Subcellular Localization and Speciation of Nickel in Hyperaccumulator and Non-Accumulator <i>Thlaspi</i>Species

TLDR

The hyperaccumulator Thlaspi goosingense Hálácsy accumulates nickel through extraordinary tolerance, yet the physiological basis of this tolerance remains unknown. The study investigates how vacuolar compartmentalization and chelation contribute to nickel tolerance in Thlaspi goosingense. X‑ray absorption spectroscopy was employed to identify the nickel‑chelating compounds in leaf tissues of both hyperaccumulator and non‑accumulator species. The hyperaccumulator accumulates about twice as much nickel in vacuoles as the non‑accumulator, with most leaf nickel bound to cell walls and the remainder complexed with citrate and histidine in vacuoles and cytoplasm, confirming that intracellular nickel is mainly stored as a Ni‑organic acid complex in the vacuole.

Abstract

Abstract The ability of Thlaspi goesingense Hálácsy to hyperaccumulate Ni appears to be governed by its extraordinary degree of Ni tolerance. However, the physiological basis of this tolerance mechanism is unknown. We have investigated the role of vacuolar compartmentalization and chelation in this Ni tolerance. A direct comparison of Ni contents of vacuoles from leaves of T. goesingense and from the non-tolerant non-accumulator Thlaspi arvense L. showed that the hyperaccumulator accumulates approximately 2-fold more Ni in the vacuole than the non-accumulator under Ni exposure conditions that were non-toxic to both species. Using x-ray absorption spectroscopy we have been able to determine the likely identity of the compounds involved in chelating Ni within the leaf tissues of the hyperaccumulator and non-accumulator. This revealed that the majority of leaf Ni in the hyperaccumulator was associated with the cell wall, with the remaining Ni being associated with citrate and His, which we interpret as being localized primarily in the vacuolar and cytoplasm, respectively. This distribution of Ni was remarkably similar to that obtained by cell fractionation, supporting the hypothesis that in the hyperaccumulator, intracellular Ni is predominantly localized in the vacuole as a Ni-organic acid complex.