Ultrastructural localization of zinc in zinc-tolerant Armeria maritima ssp halleri by autometallography

In the angiosperm plant, Armeria maritima ssp. halleri, growing on soils enriched with zinc and lead, zinc was localized at the ultrastructural level using a histochemical method called autometallography. This method is based on the conversion of available metals in the tissue to metal sulphide molecules upon which metallic silver is subsequently deposited when the tissue is placed in a solution of gum arabic containing hydroquinone and silver nitrate (physical development). We were able to demonstrate that this technique, which until now has been used only in animal tissues, also works in plant material. Tissues of plants hydroponically cultivated in the presence of 480 ppm zinc for 4 weeks, were treated with the autometallographic technique and processed for electron microscopy. In sections through roots and leaves we found electron dense grains occurring in distinct tissues and compartments. The grains contained mostly only silver, zinc, and sulphur as could be elucidated by EDXMA and ESI pictures. In roots, we found zinc enriched grains extracellularly in cell walls of rhizodermal and outer cortical cells all the way to the anticlinal walls of the endodermis, intracellularly in vacuoles of rhizodermal cells, outer cortical cells and endodermis cells, and in xylem vessels. In leaves, we found zinc grains extracellularly in all cell walls, intracellularly in xylem vessels, in vacuoles of transfer cells, parenchyma cells and gland cells, and in plasmodesmata between distinct cell types. The results are discussed in view of a) The validity of the autometallographic technique and its adaptation to plant tissue; b) Localized metal sites as a possibility of tolerance mechanism of A. m. ssp. halleri under zinc stress; and c) Supposed pathways for zinc uptake and transport within root and leaf tissues.