Role of salicylic acid in regulation of cadmium toxicity in wheat (Triticum aestivum L.)

The present study is focused on the possible mediatory role of salicylic acid (SA) under cadmium (Cd+2) toxicity. Cadmium treatments resulted in the inhibition of root dry biomass, root elongation and increased Cd accumulation in roots. Pretreatment of seeds with SA (500 μM) for 20 hrs resulted in protection against Cd, increased root dry biomass, root elongation and minimal accumulation of cadmium. Cadmium chloride (CdCl2; 0.0, 100, 400 and 1000 μM) produced a significant inhibition in chlorophyll, photosynthetic efficiency [carbon dioxide (14CO2)-fixation], relative water content (RWC) and abscisic acid (ABA) content. Pretreatment of seeds with SA alleviated the Cd negative effect on these parameters. A drop in the activities of carboxylating enzymes, phosphoenol pyruvate carboxylase (PEPC, EC 4.1.1.31), and ribulose1,5-bisphosphate carboxylase (RuBPC, EC 4.1.1.39) were observed in Cd treated plants. Pretreatment with SA ameliorated the inhibitory effect of Cd on the studied enzymatic activities. Cadmium applied to salicylic acid free plants increased the level of stress metabolites, malondialdehide (MDA), hydrogen peroxides, and free proline content. Salicylic acid pretreatment decreased the toxic effect level of cadmium manifested by lower lipid peroxidation, lesser production of H2O2 and reduction in free proline content. The ultrastructural investigation of the root cells of wheat exposed to different concentrations of Cd for 15 days was carried out. The results indicated that Cd induced several obvious ultrastructural changes such as increased vacuolation, condensed cytoplasm with increased density of the matrix, reduction of mitochondrial cristae, severe plasmolysis, highly condensed nuclear chromatin and increment of disintegrated organelles. Electron dense granules appeared between the cell wall and plasmalemma. In vacuoles, electron dense granules encircled by the membrane were aggregated and formed into larger precipitates, which increase in number and volume as a consequence of excessive Cd exposure. An important issue arising from this study was how the short-term treatment with SA affected several physiological processes, such as plant growth, photosynthesis, and antioxidant defense system. We assume that the beneficial effect of SA during an earlier growth period could be related to avoidance of cumulative damage upon exposure to cadmium. One of the important roles of SA in inducing resistance to various environmental stresses is manifested by its ability to express genes that code for pathogenesis-related proteins or defense-related enzymes. Also, SA could form a complex with Cd that may provide Cd tolerance.