Abstract
Chromium (Cr) is a toxic heavy metal whose high concentration in soil badly affects plant growth, photosynthesis, and overall yield. Metal-derived nano-particles and metal-resistant bacteria can strengthen the plant defense system against different abiotic stresses; however, little is known about the use of nanoparticles in conjunction with bacteria. This study examined the combined effect of Fe nanoparticles (Fe NPs) and a chromium-resistant bacterium Staphylococcus aureus, on rice plants grown on chromium saturated medium. Chromium stress reduced rice growth, biomass, and chlorophyll contents by causing oxidative damage leading to overproduction of electrolyte leakage, hydrogen peroxide, and malondialdehyde. Fe NPs significantly improved plant growth, biomass, yield, and photosynthetic activity by enhancing the chlorophyll contents and alleviating oxidative damage. Application of Fe NPs also reduced the uptake and accumulation of Cr in the plants by increasing the bioavailability of micronutrients to the plant. The Fe NPs decreased oxidative damage and enhanced the enzymatic and non-enzymatic activity in the plant to withstand Cr stress compared to the plants without Fe NPs treatments. The inoculation of rice plants with the chromium-resistant bacteria S. aureus further enhanced the positive impact of Fe NPs by transforming the toxic form of chromium (Cr6+) into a less toxic form of chromium (Cr3+). The bacterial inoculation reduced Cr uptake by plants through adsorption of Cr ions, resulting in decreased chromium ion bioavailability. At chromium level 100 mg/kg, the foliar application of Fe NPs from 0 to 20 mg/L increased the total chlorophyll contents from 2.8 to 3.9. The application of S. aureus further enhanced the chlorophyll contents from 4.4 to 5.4, respectively. The current study suggested that combining Fe NPs and S. aureus could be a viable strategy for reducing Cr toxicity and accumulation in rice plants and most likely other plants.
Highlights
Soil pollution, caused by the discharge of toxic metals, has become a major environmental issue worldwide
Chromium (Cr) is a toxic metal element whose chemical compounds are widely distributed in the atmosphere due to industrial waste, primarily tannery effluents, metallurgy, and rapid urbanization [1,2]
Our findings suggest that foliar application of Fe nanoparticles (Fe NPs) combined with bacterial inoculation can help rice plants cope with Cr stress by regulating the plant’s defense system
Summary
Soil pollution, caused by the discharge of toxic metals, has become a major environmental issue worldwide. Chromium (Cr) is a toxic metal element whose chemical compounds are widely distributed in the atmosphere due to industrial waste, primarily tannery effluents, metallurgy, and rapid urbanization [1,2]. Cr compounds reach our food chains; their high persistence and soil availability can be absorbed or stored in plants (directly or indirectly), causing harmful effects on human health when the latter are consumed [3]. Chromium can travel to various plant parts and can be accumulated there before being consumed by animals and humans [4]. Further chromium exposure slows soil microbial activity, inhibits enzyme activity, and triggers the formation of reactive oxygen species (ROS), leading to plant oxidative stress [6]. Chromium can lead to many problems in human biological systems, including death in those exposed to it
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