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Published on October 30, 2009; 10.1104/pp.109.146126

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Received August 12, 2009
Accepted October 29, 2009

Grain unloading of arsenic species in rice (Oryza sativa L.)

Anne-Marie Carey , Kirk G. Scheckel , Enzo Lombi , Matt Newville , Yongseong Choi , Gareth J. Norton , John M. Charnock , Joerg Feldmann , Adam H. Price , and Andrew A. Meharg *

Institute of Biology and Environmental Sciences, University of Aberdeen, Cruickshank Building, St Machar Drive, Aberdeen, AB24 3UU, UK; National Risk Management Research Laboratory, US Environmental Protection Agency, 5995 Centre Hill Avenue, Cincinnati, Ohio 45224, USA; Centre for Environmental Risk Assessment and Remediation, University of South Australia, Building X, Mawson Lakes Campus Mawson Lakes, South Australia, SA-5095 Australia and CRC CARE, PO Box 486, Salisbury, South Australia 5106, Australia; GSECARS Advanced Photon Source, Building 434A, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439; Centre for Molecular Environmental Science, University of Manchester, Oxford Road, Manchester, M13 9PL, UK; CCLRC Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, UK; Department of Chemistry, University of Aberdeen, Meston Building, Meston Walk, Aberdeen, AB24 3TU, UK

* Corresponding author; email: a.meharg{at}abdn.ac.uk.

Rice is the staple food for over half the world's population yet may represent a significant dietary source of inorganic arsenic (As), a non-threshold, class 1 human carcinogen. Rice grain As is dominated by the inorganic species, and the organic species dimethylarsinic acid (DMA). To investigate how As species are unloaded into grain rice, panicles were excised during grain filling and hydroponically pulsed with arsenite, arsenate, glutathione complexed As (As(GS)3), or DMA. Total As concentrations in flag leaf, grain and husk, were quantified by ICP-MS and As speciation in the fresh grain was determined by X-ray absorption near edge spectroscopy (XANES). The roles of phloem and xylem transport were investigated by applying a ± stem girdling treatment to a second set of panicles, limiting phloem transport to the grain in panicles pulsed with arsenite or DMA. The results demonstrate that DMA is translocated to the rice grain with over an order magnitude greater efficiency than inorganic species and is more mobile than arsenite in both the phloem and the xylem. Phloem transport accounted for 90% of arsenite, and 55% of DMA, transport to the grain. Synchrotron X-ray fluorescence (S-XRF) mapping and fluorescence microtomography revealed marked differences in the pattern of As unloading into the grain between DMA and arsenite challenged grain. Arsenite was retained in the ovular vascular trace and DMA dispersed throughout the external grain parts and into the endosperm. This study also demonstrates that DMA speciation is altered in planta, potentially through complexation with thiols.






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