title: Regulation of hormone metabolism in Arabidopsis seeds: phytochrome regulation of abscisic acid metabolism and abscisic acid regulation of gibberellin metabolism.
abstract: 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.
authors.affiliation: Growth Regulation Research Group, RIKEN Plant Science Center, Suehiro-cho 1-7-22, Tsurumi, Yokohama, Kanagawa 230-0045, Japan.
authors.forename: Mitsunori
authors.initials: M
authors.lastname: Seo
authors.forename: Atsushi
authors.initials: A
authors.lastname: Hanada
authors.forename: Ayuko
authors.initials: A
authors.lastname: Kuwahara
authors.forename: Akira
authors.initials: A
authors.lastname: Endo
authors.forename: Masanori
authors.initials: M
authors.lastname: Okamoto
authors.forename: Yukika
authors.initials: Y
authors.lastname: Yamauchi
authors.forename: Helen
authors.initials: H
authors.lastname: North
authors.forename: Annie
authors.initials: A
authors.lastname: Marion-Poll
authors.forename: Tai-Ping
authors.initials: TP
authors.lastname: Sun
authors.forename: Tomokazu
authors.initials: T
authors.lastname: Koshiba
authors.forename: Yuji
authors.initials: Y
authors.lastname: Kamiya
authors.forename: Shinjiro
authors.initials: S
authors.lastname: Yamaguchi
authors.forename: Eiji
authors.initials: E
authors.lastname: Nambara
mesh_terms: D000040:Abscisic Acid / Q000378:metabolism*; D017360:Arabidopsis / Q000196:embryology* / Q000235:genetics; D001483:Base Sequence; D017931:DNA Primers; D018525:Germination; D005875:Gibberellins / Q000096:biosynthesis / Q000378:metabolism*; D010834:Phytochrome / Q000378:metabolism*; D010937:Plant Growth Regulators / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012639:Seeds / Q000378:metabolism*
chemical_list: D017931:DNA Primers; D005875:Gibberellins; D010937:Plant Growth Regulators; D010834:Phytochrome; D000040:Abscisic Acid