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Could corn fix its own nitrogen?

The day that corn could fix its own nitrogen would be sweet, indeed. It would save growers plenty of money on fertilizer and it would help the environment by decreasing opportunities for nitrogen (N) runoff and volatilization. It would also revolutionize food production in parts of the world where farmers cannot afford high levels of industrial inputs. 

Despite limited success in finding bacterial strains that boosted corn yields, researchers may have greater success in genetically engineering N-fixing bacteria and corn that work together.    Photo by Ralph Pearce

Legumes such as beans, peas and alfalfa have long been known for their symbioses with N-fixing root nodule bacteria. Less well known is the discovery by researchers in Brazil that some sugar cane varieties receive a majority of their required N from bacteria. In sugar cane, the bacteria live within the plant roots, stalks, and leaves, in the spaces between the plant cells, rather than forming root nodules. This discovery has spurred people to look for similar bacteria that might provide N to other important grass crops such as wheat and corn.

In the mid-1990s, Dr. Eric Triplett, who was then a microbial ecologist at the University of Wisconsin, began looking for N-fixing bacteria in North American grasses. He tested a number of bacterial strains, collected from Wisconsin and other areas of the world, for their effect on various maize and wheat lines. He found some interesting results, but not the ones he had hoped to get. “The bottom line was that it’s fairly easy to find bacterial strains that can increase the yield of maize in a field by five or 10 percent,” says Triplett, who is now at the University of Florida. “The problem is, we couldn’t find any evidence that the yield increase was because of N fixation because these plants never showed relief of N deficiency symptoms.”

He thinks the yield increase in corn may have had more to do with plant hormone production, explaining, “A lot of these bacteria make plant hormones, especially auxins, and they could be playing a role.” Although Triplett did not test this idea, it has been noted in other work on similar plant–bacteria associations. 
Triplett also experimented with wheat and bacteria in the greenhouse, again with mixed results. “We did find one strain of bacteria that did improve N deficiency symptoms in wheat, repeated that experiment three times, and showed that we did get a little bit of N fixation in the greenhouse on one line of wheat with one bacterium,” he says.  “But we came to Florida, and we couldn’t reproduce it here.”

He does not know why, but thinks the hotter climate in Florida could be a factor. Also problematic, the effective bacterial strain was weakly pathogenic in mice, hindering its potential commercialization. “We’ve since moved on to other things,” says Triplett. “But I really think the best way to go after this, if we’re going to solve the problem, is to put the N-fixation genes directly in plants.”

Genetic engineering
Rather than insert the N-fixing genes directly into corn, Dr. Kaustubh Bhalerao thinks that the relatively new field of synthetic biology will eventually engineer corn and N-fixing bacteria that work together. Bhalerao, whose first degree was in civil engineering, is a synthetic biologist at the University of Illinois at Urbana-Champaign. He says that much like electrical circuits in engineering, synthetic biology aims to design libraries of various genes and their trigger mechanisms that can be inserted into a variety of organisms. “Genetic engineering so far has been about pulling out genes from one organism and putting them into another and trying to import just the functionality of those genes into the new plant,” he says. “Synthetic biology takes it a step further in trying to bring in the control mechanisms that govern that functionality along with it. So we have more control on when the functionality is expressed, how much, and under what conditions.”

For example, his laboratory has developed the equivalent of amplifiers that work in bacteria. “Just like you could hook up an electrical amplifier to your iPod and make it sound louder, you could take this little amplifier module and stick it inside a gene regulatory system and change how the signal gets transmitted across that amplifier. So a small signal can get translated to a larger output downstream of the amplifier,” he adds.

Bhalerao thinks that corn could be engineered to produce soybean-like chemical signals to attract N-fixing root nodule bacteria. Conversely, N-fixing bacteria could be engineered to respond to chemicals the corn already produces. Bhalerao and five other investigators from four other universities are in the first year of a three-year grant from the US National Science Foundation to begin exploring these possibilities. Although he does not expect to have a prototype allowing corn to fix its own N by the end of the grant, he hopes that they will have answered a number of questions about its feasibility.

Canadian contribution
A group of Canadian collaborators, including Drs. Lining Tian and Lana Reid from Agriculture and Agri-Food Canada (AAFC), Dr. Peter Pauls from the University of Guelph, and Dr. Zhongmin Dong from St. Mary’s University in Halifax, have also been exploring the possibility of introducing N fixation into corn. With funding from AAFC, Pioneer Hi-Bred Canada, and the Ontario Ministry of Agriculture, Food, and Rural Affairs, they have started by testing the ability of a major N-fixing bacterium from sugar cane to colonize Canadian sweet corn and grain corn hybrids. They think that introducing a bacterium from a related grass like sugar cane is a better possibility than getting root nodule bacteria to work with corn.

“The N fixation in legumes is by rhizobia. But rhizobia are very species specific. Also, before N fixation can take place, they must form a nodule structure, and that is a very complicated situation,” explains Tian. “So transferring rhizobia to fix N in cereals might be very difficult.”

In their greenhouse tests, the sugar cane bacterium colonized 11 of 17 grain corn lines and nine of 10 sweet corn lines tested. It proliferated and moved throughout the plant tissue during one month of plant growth. Now that the researchers know they can get the bacterium to colonize some Canadian corn hybrids, one of the next steps will be to see if they can detect any N-fixing activity in the plants. They also hope to get funding to investigate how the plant and bacterial genes are involved in the N fixation.

Would it be better to get the bacteria to grow in the plant or to take the genes from the bacteria and insert them into the plant? “It’s hard to say,” says Tian. “Let’s put it this way. If the bacteria can be simply introduced into the corn plant and can fix N in the corn plant, that would be a very simple solution. But for the long term, we’re trying to make full use of this bacterium for N fixation in different cereal plants, so it’s nice to understand the mechanism: how many and which genes are involved in N fixation.”