Phytohormone and phytochelatin metabolism in Arabidopsis thaliana treated with cadmium, copper and zinc in different combinations

Excess cadmium (Cd), copper (Cu) and zinc (Zn) are known to induce stress effects in plants. However, while Cu and Zn are part of many cell macromolecules, plants have no known metabolic function for Cd. Arabidopsis thaliana L. can be considered a model plant for -studies on metal homeostasis/detoxification, as its biochemical, physiological and morphological features are strongly affected by metal exposure. In this work, seedlings of A. thaliana were exposed to Cd, Cu and Zn at concentrations of 10, 5 and 150 μM, respectively. After 12 days of exposure to metals, applied separately or in combinations, plant shoots and roots were sampled and analyzed for morphological analysis (microscopy), metal content (ICP-MS), phytohormone and phytochelatin (PC) levels (LC-MS), and expression of the genes involved in phytohormone and PC metabolism (q-PCR). Microscopic analysis revealed that the root morphology, in terms of root apex number, branching degree and mean diameter, was strongly affected by metal exposure, both alone or in combination. The differentiated growth patterns observed in shoots and roots were accompanied by different levels and ratios of auxins and cytokinins and the main regulatory genes involved in auxin and cytokinin synthesis (NIT1 and IPT7) and degradation (AAO1 and CKX1) were regulated by the metal treatments. Phytochelatin synthesis was significantly induced by all metals, but the expression of the genes involved in glutathione and PC biosynthesis (GSH1, GSH2, PCS1 and PCS2) was not influenced. This study could reveal cross-talks between phytohormones and PC metabolism in A. thaliana plants growing in a metal multi-pollution context.