Mitigation of osmotic and salt stresses by abscisic acid through reduction of stress-derived oxidative damage in Chlamydomonas reinhardtii

In the green alga, Chlamydomonas reinhardtii, externally added abscisic acid (ABA) partly released the growth suppression caused by osmotic stress with sorbitol and salt stress with NaCl, in the same way that paraquat did in a previous study. In cells treated with ABA for 24h, however, the damage caused by a short-period exposure to these water stresses was not reduced in comparison with non-treated cells. Intracellular levels of reactive oxygen species (ROS) in stress-exposed cells were significantly higher than in non-stress-exposed cells. ABA treatment markedly reduced this ROS generation and enhanced gene expression of the antioxidant enzymes, catalase (CAT) and ascorbate peroxidase (APX), in preference to increasing the activities of these enzymes. These results suggest that ABA might only induce response reactions which reduce the oxidative damage derived from exposure to osmotic and salt stresses in C.reinhardtii, but did not act to induce any specific reactions to reduce the damage caused directly by these water stresses. This mitigation of oxidative damage by ABA, probably by the elimination of ROS. might result in the above-mentioned release of growth suppression. (C) 2004 Elsevier Ireland Ltd. All rights reserved.