Cellular-level imaging reveals distinct thallium detoxification strategies in Silene latifolia accessions

The facultative thallium (Tl) hyperaccumulator Silene latifolia can attain the highest known concentrations of Tl of any plant species. This trait is, however, differentiated at the population level with only two known Tl-tolerant and hyperaccumulating accessions occurring at metalliferous sites. This study aimed to reveal the internal dis tribution of Tl in plant organs and tissues. Silene latifolia subsp. alba from Tl-hyperaccumulating and Tl-sensitive accessions were grown hydroponically under different Tl treatments and analyzed to determine Tl accumulation. Plant organs were investigated using synchrotron X-ray fluorescence microtomography to determine cellular- level distribution of Tl and other elements. The results reveal markedly different Tl distribution patterns, with mainly vacuolar localization in the epidermis of the hyperaccumulating population and a more even distribution across various cell types in the sensitive population. The differences were found even at the same applied concentrations (5 μ M), being more pronounced in the Tl-hyperaccumulating plants at the highest treatment (100 µM of Tl). Elemental analysis showed Tl concentrations > 100 µg g ments, reaching up to 6910 µg g 1 1 in both accessions under all treat in Tl-hyperaccumulating plants treated with 100 µM of Tl. Compartmen talization in the vacuoles of epidermal cells proved to be a key mechanism for Tl hyperaccumulation in the tolerant accession, acting in both detoxification and offers protection against herbivory. In the sensitive acces sion, the localization of Tl in the mesophyll explains the toxic effect. Differences in Tl localization are of sig nificant importance for deciphering the molecular and genetic bases of this extraordinary phenomenon.