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REGULATION OF PLANT HORMONE BIOSYNTHESES 5 MEP pathway was uncovered recently. The incorporation of 14 C-labeled MVA into the iP element of tRNA in vivo has been demonstrated in tobacco pith tissue (CHEN and HALL 1969). Also, 13 C- labeled MVA was incorporated into iP and trans-zeatin riboside (tZR) in vitro in the endosperm of Sechium edule (PIAGGESI et al. 1997). In addition, there are some reports that indicate that CK levels are reduced in plants when the MVA pathway is limited (ÅSTOT et al. 2000). On the other hand, the contribution of the MEP pathway to CK biosynthesis has never been issued before. It should be noted that the incorporation of MVA does not exclude a potential role of the MEP pathway in the biosynthesis of CKs because it has often been observed that isoprene units from both MVA and MEP pathways are incorporated into a single downstream isoprenoid (KASAHARA et al. 2002; HEMMERLIN et al. 2003). Thus, a possible contribution of the MEP pathway to the biosynthesis of CKs needs to be examined to better understand how CKs are synthesized in plant. In order to selectively label metabolites from the MVA or MEP pathways with 13 C in vivo, we have previously carried out feeding of [1- 13 C] 1-deoxy-D-xylulose (DX) or [2- 13 C]mevalonolactone (MVL) to Arabidopsis seedlings. DX is converted into an MEP pathway intermediate 1-deoxy-Dxylulose 5-phosphate (DXP) by phosphorylation. Therefore, exogenous DX is able to complement the albino phenotype of the cla1-1 mutant (ESTEVEZ et al. 2000), which is defective in DXP in the MEP pathway. Similarly, the growth inhibition due to a block in the MVA pathway by mevastatin (an inhibitor of HMG-CoA reductase) is rescued by exogenous application of MVL (KASAHARA et al. 2002). Efficient 13 C-labeling of metabolites from the MVA or MEP pathways was thus achieved by feeding 13 C-labeled DX and MVL to the cla1-1 mutant and mevastatin-treated plants, respectively. These 13 C-labeling systems allowed us to determine contribution of the MVA and MEP pathways to the biosynthesis of GAs by gas chromatography-mass spectrometry (KASAHARA et al. 2002). We studied the biosynthesis route for the prenyl moiety of CKs using the 13 C-labeled tracers in Arabidopsis seedlings. Our data demonstrate that the prenyl side chain of tZ- and iP-type CKs are mainly produced through the MEP pathway (Fig. 3), whereas a large fraction of cZ derivatives is synthesized through the MVA pathway. We also show the subcellular location of AtTPTs produced as GFP-fusion proteins. Based on these data, we proposed a crucial role of the plastid-localized MEP pathway in CK biosynthesis (KASAHARA et al. 2004). Cloning of P450 genes involved in abscisic acid degradation (Tetsuo KUSHIRO, Masanori OKAMOTO, Kazumi NAKABAYASHI and Eiji NAMBARA) Abscisic acid (ABA) controls numerous aspects of plant life cycle including seed dormancy, germination and adaptive responses to environmental stresses (ZEEVAART and CREELMAN, 1988). ABA-deficient mutants from several plant species show reduced seed dormancy and wilty phenotype (MCCARTY 1995). ABA content increases during seed development or when a plant is subjected to various stresses such as osmotic stress, while it rapidly decreases during subsequent
