Hydrogen-rich water confers plant tolerance to mercury toxicity in alfalfa seedlings

• HRW confers alfalfa tolerance to Hg toxicity.
• HRW alleviates growth stunt and oxidative stress triggered by Hg.
• HRW results in the reestablishment of redox homeostasis.
• HRW leads to less Hg contents in alfalfa seedling roots under Hg exposure.

In this report, the effect of hydrogen-rich water (HRW), which was used to investigate the physiological roles of hydrogen gas (H2) in plants recently, on the regulation of plant adaptation to mercury (Hg) toxicity was studied. Firstly, we observed that the exposure of alfalfa seedlings to HgCl2 triggered production of reactive oxygen species (ROS), growth stunt and increased lipid peroxidation. However, such effects could be obviously blocked by HRW. Meanwhile, significant decreases in the relative ion leakage and Hg accumulation were observed. Hg-induced increases in total and isozymatic activities of superoxide dismutase (SOD) were significantly reversed by HRW. Further results suggested that HRW-induced the activities of guaiacol peroxidase (POD) and ascorbate peroxidase (APX), two hydrogen peroxide-scavenging enzymes, was at transcriptional levels. Meanwhile, obvious increases of the ratios of reduced/oxidized glutathione (GSH), homoglutathione (hGSH), and ascorbic acid (AsA) and corresponding gene expression were consistent with the decreased oxidative damage in seedling roots. In summary, the results of this investigation indicated that HRW was able to alleviate Hg toxicity in alfalfa seedlings by (i) alleviating growth stunt and reducing Hg accumulation, and (ii) avoidance of oxidative stress and reestablishment of redox homeostasis.