Absence of cospeciation between the uncultured Frankia microsymbionts and the disjunct actinorhizal Coriaria species.

Imen Nouioui,Faten Ghodhbane-Gtari,Maher Gtari,Philippe Normand,Abdellatif Boudabous,Maria P Fernandez

doi:10.1155/2014/924235

Abstract

Coriaria is an actinorhizal plant that forms root nodules in symbiosis with nitrogen-fixing actinobacteria of the genus Frankia. This symbiotic association has drawn interest because of the disjunct geographical distribution of Coriaria in four separate areas of the world and in the context of evolutionary relationships between host plants and their uncultured microsymbionts. The evolution of Frankia-Coriaria symbioses was examined from a phylogenetic viewpoint using multiple genetic markers in both bacteria and host-plant partners. Total DNA extracted from root nodules collected from five species: C. myrtifolia, C. arborea, C. nepalensis, C. japonica, and C. microphylla, growing in the Mediterranean area (Morocco and France), New Zealand, Pakistan, Japan, and Mexico, respectively, was used to amplify glnA gene (glutamine synthetase), dnaA gene (chromosome replication initiator), and the nif DK IGS (intergenic spacer between nifD and nifK genes) in Frankia and the matK gene (chloroplast-encoded maturase K) and the intergenic transcribed spacers (18S rRNA-ITS1-5.8S rRNA-ITS2-28S rRNA) in Coriaria species. Phylogenetic reconstruction indicated that the radiations of Frankia strains and Coriaria species are not congruent. The lack of cospeciation between the two symbiotic partners may be explained by host shift at high taxonomic rank together with wind dispersal and/or survival in nonhost rhizosphere.

Highlights

The genus Frankia comprises nitrogen-fixing actinobacteria that are able to induce perennial root nodules on woody dicotyledonous plants called actinorhizals [1]
DNAs from both Coriaria hosts and Frankia microsymbionts were characterized on the same root nodule tissues
Cospeciation has been postulated to have occurred in some Frankia actinorhizal host plants, in particular in the Casuarina-Frankia cluster 1b [18] but not in Alnus-infective and Elaeagnus-infective Frankia strains where many isolates able to fulfill Koch’s postulates have been obtained

Summary

Introduction

The genus Frankia comprises nitrogen-fixing actinobacteria that are able to induce perennial root nodules on woody dicotyledonous plants called actinorhizals [1]. The actinorhizal plant families belong to three dicotyledonous orders: Fagales (Betulaceae,Casuarinaceae, and Myricaceae), Rosales (Elaeagnaceae, Rhamnaceae, and Rosaceae), and Cucurbitales (Coriariaceae and Datiscaceae) [2]. Analysis of the molecular phylogeny of members of Frankia genus consistently identifies four main clusters regardless of the typing locus used [3]. Cluster 1 includes Frankia strains in association with Betulaceae, Myricaceae, and Casuarinaceae. Cluster 2 contains Frankia nodulating species from the Coriariaceae, Datiscaceae, and Rosaceae families as well as Ceanothus of the Rhamnaceae. Frankia strains in cluster 3 form effective root nodules on plants from members of the Myricaceae, Rhamnaceae, Elaeagnaceae, and Gymnostoma of the Casuarinaceae

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