A symbiosis of legumes and rhizobia

"Nitrogen, nitrogen everywhere, but not a drop to drink," could be a plant's plaintive song. The atmosphere we breathe is 78% nitrogen, but plants are incapable of putting the elemental nitrogen to use. Instead, they need ammonia or nitrate and depend on the useful nitrogen they can suck out of dead plants and animals as part of the nitrogen cycle.

Nitrogen-fixing bacteria are the flip side of the coin. These microorganisms can take the nitrogen from the air and turn it into a useful form, but the process takes up vast quantities of energy. Some bacteria species are able to scavenge the energy on their own, but others have opted to team up with nitrogen-hungry plants.

The best-known symbiosis is between rhizobia bacteria and legumes. It all begins when a bacterium senses flavonoids given off by the legume's roots. "Home for sale!" the flavonoids say, and the bacterium secretes a chemical in reply --- "I'd like to move in." "Great!" says the root, and it curls its tiny root hair around the bacterium to make a safely enclosed root nodule. The plant fills the nodule with carbohydrates (free energy!), proteins, and oxygen, and the bacterium responds by fixing atmospheric nitrogen into ammonia to feed the plant. The pair lives happily ever after.

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This post is part of our Nitrogen Fixing lunchtime series. Read all of the entries:

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