Excess copper inhibits the growth of rice seedlings by decreasing uptake of nitrate

Abstract

Copper (Cu) toxicity has a deleterious effect on plant growth, and different nitrogen (N) forms have significantly different impacts on the uptake and accumulation of heavy metals by plants. However, it remains unclear how excess Cu inhibits the growth of rice seedlings under different N forms. Here, we examined the mechanism of Cu toxicity inhibiting the growth of rice supplied with different N forms. Rice seedlings were grown in a nutrient solution with 0.81 mmol L^-1 N, as ammonium (NH₄⁺), nitrate (NO₃^-) and NH₄⁺ + NO₃^-, or without N (0 N) in the presence of 0.2 μmol L^-1 CuSO₄ or 10 μmol L^-1 CuSO₄. The inhibition of shoot growth under excess Cu was more pronounced in plants that were supplied with NO₃^- than NH₄⁺; such inhibition was not induced by higher Cu concentration in shoots. Applied with NO₃^- alone increased solution pH value up to 6.2, but supplied with NH₄⁺ alone and NH₄⁺ + NO₃^- decreased solution pH value to 4.0 and 4.2, respectively. The increment of solution pH reduced Cu concentration in shoots of rice supplied with NO₃^- alone. Copper toxicity decreased NO₃^- concentrations in rice seedlings that were supplied with NO₃^- alone but increased the NH₄⁺ concentrations in plants that were supplied with NH₄⁺ or NH₄⁺ + NO₃^-. High Cu levels reduced the uptake of NO₃^- in roots by the analysis of net NO₃^- flux and NO₃^- assimilation enzymes activity. Under excess Cu, the transcript levels of OsNPF6.5, OsNPF2.2 and OsNPF2.4 genes were suppressed, while OsNRT2.1, OsNRT2.2 and OsNAR2.1 were raised in roots. In conclusion, Cu toxicity inhibits NO₃^- uptake and upward translocation by modulating the expression level of NO₃^- transporter genes. The reduction in the concentrations of NO₃^- and total N decreased shoot growth of rice seedlings when N was supplied as NO₃^-. Hence, rice seedlings supplied with NO₃^- had lower shoot biomass than those with NH₄⁺ under Cu stress.

Keywords: Copper toxicity; Nitrate transporter; Nitrate uptake; Nitrogen forms; Oryza sativa.