title: Grain unloading of arsenic species in rice.
abstract: Rice (Oryza sativa) is the staple food for over half the world's population yet may represent a significant dietary source of inorganic arsenic (As), a nonthreshold, 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, or DMA. Total As concentrations in flag leaf, grain, and husk, were quantified by inductively coupled plasma mass spectroscopy and As speciation in the fresh grain was determined by x-ray absorption near-edge spectroscopy. 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 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.
authors.affiliation: Institute of Biology and Environmental Sciences, University of Aberdeen, Aberdeen AB24 3UU, United Kingdom.
authors.forename: Anne-Marie
authors.initials: AM
authors.lastname: Carey
authors.forename: Kirk G
authors.initials: KG
authors.lastname: Scheckel
authors.forename: Enzo
authors.initials: E
authors.lastname: Lombi
authors.forename: Matt
authors.initials: M
authors.lastname: Newville
authors.forename: Yongseong
authors.initials: Y
authors.lastname: Choi
authors.forename: Gareth J
authors.initials: GJ
authors.lastname: Norton
authors.forename: John M
authors.initials: JM
authors.lastname: Charnock
authors.forename: Joerg
authors.initials: J
authors.lastname: Feldmann
authors.forename: Adam H
authors.initials: AH
authors.lastname: Price
authors.forename: Andrew A
authors.initials: AA
authors.lastname: Meharg
mesh_terms: D001151:Arsenic / Q000032:analysis / Q000378:metabolism*; D001692:Biological Transport; D002101:Cacodylic Acid / Q000378:metabolism; D012275:Oryza / Q000378:metabolism*; D052585:Phloem; D012639:Seeds / Q000737:chemistry / Q000378:metabolism*; D052584:Xylem
chemical_list: D002101:Cacodylic Acid; D001151:Arsenic