6 Yuji KAMIYA Fig. 3. Origin of side chains of trans-zeatin and cis-zeatin in Arabidopsis. cZ, cis-zeatin; cZR, cis-zeatin riboside; cZRMP, cis-zeatin ribosidemonophosphate; DMAPP, dimethylallyl diphosphate; iP, isopentenyladenine; iPR, isopentenyladenine riboside; iPRMP, isopentenyladenine riboside monophosphate; MEP, methylerythritol phosphate; MVA, mevalonate germination or the recovery from stress. ABA content is determined by the balance between biosynthesis and catabolism. When endogenous ABA levels is maintained high, both ABA biosynthesis and catabolism are active (HARRISON and WALTON 1975; ZEEVAART 1980; PIERCE and RASCHKE, 1981). Constitutive expression of ABA biosynthetic gene in transgenic plants exhibits a more prominent accumulation of the catabolites compared to a moderate increase in ABA contents (QIN and ZEEVAART, 2002). Recently, most of ABA biosynthetic genes have been identified (SCHWARTZ et al. 2003; SEO and KOSHIBA 2002). However, molecular mechanisms underlying ABA catabolism remain poorly understood. ABA is catabolized into inactive forms either by oxidation or conjugation (MILBORROW 1969; MILBORROW 1975; WALTON and SONDHEIMER 1972; SONDHEIMER et al. 1974; XU et al. 2002; see review; CUTLER and KROCHKO 1999) (See Fig. 4). The predominant pathway for ABA catabolism is the oxidative pathway, which is triggered by hydroxylation at C-8’ to produce 8’- hydroxy ABA. The 8’-hydroxy ABA is subsequently isomerized spontaneously to form phaseic acid (PA) (MILBORROW et al. 1988). Biological activity of PA is significantly less than that of ABA, therefore, the major regulatory step in inactivation is likely to be 8’-hydroxylation of ABA (ARAI et al. 1999). This reaction is known to be catalyzed by a cytochrome P450 monooxygenase (P450) (GILLARD and WALTON, 1976; KROCHKO et al. 1998). However, the gene encoding ABA 8’-hydroxylase remained elusive. It is necessary to identify this gene to understand the molecular mechanism controlling the hormonal level of ABA. Tetsuo KUSHIRO, Masanori OKAMOTO (Ph.D. student from the Tokyo Metropolitan Univ., Prof. Tomokazu KOSHIBA), Kazumi NAKABAYASHI
Page 1 and 2: ISSN 0435-1096 Gamma Field Symposia
Page 3 and 4: The lecturers and the members of th
Page 5 and 6: KANEKO, T. KARITA, E. KASHIMA, M. K
Page 7 and 8: TANO, S. TSUCHIDA, Y. TSUTSUMI, N.
Page 9 and 10: The Symposium Committee Shigeki NAG
Page 11 and 12: CONTENTS Y. KAMIYA Biosyntheses and
Page 13 and 14: 2 Yuji KAMIYA Fig. 1. Gibberellin b
Page 15: 4 Yuji KAMIYA using the receptor pr
Page 19 and 20: 8 Yuji KAMIYA References AKIYOSHI,
Page 21 and 22: 10 Yuji KAMIYA SHINOMURA, T. (1997)
Page 23 and 24: 12 Yuji KAMIYA 8’ 8’ 273
Page 25 and 26: 14 Motoyuki ASHIKARI, Hironori ITOH
Page 37 and 38: 26 Atsuhiro OKA and how their signa
Page 39 and 40: 28 Atsuhiro OKA At almost the same
Page 41 and 42: 30 Atsuhiro OKA division for xylem
Page 43 and 44: 32 Atsuhiro OKA the RR domain in th
Page 45 and 46: 34 Atsuhiro OKA Fig. 4. Cross-talk
Page 47 and 48: 36 Atsuhiro OKA OKA, A., SAKAI, H.
Page 49 and 50: 38 Atsuhiro OKA ARR4 PHYB PHYBP
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Page 53 and 54: ETHYLENE-SIGNALING PATHWAY 43 signa
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MECHANISM OF BRASSINOSTEROID SIGNAL
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68 Yoshikatsu MATSUBAYASHI are grow
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70 Yoshikatsu MATSUBAYASHI Fig. 3.
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76 Yoshikatsu MATSUBAYASHI 33. Yang
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78 Yoshikatsu MATSUBAYASHI GUS
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80 BRI1
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86 PSK 23 BIL1/CFP
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