Regulation of hormone metabolism in Arabidopsis seeds: phytochrome regulation of abscisic acid metabolism and abscisic acid regulation of gibberellin metabolism

Authors

  • Mitsunori Seo,

    1. Growth Regulation Research Group, RIKEN Plant Science Center, Suehiro-cho 1-7-22, Tsurumi, Yokohama, Kanagawa 230-0045, Japan,
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    • Present address: Laboratoire de Biologie des Semences, UMR 204 INRA – INAPG, Institut Jean-Pierre Bourgin, 78026 Versailles Cedex, France.

  • Atsushi Hanada,

    1. Growth Regulation Research Group, RIKEN Plant Science Center, Suehiro-cho 1-7-22, Tsurumi, Yokohama, Kanagawa 230-0045, Japan,
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  • Ayuko Kuwahara,

    1. Growth Regulation Research Group, RIKEN Plant Science Center, Suehiro-cho 1-7-22, Tsurumi, Yokohama, Kanagawa 230-0045, Japan,
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  • Akira Endo,

    1. Growth Regulation Research Group, RIKEN Plant Science Center, Suehiro-cho 1-7-22, Tsurumi, Yokohama, Kanagawa 230-0045, Japan,
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  • Masanori Okamoto,

    1. Growth Regulation Research Group, RIKEN Plant Science Center, Suehiro-cho 1-7-22, Tsurumi, Yokohama, Kanagawa 230-0045, Japan,
    2. Department of Biological Sciences, Tokyo Metropolitan University, Minami-Ohsawa 1-1, Hachioji, Tokyo 192-0397, Japan,
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  • Yukika Yamauchi,

    1. Growth Regulation Research Group, RIKEN Plant Science Center, Suehiro-cho 1-7-22, Tsurumi, Yokohama, Kanagawa 230-0045, Japan,
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    • Present address: National Institute of Agrobiological Sciences (NIAS), 2-1-2 Kannondai, Tsukuba, Ibaraki 305-8602, Japan.

  • Helen North,

    1. Laboratoire de Biologie des Semences, UMR 204 INRA – INAPG, Institut Jean-Pierre Bourgin, 78026 Versailles Cedex, France, and
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  • Annie Marion-Poll,

    1. Laboratoire de Biologie des Semences, UMR 204 INRA – INAPG, Institut Jean-Pierre Bourgin, 78026 Versailles Cedex, France, and
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  • Tai-ping Sun,

    1. Developmental, Cell and Molecular Biology Group, Department of Biology, Duke University, Box 91000, Durham, NC 27708-1000, USA
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  • Tomokazu Koshiba,

    1. Department of Biological Sciences, Tokyo Metropolitan University, Minami-Ohsawa 1-1, Hachioji, Tokyo 192-0397, Japan,
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  • Yuji Kamiya,

    1. Growth Regulation Research Group, RIKEN Plant Science Center, Suehiro-cho 1-7-22, Tsurumi, Yokohama, Kanagawa 230-0045, Japan,
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  • Shinjiro Yamaguchi,

    1. Growth Regulation Research Group, RIKEN Plant Science Center, Suehiro-cho 1-7-22, Tsurumi, Yokohama, Kanagawa 230-0045, Japan,
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  • Eiji Nambara

    Corresponding author
    1. Growth Regulation Research Group, RIKEN Plant Science Center, Suehiro-cho 1-7-22, Tsurumi, Yokohama, Kanagawa 230-0045, Japan,
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*(fax +81 45 503 9665; e-mail nambara@postman.riken.jp).

Summary

In a wide range of plant species, seed germination is regulated antagonistically by two plant hormones, abscisic acid (ABA) and gibberellin (GA). In the present study, we have revealed that ABA metabolism (both biosynthesis and inactivation) was phytochrome-regulated in an opposite fashion to GA metabolism during photoreversible seed germination in Arabidopsis. Endogenous ABA levels were decreased by irradiation with a red (R) light pulse in dark-imbibed seeds pre-treated with a far-red (FR) light pulse, and the reduction in ABA levels in response to R light was inhibited in a phytochrome B (PHYB)-deficient mutant. Expression of an ABA biosynthesis gene, AtNCED6, and the inactivation gene, CYP707A2, was regulated in a photoreversible manner, suggesting a key role for the genes in PHYB-mediated regulation of ABA metabolism. Abscisic acid-deficient mutants such as nced6-1, aba2-2 and aao3-4 exhibited an enhanced ability to germinate relative to wild type when imbibed in the dark after irradiation with an FR light pulse. In addition, the ability to synthesize GA was improved in the aba2-2 mutant compared with wild type during dark-imbibition after an FR light pulse. Activation of GA biosynthesis in the aba2-2 mutant was also observed during seed development. These data indicate that ABA is involved in the suppression of GA biosynthesis in both imbibed and developing seeds. Spatial expression patterns of the AtABA2 and AAO3 genes, responsible for last two steps of ABA biosynthesis, were distinct from that of the GA biosynthesis gene, AtGA3ox2, in both imbibed and developing seeds, suggesting that biosynthesis of ABA and GA in seeds occurs in different cell types.

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