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 distribution 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 μM), being more pronounced in the Tl-hyperaccumulating plants at the highest treatment (100 µM of Tl). Elemental analysis showed Tl concentrations > 100 µg g^−1 in both accessions under all treatments, reaching up to 6910 µg g^−1 in Tl-hyperaccumulating plants treated with 100 µM of Tl. Compartmentalization 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 accession, the localization of Tl in the mesophyll explains the toxic effect. Differences in Tl localization are of significant importance for deciphering the molecular and genetic bases of this extraordinary phenomenon.
Regini et al. (Thu,) studied this question.