Detection and quantification of ligands involved in nickel detoxification in a herbaceous Ni hyperaccumulator Stackhousia tryonii Bailey

Abstract

Field-collected, young plants of Ni hyperaccumulator Stackhousia tryonii, grown in a glasshouse for 20 weeks, were exposed to low- (available Ni concentration in the native serpentine soil, i.e. 60 microg g(-1) dry soil) and high- (external application of 1000 ppm) Ni concentrations in the substrate. Nickel concentration in the freeze-dried leaf tissues increased from 3700 microg g(-1) to 13 700 microg g(-1) with soil Ni supplementation, of which >60% was extracted with dilute acid (0.025 M HCl). Nickel supplementation also elicited a 575%, 211%, and 37% increase in the final concentrations of oxalic, citric, and malic acids, respectively, in leaf tissues. Malic acid was the dominant organic acid, followed by citric and oxalic acids. The molar ratio of Ni to malic acid was 1.0, consistent with a role for malate as a ligand for Ni in hyperaccumulating plants, supporting detoxification/transport and storage of this heavy metal in S. tryonii. The total amino acid concentrations in the xylem sap did not change with Ni supplementation (21.7+/-3.7 mM and 17.9+/-5 mM, respectively, for low- and high-nickel-treated plants). Glutamine was the major amino acid in both the low- and high-Ni-treated plants. The concentration of glutamine decreased by >60%, with a corresponding increase in alanine, aspartic acid, and glutamic acid, on exposure to high Ni. A role of amino acids in Ni complexation and transport in S. tryonii is not immediately apparent.

Heavy Metals in Crop Plants: Transport and Redistribution Processes on the Whole Plant Level

“…The availability of complexing agents in a plant cell may be relevant for the retention in the cell (e.g., transport across the tonoplast and storage in the vacuole), the transfer to neighbor cells through plasmodesmata and the release into the apoplast for xylem or phloem loading. Sensitive localization techniques for heavy metals and metabolic studies for ligand availability may serve as a basis for more detailed studies [59,[62][63][64][65][66]71,78,111].…”

“…These elements were present in soluble form, but only a minor portion reached the vascular cylinder suggesting that compartmentation played a major role [58][59][60]. The abundance of ligands and the formation of heavy metal complexes with organic acids [62,63], with phytochelatin [64] or with nicotianamine [65] were found to be important for the retention in the roots. Good evidence was presented for the involvement of Ni-histidine complexes in the vacuolar compartmentation of Ni and as a consequence in the retention of this heavy metal in roots [59].…”

“…Ligands were found to be important in hyperaccumulators for vacuolar sequestration and for the transport of heavy metals in the xylem [59,62,65]. Genetic engineering of the metabolism may lead to increased concentrations of ligands important for xylem loading and to decreased levels of ligands leading to vacuolar sequestration causing a retention of heavy metals in the roots [59,62,65,105,106].…”

“…Genetic engineering of the metabolism may lead to increased concentrations of ligands important for xylem loading and to decreased levels of ligands leading to vacuolar sequestration causing a retention of heavy metals in the roots [59,62,65,105,106]. Transporters for heavy metals or heavy metal complexes represent other targets for classical breeding and genetic engineering to improve properties of plants envisaged for phytoremediation [107,108].…”

The changes of contents of selected free amino acids associated with cadmium stress in Noccaea caerulescens and Arabidopsis halleri

“…metallothioneins and phytochelatins), and nitrogen donor ligands (e.g. amino acids) (Bhatia et al 2005).…”