The Family Frankiaceae

The family Frankiaceae, within the order Actinomycetales, contains bacteria isolated mainly from root nodules and occasionally from soil. Members of the genus Frankia have been found associated with the roots of 23 genera of dicots belonging to eight families. Historically, strains isolated in pure culture were grouped into two physiological categories, those that use carbohydrates and those that do not. Newer genomic information indicated that frankiae in general differ markedly in their complements of genes. Besides physiological grouping, these isolates were placed into four plant-compatibility groups (1-infective on Alnus and Myrica, 2-infective on Casuarina and Myrica, 3-infective on Elaeagnaceae and Myrica, 4-infective only on Elaeagnaceae). A 16S rRNA gene-based phylogenetic study, comprising non-isolated endophytes, yielded four clusters or clades, three of which are symbiotic (1-infective on Alnus and Casuarinaceae except Gymnostoma, 2-non-isolated strains in nodules of Rosaceae-Datisca-Coriaria-Rhamnaceae, 3-infective on Elaeagnaceae and Gymnostoma) and a fourth cluster that groups non-infective and non-effective strains. These groupings have been confirmed on the whole by analysis of other loci. DNA-DNA hybridization studies have yielded 12–15 genospecies, only one of which has been named, Frankia alni; one Candidatus Frankia datiscae was recently named to accommodate the genome of an endophyte in nodules of Datisca glomerata. The family Frankiaceae is close to Acidothermus, Cryptosporangium, Geodermatophilaceae (Geodermatophilus, Modestobacter, Blastococcus), Nakamurella, Sporichthya, and Fodinicola and was grouped into suborder Frankineae. A recent rearrangement has resulted in the elevation of suborder Frankineae to order Frankiales (Normand and Benson 2012b) containing families Acidothermaceae, Cryptosporangiaceae, Frankiaceae, Geodermatophilaceae, Nakamurellaceae, and Sporichthyaceae as well as the incertae sedis Fodinicola feengrottensis.