Submitted to: Phytochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/26/2009
Publication Date: 2/15/2010
Citation: Mcnear, D.H., Chaney, R.L., Sparks, D.L. 2010. The Metal Hyperaccumulator Alyssum murale Uses Nitrogen and Oxygen Donor Ligands for Ni Transport and Storage. Phytochemistry. 71:188-200. Interpretive Summary: Rare plant species have the ability to accumulate 100-1000 fold higher Ni concentration in their leaves than most species; they have been called hyperaccumulators. This rare ability has been developed to phytoextract Ni and Co from contaminated or mineralized soils. Plant biomass with over 2% Ni (dry weight basis) can be an important source of commercial Ni ore, or of Ni fertilizer. Thus study of how and why the plants achieve the remarkable hyperaccumulation has become the subject of basic studies by several research groups. In cooperation with McNear and Sparks at the University of Delaware, the forms of Ni translocated by Alyssum murale were examined using diverse techniques including X-ray Absorption Spectrometry. Isolated xylem sap from solution or soil grown plants was examined for concentrations of organic and amino acids. Forms of Ni in the plant tissues were examined using XAS and associated methods which can identify the chemical bonds within the tissue between bound molecules and Ni. In the leaf dermal tissue, Ni is present as malate chelates, and some associated with sulfate. In the stem xylem exudate, Ni was present both as the histidine chelate and the malate chelate and perhaps some other low molecular weight organic acids. Thus, Ni is pumped into the xylem where it is largely associated with histidine, and then stored in leaf cell vacuoles as the malate chelate or sulfate complex. These findings clarify the chemical forms of Ni in the hyperaccumulator Alyssum murale, and suggest processes in roots and leaves required to achieve hyperaccumulation.
Technical Abstract: The Kotodesh genotype of the nickel (Ni) hyperaccumulator Alyssum murale was examined to determine the compartmentalization and internal speciation of Ni, and other elements, in an effort to ascertain the mechanism used by this plant to tolerate extremely high shoot Ni concentrations. Plants were grown either hydroponically or in Ni enriched soils from an area surrounding an historic Ni refinery in Port Colborne, Ontario, Canada. Electron probe microanalysis (EPMA) and synchrotron based micro x-ray fluorescence (micro-SXRF) spectroscopy were used to determine the metal distribution and co-localization and synchrotron x-ray spectroscopy and ATRFTIR were used to determine the Ni speciation in plant parts and extracted sap. Nickel is concentrated in the dermal leaf and stem tissues of A. murale bound primarily to malate along with other low molecular weight organic ligands and possibly counter anions (e.g., SO42-). Ni is present in the plant sap and vasculature bound to histidine, malate and other low molecular weight compounds. The data presented herein supports a model in which Ni is transported from the roots to the shoots complexed with histidine and stored within the plant leaf dermal tissues complexed with malate, and other low molecular weight organic acids or counter ions.