CE20 - Biologie des animaux, des organismes photosynthétiques et des microorganismes

Sinorhizobium cell cycle regulation for efficient legume symbiosis – Sincerely

Submission summary

Sincerely is a fundamental research project that is focused on the newly discovered essential cell cycle regulator FcrX in the model symbiotic bacterium Sinorhizobium meliloti. Our unpublished data shows that FcrX controls both CtrA, the master cell cycle regulator, as well as the tubulin-like protein FtsZ, which is central for the assembly of the cytokinesis machinery, the divisome. The project will decipher the molecular mechanisms underlying FcrX action on each process and moreover, it will explore how FcrX engineering can be exploited to improve the nitrogen-fixing symbiosis of S. meliloti with plants of the Medicago genus. In nature, nitrogen is often a limiting factor for plant growth, causing the massive use of chemical nitrogen fertilizers in agriculture, with tremendous ecological drawbacks such as water eutrophication and soil erosion. Moreover, industrial nitrogen fertilizers are produced through the Haber-Bosch process, a highly energy-demanding process representing ca. 1% of the world fossil energy consumption. Plants belonging to the legume family have evolved a symbiosis with bacteria able to fix atmospheric nitrogen to circumvent nitrogen limitation. However, the nitrogen fixing capacity of the bacterial partner is a critical aspect of the interaction that directly constraints the output of the symbiosis and the yield of the host plant. In many cases, the endogenous population of rhizobia residing in a field comprises cheaters and bacteria that have a low potential for nitrogen fixation. In modern agriculture, the use of commercial “elite” rhizobial inoculants has become a key step to ensure a high yield and a high protein content in the seeds of pulses. During the symbiotic interaction between plants of the Medicago genus and Sinorhizobium meliloti, plants form symbiotic organs called nodules where bacteria differentiate into nitrogen-fixing bacteroids, involving a rewiring of the bacterial cell cycle that becomes directed towards endoreduplication (repeated genome replications without intervening cell division) and a strong cell enlargement (up to ten times their original size). This endosymbiont differentiation is associated with a downshift of the CtrA master regulator and the absence of divisome assembly. This process is accompanied with an increase in symbiotic efficiency. Thus, the bacterial cell cycle regulation is at the heart of the symbiosis and determines the symbiotic performance of rhizobia. The present proposal aims at understanding the molecular mechanisms by which FcrX regulates the cell cycle and controls its coordination with cell division, by combining the expertise of two research groups having complementary skills in biochemistry, structural biology, microbiology, plant biology, cellular biology and genetics. The newly acquired knowledge of the architecture of the cell cycle regulatory network will inform us on strategies to engineer rhizobial strains with improved symbiotic capacities, ultimately leading to the development of next generation inoculants for agriculture that can improve legume yields.

Project coordinator

Monsieur Peter Mergaert (Institut de Biologie Intégrative de la Cellule)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.


I2BC Institut de Biologie Intégrative de la Cellule
IP-UMS IP-Unité de Microbiologie structurale

Help of the ANR 449,747 euros
Beginning and duration of the scientific project: December 2021 - 42 Months

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