Tag Archives: Actinobacteria

Friday Fellow: Alder-Root Bacterium

by Piter Kehoma Boll

The success of many plant species wordlwide is the result of their association with different organisms in their root systems, such as fungi and bacteria. Among bacteria, the most widely known group of root-associated species is that of the so-called rhizobia, bacteria that are associated with the root of legumes (plants of the family Fabaceae).

However, this kind of association evolved independently many times in many lineages of plants and many lineages of bacteria. Today I’ll present you a bacterium that is not closely related to the rhizobia, but acts similarly. Known scientifically as Frankia alni, it does not have a common name, but I decided to call it the alder-root bacterium.

As both its scientific name and its brand-new common name imply, the alder-root bacterium is a species associated with the roots of trees in the genus Alnus, commonly known as alder in English. It belongs to the phylum Actinobacteria and the order Actinomycetales, having a filamentous growth that produces a structure similar to the mycelia of fungi.


The hyphae that form the mycelium of Frankia alni. Photo by Wikimedia user Kkucho.*

The bacteria do not penetrate the cell membranes of the host plant at first, but remain inside a structure that develops from the plant cell wall. From there, they stimulate cell division and the production of nodules that grow from the rooth and inside of which they migrate, entering the nodule’s cells.


A nodule caused by Frankia alni on the root of an alder species. Photo by Gerhard Schuster.*

Through a complex biochemical process that I won’t present here in detail, the alder-root bacteria can capture nitrogen from the atmosphere and synthesize aminoacids from it, part of which is shared with the host. As a result, alder trees that contain nodules of alder-root bacteria are able to grow in nitrogen-poor soils and eventually enrich that soil and allow other plants to establish.


Sporangia of Frankia alni. Photo by Wikimedia user Kkucho.*

In order to disperse themselves through the environment and find new hosts, the alder-root bacteria produce spores inside sporangia. Once released, the spores may migrate, probably through water, to new localities where they germinate and restart the cycle.

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Benson, D. R.; Silvester, W. B. (1993) Biology of Frankia strains, actinomycete symbionts of actinorhizal plantsMicrobiology and Molecular Biology Reviews 57(2): 293–319.

Wikipedia. Frankia alni. Available at: < https://en.wikipedia.org/wiki/Frankia_alni >. Access on March 13, 2018.

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Filed under Bacteria, Friday Fellow