Rhizobia are Gram negative bacteria, living in soil and able to establish the nitrogen fixing symbiosis with leguminous plants [[i]]. During this mutual interaction, bacteria bring ammonia directly transformed from atmospheric N2. In turn the plant provides hydrocarbons and develops nodules to host the bacteria. During the early stages of establishing symbiosis, a molecular dialogue takes place. The plant exudes flavonoids and the rhizobia respond by secreting Nod Factors [[ii]].
Nod factors play a major role for partner recognition and trigger the organogenesis leading to the nodule. When the bacteria progress within the infection threads and invade the nodule, their surface polysaccharides are essential [[iii]]. EPS has been demonstrated as being essential for nodulation of the host plants forming indeterminate nodules.
Sinorhizobium meliloti in interaction with Medicago sativa forms indeterminate nodules. These rhizobia produce two types of exopolysaccharides (I and II). The EPSI exhibit a crucial symbiotic activity. These are constituted by many repetitions of a 8-mer of hexose-subunits put together in a linear/branched form. This backbone is decorated with O-acetyl, -succinyl and -pyruvyl groups [[iv]]. EPS are known to be pro-infectious in the early steps of the symbiotic interaction. However, recently, genes implied in the negative regulation of infection have been discovered and have been suspected to be implied in EPS biosynthesis.
We will present here -based on ESI-MS/MS and on HR-MAS NMR analyses- the composition, the degree of decoration and the yield of EPS produced by different Sm1021 strains affected or not in the negative regulation process. These data, allow highlighting the later role of EPS for the negative regulation.
[i] Spaink HP (2000). Ann. Rev. Microbiol. 54:257.
[ii] Dénarié J et al. (1992). Annu. Rev. Microbiol. 46: 497.
[iii] Fraysse N et al.(2003). Eur. J. Biol. Chem. 270:1365.
[iv] Wang LX et al.(1999) J. Bacteriol 181: 6788.
