Investigation of the ameliorative effects of thiourea application on maize (zea mays L.) seedlings exposed to lead stress

In this study, the adverse effects of lead (Pb) on growth, chlorophyll content, lead accumulation, lipid peroxidation, antioxidant enzyme activities, and transcriptome changes in maize (Zea mays L. cv. Capuzi) seedlings were investigated, along with the potential of thiourea (TU) to alleviate these negative effects. Maize seedlings were subjected to four treatments: control, TU, Pb, and TU+Pb, and physiological, biochemical, and molecular parameters were evaluated. Pb exposure caused significant reductions (p <0.05) in both fresh and dry weights of shoots and roots. This Pbinduced reduction was significantly mitigated by TU application, with a particularly notable 65.50% improvement in root dry weight. Pb accumulated at higher levels in roots than in shoots of maize seedlings exposed to Pb stress. This Pb accumulation was significantly reduced (18.77%) only in shoot tissues with TU application. Furthermore, Pb stress significantly decreased chlorophyll content (63.4%), whereas TU application substantially improved chlorophyll levels (60.0%). Malondialdehyde (MDA), an indicator of lipid peroxidation, increased with Pb treatment but was reduced to control levels with TU application. Pb exposure increased the activities of antioxidant enzymes [superoxide dismutase (SOD), ascorbate peroxidase (APX), and guaiacol peroxidase (POD)] except for catalase (CAT). In contrast, in the TU+Pb treatment compared to Pb alone, activities of SOD, CAT, and APX decreased, while POD activity increased. Differentially expressed genes (DEGs) were identified in the leaf tissues of maize under control, Pb, and TU+Pb treatments. In this study, RNAseq analysis was performed on the leaf tissues of maize seedlings exposed to control, Pb, and TU+Pb treatments to investigate the response mechanisms to Pb toxicity and the ameliorative effects of thiourea (TU). The analysis identified 204 upregulated and 357 downregulated differentially expressed genes (DEGs) in the control vs. Pb comparison; 1,037 upregulated and 718 downregulated DEGs in the control vs. TU+Pb comparison; and 171 upregulated and 175 downregulated DEGs in the Pb vs. TU+Pb comparison. Among these, 253 DEGs were commonly expressed across treatments and highlighted as important candidates reflecting the regulatory role of TU in alleviating Pb stress. Functional enrichment analyses revealed that Pb stress suppressed genes associated with photosynthesis, carbon fixation, and energy metabolism, whereas TU+Pb treatment activated pathways related to sulfur and glutathione metabolism, small molecule processes, and antioxidant defense. The reliability of RNAseq results was confirmed by qRTPCR analysis of ten genes, showing expression profiles consistent with RNAseq data. Overall, TU, as a bioregulator, was found to mitigate the adverse effects of Pb stress by exerting protective and regulatory impacts on growth, pigment levels, and oxidative stress parameters, thereby promoting early growth and development in maize seedlings. These findings demonstrate that TU application plays a significant role in enhancing maize tolerance to Pb toxicity by supporting growth, pigment stability, and redox homeostasis.