Arabidopsis thaliana Metallothioneins differential expression in Fe and Ag-induced stress: An organ-specific analysis

Pollution of soils by heavy metals (HM) is a concerning result of anthropogenic activities. Iron (Fe) and silver (Ag) are HM-deemed essential and non-essential for plants and can induce toxicity when in excess. Metallothioneins (MTs) are small Cys-rich proteins involved in HM binding and oxidative stress mitigation. This study focused on Arabidopsis thaliana MTs types I, II, and III involvement in response to increasing concentrations of Fe and Ag in an organ-specific way - shoots and roots - through RT-qPCR analysis 21 days after germination, as well as biometric and biochemical assessments. The in vivo heterologous expression of AtMT2b in Nicotiana tabacum leaves was performed. Both HM reduced plant growth, with Fe accumulating dose-dependently in both shoots and roots, while Ag mainly accumulated in roots, albeit only at the highest concentration in shoots. Fe exposure caused little change in MT expression, whereas Ag strongly induced it in shoots and, to a lesser extent, in roots (particularly AtMT1a and AtMT1c). Biochemical analyses revealed distinct stress responses: Fe increased lipid peroxidation (MDA) and antioxidant compounds (GSH, thiols) mainly in shoots, while Ag triggered oxidative metabolism predominantly in shoots, with elevated HO, MDA, GSH, thiols, and proline levels. Both HM reduced photosynthetic pigments, notably -carotenes with Fe, and chlorophylls, lutein, and -carotenes with Ag. In vivo localisation showed AtMT2b to be cytosolic. Overall, Fe and Ag induced stress in A. thaliana with organ-specific responses, and MTs played a minor role in Fe tolerance but were strongly activated by Ag, especially in shoots.