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First published online August 18, 2006; 10.1104/pp.106.081158 Plant Physiology 142:124-134 (2006) © 2006 American Society of Plant Biologists Spatial Imaging, Speciation, and Quantification of Selenium in the Hyperaccumulator Plants Astragalus bisulcatus and Stanleya pinnata1Biology Department, Colorado State University, Fort Collins, Colorado 80523 (J.L.F., L.H.Z., E.A.H.P.-S.); Advanced Light Source, Lawrence Berkeley Laboratory, Berkeley, California 94720 (M.A.M., S.F.); and Cross Institute Programme for Sustainable Soil Function, Rothamsted Research, Harpenden, Herts AL5 2JQ, United Kingdom (S.P.M.)
Astragalus bisulcatus and Stanleya pinnata hyperaccumulate selenium (Se) up to 1% of plant dry weight. In the field, Se was mostly present in the young leaves and reproductive tissues of both hyperaccumulators. Microfocused scanning x-ray fluorescence mapping revealed that Se was hyperaccumulated in trichomes in young leaves of A. bisulcatus. None of 10 other elements tested were accumulated in trichomes. Micro x-ray absorption spectroscopy and liquid chromatography-mass spectrometry showed that Se in trichomes was present in the organic forms methylselenocysteine (MeSeCys; 53%) and
1 This work was supported by the National Science Foundation (grant no. IOB–0444471 to E.A.H.P.-S.). The operations of the Advanced Light Source at Lawrence Berkeley National Laboratory are supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy (contract no. DE–AC02–05CH11231). Rothamsted Research receives grant-aided support from the U.K. Biotechnology and Biological Sciences Research Council. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Elizabeth A.H. Pilon-Smits (epsmits{at}lamar.colostate.edu). www.plantphysiol.org/cgi/doi/10.1104/pp.106.081158 * Corresponding author; e-mail epsmits{at}lamar.colostate.edu; fax 970–491–0649. Received March 29, 2006; accepted July 20, 2006. This article has been cited by other articles:
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-glutamyl-MeSeCys (47%). In the young leaf itself, there was 30% inorganic Se (selenate and selenite) in addition to 70% MeSeCys. In young S. pinnata leaves, Se was highly concentrated near the leaf edge and surface in globular structures that were shown by energy-dispersive x-ray microanalysis to be mainly in epidermal cells. Liquid chromatography-mass spectrometry revealed both MeSeCys (88%) and selenocystathionine (12%) inside leaf edges. In contrast, both the Se accumulator Brassica juncea and the nonaccumulator Arabidopsis thaliana accumulated Se in their leaf vascular tissues and mesophyll cells. Se in hyperaccumulators appears to be mobile in both the xylem and phloem because Se-treated S. pinnata was found to be highly toxic to phloem-feeding aphids, and MeSeCys was present in the vascular tissues of a S. pinnata young leaf petiole as well as in guttation fluid. The compartmentation of organic selenocompounds in specific storage areas in the plant periphery appears to be a unique property of Se hyperaccumulators. The high concentration of Se in the plant periphery may contribute to Se tolerance and may also serve as an elemental plant defense mechanism. 
