Sunflower cotyledons cope with copper stress by inducing catalase subunits less sensitive to oxidation.

Copper is an essential trace element for living organisms, in excess, can be toxic to the cell because of its capacity to generate reactive oxygen species (ROS). Catalase (CAT) catalyzes the dismutation of hydrogen peroxide into water and dioxygen and in plants it is located in peroxisomes and glyoxysomes. Different metals can induce changes in CAT activity, but the mechanism underlying its changes is unclear. After 4h of treatment with 5 and 10 μM CuCl(2) a decrease in the specific CAT activity was detected in sunflower cotyledons of post-germinative heterotrophic seedlings. At 8h of treatment, 5 μM Cu(2+) produced an induction of CAT activity while only a complete recovery to control values was observed for 10 μM Cu(2+) treated seedlings. These activity variations were not related to the level of CAT protein expression, but they did correlate with the oxidative state of the CAT protein. This indicates that the mechanism of CAT inactivation by Cu(2+) involves oxidation of the protein structure. The level of the mRNA of CATA3 and CATA4 increased with the presence of the metal after 4h of exposure. These CAT genes code for the synthesis of CAT subunits less sensitive to oxidation, which would prevent the copper-induced oxidative inactivation of CAT.