New unexpected functions for ACC deaminase genes in Sinorhizobium meliloti

Mutualistic cooperation is one of the most fascinating issue in evolutionary biology and legume-rhizobia symbiosis represent models of cross-kingdom mutualism. However, not all strains of the same rhizobial species have the same mutualistic phenotype, specifically they show different symbiotic performances and up to now only a few studies addressed the genetic basis of these differences (Galardini et al. 2011). In this context one of the most intriguing gene is that encoding the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase (acdS) present in the dispensable genome of the model species Sinorhizobium meliloti. AcdS is supposed to be involved in the sequestering and cleaving of plant-produced ACC, the precursor of the plant stress hormone ethylene (Glick 2005). However, the function of acdS in symbiotic bacteria has not been fully clarified, especially in relation to the mutualistic behavior of rhizobial strains. Indeed,few data are available on the effect of such gene in symbiotic competitiveness (Ma et al. 2004) and then on the selective benefit it may confer. To clarify this issue, an extensive phylogenetic and comparative genomic analysis of acdS orthologs was performed in genomes of S. meliloti strains and functional studies were carried out by expressing acdS from natural strains, in the model strain S. meliloti Rm1021, which lacks acdS gene. Then, the symbiotic and endophytic phenotypes of recombinant vs the parental strain were evaluated with respect to competition for root nodule occupancy, plant colonization and modulation of ethylene production by the host plant. Additionally, phenotype microarray experiments were performed to investigate the metabolic function carried out by AcdS. Data showed that the acdS orthologs present in different S. meliloti strains are polyphyletic and may indeed derive from different alphaproteobacteria representatives. No increase in fitness for nodule occupancy was found in the Rm1021 strain expressing acdS compared to the parental one, as well as faint effects on the modulation of plant ethylene levels were observed. Surprisingly, AcdS was shown to confer the ability to utilize formamide and some dipeptites as sole nitrogen source. We conclude that acdS in S. meliloti could be more related to the exploitation of unusual nitrogen sources, in connection with rhizospheric colonization or endophyitic life-style (Pini et al. 2012) than to the symbiotic interaction. 1) Galardini et al. 2011. BMC Genomics 12: 253. 2) B.R. Glick 2005 FEMS Microbiol. Lett. 251: 1–7. 3) Ma et al. 2004 Appl. Environ. Microbiol. 70: 5891-5897. 4) Pini et al. 2012 BMC Microbiol. 12: 78.