Bacteria-Based MnO Nanoparticles Alleviate Lead Toxicity in Tomato Seedling Through Improving Growth Attributes and Enhanced Gene Expression of Candidate Genes

Lead (Pb) is a well-recognized toxic trace element that is harmful for both plants and humans. In this study, the research work strived to fabricate nanoparticles (NPs) incorporating ‘features’ of Bacillus subtilis and manganese oxide (MnO). MnO-NPs were synthesized in the filtrate of B. subtilis and used for the seed priming of tomato, to mitigate lead (Pb) toxicity. Fourier transform infrared spectrum (FTIR) described the presence of carboxylic acid, alkenes, and alkyl halides on the surface of MnO-NPs, that serve as stabilizing and reducing agents. X-ray diffraction revealed MnONPs as 22 nm crystalline structures while scanning electron microscopy (SEM) displayed nano-flower structure of MnO-NPs. The energy dispersive X-ray (EDX) analysis determined the mass percentage of manganese (38.27%) and oxygen (36.91%). Before sowing under Pb stress conditions, tomato seeds were primed with three different concentrations (0.25 mg/mL, 2.5 mg/mL and 5.0 mg/mL) of MnO-NPs. In comparison to control, different concentrations of MnO-NPs displayed variable improvement in growth attributes of tomato seedlings including shoot and root lengths, plants fresh and dry weights, chlorophyll and carotenoid contents, relative water content, and proline and sugar accumulation. Nano-priming also decreased relative electrolyte leakage and the production of malondialdehyde and hydrogen peroxide. Seed priming positively influenced tomato seedling and enhanced the gene expression of chalcone synthase and phenylalanine ammonia lyase enzymes, while the expression of chlorophyllase was reduced. These findings suggested that MnO-NPs help tomato seedlings to resist the absorption and accumulation of Pb in the roots and leaves.