Heavy Metals Hyperaccumulation and the Role of Stress-Induced Proteins in Phytoremediation Mechanism in Selected Qatari Plants

Abstract

Progressive pollution of the environment by trace and heavy metals pose a significant risk and causes diverse human diseases. Current world initiatives to remove these toxic substances from the environment are costly and might add to the pollution. New emerging technology is phytoremediation by diverse plant species, a friendly and less expensive approach as compared with traditional methods. Lead (Pb), the second most toxic heavy metal on earth, generally slow or inhibit growth in most plant species at concentrations higher than 30 mg/kg in the soil. Some plants demonstrate the capacity to accumulate Pb in high concentration. Current work in this area is invested in elucidating mechanisms that underpin metal uptake in plants to optimize the process for large-scale application in polluted environments. In this study, we screened eight (8) plants species for Cd, Cr, Cu, Ni, and Pb accumulation and showed that T. qataranse, an undershrub plant accumulates higher concentration of Cd, Cr, Cu, Ni and Pb than the soil. Pb treatment at varying levels (25, 50 and 100 mg/kg) in greenhouse conditions showed that the toxic metal stimulated seed germination and seedlings growth. It concentrates on the plant tissues by more than 270 % than the metal hyperaccumulation threshold of 1000 mg/kg Pb in plants. Up to 2,784 and 1141.6 mg/kg Pb accumulates in the plant root and shoot, respectively. Evaluation of the activities of antioxidant enzymes (CAT, SOD, APX, GPX, and GR), and enrichment analysis of differentially expressed proteins due to Pb stress provides insight into the mechanism of Pb tolerance and uptake. The plant showed higher activities of these enzymes with increasing Pb concentration, suggesting the crucial role of the plant antioxidative system in scavenging ROS. A total of eighty-six (86) differentially expressed proteins, the majority of which functions in ion and protein binding, antioxidant activity, transport, and response to stress were identified. Essential stress regulating metabolic pathways, including glutathione metabolism, cellular response to stress, and regulation of HSF1-mediated heat shock response, were also enriched. Of the 86 identified proteins, enrichment analysis showed six (6) proteins with unknown function are potentially novel Pb chelators. Therefore, the antioxidative system, over-expressed stress response, and induced metal-binding proteins (phytochelatins and glycine-rich proteins), respectively, regulate Pb tolerance and detoxification in T. qataranse.