Exogenous cysteine alleviates mercury stress by promoting antioxidant defence in maize (Zea mays L.) seedlings

Abstract

Introduction Higher plants develop several adjustments to almost all different environments. Due to their immobile structure, plants are constantly subjected to abiotic and biotic stress factors that negatively affect plant growth, development, and reproduction (Rejeb et al., 2014). The impact of unfavourable conditions on the ecosystem is increasing at an alarming rate due to the rapid increase in soil and water pollution and destruction of vegetation through the excessive release of wastes produced by human activities such as industrialization and urbanization (Nagajyoti et al., 2010; Mantri et al., 2012). Heavy metals are important environmental pollutants and their toxicity is of increasing importance due to ecological, evolutionary, nutritional, and environmental reasons. They have the greatest availability in soil and water ecosystems and present a relatively small proportion in the atmosphere as particulates or vapours. The extent of heavy metal toxicity in plants differs according to the plant species, metal specificity, concentration, chemical form, exposure type, and duration (Nagajyoti et al., 2010; Yadav, 2010). Mercury is known to be one of the most important toxic heavy metals in nature (Tchounwou et al., 2012). The source of natural mercury is from "hot spots" through natural processes such as hot springs or volcanic explosions. Although mercury is present in the earth's crust in low concentrations, human activities such as mining, burning of fossil fuels, release of industrial wastes into aquatic environments cause an increase in the mercury level in the atmosphere, soil, and water (Boening, 2000; Patra and Sharma, 2000; Lopes et al., 2013; Mahbub et al., 2017). Besides, the widespread use of mercury in chlorinealkali processes, dental amalgams, thermometers, and fungicides has contributed significant quantities of Hg into the aquatic and terrestrial ecosystems (Jarup, 2003). When mercury is released into the air, it can travel long distances and then be deposited into the water and ground.

Exogenous Cysteine Improves Mercury Uptake and Tolerance in Arabidopsis by Regulating the Expression of Heavy Metal Chelators and Antioxidative Enzymes

“…Hg also upregulated GS1, GS2, MT2a, and MT3, implying that Hg stimulates the transcriptional regulation for synthesis of GSH and MTs. Recent study has shown that exogenous Cys increased the GSH content of maize seedlings in presence of Hg (Aysin et al, 2020). Our results also revealed that exogenous Cys further increased the expression of GSH synthesis related genes GS1 and GS2.…”

“…ROS scavenging is accompanied by an antioxidative defense system that comprises both enzymatic and nonenzymatic antioxidants, which maintain the redox state in plants. It has previously been shown that exogenous Cys increases the expression of SOD and CAT, but not GR, under Hg stress in maize seedlings (Aysin et al, 2020). Our results also revealed a variable modulation of the transcript levels of various enzymatic antioxidants by exogenous Cys depending on the exposure period.…”

“…However, exogenous Cys supplementation effectively suppressed the H 2 O 2 amount and cell damage (Figure 6). Similar positive effects of exogenous Cys were also observed in the maize seedlings under Hg stress (Aysin et al, 2020). Therefore, Cys maybe directly involved in ROS scavenging, thereby promoting seedling growth under Hg-induced oxidative stress.…”

“…in cobalt-stressed Ocimum basilicum (Azarakhsh et al, 2015) but increased those of CAT and POD in chromium-treated maize (Terzi and Yıldız, 2021). In a study by Aysin et al (2020), the treatment of Hg-stressed maize with exogenous Cys increased the expression of SOD and CAT as well as the activities of SOD, peroxidase, and CAT but not those of APX and GR. As there is scant information available to link Cys to ROS scavenging in plants during their response to Hg stress, the elucidation of the role that this amino acid plays in the regulation of antioxidative enzymes in plants under Hg stress would add valuable knowledge to this field of study.…”

“…Recent studies have shown that exogenous Cys treatment could increase lead and chromium tolerance in Iris and maize seedlings, even under the condition of high accumulation of these two heavy metals (Yuan et al, 2015;Terzi and Yıldız, 2021). Additionally, exogenous Cys increased the tolerance of maize plants to Hg but also decreased Hg accumulation (Aysin et al, 2020). Although these studies have clearly pointed to the importance of Cys in heavy metal tolerance-despite the different patterns of heavy metal accumulation observed according to the heavy metal type and plant species-our current knowledge on the potential role that exogenous Cys plays in the plant mechanism of Hg accumulation under Hg stress remains limited.…”