Oxygen sensing and adaptation to hypoxia in Medicago truncatula nodules

除了碳和氧，植物还需要从根系生长的土壤中提取养分。氮和磷是作物生长和产量的主要限制元素，因此农民向土壤中补充肥料。然而，土壤施肥引起可持续性问题，因为植物可利用的氮形态、硝酸盐和氨需要来自不可再生资源的高能量投入才能合成。此外，肥料受到雨水的淋滤，当它们到达水体时，对人类的消费和富营养化构成威胁。土壤施肥的另一种选择，或至少是一种整合，是促进与能够进行固氮的微生物的共生相互作用，以换取植物的碳。来自豆科植物家族的几个物种可以建立这样的相互作用。因此，属于这一类的作物不仅在氮素不足的情况下也能茁壮成长，而且还被轮作用来补充土壤中的氮。固氮细菌需要高能量，但也需要非常低的含氧量，因为这种分子对进行这种反应的酶是有毒的。为了适应这些需要，豆科植物的根形成了一个特殊的器官，称为结节，以容纳细菌。在根瘤中，自由氧的扩散受到严重限制，并被引导到产生能量的细胞器，防止对固氮酶的破坏。我们想了解这个器官中的细胞如何应对低氧条件，以及它们如何利用氧气可用性信息与固氮细菌建立共生关系。最后，我们计划研究当环境中氧气缺乏时，如土壤内涝时，结核是如何感知和响应的。该项目产生的知识将有助于培育新的豆科植物品种，即使在土壤低氧的情况下也能支持固氮。我们的研究结果也将支持其他研究人员设计策略，在其他植物物种中设计这一过程。

Apart from carbon and oxygen, plants need to extract their nutrients from the soil where their roots grow. Nitrogen and phosphorous are the main limiting elements for crop growth and yield and thus farmers replenish the soil with fertilizers. However, soil fertilization causes sustainability problems, since the nitrogen forms usable by plants, nitrate and ammonia, require high energy inputs from non-renewable resources to be synthesized. Moreover, fertilisers are subjected to leaching by rainwater and when they reach waterbodies they pose threats for human consumption and eutrophication. An alternative, or at least an integration, to soil fertilization is the promotion of symbiotic interactions with microorganisms able to carry out nitrogen fixation in exchange for carbon from the plant. Several species from the plant family of legumes can establish such an interaction. Thus, crops belonging to this group not only thrive even when nitrogen is limiting, but they are also used in rotations to replenish soil with nitrogen. The nitrogen fixing bacteria require high energy but also very low oxygen levels, since this molecule is poisonous for the enzyme that carries out this reaction. To accommodate these needs, legume roots form a specific organ, called nodule, to host the bacteria. In the nodule, the diffusion of free oxygen is severely limited, and channelled towards energy producing organelles, preventing damage to the nitrogen-fixing enzymes. We want to understand how the cells in this organ cope with low oxygen conditions, and how they use the information about oxygen availability to establish symbiosis with nitrogen-fixing bacteria. Finally, we plan to study how the nodule senses and responds when oxygen is poorly available in the environment, as it happens in case of soil waterlogging. The knowledge generated with this project will support the breeding of new legume varieties, able to support nitrogen fixation even in case of low oxygen in soil. Our results will also support other researchers to devise strategies to engineer this process in other plant species.