Heavy metals effects on proteolytic system in sunflower leaves

Plant proteolytic system includes proteases, mainly localized inside the organelles, and the ubiquitin–proteasome pathway in both, the cytoplasm and the nucleus. It was recently demonstrated that under severe Cd stress sunflower (Helianthus annuus L.) proteasome activity is reduced and this results in accumulation of oxidized proteins. In order to test if under other heavy metal stresses sunflower proteolytic system undergoes similar changes, an hydroponic experiment was carried out. Ten days old sunflower plants were transferred to hydroponic culture solutions devoid (control) or containing 100 μM of AlCl3, CoCl2, CuCl2, CrCl3, HgCl2, NiCl2, PbCl2 or ZnCl2 and analyzed for protein oxidative damage and proteolytic activities. After 4 days of metal treatment, only Co2+, Cu2+, Hg2+, and Ni2+ were found to increase carbonyl groups content. Except for Al3+ and Zn2+, all metals tested significantly reduced all proteasome activities (chymotrypsin-like, trypsin-like and PGPH) and acid and neutral proteases activities. The effect on basic proteases was more variable. Abundance of 20S protein after metal treatments was similar to that obtained for control samples. Co2+, Cu2+, Hg2+, Ni2+, Cr3+, and Pb2+ induced accumulation of ubiquitin conjugated proteins. It is concluded that heavy metal effects on proteolytic system cannot be generalized; however, impairment of proteasome functionality and decreased proteases activities seem to be a common feature involved in metal toxicity to plants.