No abstract

Hairy roots were used to investigate cadmium uptake by Thlaspi caerulescens, a metal hyperaccumulator plant with potential applications in phytoremediation and phytomining. Experiments were carried out in nutrient media under conditions supporting root growth. Accumulation of Cd in short-term (9-h) experiments varied with initial medium pH and increased after treating the roots with H(+)-ATPase inhibitor. The highest equilibrium Cd content measured in T. caerulescens roots was 62,800 microg g(-1) dry weight, or 6.3% dry weight, at a liquid Cd concentration of 3710 ppm. Cd levels in live T. caerulescens roots were 1.5- to 1.7-fold those in hairy roots of nonhyperaccumulator species exposed to the same Cd concentration, but similar to the Cd content of autoclaved T. caerulescens roots. The ability to grow at Cd concentrations of up to 100 ppm clearly distinguished T. caerulescens hairy roots from the nonhyperaccumulators. The specific growth rate of T. caerulescens roots was essentially unaffected by 20 to 50 ppm Cd in the culture medium; in contrast, N. tabacum roots turned dark brown at 20 ppm and growth was negligible. Up to 10,600 microg g(-1) dry weight Cd was accumulated by growing T. caerulescens hairy roots. Measurement of Cd levels in whole roots and in the cell wall fraction revealed significant differences in the responses of T. caerulescens and N. tabacum roots to 20 ppm Cd. Most metal was transported directly into the symplasm of N. tabacum roots within 3 days of exposure; in contrast, T. caerulescens roots stored virtually all of their Cd in the wall fraction for the first 7 to 10 days. This delay in transmembrane uptake may represent an important defensive strategy against Cd poisoning in T. caerulescens, allowing time for activation of intracellular mechanisms for heavy metal detoxification.

Hairy roots were used to investigate nickel uptake by the hyperaccumulator species, Alyssum bertolonii, A. tenium, and A. troodii. The Ni biosorption capacity of A. tenium hairy roots was lower than for other types of biomass such as bacteria and algae; in short-term (9-h) equilibrium studies, the highest Ni content measured in the roots was 17 500 microg g(-1) dry weight at a liquid concentration of about 4000 ppm. Using long-term hairy root cultures, it was demonstrated that Ni tolerance and hyperaccumulation do not necessarily depend on the presence of shoots or root-shoot translocation. A. bertolonii hairy roots remained healthy in appearance and continued to grow in the presence of 20-100 ppm Ni, accumulating up to 7200 microg g(-1) dry weight Ni. In contrast, hairy roots of Nicotiana tabacum turned dark brown at 20 ppm Ni and growth was negligible. The ability to grow at high external Ni concentrations allowed hyperaccumulator hairy roots to remove much greater amounts of heavy metals from the culture liquid than nonhyperaccumulator hairy roots, even though biomass Ni concentrations were similar. Although hairy roots proved to be a useful tool for investigating Ni hyperaccumulation, there were significant differences in the Ni uptake capacity of hairy roots and whole plants. Regenerated plants of A. tenium were much more tolerant of Ni and capable of accumulating higher Ni concentrations than hairy roots of this species.

Hairy roots were used to investigate cadmium uptake by Thlaspi caerulescens, a metal hyperaccumulator plant with potential applications in phytoremediation and phytomining. Experiments were carried out in nutrient media under conditions supporting root growth. Accumulation of Cd in short‐term (9‐h) experiments varied with initial medium pH and increased after treating the roots with H+‐ATPase inhibitor. The highest equilibrium Cd content measured in T. caerulescens roots was 62,800 μg g−1 dry weight, or 6.3% dry weight, at a liquid Cd concentration of 3710 ppm. Cd levels in live T. caerulescens roots were 1.5‐ to 1.7‐fold those in hairy roots of nonhyperaccumulator species exposed to the same Cd concentration, but similar to the Cd content of autoclaved T. caerulescens roots. The ability to grow at Cd concentrations of up to 100 ppm clearly distinguished T. caerulescens hairy roots from the nonhyperaccumulators. The specific growth rate of T. caerulescens roots was essentially unaffected by 20 to 50 ppm Cd in the culture medium; in contrast, N. tabacum roots turned dark brown at 20 ppm and growth was negligible. Up to 10,600 μg g−1 dry weight Cd was accumulated by growing T. caerulescens hairy roots. Measurement of Cd levels in whole roots and in the cell wall fraction revealed significant differences in the responses of T. caerulescens and N. tabacum roots to 20 ppm Cd. Most metal was transported directly into the symplasm of N. tabacum roots within 3 days of exposure; in contrast, T. caerulescens roots stored virtually all of their Cd in the wall fraction for the first 7 to 10 days. This delay in transmembrane uptake may represent an important defensive strategy against Cd poisoning in T. caerulescens, allowing time for activation of intracellular mechanisms for heavy metal detoxification. © 2000 John Wiley & Sons, Inc. Biotechnol Bioeng 67: 607–615, 2000.