Exploring the genetic basis of symbiotic performance in rhizobia: the acdS gene in Sinorhizobium meliloti

Mutualistic cooperation is one of the most fascinating issue in evolutionary biology and legume-rhizobia symbiosis represent a model example of cross-kingdom mutualism. However, not all strains of the same rhizobial species have the same mutualistic phenotype, that is they show different symbiotic performances and few studies only are addressing the genetic basis of different performances. 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. However, the role of acdS in the mutualistic behavior of strains of S. meliloti is still obscure as well as the selective benefit conferred. To clarify this issue, an ortholog of acdS was cloned and expressed in the model strain S. meliloti Rm1021. The recombinant strain was then tested to evaluate its symbiotic phenotype with respect to competition for root nodule occupancy and modulation of ethylene production by the host plant. Data showed that acdS may provide a selective advantage, but a tradeoff related to the decreased ethylene levels may reduce also the overall fitness of plants, then providing a balancing selection effect at the rhizobial population level.