National Synchrotron Light Source Seminar

Metal hyperaccumulator plants are used in commercial phytoremediation and phytomining operations to recover metals of economic value from enriched soils or sediments. Information regarding the distribution and speciation of metals at the micron scale is needed to locate the phenomena of hyperaccumulation in tissues, cells, and organelles in order to understand the mechanism of (hyper)tolerance and ultimately improve phytoextraction efficiency. Metal (hyper)tolerance in mature plants occurs via chemical binding of metals (i.e., chelation) and intracellular compartmentalization. X-ray fluorescence microprobe was used to investigate metal localization and speciation in situ because standard sample preparation methods (e.g. embedding) cause re-distribution of metals or alter their speciation. In seeds of nickel (Ni) hyperaccumulator Thlaspi goesingense, Ni was preferentially localized in the embryonic tissue with the greatest enrichment in the cotyledons (leaves), radicle (root), and hypocotyl (stem); nickel was nearly absent from the testa (seed coat). Nickel within the embryo was spatially correlated with sulfur (S) and closely associated with manganese (Mn). For intact seeds, fCMT revealed preferential localization of Ni in the epidermis of the rudimentary plants. High-resolution images of seed cross-sections showed Ni compartmentalized preferentially within the single layer of epidermal cells. In cotyledons, epidermal cells contained circa 8-fold more Ni than immediately adjacent palisade cells and circa 6-fold more Ni than (spongy) mesophyll cells. Nickel K-edge XAS measurements on (oriented) seeds and single epidermal cells exposed in cross-section revealed the storage form of Ni was a nickel-sulfur nanocluster resembling alpha nickel sulfide (-NiS). Intracellular compartmentalization and chemical binding of Ni in the seed embryo provides evidence that these rudimentary plants have developed a (hyper)tolerance mechanism for metal homeostasis. Ne