Biochemical and proteomic analysis of thioürea-induced cadmium tolerance in maize (Zea mays L.) seedlings

The pollution of soil with cadmium (Cd) is an important environmental problem worldwide because of its high toxicity to plants. Due to its ease of mobility, Cd is readily taken up by plants and negatively affect crop yield. Thiourea (TU) is a sulfur containing plant growth regulator and it is involved in a series of developmental, physiological, and biochemical changes. Although exogenous TU application has been found to enhance the growth and tolerance of plants under stressful conditions, the molecular regulatory mechanisms of TU on Cd tolerance in maize (Zea mays L.) seedlings remain unclear. Therefore, the current research was performed to evaluate the ameliorative effect of TU application by physiological and proteomic analyses in maize seedlings under Cd stress. For this purpose, five-day-old maize seedlings grown in hydroponic culture were exposed to different treatments of TU and CdCl2 added to the nutrient solution: control (only nutrient solution), TU (35 μM), Cd (50 μM), and TU+Cd (35 μM TU and 50 μM Cd). The results revealed that Cd stress decreased seedling growth and chlorophyll content, while TU application under Cd stress conditions increased growth and photosynthetic pigments. Compared to Cd, TU+Cd application reduced Cd accumulation in roots and leaves of maize seedlings. On the other hand, it was determined that TU+Cd application reduced MDA production, probably by regulating the activities of antioxidant enzymes. Cd-induced inhibition of catalase (CAT) activity was reduced by TU+Cd application. In addition, TU+Cd application caused an additional increase in ascorbate peroxidase (APX) activity, while it caused a decrease in guaiacol peroxidase (POD) activity. 1573 differentially expressed proteins (DEPs) were identified in leaves exposed to Cd and/or TU+Cd treatments compared to control by high-throughput proteome analysis (nLC˗MS/MS). Among these DEPs, 1237 proteins were up−regulated while 336 proteins were down−regulated. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways analysis revealed that most of the DEPs were involved in citrate cycle (TCA cycle), photosynthesis, glutathione metabolism, cysteine and methionine metabolism, pyruvate metabolism, glycolysis/gluconeogenesis, metabolic pathways, and biosynthesis of secondary metabolites. In conclusion, it can be suggested that DEPs detected by proteomic analyses in maize seedlings have important roles in TU-induced Cd tolerance.