Proteolytic system in sunflower (Helianthus annuus L.) leaves under cadmium stress.

The effect of oxidative stress induced by cadmium on growth parameters and on the balance between protein synthesis and degradation was studied in sunflower (Helianthus annuus L.) leaves. Plants were germinated for 10 days and then transferred to hydroponic medium devoid (control) or containing 100, 200 and 300μM CdCl2. Analyses were performed between days 0 and 4 of Cd-treatment. All Cd(2+) concentrations significantly reduced leaf area and, fresh and dry weight, but leaf relative water content only decreased with 200 and 300μM Cd(2+). Control and treated plants had similar soluble protein content and showed the same rate of soluble protein labeling under the assay conditions. Although protease activity increased with cadmium treatment, proteasome activity was significantly inhibited. Expression of 20S proteasome remained similar to controls in cadmium treated plants. Cadmium caused an increase in ubiquitin-conjugated proteins and carbonyl groups content of treated plants, compared to control values. Cadmium induced an increase in protease specific activity; nevertheless, this increase was not relevant enough to avoid accumulation of oxidized proteins. Oxidation of proteins is one of the most important effects of cadmium treatment. The results presented here provide evidence for the role of the proteolytic system in sunflower plants subjected to cadmium stress.