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Abstract

The symbiotic interaction between the bacterial species Sinorhizobium meliloti and the leguminous plant Medicago truncatula leads to the formation of a new root organ, the nodule, where differentiated bacteria convert the atmospheric nitrogen (N₂) in ammonium (NH₄⁺). To better understand the intracellular lifestyle adaptation of bacteria, we study the role of the S. meliloti VapBC Toxin-Antitoxin (TA) systems during symbiosis. These TA systems are composed of a stable toxin (VapC) and a labile antitoxin (VapB) inactivating the toxin. In response to a signal, antitoxin degradation by bacterial proteases releases the toxin, allowing a post-transcriptional regulation due to its RNase site-specific activity. Such modules were described in human pathogenic interaction as playing a role in intracellular survey and stress adaptation (acidic pH, microoxy).

A characterization of the 11 putative VapBC systems of S. meliloti encoded by the bacterial chromosome has been undertaken to analyze their symbiotic role. The functional validation of these systems allowed to define the toxic activity of all VapC proteins when sur-expressed in Escherichia coli; demonstration of the antidote effect by VapB has been initiated.

Infection of M. truncatula with bacterial mutants deficient in the VapC toxin component of VapBC modules, has shown that 4 vapC toxin mutants among 11 have different altered symbiotic phenotypes: defect in infection, early or delayed nodule senescence. To determine precisely the key steps of the symbiosis impacted by the different toxin deficient mutants, molecular and cellular approaches have been realized.

To further correlate the toxin activity to a defined biological function during symbiotic interaction, the consensus site of RNA cleavage targeted by VapC toxins will be identified. Indeed, due to the different phenotypes observed, some modules seem to promote fundamental biological functions involved in key steps of symbiosis. At the molecular level, we have already demonstrated that the VapC7 protein of the VapBC7 module, important for plant host viability, acted as a ribonuclease.

These studies will provide knowledge about the bacterial partner adaptation in planta throughout the symbiotic interaction. In agronomy, this could offer the opportunity to improve their symbiotic N₂ fixation capacity, notably under adverse environmental conditions.

Keywords: Medicago, Rhizobium, VapBC, Toxin-Antitoxin, Symbiosis