Diclofenac shifts the role of root glutamine synthetase and glutamate dehydrogenase for maintaining nitrogen assimilation and proline production at the expense of shoot carbon reserves in Solanum lycopersicum L.

The continuous increase of the human population worldwide has led to an increase of pharmaceuticals' consumption, such as diclofenac (DCF), a widely used non-steroidal anti-inflammatory drug (NSAID), that is not removed by wastewater treatment processes. Although there is some research regarding the effects of DCF on animals and aquatic invertebrates, information concerning its influence on plants' metabolism is still scarce. Through an integrated approach, using combined biochemical and molecular biology techniques, this work aimed to evaluate the phytotoxicity of DCF in Solanum lycopersicum L., focusing on the primary plant processes: nitrogen (N) assimilation and photosynthesis. The exposure of tomato plants to increasing concentrations of DCF (0, 0.5, and 5 mg L-1) revealed that glutamine synthetase (GS) was differentially affected, in an organ-dependent manner, by this contaminant at the gene expression, protein, and activity levels, with an increased activity of 0.2-fold in shoots of plants treated with the lowest concentration of DCF although a general decrease was registered for the SlGS gene family expression, revealing that post-translational regulation was in order, since GS2 polypeptide content did not change. Glutamate dehydrogenase (GDH) activity was generally enhanced, accompanied by increases of 0.4- to 1.9-fold in proline levels, revealing GDH as an important compensatory route for both N assimilation and proline production under stressful conditions. No alterations in most photosynthetic endpoints were noticed after DCF treatments, but small decreases of 0.1- to 0.8-fold in the accumulation of RuBisCO-encoding transcripts were observed, along with a reduction in starch content. Some alterations in the soluble polypeptide profile were also detected in response to DCF, evidencing the participation of some stress-related proteins in the plant's response to DCF.