The fate of arsenic, cadmium and lead in Typha latifolia: a case study on the applicability of micro-PIXE in plant ionomics

Understanding the uptake, accumulation and distribution of toxic elements in plants is crucial to the design of effective phytoremediation strategies, especially in the case of complex multi-element pollution. Using micro-proton induced X-ray emission, the spatial distribution of Na, Mg, Al, Si, P, S, Cl, K, Ca, Mn, Fe, Zn, As, Br, Rb, Sr, Cd and Pb have been quantitatively resolved in roots and rhizomes of an obligate wetland plant species, Typha latifolia, treated with a mixture of 100 μM each of As, Cd and Pb, together. The highest concentrations of As, Cd and Pb were found in the roots of the T. latifolia, with tissue-specific distributions. The As was detected in the root rhizodermis, and in the rhizome the majority of the As was within the vascular tissues, which indicates the high mobility of As within T. latifolia. The Cd was detected in the root exodermis, and in the vascular bundle and epidermis of the rhizome. The highest Pb concentrations were detected in the root rhizodermis and exodermis, and in the epidermis of the rhizome. These data represent an essential step in the resolution of fundamental questions in plant ionomics.

Use of micro-PIXE to determine spatial distributions of copper in Brassica carinata plants exposed to CuSO4 or CuEDDS

A better understanding of the mechanisms that govern copper (Cu) uptake, distribution and tolerance in Brassica carinata plants in the presence of chelators is needed before significant progress in chelate-assisted Cu phytoextraction can be made. The aims of this study were therefore to characterise (S,S)-N,N'-ethylenediamine disuccinic acid (EDDS)-assisted Cu uptake, and to compare the spatial distribution patterns of Cu in the roots and leaves of B. carinata plants. The plants were treated with 30 μM or 150 μM CuSO(4) or CuEDDS in hydroponic solution. Quantitative Cu distribution maps and concentration profiles across root and leaf cross-sections of the desorbed plants were obtained by micro-proton induced X-ray emission. In roots, the 30 μM treatments with both CuSO(4) and CuEDDS resulted in higher Cu concentrations in epidermal/cortical regions. At 150 μM CuSO(4), Cu was mainly accumulated in root vascular bundles, whereas with 150 μM CuEDDS, Cu was detected in endodermis and the adjacent inner cortical cell layer. Under all treatments, except with a H(+)-ATP-ase inhibitor, the Cu in leaves was localised mainly in vascular tissues. The incubation of plants with 150 μM CuEDDS enhanced metal translocation to shoots, in comparison to the corresponding CuSO(4) treatment. Inhibition of H(+)-ATPase activity resulted in reduced Cu accumulation in 30 μM CuEDDS-treated roots and 150 μM CuEDDS-treated leaves, and induced changes in Cu distribution in the leaves. This indicates that active mechanisms are involved in retaining Cu in the leaf vascular tissues, which prevent its transport to photosynthetically active tissues. The physiological significance of EDDS-assisted Cu uptake is discussed.

New insights into globoids of protein storage vacuoles in wheat aleurone using synchrotron soft X-ray microscopy

Mature developed seeds are physiologically and biochemically committed to store nutrients, principally as starch, protein, oils, and minerals. The composition and distribution of elements inside the aleurone cell layer reflect their biogenesis, structural characteristics, and physiological functions. It is therefore of primary importance to understand the mechanisms underlying metal ion accumulation, distribution, storage, and bioavailability in aleurone subcellular organelles for seed fortification purposes. Synchrotron radiation soft X-ray full-field imaging mode (FFIM) and low-energy X-ray fluorescence (LEXRF) spectromicroscopy were applied to characterize major structural features and the subcellular distribution of physiologically important elements (Zn, Fe, Na, Mg, Al, Si, and P). These direct imaging methods reveal the accumulation patterns between the apoplast and symplast, and highlight the importance of globoids with phytic acid mineral salts and walls as preferential storage structures. C, N, and O chemical topographies are directly linked to the structural backbone of plant substructures. Zn, Fe, Na, Mg, Al, and P were linked to globoid structures within protein storage vacuoles with variable levels of co-localization. Si distribution was atypical, being contained in the aleurone apoplast and symplast, supporting a physiological role for Si in addition to its structural function. These results reveal that the immobilization of metals within the observed endomembrane structures presents a structural and functional barrier and affects bioavailability. The combination of high spatial and chemical X-ray microscopy techniques highlights how in situ analysis can yield new insights into the complexity of the wheat aleurone layer, whose precise biochemical composition, morphology, and structural characteristics are still not unequivocally resolved.

