Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 1/15/2010
Publication Date: 2/22/2010
Citation: Punshon, T., Guerinot, M.L., Lanzirotti, A., Pinson, S.R., Tarpley, L., Salt, D.E., Zhang, M., Baxter, I. 2010. Spatially resolved elemental mapping of two U.S. rice core collection grain accessions with diverse arsenic accumulation characteristics via synchrotron x-Ray fluorescence microscopy (SXRF). Rice Technical Working Group Meeting Proceedings, Biloxi, MS, Feb. 22-25, 2010. CDROM. Interpretive Summary:
Technical Abstract: The discovery of arsenic in higher than expected concentrations in rice grown in the South Central United States and worldwide has prompted further study to ensure the safety of rice, and rice based products such as infant cereals. In the U.S. arsenic is thought to originate from former arsenical pesticides resident in the soil, that were used to control boll weevil when the land was used for cotton. However it has recently been found that arsenic uptake in rice plants occurs through the silicon transport system as a result of the size and charge similarity between arsenous acid in flooded paddy soils and silicic acid. Rice plants are now considered natural silicon accumulating plants, taking up far more silicon than other cereal crops such as oats, wheat or barley. Inorganic arsenic species (arsenate and arsenite) are considered more toxic to humans, but arsenic can also exist as monomethylarsonic acid and dimethylarsinic acid and there is little information about the uptake of these methylated forms in plants, or whether they form as a result of plant metabolic processes. Market basket surveys of U.S. rice have shown that approximately half of the arsenic in rice grain is inorganic, in comparison with rice grown in the arsenic-impacted regions of Bangladesh, which is 80% inorganic. Further testing showed that these differences in arsenic speciation were maintained when rice varieties from different countries were grown on the same arsenic-contaminated soil, strongly suggesting a genetic component in arsenic metabolism. An understanding of the concentration, tissue localization and speciation of arsenic are vital to address safety concerns, but this information can also be used to search for genes involved in arsenic uptake, transport and storage. Once genes have been found and characterized, they can be manipulated via genetic or more traditional breeding techniques to reduce their expression or their ability to transport arsenic; excluding arsenic from the grain. We analyzed two rice accessions sampled from the U.S. Rice Core Collection for the distribution and speciation of arsenic; one accession that accumulated arsenic when grown in flooded soils, and one accumulating arsenic in non-flooded soils, with the hypothesis that their arsenic characteristics may differ as a result of the contrasting arsenic speciation in flooded and unflooded soils. We analyzed sections of rice grains through the embryo and endosperm with a synchrotron x-ray fluorescence (SXRF) microprobe, at the National Synchrotron Light Source, Brookhaven National Laboratory. This technique can be compared to medical x-ray or CAT scanning, which enables images of bones or tumors to be captured without the need for surgery. In this case, different absorption characteristics of the x-rays by bones and tissues with different densities allows a structural image to be created. In SXRF, higher energy x-rays are used, causing elements to fluoresce. Measurement of the energy and intensity of this fluorescence allows us to collect elemental information in the form of maps or images on the sub-micron scale from samples on up to 10 elements at a time simultaneously, without requiring intrusive sample preparation or sectioning. We found that arsenic was present in the endosperm and embryo of both accessions, although the distribution differed between the accessions. Measurement of the speciation of arsenic in the endosperm of rice grown in unflooded paddy rice indicated that it was in a highly reduced form (arsenite) possibly involving binding to thiol groups.