Targeting key metabolic points for an enhanced phytoremediation of wastewaters pre-treated by the photo-Fenton process using Solanum nigrum L.

• Decreased metallothionein (MT) gene expression facilitated oxidative stress.
• Decreased nitrogen (N) metabolism resulted in decreased plant biomass.
• Increased glutathione biosynthesis helped to alleviate oxidative stress.
• N metabolism and MT accumulation are key points for future plant improvement.

Several physiological, biochemical and molecular biology responses were analysed in Solanum nigrum L. plants exposed for 28 days to an effluent that resulted from the photo-Fenton treatment of a highly concentrated pesticide and systemic fungicide aqueous solution, containing metalaxyl as active compound (150 mg L−1), in order to pinpoint metabolic steps for a future increase of these plants’ capacity to deal with the chemical process by-products. Although plants suffered oxidative stress, as indicated by increased membrane damage and a negative effect on plant biomass, they absorbed the excess iron and acted on the resulting by-products present in the effluent after the photo-Fenton process. Nitrogen assimilation and metallothionein gene expression were down regulated, while glutathione biosynthesis increased. These results suggest an enhanced nitrogen assimilation and/or metallothionein accumulation as relevant key points for further plant improvement in order to increase the efficiency of this innovative strategy that considers integration of the photo-Fenton process (as chemical primary treatment) with S. nigrum L. plants (as biological remediation post-treatment) for heavily polluted wastewaters.