Regulation of ABA and GA Levels During Seed Development and Germination in Arabidopsis
Regulation of ABA and GA Levels During Seed Development and Germination in Arabidopsis
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DOI：
10.1002/9780470988848.ch9
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发表时间：
2007-11
期刊：
影响因子：
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通讯作者：
Shinjiro Yamaguchi;Y. Kamiya;E. Nambara
中科院分区：
文献类型：
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作者：
Shinjiro Yamaguchi;Y. Kamiya;E. Nambara
种子萌发受发育和环境因素的调控。植物激素是重要的信号分子，在萌发过程中将环境的变化传递给种子。脱落酸（阿坝）和赤霉素（GA）被认为是拟南芥种子萌发过程中的主要激素。阿坝是一种倍半萜类激素，在胚胎发生后期是必需的，并抑制早熟种子萌发。阿坝还增加植物对诸如干旱、盐度和温度等胁迫的耐受性，并诱导叶片中的气孔关闭（Davies，2004）。阿坝水平的升高在胚胎发生的中后期对维持种子的正常发育是重要的。成熟种子中相对高水平的阿坝维持其休眠并避免早熟萌发。GA是调节种子发育和萌发以及植物生长和发育的其他方面（例如叶扩展、茎伸长和开花）的二萜激素。尽管一些报道表明乙烯和油菜素类固醇也影响拟南芥的种子萌发（Kucera等人，2005），我们在这里集中于调节种子中的阿坝和GA水平。活性激素的内源水平由其形成和转化为非活性形式的相对速率决定。在本章中，我们将生物合成称为从激素的前体产生激素的生物活性形式，将失活称为将激素的生物活性形式或其前体转化为失活形式。阿坝和GA的生物合成在质体中使用共同的前体香叶基香叶基二磷酸（GGDP）开始，其通过2-C-甲基-D-ATP磷酸（MEP）途径合成（Milborrow和Lee，1998; Hirai等人，2000; Kasahara等人，2004年）。胞质中的甲羟戊酸途径对于类固醇和三萜的生物合成是重要的，但不主要参与阿坝和GA的生物合成。拟南芥GA缺陷突变体的严重等位基因在没有外源生物活性GA的情况下不能发芽（Koornneef和货车der Veen，1980）。因此，GA是拟南芥种子萌发所必需的激素。然而，一些ABA缺陷和不敏感的突变体可以发芽的存在下，多效唑（PAC），GA生物合成抑制剂，在浓度足以抑制野生型种子发芽。这表明当阿坝生物合成或信号传导受损时，低水平的GA足以引起萌发。
Seed germination is regulated by both developmental and environmental factors. Plant hormones are important signaling molecules that transfer the changes of the environment to seeds during germination. Abscisic acid (ABA) and gibberellin (GA) are considered to be major hormones in germinating Arabidopsis thaliana seeds. ABA is a sesquiterpene hormone that is necessary during late embryogenesis and inhibits precocious seed germination. ABA also increases the tolerance of plants to stresses such as drought, salinity, and temperature, and induces stomatal closure in leaves (Davies, 2004). Elevated ABA levels are important during mid-and lateembryogenesis to maintain normal seed development. Relatively high levels of ABA in maturing seeds maintain their dormancy and avoid precocious germination. GA is a diterpene hormone that regulates seed development and germination and other aspects of plant growth and development, such as leaf expansion, stem elongation, and flowering. Although some reports suggest that ethylene and brassinosteroids also affect seed germination of Arabidopsis (Kucera et al., 2005), we focus here on regulation of ABA and GA levels in seeds. Endogenous levels of active hormones are determined by the relative rates of their formation and conversion into inactive forms. In this chapter, we refer to biosynthesis as production of bioactive forms of a hormone from its precursors, deactivation as conversion of bioactive forms or its precursors to the inactive (or less active) forms, and metabolism as both biosynthesis and deactivation.Biosynthesis of ABA and GA starts in plastids using a common precursor, geranylgeranyldiphosphate (GGDP), which is synthesized via the 2-C-methyl-D-erythritol phosphate (MEP) pathway (Milborrow and Lee, 1998; Hirai et al., 2000; Kasahara et al., 2004). The mevalonate pathway in the cytosol is important for steroid and triterpene biosynthesis, but is not principally involved in the biosynthesis of ABA and GA. Severe alleles of GA-deficient mutants of Arabidopsis cannot germinate without exogenous bioactive GAs (Koornneef and van der Veen, 1980). Therefore, GA is an essential hormone leading to germination in Arabidopsis. However, some ABA-deficient and-insensitive mutants can germinate in the presence of paclobutrazol (PAC), a GA biosynthesis inhibitor, at the concentration that is sufficient to inhibit germination of wild-type seeds. This suggests that a low level of GA is enough to cause germination when ABA biosynthesis or signaling is impaired.