Acinetobacter strains IH9 and OCI1, two rhizospheric phosphate solubilizing isolates able to promote plant growth, constitute a new genomovar of Acinetobacter calcoaceticus

Abstract

During a screening of phosphate solubilizing bacteria (PSB) in agricultural soils, two strains, IH9 and OCI1, were isolated from the rhizosphere of grasses in Spain, and they showed a high ability to solubilize phosphate in vitro. Inoculation experiments in chickpea and barley were conducted with both strains and the results demonstrated their ability to promote plant growth. The 16S rRNA gene sequences of these strains were nearly identical to each other and to those of Acinetobacter calcoaceticus DSM 30006 T, as well as the strain CIP 70.29 representing genomospecies 3. Their phenotypic characteristics also coincided with those of strains forming the A. calcoaceticus–baumannii complex. They differed from A. calcoaceticus in the utilization of l-tartrate as a carbon source and from genomospecies 3 in the use of d-asparagine as a carbon source. The 16S–23S intergenic spacer (ITS) sequences of the two isolates showed nearly 98% identities to those of A. calcoaceticus, confirming that they belong to this phylogenetic group. However, the isolates appeared as a separate branch from the A. calcoaceticus sequences, indicating their molecular separation from other A. calcoaceticus strains. The analysis of three housekeeping genes, recA, rpoD and gyrB, confirmed that IH9 and OCI1 form a distinct lineage within A. calcoaceticus. These results were congruent with those from DNA–DNA hybridization, indicating that strains IH9 and OCI1 constitute a new genomovar for which we propose the name A. calcoaceticus genomovar rhizosphaerae.