Frankia strains are soil bacteria that induce the formation of nitrogen fixing nodules on the roots of a diverse group of mostly woody dicotyledonous species from three different orders, collectively called actinorhizal plants.

Phylogenetically, Frankia strains can be grouped in four clusters, three of which represent symbiotic strains. The clusters form host specificity groups (which means that a given host plant species can only be nodulated by strains from a certain cluster, but not all strains of this cluster can nodulate all host pants of the cluster):

Cluster-1

• Cluster-1a: Fagales (actinorhizal Betulaceae, Myrica sp. and Comptonia sp. from the Myricaceae)
• Cluster-1c: Fagales (Casuarinaceae with the exception of Gymnostoma sp.)

Cluster-2

• Cucurbitales (Datiscaceae, Coriariaceae)
• Rosales (Rosaceae and Ceanothus sp. from the Rhamnaceae)

Cluster-3

• Rosales (Elaeagnaceae, actinorhizal Rhamnaceae with the exception of Ceanothus sp.)
• Fagales (Gymnostoma sp. from the Casuarinaceae, Morella sp. from the Myricaceae)

Representatives of the species Frankia irregularis from cluster-3 can nodulate host plants from the Elaeagnaceae and Rhamnaceae in the lab, but so far have only been isolated from nodules of host plants of other clusters which they cannot nodulate by themselves:

• G2 (type strain): isolated from nodules of Casuarina equisetifolia (Casuarinaceae, Fagales): Guadeloupe
• CcI149: isolated from nodules of Casuarina cunninghamiana (Casuarina, Fagales): Egypt
• R43: isolated from nodules of C. cunninghamiana (Casuarinaceae, Fagales): USA
• Tai2: isolated from nodules of Coriaria intermedia (Coriariaceae, Cucurbitales): Taiwan

These data raise several questions:

• Is F. irregularis intracellular in the nodules of Cluster-1c or Cluster-2 host plants, or is it surviving in the dead cells of the nodule periderm?
• If F. irregularis is routinely found associated with non-host plants, does it affect their growth as a rhizosphere bacterium?
• We know there are soil bacteria which can improve nodulation of actinorhizal plants by their compatible Frankia strains – these soil bacteria are called “helper strains”. Does F. irregularis play such a role?
• If F. irregularis strains are helper strains, can we identify the genes that allow them to play this role?

Contact

For more information, please contact:

Katharina Pawlowski
katharina.pawlowski@su.se