Although it’s common knowledge that there is a beneficial relationship between members of the legume family and Rhizobium bacteria, less people are aware that there is a similar relationship between the members of the birch and alder family and a bacteria called Frankia. What’s noteworthy is that in both mutualisms, legumes and birches gain more than just ammonium, which the bacteria form by converting nitrogen from the air with their special enzyme nitrogenase. (For those of you who are also salivating to find out what plants do with ammonium, they use it to give glutamate an extra amino group as it becomes glutamine. The latter yields an amino to α-ketoglutarate which regenerates glutamate as it transfers it to variable intermediates. Those finally become a variety of essential amino acids. )
Before revealing the other benefits, let’s look at the Frankia nodules that form around the roots of Alnus glutinosa, the common alder tree. They are the orange clumps you see throughout the picture below.
When alder or birch trees are still young, it’s been shown that the Frankia infection lowers their ability to produce tannins and other compounds, making them more appetizing to herbivores. But the drawback is short-lived. The synthesis of deterrents accelerates as the treelings quickly mature, thanks to the extra nitrogen from Frankia, and this allows the trees to survive.
Lima beans, which form nodules with Rhizobium, use the nitrogen-bonanza to make their leaves richer in cyanogenic glycosides that are poisonous to Mexican Bean beetles. The more nodules they have, the more protective compounds they produce. Researchers confirmed that hypothesis by chemically degrading the poisons with enzymes in closed Thunberg vessels and then using spectrophotometry to measure the amount of hydrogen cyanide HCN released.
Having more nodules also improves bean plants’ ability to make volatile organic compounds when they first get attacked by the beetles, which drives them away. Meanwhile, the more “infected” lima beans become with Rhizobium, the less extrafloral nectar they produce. This makes them less attractive to ants, who otherwise farm aphids at the expense of bean plants.
- Ballhorn, James and al. Colonization by nitrogen‑fxing Frankia bacteria causes
short‑term increases in herbivore susceptibility in red alder
(Alnus rubra) seedlings. Oecologia. 2017 https://cbs.umn.edu/sites/cbs.umn.edu/files/public/downloads/Ballhornetal2017.pdf
- Thamer, Sylvia and al. Dual benefit from a below-ground symbiosis: nitrogen fixing rhizobia promote growth and defense against a specialist herbivore in a cyanogenic plant. Plant and Soil. April 2011.
- James Mauseth. Botany, An Introduction to Plant Biology. Jones and Bartlett. 2008