Role of salicylic acid in alleviating heavy metal stress

Abstract

Both plant breeders and crop producers have an interest in finding crops capable of tolerating environmental changes with damage as little as possible. In order to develop such crops, the knowledge of plant defense mechanisms and regulatory processes is essential. The study presented in this chapter was performed to analyze the role of salicylic acid (SA) in regulation of plant growth and development, flowering, ion uptake, stomatal regulation and photosynthesis. The role of SA in development of plant resistance to different environmental stresses is described. Besides the physiological functions of SA, the general properties, biosynthesis and metabolism of this plant growth regulator are discussed. The present chapter focuses on the mechanisms of the beneficial effect of SA on maize plants exposed to toxic Cd concentrations. Exposure of plants to Cd (10, 15 and 25 μM) caused a gradual decrease in the dry weight accumulation of shoots and roots. Pretreatment of seeds with 500-μM SA for 6 h alleviated the negative effect of Cd on plant growth parameters. The same tendency was observed for the chlorophyll level. The rate of CO2 fixation was lower in Cd-treated plants, and the inhibition was partially overcome in SA-pretreated plants. A drop in the activities of carboxylating enzymes ribulose-1,5-bisphosphate carboxylase (RuBPCase) and phosphoenolpyruvate carboxylase (PEPCase) was observed for Cd-treated plants. Pretreatment with SA alleviated the inhibitory effect of Cd on the enzymes activity. In vivo the excess of Cd-induced alterations in the redox cycling of oxygen-evolving centers and the assimilatory capacity of maize leaves as revealed by changes in the termoluminescence emission. Pretreatment with SA before imposition of high concentration of Cd has a stabilizing effect on photochemical reactions. Changes in the activity of several important antioxidative enzymes, namely superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), guaiacol peroxidase (POD), glutathione reductase (GR) and glutathione-S-transferase (G-S-Tr) were measured. The presence of Cd in the nutrient solution led to disturbances in the activity of the antioxidant enzymes. Pretreatment with SA alleviated the negative effect of Cd on the studied enzymes. Our results suggest that the phytotoxicity of Cd is mainly induced by oxidative stress and SA is involved in the defense responses of maize plants to Cd exposure. This suggestion was consistent with the observed protective role of SA on the lipid membranes of Cd-treated maize plants.