Abstract
Root cell walls accumulate metal cations both during acquisition from the environment and removal from the protoplast to avoid toxicity, but molecular forms of the metals under field conditions remain elusive. We have identified how copper is bound to cell walls of intact roots of native Thlaspi arvense by combining synchrotron X-ray fluorescence and absorption techniques (XANES and EXAFS) at the nano-, micro-, and bulk scales. The plants grew naturally in sediment in a stormwater runoff basin at copper concentrations typical of urban ecosystems. About 90% of acquired copper is bound in vivo to cell walls as a unique five-coordinate Cu(II)-bis(L-histidinato) complex with one L-histidine behaving as a tridentate ligand (histamine-like chelate) and the other as a bidentate ligand (glycine-like chelate). Tridentate binding of Cu(II) would provide thermodynamic stability to protect cells against copper toxicity, and bidentate binding may enable kinetic lability along the cell wall through protein-protein docking with the non-bonded imidazole group of histidine residues. EXAFS spectra are provided as ESI to facilitate further identification of Cu-histidine and distinction of Cu-N from Cu-O bonds in biomolecules.
Highlights
The multiple roles of copper in biochemical reactions result from the redox-sensitivity of the Cu2+/Cu+ couple, which has elevated standard electrode potentials when complexed in proteins that facilitate electron transfer in cellular processes.[1]
Copper concentration is seven times higher in secondary roots (Rs, 68.3 Æ 24.6 mg kgÀ1 dry weight (DW), n = 8) than in shoots (15.3 Æ 3.3 mg kgÀ1 DW, n = 8), whereas zinc is more evenly distributed between the two organs (Rs = 265.6 Æ 64.8 mg kgÀ1 DW; shoot = 199.5 Æ 56.6 mg kgÀ1 DW) (Fig. 1)
Resistance to anthropogenic emissions of copper occurs by the root accumulating an excess and regulating the amount transported to the shoot
Summary
The multiple roles of copper in biochemical reactions result from the redox-sensitivity of the Cu2+/Cu+ couple, which has elevated standard electrode potentials when complexed in proteins that facilitate electron transfer in cellular processes.[1]. Knowledge of the location and molecular forms of copper in the root is sparse, especially at non-toxic concentrations. This information is critical for developing approaches to preserve copper homeostasis in plants as environmental conditions change, and for applying phytotechnology to remediate contaminated soil and water.[14,15] Results of previous electron microscopy studies show that copper is dominantly contained on cell walls as are other metals.[16,17,18,19] Copper may enter the cell wall through intercellular spaces during the uptake of water from the environment, and as a result of trace metal removal from the protoplast during the sequestration process for detoxifying excess copper. Copper typically is not sequestered in root cortical vacuoles, as is commonly the case for zinc and cadmium detoxification.[20,21,22] Sequestration of copper in vacuoles has been observed only in two Cu-tolerant plants, Armeria maritima sp. halleri grown in the wild on Zn-polluted soil with a root content
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