RESUMO
Changes induced by three levels of Zn toxicity in the root proteome from Beta vulgaris were studied by two dimensional gel electrophoresis. 320 spots were consistently detected and 5, 5 and 11% of them showed significant changes in relative abundance as a result of the 50, 100 and 300µM Zn treatments, respectively, when compared to controls (1.2µM Zn). Forty-four spots had consistent changes between all treatments, and 93% were identified. At low and mild Zn excess, the complex I of the mitochondrial transport chain and the oxidative phosphorylation were mildly impaired, and an effort to compensate this effect by increasing glycolysis was observed. At high Zn excess, a general metabolism shutdown occurred, as denoted by decreases in the aerobic respiration and by an impairment of the defense systems against oxidative stress. Accordingly, lipid peroxidation increased as Zn supply increased. This study suggests that metabolic changes at high Zn supply reflect cell death, while changes at low and mild Zn supplies may rather explain the metabolic reprogramming occurring upon Zn toxicity. Results also suggest that Zn competition with divalent ions including Fe may contribute to many of the Zn toxicity symptoms, especially at low and moderate Zn supplies. BIOLOGICAL SIGNIFICANCE: Results in this work provide a comprehensive overview of the effects of Zn toxicity in roots of sugar beet plants. Effects at low and mild Zn excess are similar and reflect changes in the metabolism aimed to overcome this heavy metal stress, whereas effects at high Zn supply indicate a general shutdown of the metabolism and cell death. Our results indicate that Zn toxicity elicits major impairments in the oxidative stress defense systems, possibly due to Zn competition with divalent cations including Fe, in spite that Zn is not a redox active element by itself.