Improved lateral discrimination in screening the elemental composition of buckwheat grain by micro-PIXE

The elemental composition of specific fractions of cereal and pseudocereal grains can be roughly estimated after milling. Alternatively, the elemental localization of cross-sectioned grains can be quantitatively analyzed by microproton induced X-ray emission (micro-PIXE), taking advantage of high elemental sensitivity and low lateral resolution. We present a micro-PIXE study on buckwheat (Fagopyrum esculentum) grain, with a detailed description of the elemental distributions. Elements such as Mg, P, S, K, Fe, Ni, Cu, and Zn were preferentially localized in the cotyledons and embryonic axis; however, significant amounts of K and Fe were also found in the pericarp. The aleurone layer covering the cotyledons was especially enriched in S and P, while testa, a thin layer above the aleurone did not show any significant element enrichments. The highest concentrations of Al, Si, Cl, Ca, and Ti were found in the pericarp. A detailed element localization study of pericarp layers revealed that the inner layer was enriched in K, Mn, Ca, and Fe, while the outer layer showed enrichments in Na, Mg, P, S, and Al. On the basis of the data obtained, milling techniques can be adapted to obtain milling fractions with targeted nutritional values.

Localization of aluminium in tea (Camellia sinensis) leaves using low energy X-ray fluorescence spectro-microscopy

Information on localization of Al in tea leaf tissues is required in order to better understand Al tolerance mechanism in this Al-accumulating plant species. Here, we have used low-energy X-ray fluorescence spectro-microscopy (LEXRF) to study localization of Al and other low Z-elements, namely C, O, Mg, Si and P, in fully developed leaves of the tea plant [Camellia sinensis (L.) O. Kuntze]. Plants were grown from seeds for 3 months in a hydroponic solution, and then exposed to 200 microM AlCl(3) for 2 weeks. Epidermal-mesophyll and xylem phloem regions of 20 microm thick cryo-fixed freeze-dried tea-leaf cross-sections were raster scanned with 1.7 and 2.2 keV excitation energies to reach the Al-K and P-K absorption edges. Al was mainly localized in the cell walls of the leaf epidermal cells, while almost no Al signal was obtained from the leaf symplast. The results suggest that the retention of Al in epidermal leaf apoplast represent the main tolerance mechanism to Al in tea plants. In addition LEXRF proved to be a powerful tool for localization of Al in plant tissues, which can help in our understanding of the processes of Al uptake, transport and tolerance in plants.

Spatial distribution of cadmium in leaves of metal hyperaccumulating Thlaspi praecox using micro-PIXE

* Localization of cadmium (Cd) and other elements was studied in the leaves of the field-collected cadmium/zinc (Cd/Zn) hyperaccumulator Thlaspi praecox from an area polluted with heavy metals near a lead mine and smelter in Slovenia, using micro-PIXE (proton-induced X-ray emission). * The samples were prepared using cryofixation. Quantitative elemental maps and average concentrations in whole-leaf cross-sections and selected tissues were obtained. * Cd was preferentially localized in the lower epidermis (820 microg g(-1) DW), vascular bundles and upper epidermis, whereas about twice the lower concentrations were found in the mesophyll. * Taking into account the large volume of the mesophyll compared with the epidermis, the mesophyll is indicated as a relatively large pool of Cd, possibly involved in Cd detoxification/dilution at the tissue and cellular level.