The search for the holy grail: nitrogen fixation in cereal crops

Sugar cane model holds promise


Breakthroughs in science are usually the result of labour over a lifetime or longer. Most crop research is devoted to incremental improvements, but some are devoted to transformative change. One such search is for a way to allow cereals to fix nitrogen as pulse crops do. Such a breakthrough would free producers, particularly poor farmers in the developing world, from the crushing expense of commercial fertilizers. 
The Gates Foundation funds research on nitrogen fixation. As Western Producer reporter Robert Arnason found in this second part of a special report on crop research, the foundation’s support gives security for scientists who pursue a goal that will likely take decades to achieve.

A number of words could be used to describe Giles Oldroyd’s research: visionary, mind boggling, revolutionary, fanciful and downright impossible.

The British plant biologist appreciates the praise but understands the skepticism because he also has reservations about his research: trying to turn corn into a legume.

Oldroyd is attempting to transfer the nitrogen-fixing ability of peas and soybeans into cereal crops.

The allure of nitrogen fixing cereals has enticed hundreds of plant scientists for more than a century. Who wouldn’t want to solve a problem regarded as the ultimate riddle in plant science?

Oldroyd, however, is a realist who recognizes what he’s up against.

“With the technologies we have right now, this is hard work…. I don’t think we have all the knowledge to do it right now. I don’t think we know sufficiently … the process in legumes to transfer that capability,” said Oldroyd, who works at the John Innes Centre, an independent plant science and microbiology lab in Norwich, England.

German scientists Hermann Hell­riegel and Hermann Wilfarth discovered in 1886 that legumes use atmospheric nitrogen for their growth. They didn’t know precisely how it worked but they concluded, correctly, that legumes performed the trick in tumour-like structures on their roots called nodules, where bacteria convert atmospheric nitrogen to ammonia.

Scientists have advanced their knowledge of this process in the 128 years since Hellriegel’s and Wilfarth’s eureka moment and have attempted to transfer the trait to other crops, with minimal to modest success.

Plant scientists who study nitrogen fixation caught a break in 2011 when Microsoft founder Bill Gates became intrigued by the concept.

The Bill and Melinda Gates Foundation funds agricultural development around the globe, particularly research to boost production in sub-Saharan Africa.

Allen Good, a University of Alberta plant scientist, pitched nitrogen fixing cereals to the Gates Foundation several years ago, arguing they could transform agriculture in Africa and parts of the world where farmers can’t afford fertilizer.

Good’s pitch must have resonated because it led to a meeting between foremost scientists in the field and the Gates Foundation in 2011. Gates didn’t participate in the meeting, but his employees said the billionaire is “quite interested in nitrogen.”

“You know why it’s interesting … because potentially it’s a solvable problem,” said Good, speculating on Gates’ interest.

“There are no other big problems in agriculture that have the air of possibly being solvable, other than the simple ones like … continuing to boost yields.”

Good followed up on the meeting by writing a paper that summarized potential methods to engineer nitrogen-fixing cereals.

“The question was simply: if you had to tackle this problem, how would you do it?” said Good, who studies nitrogen use efficiency and biological nitrogen fixation at the U of A.

In a paper published in Science, Future Prospects for Cereals that Fix Nitrogen, Good identified three approaches to the problem:

  • Inoculate seeds with micro-organisms called endophytes, which live between plant cells. These bacteria already exist in cereals and some have the ability to fix nitrogen.
  • Re-engineer the biology of a cereal plant so it mimics the nitrogen fixing ability of legumes.
  • Use an enzyme known as nitrogenase, which can break the triple bond that holds atmospheric nitrogen together

The Gates Foundation publicly backed its interest in nitrogen fixing cereals in 2012 by announcing $10 million US in funding for the second option.

“We need innovation for farmers to increase their productivity in a sustainable way so that they can lift themselves and their families out of poverty,” Katherine Kahn, the Gates Foundation’s senior programs officer, said in 2012.

“Improving access to nitrogen could dramatically boost the crop yields of farmers in Africa.”

The research focuses on corn, and Oldroyd leads the John Innes Centre team tackling the problem.

Oldroyd and his colleagues are concentrating on the communication between legumes and soil bacteria known as rhizobia, which co-operate with plants to fix nitrogen.

“(The plant) has to find the rhizobia in the soil and it does (it) through this chemical communication,” said Oldroyd, who studied at Stanford University and the University of California.

Oldroyd described the communication channel between soil fungi and legumes as a “signalling pathway.”

Oldroyd has to biologically construct a similar pathway in corn to duplicate legumes’ ability to fix nitrogen.

“That mycorrhizal symbiosis is ubiquitous within the plant kingdom,” he said.

“The signalling pathway is there in cereals and allows recognition of mycorrhizal fungi. What we’re attempting to do in cereals is to engineer that pathway to also allow recognition of the nitrogen fixing bacteria…. We (don’t) have to recreate a whole new signalling capability. We’re engineering a pre-existing signalling pathway.”

Good said the signalling pathway exists in corn and other cereals, but key components are missing.

“Imagine you have an old-fashioned car … and you’re missing a carburetor. If you can put the carburetor on to get the fuel in, all of a sudden you’ve got an engine that works.”

