Genome characteristics of facultatively symbiotic Frankia sp. strains reflect host range and host plant biogeography

Abstract

Soil bacteria that also form mutualistic symbioses in plants encounter two major levels of selection. One occurs during adaptation to and survival in soil, and the other occurs in concert with host plant speciation and adaptation. Actinobacteria from the genus Frankia are facultative symbionts that form N 2 -fixing root nodules on diverse and globally distributed angiosperms in the "actinorhizal" symbioses. Three closely related clades of Frankia sp. strains are recognized; members of each clade infect a subset of plants from among eight angiosperm families. We sequenced the genomes from three strains; their sizes varied from 5.43 Mbp for a narrow host range strain (Frankia sp. strain HFPCcI3) to 7.50 Mbp for a medium host range strain (Frankia alni strain ACN14a) to 9.04 Mbp for a broad host range strain (Frankia sp. strain EAN1pec.) This size divergence is the largest yet reported for such closely related soil bacteria (97.8%-98

SymRK defines a common genetic basis for plant root endosymbioses with arbuscular mycorrhiza fungi, rhizobia, and Frankia bacteria

“…Part of this signaling cascade is also involved in transduction of the symbiotic signal in AM symbioses (5). This gave rise to the hypothesis that the evolutionarily recent legume-rhizobia symbiosis reuses some of the molecular mechanisms of the more ancient AM symbiosis (16 indicate only that the Frankia symbiotic signal is likely chemically different from NFs (10)(11)(12). Here, we report the isolation and characterization of CgSymRK, a SymRK/DMI2 homolog from the actinorhizal tree C. glauca.…”

Plant nodulation inducers enhance horizontal gene transfer of Azorhizobium caulinodans symbiosis island

“…Interestingly, the Sesbania symbionts S. saheli bv sesbaniae and S. teranga bv sesbaniae bear a symbiotic plasmid and harbor nodulation genes phylogenetically unrelated to Azorhizobium nod genes (tree in ref. 38), suggesting they have a completely different origin. This finding is surprising, because these Sesbania azorhizobia and sinorhizobia have the same geographical origin and host, and suggests that other factors determine the success of the transfer.…”

Transcriptomics of Actinorhizal Symbioses Reveals Homologs of the Whole Common Symbiotic Signaling Cascade

“…Concerning the microbe, little is known about the symbiotic signals and their perception during actinorhizal symbiosis. Genome sequencing of Frankia revealed the absence of the canonical nod gene described in rhizobia (Normand et al, 2007a). Nevertheless, the Frankia root hair deformation signal shares functional similarities with the rhizobial Nod and fungal Myc factors, such as thermoresistance, a size below 1,400 D, sensitivity to some enzymes (Ceremonie, 1998), and hints that N-acetyl-D-glucosamine may be present (Ceremonie et al, 1999).…”

“…Frankia has not been genetically transformed despite repeated attempts , but the genomes of three Frankia strains were recently sequenced (Normand et al, 2007a). Frankia genome analysis revealed the absence of canonical nod genes.…”