The excessive use of synthetic nitrogen fertilizers in agriculture, while vital for plant growth, presents a sustainability challenge. In a recent review article published in the journal Trends in Microbiology on September 26, a team of bacteriologists and plant scientists has proposed an innovative approach to tackle this issue by harnessing genetic engineering to foster mutually beneficial relationships between plants and nitrogen-fixing microbes known as “diazotrophs.” These engineered partnerships could enable crops to extract nitrogen from the atmosphere, mirroring the natural associations seen in legumes and nitrogen-fixing bacteria.
The research team, led by senior author Jean-Michel Ané from the University of Wisconsin–Madison, posits that engineering diazotrophs to provide nitrogen to crops offers a promising and expedient solution to address the escalating cost and sustainability concerns linked to synthetic nitrogen fertilizers.
Diazotrophs, which include various soil bacteria and archaea, have the remarkable ability to convert atmospheric nitrogen into ammonium—a form of nitrogen that plants can utilize. Some of these microbes have established mutually beneficial relationships with plants, wherein plants provide a source of carbon and a protective, low-oxygen environment, while the microbes supply nitrogen in return. For instance, legumes house nitrogen-fixing microbes in small root nodules.
However, these mutualistic partnerships are limited to a select group of plants and crop species. To reduce the reliance on synthetic nitrogen fertilizers, there’s a pressing need for more plants to establish such associations with nitrogen fixers. Unfortunately, these natural relationships take an extensive amount of time to evolve.
Enhancing nitrogen fixation in non-legume crops remains an ongoing challenge in agriculture. Various methods have been proposed, including genetically modifying plants to produce nitrogenase (the enzyme that nitrogen fixers use to convert atmospheric nitrogen) or engineering non-legume plants to form root nodules.
The alternative approach explored in this review involves genetically engineering both plants and nitrogen-fixing microbes to facilitate mutualistic associations. In essence, plants would be engineered to become more hospitable hosts, while microbes would be engineered to release fixed nitrogen more readily upon encountering molecules secreted by the engineered plant hosts.
“Since free-living or associative diazotrophs do not altruistically share their fixed nitrogen with plants, they need to be manipulated to release the fixed nitrogen so the plants can access it,” explain the authors.
This approach hinges on bi-directional signaling between plants and microbes, a process that occurs naturally. Microbes possess chemoreceptors allowing them to detect metabolites released by plants into the soil, while plants can sense microbe-associated molecular patterns and microbe-secreted plant hormones. Genetic engineering could fine-tune these signaling pathways, enhancing communication between pairs of engineered plants and microbes.
The authors also discuss methods to optimize these engineered relationships. Given that nitrogen fixation is energy-intensive, it would be advantageous for microbes to regulate this process and produce ammonium only when necessary.
“Relying on signaling from plant-dependent small molecules would ensure that nitrogen is only fixed when the engineered strain is proximal to the desired crop species,” the authors note. “In these systems, cells perform energy-intensive fixation only when most beneficial to the crop.”
Beyond nitrogen fixation, many nitrogen-fixing microbes can offer additional benefits to plants, such as promoting growth and stress tolerance. The authors suggest that future research should focus on “stacking” these multiple benefits. However, given the energy-intensive nature of these processes, the researchers recommend developing microbial communities comprising various species, each contributing different advantages, thereby distributing the production load among multiple strains.
Acknowledging the complexity of genetic modification and the need for public acceptance, the authors emphasize transparent communication between scientists, breeders, growers, and consumers regarding the risks and benefits of these emerging technologies.
Biocontainment poses another challenge, as microbes readily exchange genetic material within and between species. The authors propose implementing control measures, potentially layered, to prevent the spread of transgenic material into native microbes in surrounding ecosystems. Measures include engineering microbes to depend on non-naturally occurring molecules or incorporating “kill switches.”
The authors advocate for testing these engineered plant-microbe mutualisms under variable field conditions to address the practical challenges posed by biotic and abiotic environmental factors, emphasizing the importance of highly replicated field trials.
In summary, genetic engineering presents a promising avenue to reduce agriculture’s reliance on synthetic nitrogen fertilizers, offering a sustainable solution to enhance crop productivity while addressing sustainability concerns.