Trichomes of Tobacco Excrete Zinc as Zinc-Substituted Calcium Carbonate and Other Zinc-Containing Compounds

Abstract

(S.F., M.A.M.) Tobacco (Nicotiana tabacum L. cv Xanthi) plants were exposed to toxic levels of zinc (Zn). Zn exposure resulted in toxicity signs in plants, and these damages were partly reduced by a calcium (Ca) supplement. Confocal imaging of intracellular Zn using Zinquin showed that Zn was preferentially accumulated in trichomes. Exposure to Zn and Zn 1 Ca increased the trichome density and induced the production of Ca/Zn mineral grains on the head cells of trichomes. These grains were aggregates of submicrometer-sized crystals and poorly crystalline material and contained Ca as major element, along with subordinate amounts of Zn, manganese, potassium, chlorine, phosphorus, silicon, and magnesium. Micro x-ray diffraction revealed that the large majority of the grains were composed essentially of metal-substituted calcite (CaCO 3 ). CaCO 3 polymorphs (aragonite and vaterite) and CaC 2 O 4 (Ca oxalate) mono-and dihydrate also were identified, either as an admixture to calcite or in separate grains. Some grains did not diffract, although they contained Ca, suggesting the presence of amorphous form of Ca. The presence of Zn-substituted calcite was confirmed by Zn K-edge micro-extended x-ray absorption fine structure spectroscopy. Zn bound to organic compounds and Zn-containing silica and phosphate were also identified by this technique. The proportion of Zn-substituted calcite relative to the other species increased with Ca exposure. The production of Zn-containing biogenic calcite and other Zn compounds through the trichomes is a novel mechanism involved in Zn detoxification. This study illustrates the potential of laterally resolved x-ray synchrotron radiation techniques to study biomineralization and metal homeostasis processes in plants.Smoking of tobacco (Nicotiana tabacum) leaves is one of the principal routes of exposure to heavy metals. Metals contained in tobacco leaves originate from root uptake and transfer to the shoots and also from deposition of aerosol particles on the leaves (Fleisher and Parungo, 1974). Efforts have been made to minimize toxic metal contents in the leaves (LugonMoulin et al., 2004). However, this is not the case for counterfeit tobacco products, which contain higher concentrations of metals than their genuine equivalents (Stephens et al., 2005). Besides its primary application for leaf production, tobacco is also a candidate for phytoextraction. Although this plant species is not a hyperaccumulator, it has several advantages, including a high biomass, moderate soil requirements, fast growth rate, and ease of harvesting. For this particular application, research is conducted with the aim of enhancing metal accumulation in the shoots (Song et al., 2003).Little is known about the mechanisms of metals accumulation and detoxification in tobacco. The exposure of plants to high concentrations of metals generally induces a stress, whose visible symptoms include an inhibition of root elongation, a decrease in shoot growth, leaf chlorosis, and necrosis of the tissues. This stress is ge...

Chemical forms of calcium in Ca,Zn- and Ca,Cd-containing grains excreted by tobacco trichomes

“…Calcium oxalate mono and 34 dihydrate were found occasionally. Calcite and vaterite were substituted by cations, probably Zn, 35 Cd and possibly Mn and Mg. Zn µEXAFS confirmed the occurrence of Zn-substituted calcite, 36 and evidenced Zn associations with other phases including organic compounds, silica and 37 phosphate (3). Cd µXANES showed that cadmium was a Ca substituent in calcite and vaterite 38 and/or sorbed on the surface of these minerals (Isaure et al, in prep.).…”

“…Xanthi). Tobacco exposed to Cd excreted Cd,Ca-containing grains 21 through leaf trichomes (1,2), and a similar excretion of Zn,Ca-containing grains was observed 22 under Zn exposure (3). Trichomes are specialized epidermal structures.…”

“…Such 24 detoxification process may have implication in human health since smoking is one of the 25 principal routes of exposure to heavy metals, and also in phytoremediation as tobacco is a 26 candidate species for phytoextraction. 27 The morphology, elemental composition, mineralogy as well as Zn and Cd speciation in the 28 grains were investigated by scanning electron microscopy coupled with energy dispersive X-ray 29 analysis (SEM-EDX), micro-X-ray fluorescence (µXRF), micro-X-ray diffraction (µXRD), Zn 30 K-edge micro extended X-ray absorption fine structure (µEXAFS) (3) and Cd L III edge micro X-31 ray absorption near edge structure (µXANES) spectroscopy (Isaure et al, in prep.). Calcium was 32 always the major component.…”

Structural analysis of metal-doped calcium oxalate

“…As both biogenic and non-biogenic crystals have been shown to serve as hosts for organic molecules 1, 2 and for metal ions, their interaction has been extensively studied. In particular, calcium-bearing minerals that plant organisms produce, [3][4][5][6][7][8][9][10] as well as calcium carbonate and hydroxyapatite, [11][12][13][14] have been shown to absorb or exchange foreign cations effectively. [14][15][16] A variety of cations have been tested for their sorption behavior especially on calcium carbonate and hydroxyapatite.…”

Comparative Metabolomics of Transgenic Tobacco Plants (Nicotiana tabacum var. Xanthi) Reveals Differential Effects of Engineered Complete and Incomplete Flavonoid Pathways on the Metabolome

“…In addition, oxalic acid accumulated at higher levels in ANR plants in a pattern similar to that of the cembranoids. Oxalate, which accumulates in glandular trichomes and idioblast cells of Nicotiana species (Choi et al 2001;Sarret et al 2006;VolkA and Franceschi 2000), is the calcium salt of oxalic acid and has important defensive activities against chewing insects (Korth et al 2006). We microscopically examined trichomes, but did not observe changes of their morphology and density on the surfaces of stems and leaves between transgenic and WT tobacco plants (Fig.…”