Role and Regulation of ACC Deaminase Gene in Sinorhizobium meliloti: Is It a Symbiotic, Rhizospheric or Endophytic Gene?

Abstract

Plant-associated bacteria exhibit a number of different strategies and specific genes allow bacteria to communicate and metabolically interact with plant tissues. Among the genes found in the genomes of plant-associated bacteria, the gene encoding the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase (<i>acdS</i>) is one of the most diffused. This gene is supposed to be involved in the cleaving of plant-produced ACC, the precursor of the plant stress-hormone ethylene toning down the plant response to infection. However, few reports are present on the actual role in rhizobia, one of the most investigated groups of plant-associated bacteria. In particular, still unclear is the origin and the role of <i>acdS</i> in symbiotic competitiveness and on the selective benefit it may confer to plant symbiotic rhizobia. Here we present a phylogenetic and functional analysis of <i>acdS</i> orthologs in the rhizobium model-species <i>Sinorhizobium meliloti</i>. Results showed that <i>acdS</i> orthologs present in <i>S. meliloti</i> pangenome have polyphyletic origin and likely spread through horizontal gene transfer, mediated by mobile genetic elements. When <i>acdS</i> ortholog from AK83 strain was cloned and assayed in <i>S. meliloti</i> 1021 (lacking <i>acdS</i>), no modulation of plant ethylene levels was detected, as well as no increase in fitness for nodule occupancy was found in the <i>acdS</i>-derivative strain compared to the parental one. Surprisingly, AcdS was shown to confer the ability to utilize formamide and some dipeptides as sole nitrogen source. Finally, <i>acdS</i> was shown to be negatively regulated by a putative leucine-responsive regulator (LrpL) located upstream to <i>acdS</i> sequence (<i>acdR</i>). <i>acdS</i> expression was induced by root exudates of both legumes and non-leguminous plants. We conclude that <i>acdS</i> in <i>S. meliloti</i> is not directly related to symbiotic interaction, but it could likely be involved in the rhizospheric colonization or in the endophytic behavior.