Mechanisms controlling the transition between nitrogen fixation and senescence in legume symbiotic nodules – STAYPINK

Nitrogen is limiting for plant growth, which explains the massive use of nitrogen fertilizers in agriculture. However, nitrogen fertilizers present a number of drawbacks, including a large waste of fossil energy for their production and use, and a strong pollution of water resources (nitrates) and atmosphere (greenhouse gas). The development of sustainable agriculture practices more respectful of our environment therefore needs to find alternatives to nitrogen fertilizers.
In contrast to other plants, legumes can grow without addition of exogenous nitrogen. Indeed, these plants are able develop a symbiotic relationship with soil bacteria called rhizobia, which possess the capacity to reduce atmospheric nitrogen into ammonium to the benefit of their host plants. Thanks to this unique property, legume plants can increase nitrogen content in the soil, which can be beneficial to other crops, like cereals, cultivated either in association or in alternance (rotation). In addition, legumes are also cultivated for their own properties and are the basis of human food and animal feed worldwide.
Biological nitrogen fixation by rhizobia takes place in specific root organs, called nodules. Like all plant organs, root nodules have a limited life-span, and eventually enter a senescence process characterized by the coordinated death of both bacteria and plant cells. This senescence process either results from nodule aging (developmental nodule senenescence) or can be triggered prematurely by adverse environmental conditions. In all cases, nodule senescence results in a decline of nitrogen fixation, characterized by a color change from pink to green (due to degradation of the plant leghemoglobin, a hallmark of active nitrogen fixation). Preventing or delaying nodule senescence (i.e. allowing nodules to stay pink) is therefore a promising strategy to prolong nitrogen fixation and thereby increase legume crop yields. However, although an impressive knowledge on the early stages of endosymbiosis has been gained in the past decades, the latest stages, notably nodule senescence, remain poorly understood. The main objective of this proposal is to describe the mechanisms acting in symbiotic nodules, both in plant and bacterial cells, to regulate the transition between an active nitrogen fixation and the onset of senescence (whether developmental or environmentally-induced).
The project will use an integrative approach carried out in parallel in the two symbionts. This objective will be achieved thanks to the specific structuration of the proposed consortium, involving a tight collaboration between plant biologists and microbiologists. In a first part of the project, a without a priori approach based on a combination of transcriptomics and genetics will allow the identification of key legume and rhizobial genes linked to nodule senescence, either developmental or environmentally induced. In a second part of the project, several biological actors/pathways already known or predicted to be involved in nodule senescence will be characterized in details: phytohormonal pathways, cysteine proteinases, nitric oxide and bacterial toxin/antitoxin systems. Connections between these actors/pathways, or with genes identified during the first part of the project, will also be looked for. Most of the studies will be conducted on the model symbiosis between Medicago truncatula and Sinorhizobium meliloti, but the ultimate goal is to provide targets to delay nodule senescence of agriculturally relevant legumes. Thus some of the strategies already known to delay nodule senescence in the model symbiosis will be transferred in a legume crop, pea, and its symbiotic partner, Rhizobium leguminosarum.