Characterization of mechanisms involved in tolerance and accumulation of Cd in Biscutella auriculata L.

Biscutella auriculata L. is one of the rare species that is able to grow in a very contaminated mining area in Villamayor de Calatrava (Ciudad Real, Spain). In an effort to understand the mechanisms involved in the tolerance of this plant to high metal concentrations, we grew B. auriculata in the presence of 125 μM Cd(NO3)2 for 15 days and analysed different parameters associated with plant growth, nitric oxide and reactive oxygen species metabolism, metal uptake and translocation, photosynthesis rate and biothiol (glutathione and phytochelatins) content. Treatment with Cd led to growth inhibition in both the leaves and the roots, as well as a reduction of photosynthetic parameters, transpiration and stomatal conductance. The metal was mainly accumulated in the roots and in the vascular tissue, although most Cd was detected in areas surrounding their epidermal cells, while in the leaves the metal accumulated mainly in spongy mesophyll, stomata and trichrome. Based on the Cd bioaccumulation (5.93) and translocation (0.15) factors, this species denoted enrichment of the metal in the roots and its low translocation to the upper tissues. Biothiol analysis showed a Cd-dependent increase of reduced glutathione (GSH) as well as the phytochelatins (PC2 and PC3) in both roots and leaves. Cd-promoted oxidative damage occurred mainly in the leaves due to disturbances in enzymatic and nonenzymatic antioxidants, while the roots did not show significant damage as a result of induction of antioxidant defences. It can be concluded that B. auriculata is a new Cd-tolerant plant with an ability to activate efficient metal-sequestering mechanisms in the root surface and leaves and to induce PCs, as well as antioxidative defences in roots.