Mechanism of CATA3 induction by cadmium in sunflower leaves

One of the main antioxidant enzymes, catalase (CAT, EC 1.11.1.6), is capable of catalyzing the dismutation of H2O2. This enzyme is involved in signal transduction pathway in plants, controlling the cellular level of this reactive oxygen species. Four different genes, CATA1–CATA4, were identified in Helianthus annuus L. cotyledons. Incubation of sunflower leaf discs with 300 and 500 μM CdCl2 under light conditions increased CATA3 transcript level. However, it was not induced by Cd2+ in etiolated plants. This Cd2+-induced increase was reverted by adding 10 mM ascorbate. Treatments with 0.4 and 10 μM rose bengal (a generator of 1O2) did not activate CATA3, but 10 μM methyl viologen (an enhancer of O2− production) and 10 mM H2O2 increased its expression. In isolated chloroplasts, Cd2+ and methyl viologen produced oxidation of the probe 2′,7′-dichlorofluorescein diacetate indicating ROS formation. Besides, Cd2+ treatment of leaf discs under light decreased CAT activity and increased carbonyl groups content, thus suggesting that enzyme inactivation could be due – in part – to a protein oxidation. These results indicate that light is involved in Cd2+-induced CATA3 enhancement, which leads to the synthesis of CAT isoforms less sensible to oxidation, and that chloroplast might be the main source of ROS responsible for this process.