Scientists peer inside nitrogen fixation

Process occurs when bacteria breach cell walls | Scientists discover that plants allow it to happen

A British discovery that explains how plants fix nitrogen has answered a 124-year-old question and might alter the future of crop agriculture.

Since an 1887 study on the nodules that form on legume roots, scientists have tried to understand how soil bacteria breach the cell walls of legumes, which is an essential step in the process of nitrogen fixation.

For much of the last century, scientists assumed the bacteria rhizobia released an enzyme that degraded the plant cells walls. Once inside the plant, the rhizobia take nitrogen from the atmosphere and supply it to the plant in a form of ammonia.

However, plant scientists at the John Innes Centre, an independent research facility in the United Kingdom, concluded last year that bacteria don’t break down cell walls. Instead, they determined that the legume is in control of the process and willingly lets the rhizobia pass through its cells.

“This is a major discovery,” said Krzysztof Szczyglowski, an Agriculture Canada plant scientist and nitrogen fixing expert in London, Ont.

“The fascination with this mechanism has been for some time now. This is an incredibly intensively studied subject and also contested subject in the past research.”

Szczyglowski said the discovery is significant because despite years of research, scientists never fully understood how the rhizobia penetrated and colonized roots.

Plants’ cell walls are hard to penetrate because they are formed from carbohydrates such as pectin, according to a news release from the John Innes Centre. Some scientists assumed rhizobia used an enzyme called pectate lyase to degrade the plant’s cell walls.

“Mostly it (pectate lyase) has been associated with pathogenic bacteria, which are trying to break the plant’s defense mechanism and enter the plant’s cells to scavenge for the nutrients,” Szczyglowski said.

Yet, John Innes scientists learned that a pectate lyase gene in a legume supplies the necessary enzyme to break down its own cell walls.

The discovery could have broad implications for agriculture because scientists might be able to transfer the nitrogen fixing abilities of legumes to other crops.

“The fact that legumes themselves call the shots is a great finding, but it also shows the complexity of the challenge to try to transfer the process to non-legumes,” said Allan Downie, lead author of the John Innes study, which was published in the Proceedings of the National Academy of Sciences.

The second author on the study is a Canadian, Jeremy Murray, who earned his PhD in plant science at the University of Guelph.

Knowing the plant is in charge of the process means researchers can now try to answer other relevant questions, Szczyglowski said.

“This mechanism of plant control, is it applicable to second symbiosis, the symbiosis of plants with (mycorrhiza) fungi?”

As well, plants usually launch defense mechanisms when invaded by a foreign body such as bacteria, which means scientists will need to understand how the plant shuts off its auto-immune response.

Although it may take a decade or longer to answer these types of questions, Szczyglowski said it could be possible to take the unique biological properties of legumes and inject them into cereals and other crops. That means farmers might soon be less dependent on man-made nitrogen.

“Most people think, based on the current information, that it is viable,” Szczyglowski said. “But it is probably a distance away.”

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