Calcium and Citrate Protect Pisum sativum Roots against Copper Toxicity by Regulating the Cellular Redox Status

The protective effects of exogenous application of calcium (Ca) or citrate against copper (Cu)-induced toxicity in pea seedlings was investigated. Seeds were germinated in distilled water (control) or aqueous solutions of Cu (200 µM) in combination with Ca (10 mM) or citrate (100 µM) for 6 days. The exposure of seeds to Cu caused high metal accumulation in tissues associated with increased activities of hydroxyl radical, superoxide anion, and thiol groups as well as the lipoxygenase activity. By contrast, Cu decreased the glutathione and cysteine contents by almost 68% and 27%, respectively. Moreover, one-dimension (1D) and two-dimension (2D) electrophoresis analyses showed that Cu induced a significant increase in the levels of thiol-containing proteins. In contrast, seed treatment with Ca or citrate ameliorated the cellular redox state of roots as reflected by lower levels of oxidative stress biomarkers. This achievement could be attributed to the reduction of Cu accumulation in the tissues. Besides, exogenous Ca and citrate mitigated the Cu-induced disruption of the glutathione and cysteine redox homeostasis via the modulation of the activities of the glutathione reductase, glutathione-S-transferase, and glutathione peroxidase enzymes. Moreover, both Ca and citrate modulated the thioredoxin and ferredoxin regeneration systems to a level close to steady-state condition. The current results evidenced that Ca and citrate are effective in imparting Cu tolerance to pea germinating seeds and suggest that Ca or citrate supplementation to Cu-polluted environment could be an economic procedure for the control of heavy metal toxicity and accumulation in crops.