Last week was Earth Day, and panels, celebrations and events were held all across the University of Chicago campus. On the Medical Center side of campus, I attended a talk that wasn’t technically a part of the Earth Day blitz, but which nevertheless offered an elegant environmental solution applying the latest genetic tools to a farming trick almost as old as agriculture itself.
For thousands of years, farmers have employed crop rotation to keep their soil from being depleted of essential nutrients, particularly nitrogen. One group of plants long known to be particularly useful for this function are the legumes, such as peanuts, alfalfa, blue bonnets (pictured at right), or soybeans. As Sharon Long, professor of biological sciences at Stanford University explained in a lecture last week, the nitrogen-fixing talent of legumes lies in a strange partnership between the plants and bacteria – a symbiotic relationship that essentially creates tumors on the plant’s roots.
This fascinating symbiosis is the focus of Long’s research, which she presented in the 5th annual Margot and Robert Haselkorn Visiting Lectureship. Robert Haselkorn, a professor of biochemistry and genetics at the University of Chicago, studies bacteria with the ability to fix nitrogen – the process of converting nitrogen into bioavailable ammonia. So Long was a logical invite, as she has dedicated her work to decoding the genes and signals that allow legumes to be nature’s soil saviors.
The environmental aspect comes in with how most modern farms replenish the nitrogen in their soil: fertilizer. To create artificial fertilizer requires extreme heat, which is typically generated with fossil fuels. Besides gobbling up a dwindling natural resource (3-5 percent of natural gas goes to fertilizer manufacture), the process also makes fertilizer prices sensitive to fluctuations in fossil fuel prices. That can lead to food shortages, Long said, as the price of fertilizer becomes too much for farmers in poorer countries to purchase and use, leading to nutrient-depleted soil.
However, legumes offer a potential natural answer to that problem, Long said. When particular species of bacteria infect the plants, they form “root nodules” – tumorous growths that enable the bacteria to harness the plant’s photosynthetic energy for the fixation of nitrogen.
“Nitrogen fixation cannot be carried out by any eukaryote; only bacteria can,” Long said. “It’s not an accident that so many legumes have big, fat, whopping, protein-rich seeds. It’s because for legumes, nitrogen is not a problem, and that’s due to this symbiosis.”
That feature was recognized as long ago as the Roman Empire, Long said, and was used by native Americans as part of the “three sisters” strategy of agriculture that planted beans, squash and maize together.
“They didn’t know why it worked, but they knew it could do something,” Long said. “We now know a lot more about how it happens, why it works and now we can start to either improve it or we can imagine extending it.”
Long’s research has applied a wide array of scientific tools to determining the intricate steps involved in the bacteria/legume symbiosis. Each legume species prefers to pair with a particular bacterial species, and genetic analysis uncovered the plant signals, called “flavonoids,” that are produced by the plants to initiate the partnership.
“It’s as though the bacteria are tasting different flavonoids and only respond to the one that tastes right,” Long explained.
Once the right connection is made, the bacteria infect hairs of the plant’s root and induce a chain reaction that creates abnormal cell growth, producing the root nodules. It’s a process quite similar to what is seen in tumor growth, Long said, but in this case both parties benefit. An intricate exchange of chemicals between plant and bacteria maintain the partnership, controlled by several genes that Long’s lab has discovered.
So now that we know the vocabulary of this fascinating and beneficial symbiosis, what can we do with it? The knee-jerk reflex is to consider genetically engineering species of corn or rice that could form an alliance with nitrogen-fixing bacteria, but Long noted that such a strategy would be technically difficult – after all, it took millions of years of evolution to perfectly calibrate the process in legumes so that both plant and bacteria benefit. Instead, Long suggested, scientists should look to improve the existing system in legumes: extend the geographic range in which the plants can grow in hostile conditions. Those small changes could allow more farmers will be able to apply their natural nitrogen-fixing power to their own soil, and make farms less reliant upon artificial fertilizers.
“I personally think that sustainability is the most important place that we should be trying to use our molecular biology,” Long said. “I think we have a lot to be done using the present technology and all this new information about what genes in the plant are responsible for nitrogen fixation.”