Cadmium-inducible expression of the ABC-type transporterAtABCC3increases phytochelatin-mediated cadmium tolerance inArabidopsis

The heavy metal cadmium (Cd) is a widespread environmental contaminant with harmful effects on living cells. In plants, phytochelatin (PC)-dependent Cd detoxification requires that PC-Cd complexes are transported into vacuoles. Here, it is shown thatArabidopsis thalianaseedlings defective in the ABCC transporter AtABCC3 (abcc3) have an increased sensitivity to different Cd concentrations, and that seedlings overexpressingAtABCC3(AtABCC3ox) have an increased Cd tolerance. The cellular distribution of Cd was analysed in protoplasts fromabcc3mutants andAtABCC3overexpressors grown in the presence of Cd, by means of the Cd-specific fluorochromes 5-nitrobenzothiazole coumarin (BTC-5N) and Leadmium™ Green AM dye. This analysis revealed that Cd is mostly localized in the cytosol ofabcc3mutant protoplasts whereas there is an increase in vacuolar Cd in protoplasts from AtABCC3ox plants. Overexpression ofAtABCC3incad1–3mutant seedlings defective in PC production and in plants treated with l-buthionine sulphoximine (BSO), an inhibitor of PC biosynthesis, had no effect on Cd tolerance, suggesting thatAtABCC3acts via PCs. In addition, overexpression ofAtABCC3inatabcc1 atabcc2mutant seedlings defective in the Cd transporters AtABCC1 and AtABCC2 complements the Cd sensitivity of double mutants, but not in the presence of BSO. Accordingly, the level ofAtABCC3transcript in wild type seedlings was lower than that ofAtABCC1andAtABCC2in the absence of Cd but higher after Cd exposure, and even higher inatabcc1 atabcc2mutants. The results point to AtABCC3 as a transporter of PC-Cd complexes, and suggest that its activity is regulated by Cd and is co-ordinated with the activity of AtABCC1/AtABCC2.