Mechanistic insights into mercury tolerance and detoxification in the potential hyperaccumulator fern Pteris vittata

Chinese brake fern (Pteris vittata) is recognized as a promising potential mercury (Hg) hyperaccumulator; however, limited understanding of its accumulation, tolerance, and detoxification mechanisms has hindered its application in phytoremediation. To address these knowledge gaps, we investigated tolerance and detoxification mechanisms of P. vittata through field investigation and pot experiments. In abandoned Hg mining areas, P. vittata exhibited robust Hg accumulation, reaching total mercury (THg) concentrations of up to 163 mg/kg in roots and 83 mg/kg in shoots. Subcellular distribution analysis showed that Hg predominantly localized to the soluble fraction and cell walls in both leaves and roots. Transmission electron microscopy (TEM) showed severe cellular and ultrastructural damage in P. vittata tissues exposed to 1000 mg/kg HgCl₂. Under HgCl₂ stress, glutathione (GSH) and malondialdehyde (MDA) contents increased, while peroxidase (POD) activity and proline (Pro) contents initially increased before subsequently declining. These responses suggest that P. vittata activates an antioxidant defense response to mitigate Hg-induced stress, though excessive Hg exposure can overwhelm these protective mechanisms, leading to oxidative imbalance. Notably, chlorophyll decline occurred earlier than reductions in pro and POD activity, suggesting that the photosynthetic system is especially vulnerable to Hg exposure. X-ray absorption near-edge structure (XANES) analysis revealed that β-HgS and Hg(SG)₂ were the predominant Hg species in both roots and shoot, implicating inorganic precipitation and thiol-mediated complexation as key detoxification pathways. Collectively, this study advances the understanding of Hg tolerance and detoxification mechanisms in P. vittata, further supporting its potential application in remediating Hg-contaminated environments.