A genetic switch lets plants accept nitrogen-fixing bacteria

Researchers are one step closer to understanding how some plants survive without nitrogen. Their work could ultimately reduce the need for artificial fertilizers in crops such as wheat, corn or rice.

“We are taking another step towards greener and more climate-friendly food production,” say Kasper Røjkjær Andersen and Simona Radutoiu, both professors of molecular biology at Aarhus University. The results are published in the journal Nature.

The two researchers conducted a new study in which they discovered an important key to understanding how we can reduce agriculture’s need for artificial fertilizers.

Plants need nitrogen to grow, a nutrient that most crops obtain only from fertilizer. Only a few plants, such as peas, clover and beans, can do without it. They live in symbiosis with special bacteria that convert nitrogen in the air into a form usable by the plant.

Today, researchers around the world are working to understand the genetic and molecular mechanisms behind this special ability, so that it can one day be transferred to crops such as wheat, barley and corn.

This would make plants self-sufficient in nitrogen and thus reduce the need for artificial fertilizers, which currently account for around 2% of total global energy consumption and emit large amounts of CO.₂.

Researchers have identified small changes in plant receptors that cause them to turn off the immune system and allow symbiosis with nitrogen-fixing bacteria.

Friend or foe?

Plants use receptors on the surface of their cells to sense signals from microorganisms in the soil.

Some bacteria emit chemicals that signal that they are “enemies” and that plants need to defend themselves. Others are “friends” who help feed.

Legumes, like peas, beans and clover, invite special bacteria into their roots. Here, bacteria transform nitrogen from the air and transmit it to the plant. This cooperation is called symbiosis and is why legumes can grow without artificial fertilizers.

Researchers have discovered that this ability is largely controlled by two amino acids, two small “building blocks” of a protein found in plant roots.

“This is a remarkable and important discovery,” emphasizes Radutoiu.

The protein in the roots functions as a “receptor” that receives signals from the bacteria. It is he who decides whether the plant should sound the alarm (immune system) or welcome the bacteria (symbiosis).

The researchers discovered a small area in the protein that they called Symbiosis Determinant 1. This area acts as a sort of switch that determines what message is sent inside the plant cell. By changing just two amino acids in this switch, researchers could obtain a receptor that normally triggers an immune response to start a symbiosis with nitrogen-fixing bacteria.

“We showed that two small changes can cause plants to change their behavior on a crucial point: from rejecting bacteria to cooperating with them,” explains Radutoiu.

Opportunities for wheat, barley and corn

In the laboratory, researchers have succeeded in modifying the Lotus japonicus plant. But the same principle has proven effective for barley.

“It is quite remarkable that we are now able to extract a receptor from barley, make small modifications to it, and then fix the nitrogen again,” says Røjkjær Andersen.

The prospects are great. If the modification can be transferred to other crops, it may eventually be possible to produce cereal plants such as wheat, corn or rice with the ability to fix nitrogen themselves, just as legumes do today.

“But first we need to find the other essential keys,” says Radutoiu. “Today, very few crops can achieve symbiosis. If we can extend this to widely used crops, it can really make a big difference on how much nitrogen to use.”