First published online February 6, 2003; 10.1104/pp.014787
Plant Physiol, March 2003, Vol. 131, pp. 1460-1467
Chemical Form and Distribution of Selenium and Sulfur in the
Selenium Hyperaccumulator Astragalus
bisulcatus1
Ingrid J.
Pickering,
Carrie
Wright,
Ben
Bubner,
Danielle
Ellis,
Michael W.
Persans,
Eileen Y.
Yu,
Graham N.
George,
Roger C.
Prince, and
David E.
Salt*
Department of Horticulture and Landscape Architecture, Purdue
University, West Lafayette, Indiana 47907 (D.E.S., D.E.); Northern
Arizona University, Flagstaff, Arizona 86011 (C.W., B.B.); Department
of Biology, University of Texas-Pan American, Edinburg, Texas 78539 (M.W.P.); Stanford Synchrotron Radiation Laboratory, Stanford
University, Stanford Linear Accelerator Center, Menlo Park, California
94025 (I.J.P., E.Y.Y., G.N.G.); and ExxonMobil Research and Engineering
Company, Annandale, New Jersey 08801 (R.C.P.).
In its natural habitat, Astragalus bisulcatus
can accumulate up to 0.65% (w/w) selenium (Se) in its shoot dry
weight. X-ray absorption spectroscopy has been used to examine
the selenium biochemistry of A. bisulcatus. High
concentrations of the nonprotein amino acid Se-methylseleno-cysteine
(Cys) are present in young leaves of A. bisulcatus, but
in more mature leaves, the Se-methylseleno-Cys concentration is lower,
and selenate predominates. Seleno-Cys methyltransferase is the enzyme
responsible for the biosynthesis of Se-methylseleno-Cys from seleno-Cys
and S-methyl-methionine. Seleno-Cys methyltransferase is
found to be expressed in A. bisulcatus leaves of all
ages, and thus the biosynthesis of Se-methylseleno-Cys in older leaves
is limited earlier in the metabolic pathway, probably by an inability
to chemically reduce selenate. A comparative study of sulfur (S) and Se
in A. bisulcatus using x-ray absorption spectroscopy indicates similar trends for oxidized and reduced Se and S species, but
also indicates that the proportions of these differ significantly. These results also indicate that sulfate and selenate reduction are
developmentally correlated, and they suggest important differences between S and Se biochemistries.
1
This research was supported by the U.S. National
Cancer Institute (grant to D.E.S.) and by the U.S. National Institutes
of Health (grants to E.Y.Y., I.J.P., and G.N.G.). During the period of this research, C.W. and D.E. were working in the
laboratory of D.E.S. as employees of NuCycle Therapy (Monmouth
Junction, NJ), which was supported by the U.S. National Cancer
Institute (Small Business Technology Transfer grant). Portions
of this research were carried out at the Stanford Synchrotron Radiation
Laboratory (SSRL), a national user facility operated by Stanford
University on behalf of the U.S. Department of Energy, Office of Basic
Energy Sciences. The SSRL Structural Molecular Biology Program is
supported by the Department of Energy, Office of Biological and
Environmental Research, and by the National Institutes of Health,
National Center for Research Resources, Biomedical Technology Program.
The sulfur K-edge x-ray absorption spectroscopy studies at SSRL were
supported by the U.S. National Institutes of Health (grant no. GM57375).
*
Corresponding author; e-mail dsalt{at}purdue.edu;
fax 765-494-0391.
© 2003 American Society of Plant Biologists
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