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

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.

Symbiotic interactions between plants and microbes are critical drivers of plant productivity and agricultural yield. The symbiotic relationship between legumes and rhizobium leads to the transformation of atmospheric nitrogen into ammonia. The utilization of this nutrient by plants promotes growth and reduces the use of artificial fertiliser. The interaction between rhizobium and legume hosts is extremely complex and involves a crosstalk between the two partners from the rhizosphere to the final stages of the nodulation process. Transcriptomics and high throughput mutagenesis studies have provided unvaluable information about the genes that contribute to symbiosis during the rhizobium-pea interaction. They revealed that a group of enzymes contributing to bacterial cell envelope remodelling (called L,D-transpeptidases; Ldts) are differentially regulated during the interaction with the legume host, with one gene being essential for bacterial viability in the early stages of nodulation. Ldts represent a diverse family of enzymes playing a pivotal role in metabolism of peptidoglycan, the major component of the bacterial cell wall. In many Gram-negative bacteria, Ldts covalently attach beta barrel proteins to peptidoglycan, thereby tethering the outer membrane and maintaining cell envelope integrity. This process plays a role in the resistance to abiotic stresses such as heat, osmotic change or acidic conditions and has been proposed to be involved in dormancy. We will leverage recent breakthroughs in the methods available to study peptidoglycan structure and remodelling during symbiosis with the aim to investigate the contribution of Ldts to the different steps of the Rhizobium symbiosis in peas and beans (which form different types of nodules). We will also explore the structure/function relationship of Ldts which represent a family of enzymes with variety of activities. This project will lay the foundation to engineer Rhizobium strains for sustainable agriculture