Mechanisms of Hg⁰ Uptake, Transport, Distribution, and Redistribution in the Leaves of Hg Bioindicator <i>Tillandsia Usneoides</i> (Spanish Moss)

Atmospheric hydrargyrum (Hg) is a neurotoxic heavy metal, and plant leaves are active Hg reservoirs. <i>Tillandsia usneoides</i> is an indicator plant for atmospheric Hg pollution; however, the uptake, transport modes, and redistribution mechanisms of Hg in <i>T. usneoides</i> are underexplored. Herein, we investigated these mechanisms and the influencing factors of Hg⁰ in <i>T. usneoides</i> at multiple levels. We found that Hg⁰ can be absorbed through both stomata and lipids, with higher Hg concentrations showing a greater tendency to be taken up by lipids. Hg passes through cell membranes via active transport, facilitated by Ca²⁺ ion channels and water channel proteins. Most Hg (50.1-97.9%) is retained in tissue cells in a low-toxicity and low-activity form (phosphate, pectinate, protein-bound and oxalate), with a small fraction located on leaf surfaces and in cuticular cells. After entering the cells, Hg was primarily retained in the cell wall (26.7-47.9%), with HC-2 demonstrating maximal retention (88.8-96.6%). As much as 61.3-91.5% of organelle-associated Hg was localized in chloroplasts. The -OH functional group in HC-2 might play an important role in Hg retention, closing a significant gap in our understanding of the underlying mechanisms. Furthermore, we discovered that after the removal of Hg stress, <i>T. usneoides</i> did not release Hg for a month. However, there was a tendency for Hg in the tissue and surface to be transported toward the cuticle. Our findings expand the understanding of plant leaf-atmosphere Hg interactions and reveal the intrinsic mechanisms of Hg detoxification in <i>T. usneoides</i>.