Oxygen sensing and adaptation to hypoxia in Medicago truncatula nodules

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.

Nodules are characterized by internal microoxic conditions, which are necessary for nitrogen fixation by bacteroids. This is likely to require adaptation of plant cells to cope with such a condition. On the other hand, external suboptimal oxygen concentration (hypoxia) impairs nodule development and functioning in several legume species. With the present proposal, we plan to use Medicago truncatula and its symbiont Sinorhizobium meliloti to study the contribution of plant and bacterial oxygen sensing to nodule development and functioning, beyond the already established regulation of nitrogen fixation genes. In addition to their role in conditions of sufficient oxygen provision through the soil, we want to understand the mechanisms by which hypoxia impact the key regulator of nodule development in Medicago, NIN. Understanding these regulatory mechanisms will allow, in the future, the deployment of breeding strategies for improved nitrogen fixation in case of soil hypoxia. It will also support the engineering of nitrogen fixing symbiosis in crop species which do not establish such an interaction with rhizobia.