Tobacco excretes zinc and cadmium in metal-substituted CaCO<,3> crystals through leaf trichomes

摘要

Recently, tobacco (Nicotiana tabacum L. cv Xanthi) was shown to develop an original mechanism of cadmium detoxification by exudation of Cd/Ca-containing particles through leaf trichomes. In the present study, the possible detoxification of Zn by a similar mechanism was tested. Plants were exposed to Zn with or without a supplement of Ca, and Zn toxicity was partly reduced by the Ca supplement. An accumulation of Zn at the tip of the trichomes and an excretion of Zn/Ca-containing grains were observed. Grains produced under Cd or Zn exposure with or without a supplement of Ca were characterized for their morphology, chemical and mineralogical composition, and local structure of Cd and Zn using electron microscopy and synchrotron techniques. In all cases, grains were aggregates of submicrometer-sized crystals and poorly crystalline material and contained Ca as major element, along with lesser amounts of Zn, Cd, Mn, K, Cl, P, Si and Mg depending on the condition tested. Micro X-ray diffraction showed that the mjor crystallized phase was substituted calcite (CaCO3). CaCO3 polymorphs (aragonite and vaterite), Ca oxalate mono- and dihydrate and amorphous Ca were also found. Zn was immobilized in the grains as a Ca substituent in calcite and associated with organic compounds, silica and phosphate. Cd was presented as a Ca substituent in magnesium calcite and vaterite. The production of metal-containing biogenic calcium carbonate minerals through leaf trichomes is a previously unknown mechanism of metal detoxificatior~ This study illustrates the potential of laterally resolved Xray synchrotron radiation techniques to study biomineralization and metal homeostasis processes in plants.