Genome-based reclassification of Azospirillum brasilense Sp245 as the type strain of Azospirillum baldaniorum sp. nov

Abstract

<i>Azospirillum</i> sp. strain Sp245^T, originally identified as belonging to <i>Azospirillum brasilense</i>, is recognized as a plant-growth-promoting rhizobacterium due to its ability to fix atmospheric nitrogen and to produce plant-beneficial compounds. <i>Azospirillum</i> sp. Sp245^T and other related strains were isolated from the root surfaces of different plants in Brazil. Cells are Gram-negative, curved or slightly curved rods, and motile with polar and lateral flagella. Their growth temperature varies between 20 to 38 °C and their carbon source utilization is similar to other <i>Azospirillum</i> species. A preliminary 16S rRNA sequence analysis showed that the new species is closely related to <i>A. brasilense</i> Sp7^T and <i>A. formosense</i> CC-Nfb-7^T. Housekeeping genes revealed that <i>Azospirillum</i> sp. Sp245^T, BR 12001 and Vi22 form a separate cluster from strain <i>A. formosense</i> CC-Nfb-7^T, and a group of strains closely related to <i>A. brasilense</i> Sp7^T. Overall genome relatedness index (OGRI) analyses estimated based on average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) between <i>Azospirillum</i> sp. Sp245^T and its close relatives to other <i>Azospirillum</i> species type strains, such as <i>A. brasilense</i> Sp7^T and <i>A. formosense</i> CC-Nfb-7^T <i>,</i> revealed values lower than the limit of species circumscription. Moreover, core-proteome phylogeny including 1079 common shared proteins showed the independent clusterization of <i>A. brasilense</i> Sp7^T, <i>A. formosense</i> CC-Nfb-7^T and <i>Azospirillum</i> sp. Sp245^T, a finding that was corroborated by the genome clustering of OGRI values and housekeeping phylogenies. The DNA G+C content of the cluster of Sp245^T was 68.4-68.6 %. Based on the phylogenetic, genomic, phenotypical and physiological analysis, we propose that strain Sp245^T together with the strains Vi22 and BR12001 represent a novel species of the genus <i>Azospirillum</i>, for which the name <i>Azospirillum baldaniorum</i> sp. nov. is proposed. The type strain is Sp245^T (=BR 11005^T=IBPPM 219^T) (GCF_007827915.1, GCF_000237365.1, and GCF_003119195.2).