Copper toxicity compromises root acquisition of nitrate in the high affinity range

Abstract

The application of copper (Cu)-based fungicides for crop protection plans has led to a high accumulation of Cu in soils, especially in vineyards. Copper is indeed an essential micronutrient for plants, but relatively high concentrations in soil or other growth substrates may cause toxicity phenomena, such as alteration of the plant's growth and disturbance in the acquisition of mineral nutrients. This last aspect might be particularly relevant in the case of nitrate $\left({\text{NO}}_{3^{-}}\right)$ , whose acquisition in plants is finely regulated through the transcriptional regulation of ${\text{NO}}_{3^{-}}$ transporters and plasma membrane H⁺-ATPase in response to the available concentration of the nutrient. In this study, cucumber plants were grown hydroponically and exposed to increasing concentrations of Cu (i.e., 0.2, 5, 20, 30, and 50 µM) to investigate their ability to respond to and acquire ${\text{NO}}_{3^{-}}$ . To this end, the kinetics of substrate uptake and the transcriptional modulation of the molecular entities involved in the process have been assessed. Results showed that the inducibility of the high-affinity transport system was significantly affected by increasing Cu concentrations; at Cu levels higher than 20 µM, plants demonstrated either strongly reduced or abolished ${\text{NO}}_{3^{-}}$ uptake activity. Nevertheless, the transcriptional modulation of both the nitrate transporter CsNRT2.1 and the accessory protein CsNRT3.1 was not coherent with the hindered ${\text{NO}}_{3^{-}}$ uptake activity. On the contrary, CsHA2 was downregulated, thus suggesting that a possible impairment in the generation of the proton gradient across the root PM could be the cause of the abolishment of ${\text{NO}}_{3^{-}}$ uptake.

Keywords: NRT genes; PM H+-ATPase; copper; cucumber; gene expression; high affinity transport system; nitrate induction.