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.
氮是植物生长必需的营养物质，但农业过度使用合成氮肥是不可持续的。美国科学家研究了利用基因工程改进植物与固氮微生物之间互惠关系的可能性，发现通过模仿豆科植物和固氮细菌之间的相互作用，有助于作物从空气中获取氮。相关观点文章近日发表于《生物学趋势》。
“通过改造作物与固氮微生物的关系，为作物提供氮，是一个有前途的、能相对快速实现的解决方案，可以解决合成氮肥的高成本和可持续性问题。”论文通讯作者、威斯康星大学麦迪逊分校的Jean-Michel Ane说。
固氮微生物是一种土壤细菌和古细菌，可以自然地将大气中的氮“固定”为铵，而铵是植物可以利用的来源。一些固氮微生物与植物形成了互惠关系，植物为它们提供碳源和安全的低氧家园，作为回报，它们为植物提供氮。例如，豆科植物将固氮微生物安置在根瘤中。然而，这种共生关系只发生在少数植物中。如果更多的植物能够与固氮微生物形成联系，就能减少对合成氮肥的需求，但这种关系需要千万年才能自然进化。
如何提高非豆科作物的固氮能力？针对这一农业领域面临的挑战，科学家已经提出了几种不同的方法，包括对植物进行基因改造使其自身产生氮酶（固氮微生物正是利用这种酶将大气中的氮转化为铵），或者使非豆科植物产生根瘤。
此外，还有改造植物和固氮微生物，以促进它们形成互惠关系。该方法目的是将植物改造成更好的寄主，固氮微生物则被改造成在遇到改造植物分泌的分子时更容易释放固定的氮。
“由于自由的固氮微生物不会‘无私地’与植物分享它们固定的氮，因此需要操纵它们释放氮，以便植物能够获得。” Ane说。
这种改造方法依赖于植物和微生物之间的双向信号，这已经自然存在了。微生物具有化学感受器，可感知植物分泌到土壤中的代谢物，而植物能够感知微生物相关分子模式及其分泌的植物激素。这些信号通路可以通过基因工程进行调整，使转基因植物和微生物之间的交流更加明确。
研究者在文中还讨论了让这些工程关系更有效的方法。由于固氮是一个能量密集过程，因此固氮微生物能够调节固氮并仅在必要时产生铵将十分重要。“依靠植物的小分子信号，可以确保只有当改造菌株接近目标作物品种时，氮才会固定。”Ane说，“在这些系统中，细胞只在对作物最有利的时候进行能量密集的固氮行为。”
除了固氮，许多固氮微生物还能为植物提供额外的好处，包括促进生长和抗逆性。作者认为，未来的研究应该集中在“叠加”这些多重好处上。然而，由于这些过程均为能源密集型的，研究人员建议开发由几个物种组成的微生物群落，每种微生物都提供不同的好处，从而“将生产负荷分散到几个菌株之中”。
研究者承认，基因改造是一个复杂的问题，在农业中大规模使用转基因生物需要公众的接受。“科学家、育种者、种植者和消费者之间需要就这些新兴技术的风险和收益进行沟通。”Ane说。
此外，由于微生物很容易在物种内部和物种之间交换遗传物质，因此需要采取措施防止转基因物质传播到周围生态系统中的原生微生物中。对此，科学家已经开发了几种生物控制方法。例如，对微生物进行改造，使它们依靠非天然存在的分子生存，这意味着它们将被限制在改造植物田地里，或者给微生物安装“杀死开关”。研究者表示，这些控制措施被多层使用可能会更有效，因为每种措施都有局限性。他们还强调有必要在作物生长的可变田间条件下测试这些经过改造的植物—微生物相互作用。
“由于环境因素及其对植物、微生物及相互作用的影响的高度可变性，该技术的实际应用及其从实验室到田间的转变仍然具有挑战性。在高度控制的环境中进行的试验往往不能很好地转化到田间，我们建议在高度重复的田间试验中进行测试。”Ane和同事在文中写道。