Ochrobactrum sp. Pv2Z2 exhibits multiple traits of plant growth promotion, biodegradation and N-acyl-homoserine-lactone quorum sensing

Abstract

Rhizosphere bacteria play a vital role in plant growth, pathogen control, biodegradation and rhizosphere signaling. A motile, rod-shaped bacterium, Pv2Z2, isolated from the nodules of the common bean grown in Tanzanian soil was characterized using a polyphasic approach. The traits assessed included the production of indole-3-acetic acid and N-acyl homoserine lactone (AHL) molecules, solubilization of insoluble phosphate and zinc compounds and biodegradation of a number of toxic compounds. The 16S rRNA sequence of Pv2Z2 (EU399793) showed 99 % homology to Ochrobactrum anthropi isolates (Accession no. AJ867292, AJ867291, AJ867290) from soil samples of wheat root. Phylogenetic analysis showed relatedness to nodulating strain Ochrobactrum cytisi rather than to the clinical/pathogenic type strain of O. anthropi. Moreover, it showed unique fingerprints in the randomly amplified polymorphic DNA (RAPD) and two primers-RAPD assays which were different from those of the pathogenic type strain of O. anthropi. The bacterium produced 6.68 μg/mL-1 indoleacetic acid in the presence of tryptophan, released 25.7 μg/mL-1 phosphorus from inorganic tri-calcium phosphate in the Pikoviskaya’s medium and solubilized zinc sulphate and zinc oxide in the LG1 medium. The production of AHLs (e.g. 3O-C7-HSL, 3OH-C7-HSL) was detected with biosensor strains CV026 using reverse phase thin layer chromatography. The bacterium was able to grow in minimal salt medium supplemented with 100 mg/L each of phenol, 2-bromophenol, 2,4-diamino phenol hydrochloride, 3,4-dimethoxy benzyl alcohol and 4-methoxy benzyl alcohol. Phenol degradation was recorded up to a level of 94 % within 12 days. Inoculation of common bean plants resulted in a significant increase in plant height, fresh/dry weight and nitrogen uptake as compared to non-inoculated plants. The data suggest that the plant growth-promoting and biodegradation potential of this bacterium may be exploited on a large scale. The capacity to produce AHL molecules by members of the Ochrobactrum genus has not been previously reported and needs to be explored in detail.