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

Titanium dioxide nanoparticles (TiO 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 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 ) of TiO NPs on cell viability, morphology, and plant growth promoting (PGP) traits of rhizobia isolated from mung bean root nodule. Two types of TiO NPs, that is, mixture of anatase and rutile, and anatase alone were used in the study. These TiO NPs were supplemented in broth along with a multifunctional isolate (Bradyrhizobium sp.) and two reference cultures. The exposure of TiO (anatase+rutile) NPs at low concentrations (less than 20.0 mg L ) 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 (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 particles. The study revealed higher reactive oxygen species (ROS) generation by the TiO (anatase) NPs as compared with TiO (anatase+rutile) NPs. Exposure to TiO NPs also altered the morphology of rhizobial cells. The findings suggest that TiO NPs could act as promoters of PGP traits of PGP bacteria when applied at appropriate lower doses.