Hyperaccumulation of cadmium by hairy roots of Thlaspi caerulescens

Hairy roots were used to investigate cadmium uptake by Thlaspi caerulescens , a metal hyperaccumulator plant with potential applications in phytoremedi ation 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 mu g g(-1) dry weight, or 6.3% dry wei ght, at a liquid Cd concentration of 3710 ppm. Cd levels in live T. caerule scens roots were 1.5- to 1.7-fold those in hairy roots of nonhyperaccumulat or species exposed to the same Cd concentration, but similar to the Cd cont ent of autoclaved T. caerulescens roots. The ability to grow at Cd concentr ations of up to 100 ppm clearly distinguished T. caerulescens hairy roots f rom the nonhyperaccumulators. The specific growth rate of T. caerulescens r oots was essentially unaffected by 20 to 50 ppm Cd in the culture medium; i n contrast, N. tabacum roots turned dark brown at 20 ppm and growth was neg ligible. Up to 10,600 mu g 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 transpor ted directly into the symplasm of N. tabacum roots within 3 days of exposur e; in contrast, T. caerulescens roots stored virtually all of their Cd in t he wall fraction for the fi rst 7 to 10 days. Th is delay in transmembrane uptake may represent an important defensive strategy against Cd poisoning i n T. caerulescens, allowing time for activation of intracellular mechanisms for heavy metal detoxification. (C) 2000 John Wiley & Sons, Inc.