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

Abstract

Atmospheric hydrargyrum (Hg) is a neurotoxic heavy metal, and plant leaves are active Hg reservoirs. Tillandsia usneoides is an indicator plant for atmospheric Hg pollution; however, the uptake, transport modes, and redistribution mechanisms of Hg in T. usneoides are underexplored. Herein, we investigated these mechanisms and the influencing factors of Hg⁰ in T. usneoides 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, T. usneoides 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 T. usneoides.