My Green Neighbor’s Ally Seemed to Come Out of Nowhere

Something happened that made me think of a classic scene from Woody Allen’s “Annie Hall”. Woody is waiting for movie tickets and he’s stuck in line with a man behind him pontificating and annoying him.

Well, yesterday morning, life was almost like that for a neighbor of mine. She was trying to convince another neighbor not to use herbicide. Clover for example, she said, makes its own fertilizer. Why kill it?

Now, I’m not the Marshall McLuhan of clover-knowledge, but it is something I’ve dabbled in for many years. And precisely at the moment she said that, by tremendous fluke, I happened to walk by and heard her. I interrupted and said, “well yeah. Rhizobium bacteria have a great mutualism going on with clover plants and the bacteria make ammonium, which plants use to make protein, nucleic acids, etc. ..

“Oh thank you” she said! , as if I had dropped out of the sky to strengthen her argument.

If I had wanted to be rude I would have persisted and pointed out that Rhizobium causes an infection which result in nodules on the roots of clover and other legumes. The bacteria not only get sugars in return for the favor of giving the plants a form of nitrogen that they can use, but from clover and other legumes, Rhizobium bacteria get leghaemoglobins which binds to oxygen, facilitating cellular respiration. Resembling our hemoglobin, leghaemoglobin at the same time prevents excess oxygen from severely slowing down the enzyme (nitrogenase) that the bacteria use to convert relatively inert atmospheric nitrogen to ammonium ion. For over 100 years some scientists have dreamed of inducing such a partnership in other plants, but they have failed to replicate or induce such a complex association.

On the left we see clover roots infected with Rhizobium. The nodules are shaped like ends of Q-tips. On the right a nodule has been dissected to reveal the reddish color of the oxygen-carrying pigment, leghemoglobin. Source:

Meanwhile, a few years ago, soy leghemoglobin was approved as a color additive, a definitely silly idea. Committed to not eating mammals, why would I— or a vegetarian —want to be reminded of blood while eating a soy product? On a more important note, I learned recently that bacteria known as Rhizobium leguminosarum are too diverse genetically to be considered a single species. It should not come as a shock because— what were the chances that no speciation occurred in bacteria, given that their symbiotic partners, the legume family, consist of 765 known genera and almost 20 000 species that are distributed throughout the world?

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