Abstract
Melatonin (MT) is a tryptophan-derived natural product that plays a vital role in plant response to abiotic stresses, including heavy metals (HMs). However, it remains elusive how exogenous MT mediates lead (Pb) accumulation and detoxification at the methylation and transcriptional levels in radish. In this study, decreased Pb accumulation and increased antioxidant enzyme activity were detected under MT treatment in radish. Single-base resolution maps of DNA methylation under Pb stress (Pb200) and Pb plus MT treatment (Pb_50MT) were first generated. The genome-wide methylation level was increased under Pb stress, while an overall loss of DNA methylation was observed under MT treatment. The differentially methylated region (DMR)-associated genes between Pb_50MT and Pb200 were uniquely enriched in ion binding terms, including cation binding, iron ion binding, and transition metal ion binding. Hyper-DMRs between Pb200 and Control exhibited a decreasing trend of methylation under Pb_50MT treatment. A few critical upregulated antioxidant genes (e.g., RsAPX2, RsPOD52 and RsGST) exhibited decreased methylation levels under MT treatment, which enabled the radish plants to scavenge lead-induced reactive oxygen species (ROS) and decrease oxidative stress. Notably, several MT-induced HM transporter genes with low methylation (e.g., RsABCF5, RsYSL7 and RsHMT) and transcription factors (e.g., RsWRKY41 and RsMYB2) were involved in reducing Pb accumulation in radish roots. These findings could facilitate comprehensive elucidation of the molecular mechanism underlying MT-mediated Pb accumulation and detoxification in radish and other root vegetable crops.
Highlights
Contamination of soil and water with heavy metals (HMs) has become an increasingly concerning problem worldwide and affects human health through the food chain[1,2]
MT-induced reduction in Pb content in radish To characterize the roles of MT under Pb stress, the Pb contents and antioxidant enzymes of radish roots and leaves treated with Pb(NO3)[2] (200 mg L−1) (Pb200) or Pb200 plus MT (0, 10, 25, 50, 100 and 150 μM) were assayed (Fig. 1)
The weights of the roots and leaves under Pb_50MT were larger than those under Pb200, and the activities of antioxidant enzymes including ascorbate peroxidase (APX) and glutathione reductase (GR) were increased under Pb stress at different melatonin concentrations, both of which reached the maximum level at 50 μM melatonin, indicating that MT treatment relieved the toxicity of Pb stress in both the leaves and roots of radish plants (Fig. 1b–e)
Summary
Contamination of soil and water with heavy metals (HMs) has become an increasingly concerning problem worldwide and affects human health through the food chain[1,2]. Excessive reactive oxygen species (ROS), and DNA damage can be induced by Pb stress, resulting in the inhibition of plant growth and development[3]. Due to the transfer of Pb2+ from contaminated agricultural soil and. The reduction of Pb accumulation is imperative to effectively prevent toxicity in foods from crop plants. DNA methylation, as an epigenetic modification, is associated with transcriptional activity and gene expression[6,7]. In plant genomes, methylated cytosines are categorized into three contexts: CG, CHG, and CHH (where H = A, T, or C). In Arabidopsis, CG and CHG methylation are maintained by a conserved DNA METHYL-
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