Introducing Biological Nitrogen Fixation to Plants

氮是作物的主要限制性养分，目前我们每年合成的肥料超过1亿吨。这彻底改变了作物产量，但付出了巨大的经济和环境代价（Good and Beatty, 2011）。然而，如果我们要在2050年养活地球上未来的90亿人口，我们将需要额外增加5100万吨的固定氮。即使这是可能的，其环境后果也将是如此严重的有害，以至于给“污染自己的巢”这句话带来了新的含义。因此，毫无疑问，我们需要解决我们固定氮的方式，要么通过生产氮基肥料，要么通过生物固氮（BNF）。这种水平的农业固定氮是不可持续的，工业固定氮（IFN）是使用Haber-Bosch工艺生产的。然而，某些微生物在环境温度和压力条件下具有固定大气氮的能力。原则上，可以将生物固定氮的能力引入植物，使它们能够在需要时直接固定大气中的氮。通过消除多余的氮流失到环境中，这将使环境几乎“不受影响”。该建议侧重于实施一系列实验，这将使我们能够确定是否有可能将生物固氮引入作物植物。IFN的生产和运输都需要大量的能源，而生物固定氮对全球氮循环的影响比IFN小得多。最近，人们对生物工程固氮谷物重新产生了兴趣，并且在过去二十年中，我们对固氮生物学的理解取得了重大进展。我们现在了解了完成BNF反应的氮酶的完整生物合成途径，并且我们能够在体外分析组装途径的每个步骤。最后，随着合成生物学的快速发展，我们现在可以合成新的基因、调节成分和途径，并将它们引入我们选择的生物体中。

Nitrogen is the main limiting nutrient for crops, and we currently synthesize over 100MMt of fertilizer annually. This revolutionized crop yield but at substantial economic and environmental costs (Good and Beatty, 2011). However, if we are to feed the future 9B people on the planet in 2050, we would need to increase the amount of fixed N by an additional 51MMt. Even if this was possible, the environmental consequences would be so significantly deleterious as to bring new meaning to the statement, “To foul your own nest”. Therefore there is no question that we need to address the ways in which we fix N, either through the production of N based fertilizers or by biological nitrogen fixation (BNF). This level of fixed N for agriculture is unsustainable, Industrially fixed N (IFN) is produced using the Haber-Bosch process. However, certain microorganisms have the ability to fix atmospheric nitrogen under ambient temperature and pressure conditions. In principle, the ability to biologically-fixed nitrogen could be introduced into plants, allowing them to fix atmospheric nitrogen directly and when needed. This would keep the environment almost “untouched”, by eliminating excess N loss into the environment. This proposal focuses on implementing a series of experiments that will allow us to determine whether it is possible to introduce biological N fixation into crop plants. IFN requires significant amounts of energy both to produce and ship, while biologically-fixed nitrogen influences the global nitrogen cycle substantially less than IFN. Recently there has been renewed interest in bioengineering N2 fixing cereals and significant advances in our understanding of the biology of N2 fixation have been made in the last two decades. We now understand the complete biosynthesis pathway for nitrogenase, the enzyme that performs the BNF reaction, and we are in a position to assay each step in the assembly pathway in vitro. Finally, with the rapid advances in synthetic biology, we can now synthesize novel genes, regulatory components and pathways and introduce them into organisms of our choice. N2-fixing cereal crops would benefit developed area farmers (ie Alberta) by lowering N fertilizer expenses while developing area farmers would have access to N2 fixing cereal plants. We want to reduce the cereal crop dependence on applied fertilizers by engineering crop plants that fix their own nitrogen to sustain growth and yield. This is a timely project that will build on decades of research on the N2 fixing bacteria/plant symbioses, the genetic and protein components of the bacterial N2 fixing nitrogenase complex, the full genomic sequences of cereal organelle genomes, and the recent development of techniques for the transient expression of multiple genes.