title: Role and Regulation of ACC Deaminase Gene inSinorhizobium 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 (acdS) 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 ofacdSin symbiotic competitiveness and on the selective benefit it may confer to plant symbiotic rhizobia. Here we present a phylogenetic and functional analysis ofacdSorthologs in the rhizobium model-speciesSinorhizobium meliloti. Results showed thatacdSorthologs present inS. melilotipangenome have polyphyletic origin and likely spread through horizontal gene transfer, mediated by mobile genetic elements. WhenacdSortholog from AK83 strain was cloned and assayed inS. meliloti1021 (lackingacdS), no modulation of plant ethylene levels was detected, as well as no increase in fitness for nodule occupancy was found in theacdS-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,acdSwas shown to be negatively regulated by a putative leucine-responsive regulator (LrpL) located upstream toacdSsequence (acdR).acdSexpression was induced by root exudates of both legumes and non-leguminous plants. We conclude thatacdSinS. melilotiis not directly related to symbiotic interaction, but it could likely be involved in the rhizospheric colonization or in the endophytic behavior.
authors.affiliation: Department of Biology, University of Florence Sesto Fiorentino, Italy.
authors.forename: Alice
authors.initials: A
authors.lastname: Checcucci
authors.affiliation: Department of Agri-food Production and Environmental Science, University of Florence Florence, Italy.
authors.forename: Elisa
authors.initials: E
authors.lastname: Azzarello
authors.affiliation: Department of Biology, University of Florence Sesto Fiorentino, Italy.
authors.forename: Marco
authors.initials: M
authors.lastname: Bazzicalupo
authors.affiliation: Consiglio Nazionale delle Ricerche (CNR), Istituto per la Valorizzazione del Legno e delle Specie Arboree Florence, Italy.
authors.forename: Anna
authors.initials: A
authors.lastname: De Carlo
authors.affiliation: Consiglio Nazionale delle Ricerche (CNR), Istituto per la Valorizzazione del Legno e delle Specie Arboree Florence, Italy.
authors.forename: Giovanni
authors.initials: G
authors.lastname: Emiliani
authors.affiliation: Department of Agri-food Production and Environmental Science, University of Florence Florence, Italy.
authors.forename: Stefano
authors.initials: S
authors.lastname: Mancuso
authors.affiliation: Department of Agri-food Production and Environmental Science, University of Florence Florence, Italy.
authors.forename: Giulia
authors.initials: G
authors.lastname: Spini
authors.affiliation: Department of Agri-food Production and Environmental Science, University of Florence Florence, Italy.
authors.forename: Carlo
authors.initials: C
authors.lastname: Viti
authors.affiliation: Department of Biology, University of Florence Sesto Fiorentino, Italy.
authors.forename: Alessio
authors.initials: A
authors.lastname: Mengoni
keywords: ACC deaminase; Sinorhizobium meliloti; acdS; endophytic colonization; ethylene; nitrogen sources; rhizosphere