Elevated CO₂ exacerbates effects of TiO₂ nanoparticles on rice (Oryza sativa L.) leaf transcriptome and soil bacteria

Elevated CO₂ affects the plant rhizosphere and can therefore affect the fate and toxicity of soil contaminants. However, little is known about how the effects of nanoparticles on plants and soil bacteria will change under future CO₂ levels. A free-air CO₂ enrichment system with two CO₂ levels (ambient, 390 μmol mol^-1; elevated, 590 μmol mol^-1) was used to investigate the responses of rice (Oryza sativa L.) and soil bacteria to titanium dioxide nanoparticles (nano-TiO₂, 0 and 200 mg kg^-1). Results showed that nano-TiO₂ alone did not significantly affect rice growth but affected soil bacteria involved in the carbon and sulfur cycles. Elevated CO₂ alone increased rice plant biomass and up-regulated genes related to ribosomes, but its combination with nano-TiO₂ down-regulated genes related to photosynthesis and photosynthetic antennae. Elevated CO₂ also exacerbated the disturbance by nano-TiO₂ to soil bacteria involved in carbon and nitrogen cycles, and consequently inhibited the rice growth. These findings provide a reference for the comprehensive evaluation for the risk of soil pollution.