Calcium promotes cadmium elimination as vaterite grains by tobacco trichomes

In tobacco plants, elimination of Zn and Cd via the production of Ca-containing grains at the top of leaf hairs, called trichomes, is a potent detoxification mechanism. This study examines how Cd is incorporated in these biominerals, and how calcium growth supplement modifies their nature. Scanning electron microscopy coupled with energy dispersive X-ray microanalysis (SEM-EDX), microfocused X-ray diffraction (l-XRD), and microfocused X-ray absorption near edge structure (l-XANES) spectroscopy were used to image the morphology of the grains, identify the crystallized mineral phases, and speciate Cd, respectively. The mineralogy of the grains and chemical form of Cd varied with the amount of Ca. When tobacco plants were grown in a nutrient solution containing 25 lM Cd and low Ca supplement (Ca/Cd = 11 mol ratio), most of the grains were oblong-shaped and low-Cd-substituted calcite. When exposed to the same amount of Cd and high Ca supplement (Ca/Cd = 131 mol ratio), grains were more abundant and diverse in compositions, and in total more Cd was eliminated. Most grains in the high Ca/Cd experiment were round-shaped and composed predominantly of Cd-substituted vaterite, a usually metastable calcium carbonate polymorph, and subordinate calcite. Calcium oxalate and a Ca amorphous phase were detected occasionally in the two treatments, but were devoid of Cd. The biomineralization of cadmium and implications of results for Cd exposure of smokers and phytoremediation are discussed. 2010 Elsevier Ltd. All rights reserved. * Corresponding author. Present address: Marie-Pierre Isaure, Institut Pluridisciplinaire de Recherche sur l’Environnement et les Matériaux/Laboratoire de Chimie Analytique Bio-Inorganique et Environnement (IPREM-UMR 5254/LCABIE), Université de Pau et des Pays de l’Adour, Hélioparc, 2 Avenue Pierre Angot, 64053 PAU Cedex 9 France. Tel.: +33 5 40 17 50 53. E-mail address: [email protected] (M.-P. Isaure). 1 Present address: Research Institute for Sustainable Humanosphere (RISH), Kyoto University Gokasho, Uji 611-0011 Kyoto Japan. 2 Present address: Laboratoire Interdisciplinaire Carnot de Bourgogne UMR 5209 CNRS and Université de Bourgogne, BP 47870, 21078 Dijon cedex, France. 3 Present address: Department of Plant Physiology, University of Bayreuth, Universitaetsstrasse 30, D-95440 Bayreuth, Germany.