Peptidoglycan remodelling during Rhizobium leguminosarum life cycle, from the rhizosphere to the formation of bacteroids

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Optimal plant growth is achieved through complex interactions with soil microorganisms including bacteria, fungi, single celled animals, and nematodes. Plants belonging to the family of Fabaceae (legumes such as peas, beans, soybeans, or lentils) rely on a mutually beneficial interaction (symbiosis) with a group of bacteria called rhizobia to utilise atmospheric nitrogen. This basic nutrient is present as a gas that makes up 78% of Earth's atmosphere but cannot be used directly by plants for the synthesis of their cellular constituents. Assimilation of nitrogen by rhizobia is therefore a critical process for plant growth, which in turn supports animal life, with a tremendous economic and environmental impact. The use of artificial nitrogen fertilisers has a major environmental cost; it accounts for nearly 50% of the fossil fuel used in agriculture and leads to eutrophication of lakes, rivers and drinking water, toxic algal blooms, and biodiversity loss. The plant-rhizobia symbiosis involves a complex molecular dialog that leads to the bacterial invasion of root tissues and the formation of root nodules. Inside these nodules, rhizobia form specialised cells called bacteroids able to transform atmospheric nitrogen into ammonia used by plants. We have identified a family of enzymes that are differentially expressed during the life cycle of the model symbiont Rhizobium leguminosarum and essential for the viability of bacteria in the nodule. The purpose of this research is to study this group of important proteins to understand how the remodelling of the bacterial cell envelope contributes to symbiosis in peas and beans. This work is an important step to engineer rhizobia for sustainable agriculture.