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We investigated the effect of cadmium (Cd) on leaves of 3-week-old Arabidopsis thaliana with a particular interest on glutathione (GSH) production and consumption, and antioxidative defence. Plants were exposed to either 1 or 10 mM Cd for 1 week. Several genes and correspondent proteins known to participate in the regulation of redox states in plants were analysed in Cdstressed and unstressed plants. Cd induced a significant increase in the messenger RNA level of genes involved in GSH synthesis (gsh1 and gsh2) and phytochelatin synthase (pcs1). We observed a significant decrease of reduced GSH in Cd-treated plants. In parallel, an increase of phytochelatin (PC), predominantly PC2 was observed. Cd treatment increased the accumulation of glutathione disulphide (GSSG), keeping the GSH/GSSG ratio lower than in control plants. The accumulation of GSSG was accompanied by a decrease of the glutathione reductase (gr) transcript level, while the activities of GR and nicotinamide nucleotide phosphate-reducing enzymes were significantly enhanced. The antioxidative defence mechanism related to the ascorbate (AsA)-GSH cycle was studied in parallel. A general increase of reactive oxygen species scavenging enzymes such as ascorbate peroxidase, catalase or superoxide dismutase was observed. Our data suggest that the plants respond to Cd stress by the AsA-GSH defence network at both transcriptional and enzymatic level. Taken together, it appears that Arabidopsis plants exposed to 1 mM Cd were able to adopt a new metabolic equilibrium, allowing them to cope with this metal. However, when exposed to 10 mM Cd, loss of cellular redox homeostasis resulted in oxidative stress and toxicity.

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Summary• We examined the subcellular cadmium (Cd) localization in roots and leaves of wild-type Arabidopsis thaliana (ecotype Columbia) exposed to environmentally relevant Cd concentrations.• Energy-dispersive X-ray microanalysis (EDXMA) was performed on high-pressure frozen and freeze-substituted tissues.• In the root cortex, Cd was associated with phosphorus (Cd/P) in the apoplast and sulfur (Cd/S) in the symplast, suggesting phosphate and phytochelatin sequestration, respectively. In the endodermis, sequestration of Cd/S was present as fine granular deposits in the vacuole and as large granular deposits in the cytoplasm. In the central cylinder, symplastic accumulation followed a distinct pattern illustrating the importance of passage cells for the uptake of Cd. In the apoplast, a shift of Cd/S granular deposits from the middle lamella towards the plasmalemma was observed. Large amounts of precipitated Cd in the phloem suggest retranslocation from the shoot. In leaves, Cd was detected in tracheids but not in the mesophyll tissue.• Extensive symplastic and apoplastic sequestration in the root parenchyma combined with retranslocation via the phloem confirms the excluder strategy of Arabidopsis thaliana .

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