Elucidating the effect of TiO₂ nanoparticles on mung bean rhizobia via in vitro assay: Influence on growth, morphology, and plant growth promoting traits

Titanium dioxide nanoparticles (TiO 2 NPs) are among the most commonly used nanomaterials and are most likely to end up in soil. Therefore, it is pertinent to study the interaction of TiO 2 NPs with soil microorganisms. The present in vitro broth study evaluates the impacts of low‐dose treatments (0, 1.0, 5.0, 10.0, 20.0, and 40.0 mg L −1 ) of TiO 2 NPs on cell viability, morphology, and plant growth promoting (PGP) traits of rhizobia isolated from mung bean root nodule. Two types of TiO 2 NPs, that is, mixture of anatase and rutile, and anatase alone were used in the study. These TiO 2 NPs were supplemented in broth along with a multifunctional isolate ( Bradyrhizobium sp.) and two reference cultures. The exposure of TiO 2 (anatase+rutile) NPs at low concentrations (less than 20.0 mg L −1 ) enhanced the cell growth, and total soluble protein content, besides improving the phosphate solubilization, Indole‐3‐acetic acid (IAA) production, siderophore, and gibberellic acid production. The TiO 2 (anatase) NPs enhanced exopolysaccharide (EPS) production by the test rhizobial cultures. The radical scavenging assay was performed to reveal the mode of action of the nano‐TiO 2 particles. The study revealed higher reactive oxygen species (ROS) generation by the TiO 2 (anatase) NPs as compared with TiO 2 (anatase+rutile) NPs. Exposure to TiO 2 NPs also altered the morphology of rhizobial cells. The findings suggest that TiO 2 NPs could act as promoters of PGP traits of PGP bacteria when applied at appropriate lower doses.