The plant may begin fixing nitrogen if scientists can convince the soil bacteria and cereals to communicate and form nodules on the roots.

“The argument is once you have a nodule, maybe you don’t need to do anything more than that,” Good said.

Unfortunately, engineering a communication pathway that stimulates nodule formation is much more complicated than installing a carburetor in a 1969 Camaro.

Manish Raizada, a University of Guelph plant scientist, has confidence in the John Innes Centre, but he questions the basic premise of Oldroyd’s research.

“The folks at John Innes … these guys are really, really good. They’re among the best scientists in the world in this area,” said Raizada, who studies nitrogen fixation and nitrogen use efficiency.

“(But) In terms of their final objective, yes, I am very skeptical.”

Raizada said dozens of genes play a role in the pathway, and how are scientists going to figure out how to make nodules on corn roots if Mother Nature couldn’t do it?

“If you look at the plants that form nodules … it’s a very narrow group.… Other plants really don’t seem to be able to do this,” he said.

“Nitrogen is the limiting factor for growth of all plants on the planet. That means for millions and millions of years, there would have been strong natural selection for microbes that could do this…. If nature couldn’t do it after 100 million years, that’s a lot of generations (of plants). Then we have to be very, very skeptical.”

Raizada is pursuing a different approach to achieve the same goal. He’s following a model based on successes with sugar cane in Brazil, where growers inoculate the soil with bacteria that fix nitrogen for the cane.

“It’s estimated today that Brazil saves $1 billion in nitrogen fertilizer costs (annually) because of these bacteria,” Raizada said.

“It’s one of the reasons why ethanol from sugar cane is actually financially viable … because they don’t have to dump on nitrogen.”

Figures vary widely on how much nitrogen the bacteria, known as endophytes, fix for sugar cane. Estimates range from five to 50 pounds per acre.

Raizada and his team want to replicate the sugar cane model in corn. They have identified 30 endophytes in corn capable of fixing nitrogen.

“We looked at ancestral corn that grows in the wild, all the way from Central America to (Canada),” he said.

“We suspect they fix at a very low level, but the next number of years we’ll be doing systemic testing on it.”

If they can isolate bacteria suitable for modern corn varieties, the idea is to apply the bacteria to corn seed, much like inoculants on soybeans.

Several challenges are associated with this approach. For one, some scientists wonder if the endophytes are actually producing nitrogen for sugar cane. They might be acting more like plant growth hormones.

Raizada said the endophytes might fix nitrogen for sugar cane but not other crops.

“It’s sugar for nitrogen,” he said.

“The plant, through photosynthesis, feeds the bacteria sugar and it fixes in return. In sugar cane it’s easy to imagine how this happens because the cane is very high in sugar. That might be why this seems to work in sugar cane.”

Oldroyd and Raizada may be tackling the problem from different perspectives, but they agree on a broader topic: engineering nitrogen fixing cereals is exceptionally complex and anyone promising to deliver it in few years is a charlatan.

“When I talk to the press, I always get asked: how long is it going to take? There is no answer to that. We are working in the unknown,” Oldroyd said.

“It could take years. It could take 50 years…. I don’t know whether it’s possible to achieve it within my career. It might not be.”

Scientists made comments in the 1990s suggesting nitrogen fixing cereals were just around the corner.

“Overstatements from scientists and from the press don’t help at all,” Oldroyd said.

Raizada was more specific in his criticism. Researchers at the University of Nottingham said last year that they had discovered a specific strain of bacteria that could colonize the roots of dozens of crops, thus permitting those crops to fix nitrogen.

Raizada said the claim is unsubstantiated.

“I have tried to chase down the scientific literature on this. I could only find one paper from a very long time ago,” he said.

“They’re claiming this bacteria can colonize, I think, 60 different species of crops. That is a remarkable claim…. We have hundreds of species of bacteria … in my lab…. Most of those bacteria will only infect, or colonize, a few species.”

He said it’s maddening to see headlines claiming researchers have solved the nitrogen fixation riddle because the science simply isn’t there.

“What this does … it creates false hope. Funders and grower groups start investing, they get their hopes up and then it’s going to be dashed,” he said.

“And when it gets dashed, a lot of other researchers, who are doing long, painful, very difficult serious work, they’re the ones who suffer.”

Oldroyd said scientists should promote modest goals. If it is possible to engineer corn that works in symbiosis with rhizobia, it’s not going to replace 200 lb. of applied nitrogen.

“You can probably get primitive, more rudimentary structures on the cereal root that will fix nitrogen at some level, but they won’t fix nitrogen at the level to replace fertilizers,” he said.

Still, 25 to 50 lb. of nitrogen could dramatically improve yields for farmers in Africa.

Private companies, with the exception of Monsanto, aren’t publicly pursuing the technology because the research is too risky and the return on investment may take decades, Good said.

Oldroyd hopes the technology is developed during his lifetime, but his primary task is charting a path for future scientists.

“I don’t feel it’s my role to deliver nitrogen fixing cereals to farmers,” he said.

“I feel it is my role in the public sector to show what is possible, and to essentially lower the risk (for) the companies.”

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