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RESEARCH ARTICLEOpen AccessGenome-wide transcriptional responses of twometal-tolerant symbiotic Mesorhizobium isolatesto Zinc and Cadmium exposureGéraldine Maynaud1, Brigitte Brunel2, Damien Mornico3, Maxime Durot3, Dany Severac4, Emeric Dubois4,Elisabeth Navarro5, Jean-Claude Cleyet-Marel1and Antoine Le Quéré5*AbstractBackground: Mesorhizobium metallidurans STM 2683Tand Mesorhizobium sp. strain STM 4661 were isolated fromnodules of the metallicolous legume Anthyllis vulneraria from distant mining spoils. They tolerate unusually highZinc and Cadmium concentrations as compared to other mesorhizobia. This work aims to study the geneexpression profiles associated with Zinc or Cadmium exposure and to identify genes involved in metal tolerance inthese two metallicolous Mesorhizobium strains of interest for mine phytostabilization purposes.Results: The draft genomes of the two Mezorhizobium strains were sequenced and used to map RNAseq dataobtained after Zinc or Cadmium stresses. Comparative genomics and transcriptomics allowed the rapid discovery ofmetal-specific or/and strain-specific genes. Respectively 1.05% (72/6,844) and 0.97% (68/6,994) predicted CodingDNA Sequences (CDS) for STM 2683 and STM 4661 were significantly differentially expressed upon metal exposure.Among these, a significant number of CDS involved in transport (13/72 and 13/68 for STM 2683 and STM 4661,respectively) and sequestration (15/72 and 16/68 for STM 2683 and STM 4661, respectively) were identified. ThirteenCDS presented homologs in both strains and were differentially regulated by Zinc and/or Cadmium. For instance,several PIB-type ATPases and genes likely to participate in metal sequestration were identified. Among theconserved CDS that showed differential regulation in the two isolates, we also found znuABC homologs encodingfor a high affinity ABC-type Zinc import system probably involved in Zinc homeostasis. Additionally, global analysessuggested that both metals also repressed significantly the translational machinery.Conclusions: The comparative RNAseq-based approach revealed a relatively low number of genes significantlyregulated in the two Mesorhizobium strains. Very few of them were involved in the non-specific metal response,indicating that the approach was well suited for identifying genes that specifically respond to Zinc and Cadmium.Among significantly up-regulated genes, several encode metal efflux and sequestration systems which can beconsidered as the most widely represented mechanisms of rhizobial metal tolerance. Downstream functionalstudies will increase successful phytostabilization strategies by selecting appropriate metallicolous rhizobial partners.Keywords: Mesorhizobium, Metal tolerant rhizobia, Zinc, Cadmium, Comparative genomics and transcriptomics,RNAseq, znuABC* Correspondence: antoine.le-quere@ird.fr5IRD, Laboratoire des Symbioses Tropicales et Méditerranéennes UMR113,IRD/INRA/CIRAD/Montpellier SupAgro/Université Montpellier II, MontpellierF34000, FranceFull list of author information is available at the end of the article© 2013 Maynaud et al.; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the CreativeCommons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, andreproduction in any medium, provided the original work is properly cited.Maynaud et al. BMC Genomics 2013, 14:292http://www.biomedcentral.com/1471-2164/14/292