Organic acid complexation, heavy metal distribution and the effect of ATPase inhibition in hairy roots of hyperaccumulator plant species.

Heavy metal uptake and distribution were investigated in hairy roots of the Cd hyperaccumulator, Thlaspi caerulescens, and the Ni hyperaccumulator, Alyssum bertolonii. Hairy roots of both species contained high constitutive levels of citric, malic and malonic acids. After treatment with 20 ppm Cd or 25 ppm Ni, about 13% of the total Cd in T. caerulescens roots and 28% of the total Ni in A. bertolonii were associated with organic acids. T. caerulescens and A. bertolonii hairy roots remained healthy and grew well at high concentrations of Cd and Ni, respectively, whereas hairy roots of the non-hyperaccumulator, Nicotiana tabacum, did not. Most of the Cd in T. caerulescens and N. tabacum roots was localised in the cell walls. In contrast, 85-95% of the Ni in A. bertolonii and N. tabacum was associated with the symplasm. Growth of T. caerulescens and A. bertolonii hairy roots was severely reduced in the presence of diethylstilbestrol (DES), an inhibitor of plasma membrane H(+)-ATPase. Treatment with DES increased the concentration of Cd in the symplasm of T. caerulescens about 6-fold with retention of root viability, whereas viability and Ni transport across the plasma membrane were both reduced in A. bertolonii. These results suggest that the mechanisms of Cd tolerance and hyperaccumulation in T. caerulescens hairy roots are capable of withstanding the effects of plasma membrane depolarisation, whereas Ni tolerance and hyperaccumulation in A. bertolonii hairy roots are not.