An X-ray absorption spectroscopy investigation of speciation and biotransformation of copper in Elsholtzia splendens

Abstract

Elsholtzia splendens is a Cu-tolerant plant growing in copper mine areas in the south of China. In this study, X-ray absorption spectroscopy (XAS) was used to investigate the Cu speciation and biotransformation in E. splendens with 300 μM Cu treatment from 10 days to 60 days. The results showed that 300 μM Cu was phytotoxic to E. spendens. The Cu Kedge X-ray absorption near edge structure (XANES) revealed that most copper in roots, stems and leaves exists as divalent Cu. Cu speciation changed depending on the treatment time, but there was no unidirectional trend in roots, stems, and leaves. The percentages of potential Cu ligands in all samples were estimated by fitting the XANES spectra with linear combinations. Most Cu in roots, stems and leaves was bound with cell wall and histidine (His)-like ligands, while a minor proportion of the Cu was bound to oxalate and glutathione-like ligands. The fitting results of Cu K-edge extended X-ray absorption fine structure (EAXFS) showed that nitrogen/oxygen (N/O) ligands were dominant in roots, stems and leaves of the plant, while S ligands were rare. All these results suggest that Cu bound by N/O ligands plays a key role in Cu detoxification of E. splendens, and a role for classical metal-detoxifying S ligands, such as metallothioneins and phytochelatins, in Cu detoxification of E. splendens is not supported in the present study. Due to the phytotoxicity of 300 μM Cu to E. splendens, the question of whether S ligands play a significant role in Cu detoxification in E. splendens exposed to lower levels of Cu should be further studied.

In Situ Distribution and Speciation of Toxic Copper, Nickel, and Zinc in Hydrated Roots of Cowpea

“…For example, in the roots, stems, and leaves of E. splendens (a Cutolerant plant) grown in 300 mM Cu for 10 to 60 d, most Cu was bound to O-containing ligands in the cell wall (Shi et al, 2008). Similarly, Kü pper et al (2009) reported that, in frozen, hydrated shoots of Crassula helmsii grown for 8 d at 10 mM Cu, Cu was bound by O ligands (such as organic acids).…”

“…To our knowledge, no studies meeting all of these criteria have investigated the speciation of Cu, Ni, or Zn in plant roots. For example, many studies use high and environmentally irrelevant concentrations of metals, particularly for Cu where the higher concentrations are most likely used to increase plant uptake and hence improve the signal-noise ratio within analyses ; the extended x-ray absorption fine structure (EXAFS) analyses of Sahi et al (2007) used 630 mM Cu while those of Shi et al (2008) used 300 mM Cu. In contrast, studies have shown Cu to be toxic at concentrations ,10 mM (Kopittke et al, 2010).…”

Complexation and Toxicity of Copper in Higher Plants. II. Different Mechanisms for Copper versus Cadmium Detoxification in the Copper-Sensitive Cadmium/Zinc Hyperaccumulator Thlaspi caerulescens (Ganges Ecotype)

“…In their EXAFS measurements on samples frozen in liquid nitrogen, they found evidence for a Cu(II)-phytochelatin complex involved in transport and an unknown Cu complex involved in Cu storage. A second study using the same sample preparation method (that due to the formation of a gas layer around the sample slows freezing, resulting in ice formation with the risk of membrane damage) investigated the Cu-tolerant Cu-excluder plant Elsholtzia splendens (Shi et al, 2008). Unfortunately, these authors applied very high, physiologically not relevant Cu levels (300 mM) in their nutrient solution in order to force enough Cu into the tissues to get an acceptable signal-noise level in their EXAFS data within the limited synchrotron beam time.…”

Sorption and Distribution of Copper in Unsaturated Pseudomonas putida CZ1 Biofilms as Determined by X-Ray Fluorescence Microscopy

“…Reference spectra were obtained from pure compounds of Cu-citrate, Cu 3 (PO 4 ) 2 and CuS. Spectra were also collected from a Cu-glutathione reference sample which was prepared as described previously (28). A Cu-cysteine reference sample containing 5 mM copper was prepared by adding the appropriate volumes of a 100 mM CuSO 4 solution to a 50 mM cysteine solution.…”

“…The data were analyzed with the WinXAS version 3.1 program as previously described (8,28). The spectra were normalized by fitting first-and second-order polynomial functions to the pre-and postedge regions, respectively.…”