Nitrogen is an essential nutrient for plant growth, but excessive use of synthetic nitrogen fertilizers in agriculture is unsustainable. American scientists have studied the possibility of using genetic engineering to improve the mutualistic relationship between plants and nitrogen-fixing microorganisms, and found that by mimicking the interaction between legumes and nitrogen-fixing bacteria, it helps crops obtain nitrogen from the air. A related opinion article was recently published in Trends in Biology.
"Providing nitrogen to crops by modifying the relationship between crops and nitrogen-fixing microorganisms is a promising and relatively quickly implemented solution that can solve the high cost and sustainability issues of synthetic nitrogen fertilizers." Corresponding author of the paper, McGrady of the University of Wisconsin-Madison said Jean-Michel Ane of the University of Johnson & Johnson.
Nitrogen-fixing microorganisms are a type of soil bacteria and archaea that naturally "fix" atmospheric nitrogen into ammonium, a source that plants can utilize. Some nitrogen-fixing microorganisms form mutualistic relationships with plants, which provide them with a carbon source and a safe, low-oxygen home, and in return provide nitrogen to the plant. For example, leguminous plants house nitrogen-fixing microorganisms in their root nodules. However, this symbiotic relationship only occurs in a few plants. If more plants could form relationships with nitrogen-fixing microbes, it would reduce the need for synthetic nitrogen fertilizers, but it would take tens of millions of years for this relationship to evolve naturally.
How to improve the nitrogen fixation capacity of non-legume crops? Scientists have proposed several different approaches to address this agricultural challenge, including genetically modifying plants to produce their own nitrogen enzymes (which nitrogen-fixing microorganisms use to convert atmospheric nitrogen into ammonium), Or nodulate non-leguminous plants.
Additionally, plants and nitrogen-fixing microorganisms are engineered to facilitate their mutualistic relationships. The goal is to engineer plants into better hosts, and nitrogen-fixing microbes to be engineered to more easily release fixed nitrogen when exposed to molecules secreted by the engineered plants.
"Since free nitrogen-fixing microorganisms do not 'selflessly' share their fixed nitrogen with plants, they need to be manipulated to release the nitrogen so that it is available to plants," Ane said.
This engineering approach relies on bidirectional signaling between plants and microorganisms, which already exists naturally. Microorganisms have chemoreceptors that sense metabolites secreted by plants into the soil, and plants are able to sense microorganism-related molecular patterns and the phytohormones they secrete. These signaling pathways can be tuned through genetic engineering, allowing clearer communication between transgenic plants and microorganisms.
The researchers also discuss ways to make these engineering relationships more effective. Since nitrogen fixation is an energy-intensive process, it will be important for nitrogen-fixing microorganisms to be able to regulate nitrogen fixation and produce ammonium only when necessary. "Relying on small molecule signals from the plant can ensure that nitrogen is fixed only when the engineered strain is close to the target crop species," Ane said. "In these systems, the cells only engage in energy-intensive nitrogen-fixing behavior when it is most beneficial to the crop." ."
In addition to nitrogen fixation, many nitrogen-fixing microorganisms provide additional benefits to plants, including growth promotion and stress tolerance. The authors believe that future research should focus on "stacking" these multiple benefits. However, since these processes are energy-intensive, the researchers recommend developing microbial communities composed of several species, each offering different benefits, to "spread the production load among several strains."
Researchers acknowledge that genetic modification is a complex issue and that large-scale use of GMOs in agriculture requires public acceptance. "There needs to be communication between scientists, breeders, growers and consumers about the risks and benefits of these emerging technologies," Ane said.
Furthermore, because microorganisms readily exchange genetic material within and between species, measures are needed to prevent the spread of genetically modified material to native microorganisms in the surrounding ecosystem. In response, scientists have developed several biological control methods. For example, engineering microorganisms to survive on non-naturally occurring molecules would mean they would be confined to fields of engineered plants, or equipping microbes with "kill switches." Researchers say these controls may be more effective if used in multiple layers, as each has limitations. They also highlight the need to test these engineered plant-microbe interactions under variable field conditions where crops are grown.
"The practical application of this technology and its translation from laboratory to field remain challenging due to the high variability of environmental factors and their effects on plants, microorganisms and their interactions. Trials conducted in highly controlled environments often cannot Translates well to the field and we recommend testing it in highly repetitive field trials," write Ane and colleagues.