ZnO nanoparticles and root colonization by a beneficial pseudomonad influence essential metal responses in bean (Phaseolus vulgaris).

Nanoparticles (NPs) incorporated into commercial products are reactive on plants. Here, the influence of a root-associated bacterium, Pseudomonas chlororaphis O6 (PcO6) on the responses of bean (Phaseolus vulgaris) to commercial ZnO nanoparticles (NPs) was examined. ZnO NPs (250-1000 mg Zn/kg) significantly (p = 0.05) impacted root elongation after 7 days; only at 1000 mg/kg was shoot growth significantly inhibited. Zn solubilized from ZnO NPs correlated with root growth inhibition (r(2 )= 0.8709); solubility of Fe (r(2 )= 0.916) and Mn (r(2 )= 0.997), and shoot accumulation of Zn (r(2 )= 0.9095), Fe (r(2 )= 0.9422) and Mn (r(2 )= 0.789). Root ferric reductase activity diminished 31% in NP-exposed plants. Amendments with Zn ions at 6 mg/kg, corresponding to Zn solubilized from the NPs, did not replicate the responses, suggesting a nano-specific contribution of the ZnO. Neither NPs (500 mg Zn/kg) nor Zn ions affected root colonization by PcO6. Siderophore production by PcO6 increased 17% by exposure to NPs and 11% with Zn ions (18 mg/kg). PcO6 restored plant ferric reduction under NP exposure, but decreased uptake of Zn and Fe, 58 and 18%, respectively, suggesting soil bacteria could reduce plant accumulation of metals under toxic exposure levels, while negatively affecting uptake of essential elements. Collectively, these findings demonstrated that growth and balance of essential metals in bean exposed to ZnO NPs were influenced by the NPs and bacterial colonization of NP-exposed roots, indicating subtle effects of NPs in plant nutrition.