Diazotrophic bacteria of the genus Azospirillum colonize roots of important cereals and grasses, and promote plant growth by several mechanisms, notably phytohormone synthesis. The genomes of several Azospirillum strains belonging to different species, isolated from various host plants and locations, were recently sequenced and published. In this review, the most relevant features revealed by comparative genome analyses are highlighted. Genome size ranges from 6.5 Mbp to 7.6 Mbp and all strains are composed of multiple circular replicons, the biggest replicon (ca. 3 Mpb) having a typical chromosomal OriC replication origin and all the other replicons (chromids or plasmids) having repABC/parAB plasmid‐type replication systems. Alignments of Azospirillum genomes revealed poor synteny, indicating the occurrence of numerous genomic rearrangements.
The genus Azospirillum belongs to the family Rhodospirillaceae whose nearly all known other representatives live in aquatic habitats and display genome sizes ≤ 5 Mpb. While most Azospirillum housekeeping genes have orthologs in the genomes of close aquatic relatives, this lineage has obtained nearly half of its genome from terrestrial organisms and plant‐associated bacteria, notably from Rhizobiales and Burkholderiales. The majority of genes encoding functions critical for association with plants are among the horizontally transferred genes.
The Azospirillum core genome was established with the genomes of strains A. brasilense Sp245, A. brasilense CBG497, A. lipoferum 4B, and Azospirillum sp. B510. It consists of 2,328 proteins, mainly chromosomally encoded and of ancestral origin (74%), and includes nitrogen‐fixation genes shared with some aquatic relatives. Many genes involved in plant growth promotion and rhizosphere competence are restricted to a particular strain and/or species, strongly suggesting niche‐specific adaptation.