Frankia sp. EAN1pec
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Biology of Frankia

Frankia sp. strains comprise a distinct lineage among the high Mol % G+C Gram positive Actinobacteria . They are filamentous "euactinomycetes" that grow by hyphal branching and tip extension and thus resemble the antibiotic-producing Streptomyces sp.   Frankiae are developmentally complex, producing three cell types during growth:   vegetative hyphae, sporangiospores and lipid-enveloped cellular structures known as "vesicles". The latter develop during N-starvation and contain the O2-labile nitrogenase. In symbiosis, they supply sufficient combined nitrogen so that the plant can grow without added nitrogen.

Physiologically, Frankia strains grow slowly in defined nutrient media.   They grow best on short-chain fatty acids, dicarboxylic acids or other low MW organic acids (propionate, succinate, pyruvate, etc.) plus ammonia, or N2, as the nitrogen source.   Some strains including EAN1pec grow on simple sugars.   Frankiae thus have all housekeeping genes necessary for saprophytic existence plus genes for sporulation, vesicle development, symbiosis, N2 fixation and secondary metabolite production.

Three phylogenetic groups, designated Group I, II and III, have been identified for Frankia .   The basal Group I strains infect Ceanothus sp. of the Rhamnaceae, the Coriariaceae, Datiscaceae and Rosaceae.   No isolates have been obtained from Group I.   Phylogenetic Group II strains infect members of the "higher" hamamelidid families Betulaceae, Casuarinaceae and Myricaceae.   Group III strains are more promiscuous, infecting many hamamelidids, all members of the Elaeagnaceae and Rhamnaceae and occasionally are atypical inhabitants of root nodules in the Rosaceae, Coriariaceae and Ceanothus .

Frankia sp. strain EAN1pec was isolated in 1978 at the Kettering Research Laboratory (Ohio)   by Maurice Lalonde.   EAN1pec grows slowly on dicarboxylic acids (such as succinate or malate), sugars including fructose, and sugar alcohols (i.e. sorbitol and mannitol).   It is resistant to lincomycin, kasugamycin, novobiocin, and, like many actinomycetes, to nalidixic acid.   This strain is also resistant to elevated levels of lead, chromate, and arsenate.   EAN1pec belongs to phylogenetic Group III and will nodulate Elaeagnus ssp., Hippophaë sp., Shepherdia spp, and potentially many hamamelidids. The Elaeagnaceae is a temperate family of trees and shrubs. Elaeagnus is found throughout North America and Eurasia, although some members of the genus have been widely cultivated and used in land reclamation outside their native range.  Elaeagnus has about 45 described species.  Some species are invasive in Australia and western North America.  Shepherdia is composed of three species, restricted to North America.   Hippophae is native to central Asia, distributed from the North Sea to the Black Sea and east to the Himalayas .

  The Global Importance of the Actinorhizal Symbiosis

The importance of Frankia sp. strains derives from their ability to form nitrogen-fixing root nodule symbioses on certain woody angiosperms, termed "actinorhizal plants". Actinorhizal plants are classified in eight plant families, 24 genera and more than 200 species.   These symbioses add a large proportion of new nitrogen to several ecosystems .   They constitute the major N2-fixing symbioses in temperate forests ( Alnus ), dry chaparral and matorral ( Ceanothus, Trevoa, Talguenea, Chamaebatia, Cercocarpus, Purshia ), coastal dunes ( Casuarina, Myrica, Hippophae ), alpine communities ( Alnus ) and in colder regions ( Alnus, Dryas ) such as in Scandinavia, Canada, Alaska or New Zealand where legumes are insignificant or absent. Actinorhizal symbioses are thus key to establishing and maintaining many forests and shrublands, particularly on occasions where climatic or anthropogenic changes disrupt ecosystems.

For more information see:   http://web.uconn.edu/mcbstaff/benson/Frankia/FrankiaHome.htm#

Relevant Publications

Benson, D.R. and W.B. Silvester, Biology of Frankia strains, actinomycete symbionts of actinorhizal plants. Microbiol. Rev., 1993. 57 : p. 293-319.

Lalonde, M., Calvert, H.E., and Pine, S . . Isolation and use of Frankia strains in actinorhizae formation, In A.H. Gibson and Newton, W.E.(ed.) Current Perspectives in Nitrogen Fixation. 1981 p.296-299. Australian Academy of Science, Canberra.