Cadmium impairs iron homeostasis in Arabidopsis thaliana by increasing the polysaccharide contents and the iron-binding capacity of root cell walls

Iron (Fe) deficiency is a well-known symptom of cadmium (Cd) toxicity. Here, the mechanisms underlying Cd impairment of Fe homeostasis in Arabidopsis were investigated. Arabidopsis plants were subjected to 0 (CK) or 10 mu M Cd treatments. After the treatments period, Fe concentrations, expression levels of Fe uptake- and translocation-related genes, polysaccharide concentrations, pectin methylesterase activities (PME) and the degree of pectin methylation (DM) were analyzed. Fe concentrations in Cd-treated plants were lower in shoots, but higher in roots, especially in the root apoplasts, compared with the CK, and their leaves developed Fe-deficiency-like symptoms. However, Cd stimulated the root's Fe uptake activity and the expression of genes involved in Fe acquisition. In both treatments, the citrate concentrations and expression levels of Fe translocation-related gene were comparable. The Cd treatment significantly increased pectin and hemicelluloses concentrations in cell walls, while significantly decreased the DM by increasing the PME activity, which led to a higher binding capacity of the cell wall to Fe. The Cd-induced Fe accumulation in roots is mediated by increasing polysaccharide concentrations and decreasing the DM, which increases the Fe retention in roots and hampers its translocation to shoots.