae). Comparison of these sequences and other published sequences of Frankia 16S rDNA rev eals that the microsymbionts and isolated strains from the two plant f amilies form a distinct phylogenetic clade, except for those from C. a mericanus. All sequences in the clade have a common 2-base deletion co mpared with other Frankia strains. Sequences from C. americanus nodule s lack the deletion and cluster with Frankia strains infecting plants of the family Rosaceae. Published plant phylogenies (based on chloropl ast rbcL sequences) group the members of the families Elaeagnaceae and Rhamnaceae together in the same clade. Thus, with the exception of C. americanus, actinorhizal plants of these families and their Frankia m icrosymbionts share a common symbiotic origin.