Identification of N2-fixing plant- and fungus-associated Azoarcus species by PCR-based genomic fingerprints

Abstract

Most species of the diazotrophic Proteobacteria Azoarcus spp. occur in association with grass roots, while A. tolulyticus and A. evansii are soil bacteria not associated with a plant host. To facilitate species identification and strain comparison, we developed a protocol for PCR- generated genomic fingerprints, using an automated sequencer for fragment analysis. Commonly used primers targeted to PEP (repetitive extragenic palindromic) and ERIC (enterobacterial repetitive intergenic consensus) sequence elements failed to amplify fragments from the two species tested. In contrast, the BOX-PCR assay (targeted to repetitive intergenic sequence elements of Streptococcus) yielded species-specific genomic fingerprints with some strain-specific differences. PCR profiles of an additional PCR assay using primers targeted to tRNA genes (tDNA-PCR, for tRNA(IIe)) were more discriminative, allowing differentiation at species-specific (for two species) or infraspecies-specific level. Our protocol of several consecutive PCR assays consisted of 16S ribosomal DNA (rDNA)-targeted, genus-specific PCR followed by BOX- and tDNA-PCR; it enabled us to assign new diazotrophic isolates originating from fungal resting stages (sclerotia) to known species of Azoarcus. The assignment was confirmed by phylogenetic analysis of 16S rDNA sequences. Additionally, the phylogenetic distances and the lack of monophyly suggested emendment of the genus Azoarcus: the unnamed species Azoarcus groups C and D and a new group (E) of Azoarcus, which was detected in association with fungi, are likely to have the taxonomic rank of three different genera. According to its small subunit rRNA, the sclerotium- forming basidiomycete was related to the Ustilagomycetes, facultatively biotrophic parasites of plants. Since they occurred in a field which was under cultivation with rice and wheat, these fungi might serve as a niche for survival for Azoarcus in the soil and as a source for reinfection of plants.