A new research project funded by the Alberta Wheat Commission and the Saskatchewan Wheat Development Commission will try to answer a question that has bedevilled plant scientists for years: can cereal crops be made to fix their own nitrogen the way legumes do?
The AWC is spending $100,000 to have Agriculture and Agri-Food Canada scientists at Lethbridge, Alta., isolate triticale cells that fix atmospheric nitrogen and then regenerate entire nitrogen-fixing plants from those cells.
The ultimate goal is to produce nitrogen-fixing wheat, said Alicja Ziemienowicz, one of the AAFC researchers.
“Once we obtain nitrogen-fixing triticale, we will transfer this trait into wheat, using inter-species breeding techniques,” Ziemienowicz said in an email.
If successful, the project could have a significant dual benefit. It would be a money saver for farmers who currently spend up to 20 per cent of their cereal production costs on synthetic nitrogen fertilizer. (That’s not counting expenses for fuel, machinery and labour for applying it.) Also, reducing nitrogen fertilizer use would benefit the environment by reducing emissions of nitrous oxide (N20), a greenhouse gas connected to global warming.
“Generation of nitrogen-fixing cereal crops will contribute to increasing farmers’ income and agricultural sustainability,” Ziemienowicz said.
Can cereals act like pulses?
At first glance, the idea of getting cereal plants to partner with bacteria to create usable nitrogen seems a contradiction in terms, like turning a sheep into a goat or putting photosynthesis into a cow.
But Ziemienowicz said the technology is partially available now and she believes additional technologies can be developed to produce plants with this new trait.
There are three biotech approaches for achieving biological nitrogen fixation (BNF) in cereals. All require genetic engineering of bacteria, plants or both:
- Convincing rhizobia (nitrogen-fixing bacteria in the soil) to interact with cereals the way they do with legumes.
- Improving bacteria living inside cereal plants (endophytes) or in the vicinity of plant roots (rhizosphere) to form associations with cereals.
- Transferring bacterial nitrogen fixation (nif) genes directly into the plant.
Ziemienowicz acknowledges the numerous scientific challenges in generating a nitrogen-fixing cereal.
“The first one is to deliver and successfully express in plants at least 16 bacterial nif genes; usually we deliver just one to two genes,” she said. “Second, to make nitrogenase (the enzyme which converts atmospheric nitrogen into ammonia) active in plants.
“Third, cereals are not as easy to transform as (some other) plants, especially if we want to deliver nif genes to the place where nitrogenase will be active. Fourth, most gene-delivery procedures require regeneration of plants from cells or tissues. These procedures have been developed for many cereal crops but they don’t work equally well in all species.”
For that reason, researchers are working with triticale because these procedures “work better in triticale than in wheat,” Ziemienowicz said.
The GMO problem
Even if Ziemienowicz’s team manages to overcome all these barriers, there’s another challenge to nitrogen-fixing cereals involving, not science, but politics.
To achieve N-fixing cereals, scientists must use transgenesis (introducing a new gene into an organism to exhibit a new property). Transgenic wheat is currently not grown in Canada (witness Roundup Ready wheat) because of widespread opposition by customers. But Ziemienowicz hopes that may change by the time an N-fixing cereal gets generated and commercialized.
Nitrogen-fixing cereal crops have been a Holy Grail for plant scientists ever since the discovery in 1917 of the symbiosis (interaction) between nitrogen-fixing bacteria and legumes. The idea of weaning cereals off nitrogen fertilizer has intrigued researchers ever since, although progress in that direction has been limited.
Kevin Vessey, a professor of plant biology at St. Mary’s University in Halifax, did some research on N-fixation several years ago when he was at the University of Manitoba. He says scientists have learned a lot about nitrogen fixation in the last 50 years. The actual advances toward nitrogen-fixing cereals? Not so much.
“I’m not sure we’re any closer to achieving nitrogen fixation in cereals,” Vessey said. “But, like I say, never say never.”
There is, of course, a way for farmers to take advantage of nitrogen fixation right now through crop rotation. That involves planting cereal crops after alfalfa or other legumes to utilize residual soil nitrogen. This is a technology that has existed for centuries. Even the ancient Romans, without understanding what nitrogen fixation was, found that planting a cereal crop after fababeans helped the cereal grow better.
GM not a magic solution
The main reason for the slow progress in breeding nitrogen-fixing cereals is that the process is a lot more complicated than originally thought, said Vessey. When molecular biology was coming of age in the 1970s, people thought all they had to do was take genes from bacteria that fix nitrogen, put them in the plant, have them expressed in the plant and, presto… nitrogen-fixing wheat.
Turns out it’s not that simple.
“Back in the ’70s and ’80s, we thought all we’d have to do was a little genetic engineering and everything would work. Well, it doesn’t,” Vessey said.
Vessey did have some initially promising results back in 2001. He and his colleagues at the University of Manitoba looked at the potential of a nitrogen-fixing bacterium called Gluconacebacter diazotrophicus, discovered in sugar cane by Brazilian scientists in 1988. It was hoped that some strain of wheat might be able to adapt to that bacterium because sugar cane and wheat are both grass plants.
Vessey’s team used bacteria from sugar cane, including Gluconacetobacter. They learned a lot about how the bacterium worked. Ultimately, however, efforts to get it to work in wheat were largely unsuccessful.
Studies show that Azospirillium, another free-living nitrogen-fixing bacterium also found in many plants, can have some effect in wheat. Identified in the 1970s and widely used in South America as a seed treatment, Azospirillium can produce a yield increase of 9.5 per cent in summer cereals and up to 14 per cent in winter cereals.
A U.K. company, Azotic Technologies, is marketing the use of these so-called “associative N2 fixers” in a variety of crops, including cereals.
But although these bacteria have been shown to fix some amount of N2 in cereals, it’s nothing compared to the levels seen in legumes, said Vessey.
“So we do have nitrogen-fixing wheat but the benefits are small,” he said. “It makes some difference. It just doesn’t go the whole way.”
So what happens now?
Although progress is slow and success so far is limited, Vessey said research will definitely continue because the potential payoffs are major. Scientists will continue to look for different strains of associative fixing bacteria to find one that works efficiently on a range of plants.
“I can guarantee people will continue — and it’s happening today — to look for these plant growth-promoting rhizobacteria,” Vessey said.