Subcellular localization and speciation of nickel in hyperaccumulator and non-accumulator Thlaspi species

The ability of Thlaspi goesingense Halacsy to hyperaccumulate Ni appears to be governed by its extraordinary degree of Ni tolerance. However, the phys iological basis of this tolerance mechanism is unknown. We have investigate d the role of vacuolar compartmentalization and chelation in this Ni tolera nce. A direct comparison of Ni contents of vacuoles from leaves of T. goesi ngense and from the non-tolerant non-accumulator Thlaspi arvense L. showed that the hyperaccumulator accumulates approximately 2-fold more Ni in the v acuole than the non-accumulator under Ni exposure conditions that were non- toxic to both species. Using x-ray absorption spectroscopy we have been abl e to determine the likely identity of the compounds involved in chelating N i within the leaf tissues of the hyperaccumulator and non-accumulator. This revealed that the majority of leaf Ni in the hyperaccumulator was associat ed with the cell wall, with the remaining Ni being associated with citrate and His, which we interpret as being localized primarily in the vacuolar an d cytoplasm, respectively. This distribution of Ni was remarkably similar t o that obtained by cell fractionation, supporting the hypothesis that in th e hyperaccumulator, intracellular Ni is predominantly localized in the vacu ole as a Ni-organic acid complex.