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REGULATION OF PLANT HORMONE BIOSYNTHESES 3 chilling treatment in published studies also involved a longer imbibition period. Yukika YAMAUCHI and Shinjiro YAMAGUCHI have worked intensively about the effect of cold treatment. We first carried out large-scale expression analysis during imbibition of after-ripened Arabidopsis dry seeds at 4. This investigation indicates that a number of GA-related genes are differentially expressed between dry and cold-treated seeds. Our GC-MS and reverse transcription- PCR analyses show that GA biosynthesis is activated in response to low temperature in darkimbibed seeds, and that the effect of temperature is targeted to particular GA biosynthesis genes. Using a loss-of-function mutant of the cold-inducible AtGA3ox1 gene, we show that this gene is required for cold-promoted synthesis of active GAs and seed germination. Our results suggest that germination of Arabidopsis seeds is stimulated in response to low temperature in part through modulating GA biosynthesis (Fig. 2). Furthermore, we show that cold treatment increases the number of cell types accumulating the AtGA3ox1 transcript detectable by in situ hybridization analysis, suggesting a complex regulatory mechanism by which the spatial distribution of GA biosynthesis is determined (YAMAUCHI et al. 2004). Biosynthesis of prenyl side chain of cytokinins (Hiroyuki KASAHARA, Kentaro TAKEI, Shinjiro YAMAGUCHI and Hitoshi SAKAKIBARA) Cytokinins (CKs) have many physiological roles in plants, such as promotion of cell division and shoot formation in the presence of auxin, release of lateral buds from apical dominance, stimulation of chloroplast development, and delay of senescence. The biological activity, biosynthesis and metabolism of CKs have been well studied (MOK and MOK, 2001). Recently, a CK receptor has been identified in Arabidopsis (INOUE et al. 2001; YAMADA et al. 2001). Functional analysis Fig. 2. In Arabidopsis germinating seeds, GA3ox1 gene is regulated by light, temperature and negative feed back. Red and blue and arrows indicate effects of light and cold temperature respectively.
4 Yuji KAMIYA using the receptor protein expressed in Escherichia coli and yeast have indicated that free CKs, transzeatin (tZ) and isopentenyl adenine (iP), are active CK species in Arabidopsis (INOUE et al. 2001). Most of CKs identified from plants are derivatives of N 6 -prenylated adenine. Plants have two possible biosynthetic pathways for the production of the side chain of CKs, namely mevalonate (MVA) pathway in the cytosol and methylerythritol phosphate (MEP) pathway in plastids (LICHITENTHALER 1999; ROHMER 2003). Both pathways supply common precursors, isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). Although the MVA and MEP pathways are localized in different subcellular compartments, there is some exchange of common precursor(s) between the two pathways (KASAHARA et al. 2002). Therefore, application of a precursor specific to the MEP pathway can suppress the growth inhibition caused by a block in the MVA pathway, and vice versa (HEMMERLIN et al. 2003). Hiroyuki KASAHARA and Shinjiro YAMAGUCHI in collaboration with Kentaro TAKEI and Hitoshi SAKAKIBARA worked about the prenylation of CK side chain. CK biosynthesis pathway in plants has initially been deduced on the basis of bacterial enzymes. The formation of N 6 -prenylated adenine from AMP was first demonstrated using cellfree extracts of a slime mold, Dictyostelium discoideum (TAYA et al. 1978). A CK biosynthesis gene TMR, which encode AMP:DMAPP-isopentenyltransferase (AMP:DMAPP-IPT), was isolated from Ti-plasmid of Agrobacterium tumefaciens (BARRY et al. 1984; AKIYOSHI et al. 1984). Because CK levels were elevated in transgenic plants that overproduce bacterial AMP: DMAPP- IPT, the formation of isopentenyl adenosine monophosphate (iPRMP) from AMP and DMAPP is likely to be a committed step in CK biosynthesis. Subsequent formation of iP from iPRMP requires 5’-nucleotidase and adenosine nucleosidase. The conversion of iP into tZ is catalyzed by trans-hydroxylase, which is probably a P450 monooxygenase (CHEN and LEISNER 1984). Recently, a search for Arabidopsis genes that are homologous to bacterial AMP:DMAPP-IPT has identified nine AtIPT genes (TAKEI et al. 2001; KAKIMOTO 2001). Unlike bacterial IPTs, seven AtIPTs were able to transfer DMAPP not only to AMP, but also to ADP and ATP, to give corresponding nucleotide CKs in vitro. Thus, the AMP/ADP/ATP-dependent pathway has been proposed for the biosyntheses of iP/tZ in plants. On the other hand, the tRNA-dependent pathway has been also proposed for the biosynthesis of CKs in plants because tRNAs in bacteria, yeast and plants contain a N 6 -prenylated adenine moiety, which, by hydrolysis, is capable of forming CKs (MURAI 1994). Among nine IPT-related sequences in Arabidopsis, two AtIPT genes encode (putative) tRNA-isoprenyltransferases (tRNA- IPTs). The prenylated adenine element in tRNA is usually consisted of iP and cis-zeatin (cZ), the cis-isomer of tZ. Therefore, the tRNA-dependent pathway has not been considered as the main route to tZ. However, the occurrence of cis-trans isomerase activity in Phaseolus vulgaris immature seeds suggested that the tRNA-dependent pathway might also contribute to the biosynthesis of tZ through cZ (BASSIL et al. 1993). The MVA pathway had been considered as the sole route providing DMAPP to CKs until the
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: 2 Yuji KAMIYA Fig. 1. Gibberellin b
Page 17 and 18: 6 Yuji KAMIYA Fig. 3. Origin of sid
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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