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Published on August 18, 2006; 10.1104/pp.106.081158

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Received March 29, 2006
Accepted July 20, 2006

Spatial imaging, Speciation and Quantification of Selenium in the Hyperaccumulator Plants Astragalus Bisulcatus and Stanleya Pinnata

John L. Freeman , Li Hong Zhang , Matthew A. Marcus , Sirine Fakra , Steve P. McGrath , and Elizabeth A.H. Pilon-Smits *

Biology Department, Colorado State University, Fort Collins, CO 80523, U.S.A.
Advanced Light Source, Lawrence Berkeley Laboratory, Berkeley, CA 94720, U.S.A.
Cross Institute Programme for Sustainable Soil Function, Rothamsted Research, Harpenden, Herts, AL5 2JQ, UK

* Corresponding author; email: epsmits{at}lamar.colostate.edu.

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. Micro focused scanning x-ray fluorescence mapping (µ-SXRF) revealed that Se was hyperaccumulated in trichomes in young leaves of A. bisulcatus. None of ten other elements tested were accumulated in trichomes. Micro x-ray absorption spectroscopy (µ-XAS) and LC-MS showed that Se in trichomes was present in the organic forms methylselenocysteine (MeSeCys, 53%) and {gamma}-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 (EDXMA) to be mainly in epidermal cells. LC-MS revealed both MeSeCys (88%) and selenocystathionine (12%) inside leaf edges. In contrast, both the Se accumulator Brassica juncea and the non-accumulator A. thaliana accumulated Se in their leaf vascular tissues and mesophyll cells. Selenium in hyperaccumulators appears to be mobile in both the xylem and phloem, since 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.




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