Exogenously applied auxins and cytokinins ameliorate lead toxicity by inducing antioxidant defence system in green alga Acutodesmus obliquus

The effects of auxins (IAA, IBA, PAA) and cytokinins (tZ, Kin, DPU) on the growth, oxidative damage, level of antioxidants and the activity of antioxidant enzymes as well as the contents of proteins and photosynthetic pigments in green alga Acutodesmus obliquus were investigated under 100 μM lead (Pb) stress. Heavy metal induced oxidative damage as evidenced by a decrease in cell number and reduction in the contents of proteins and chlorophylls as a consequence of an increase in reactive oxygen species (ROS) formation and lipid peroxidation. The application of exogenous auxins and cytokinins modulated biosorption of Pb by algal cells significantly alleviated the growth inhibition and stimulated the accumulation of proteins, chlorophylls and carotenes. Phytohormones also activated the xanthophyll cycle which is extensively involved in the protection of the photosynthetic apparatus in adverse environmental conditions. The reduction in oxidative stress caused by the presence of toxic Pb was observed in algal cultures treated with phytohormones. Cytokinins were more effective in lowering hydrogen peroxide and lipid peroxidation levels in comparison with auxins. This improving effect of cytokinins seems to be mediated by a decrease in Pb accumulation by algal cells, whereas auxins promoted metal uptake. Importantly, auxins and cytokinins enhanced the redox status of algal cells inducing the increase in the content of antioxidants (ascorbate, glutathione, and proline) and in the activity of antioxidant enzymes (superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase) involved in ROS scavenging. The results of the present study strongly suggest that exogenous auxins and cytokinins enhanced the resistance of microalga A. obliquus against Pb toxicity through the activation of the antioxidant defence system.