Effect of Nickel Stress on Nitrogen Metabolism in Cucumber Plants

Excessive concentrations of nickel (Ni) are phytotoxic, leading to disturbances in plant cell structure and function. Although some attempts have been made to elucidate the Ni impact on plant metabolism, the effect of this metal on nitrogen assimilation and transformation of nitrogen compounds still remains poorly understood. The objective of our study was to gain a better insight into the Ni influence on nitrogen metabolism in cucumber plants. Nitrogen metabolism-related enzyme activities and selected metabolite contents were assayed using spectrophotometric methods. Additionally, in the leaves, nitrogen assimilation-involved gene expression was analyzed using quantitative real-time PCR. Nickel treatment resulted in a decline in NO3- content in the leaf and NH4+ content in the root. In the leaf, ferredoxin-dependent glutamate synthase (Fd-GOGAT) activity decreased, while NADH-dependent glutamate synthase (NADH-GOGAT) and glutamate dehydrogenase (GDH) activities increased. The GDH activity showed increases in both its aminating (NADH-GDH) and deaminating (NAD-GDH) functions. The activities of the other enzymes involved in nitrogen assimilation were not influenced by Ni stress. In the root, the activities of most enzymes were downregulated by Ni treatment except for NADH-GDH and NAD-GDH activities which showed increases. While glutamate content remained unaltered after Ni exposure in the leaf, in the root it was slightly lowered. In contrast to the leaf, showing accumulation of non-protein thiols and proline, in the root, these compound contents were markedly decreased. Our study revealed an organ-specific response of cucumber plants to Ni treatment. Accumulation of glutamate derivatives involved in response to heavy metal stress without significant changes in glutamate content may suggest that in the leaf, the induction of NADH-GOGAT and NADH-GDH activities efficiently compensates for the reduced Fd-GOGAT activity. Additionally, the increased NADP-ICDH activity may support glutamate production by providing 2-oxoglutarate for reactions catalyzed by NADH-GOGAT and NADH-GDH.