Cysteine-induced upregulation of nitrogen metabolism-related genes and enzyme activities enhance tolerance of maize seedlings to cadmium stress

• Cadmium stress disturbs nitrogen metabolism in plants.
• Cysteine-induced cadmium tolerance is linked to modulation of nitrogen metabolism.
• Cysteine activates nitrogen metabolism at the level of biochemical and molecular.
• There is an important correlation between gene expressions and activities of enzymes involved in nitrogen metabolism.

We aimed to determine the effects of cysteine (Cys) on nitrogen metabolism of maize seedlings exposed to cadmium (Cd) stress at the biochemical and molecular levels. Cd stress significantly reduced root-leaf lengths and dry weight, whereas Cys application partially mitigated these reductions. Correspondingly, the declines occurring in chlorophyll content under the stress were also markedly mitigated with Cys supplementation. When compared to the control, although significant reductions were recorded in the activities of nitrate reductase, nitrite reductase, glutamine synthase (GS) and glutamate synthase (GOGAT) in Cd-applied seedlings, a marked rise was determined in glutamate dehydrogenase (GDH) activity. Similarly, while nitrate and nitrite contents dropped with the application of Cd, a remarkable elevation of ammonium content took place. Cd-induced declines detected in enzyme activity and nitrate and nitrite contents were reversed with Cys application, and this was in contrast to the decrease in ammonia content. Versus the other enzymes studied, the Cd-induced enhancement recorded in GDH activity was reversed by Cys application. Therefore, an important positive correlation was established between activity and gene expression levels of all enzymes studied. The reductions noted in oxidant content and membrane damage with Cys application demonstrated that the soothing effect of Cys arises from mitigation of oxidative damage through the direct and/or indirect influence of cysteine. Taken together, it is possible to say that the repressive effect of Cd on nitrogen metabolism at the level of the enzyme and gene was significantly alleviated by Cys, and hence an important contribution to enhancing plant tolerance in face of Cd stress was noted.