Possible chemical forms of cadmium and varietal differences in cadmium concentrations in the phloem sap of rice plants (Oryza sativa L.)

Authors

  • Mariyo KATO,

    1. Department of Applied Biological Chemistry, The University of Tokyo, Tokyo 113-8657
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    • Present addresses: Rohto Pharmaceutical Company, 6-5-4, Kunimidai, Kizugawa, Kyoto 619-0216, Japan.

  • Satoru ISHIKAWA,

    1. Soil Environment Division, National Institute for Agro-Environmental Sciences, Tsukuba, Ibaraki 305-8604
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  • Kazumi INAGAKI,

    1. National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8563
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  • Koichi CHIBA,

    1. National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8563
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  • Hiroaki HAYASHI,

    1. Department of Applied Biological Chemistry, The University of Tokyo, Tokyo 113-8657
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    • 694 Futago, Aki, Kunisaki, Oita 873-0356, Japan.

  • Shuichi YANAGISAWA,

    1. Department of Applied Biological Chemistry, The University of Tokyo, Tokyo 113-8657
    2. The Core Research for Evolutional Science and Technology, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan
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  • Tadakatsu YONEYAMA

    1. Department of Applied Biological Chemistry, The University of Tokyo, Tokyo 113-8657
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T. YONEYAMA, Tsukuho 3-3-12, Tsukuba, Ibaraki 300-3257, Japan. Email: tadakatsu_yoneyama@opal.ocn.ne.jp

Abstract

In rice (Oryza sativa L.), cadmium (Cd), a toxic heavy metal, is found in phloem sap and eventually accumulates in the grains. To further characterize phloem-transported Cd, the chemical forms of Cd and other metals and varietal differences in phloem sap Cd concentrations were investigated in young rice plants. The size-exclusion chromatography elution times for Cd-bound compounds indicated that phloem Cd in cv. Nipponbare exists mainly as an approximately 13 kDa complex. Protease digestion of rice phloem sap reduced the bound Cd content from 92 to 19%. The remaining Cd may bind to low-molecule SH compounds. An experiment examining in vitro addition of Cd2+ to phloem sap from non-Cd-treated plants revealed that rice phloem sap constitutively contains Cd chelators. The major Cd peak is distinguishable from those of Fe, Zn, Cu, Mn, Ni and Co, which probably bind to nicotianamine, 2′-deoxymugineic acid, citrate and histidine. The Cd concentrations of the phloem saps in three varieties (Milyang 23, LAC 23 and Koshihikari) grown under the same soil conditions were correlated with their grain Cd concentrations, which had been reported previously, whereas the concentrations of the xylem saps were not. In conclusion, rice phloem sap Cd differs from other metals as it may bind to a novel approximately 13 kDa protein and SH compounds, and the concentration of Cd in rice phloem sap may be a key determinant of its grain content.

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