脱落酸代谢与信号传递及其调控种子休眠与萌发的分子机制

doi:10.3864/j.issn.0578-1752.2020.05.001

摘要/Abstract

摘要：

种子休眠是许多植物在长期系统发育进程中获得的一种适应环境变化的特性,是调控种子萌发和幼苗形成的最适时空分布的一种有效方式,也是物种成功繁衍与传播的一种选择性策略。种子休眠与萌发的激素调控可能是一种高度保守的机制,其中脱落酸（ABA）在种子休眠解除与萌发中起关键作用,赤霉素（GA）在休眠被解除后促进种子萌发。ABA在种子休眠与萌发中的作用主要受ABA代谢（生物合成和分解代谢）和信号传递途径的调控。为此,本文在综述ABA代谢和信号传递研究进展的基础上,阐述了ABA在种子发育、休眠与萌发中的作用,以及种子休眠特异性基因DOG1（萌发延迟1）与ABA信号组分的关系。研究表明,C40环氧类胡萝卜素是ABA生物合成的前体,玉米黄质环氧化酶和9-顺式-环氧类胡萝卜素二加氧酶是ABA生物合成的主要调节酶;ABA的分解代谢包括羟基化作用和与葡萄糖结合,CYP707A家族催化ABA C-8'位置上的羟基化作用,这是ABA分解代谢的重要步骤。在核心ABA信号传递途径中,ABA与PYR/PYL/RCAR受体结合并触发受体发生构象变化,从而允许受体-ABA复合物与2C类蛋白磷酸酶（PP2C）结合并抑制其活性,导致激酶如蔗糖非发酵-1相关的蛋白激酶2（SnRK2）的去抑制和活化。然后,这些激酶磷酸化和活化转录因子（transcription factors,TF）,TF与靶启动子结合和诱导下游的ABA反应基因表达。ABA在种子成熟中后期积累,合子组织中合成的ABA诱导初生休眠和促进种子成熟;在发育中积累和在干种子中存留的ABA含量在种子吸胀初期下降。ABA是种子休眠诱导和维持的正调控因子,是萌发的负调控因子。DOG1在种子成熟过程中表达和发挥作用,其表达受可变剪接和可变多腺苷酸化调控。反义DOG1是种子休眠的一种抑制因子,通过干扰转录和转录延伸负调控DOG1的表达和种子休眠。种子的休眠与萌发除了被核心ABA信号途径调控外,也被DOG1-AHG1（ABA过敏感萌发1）/AHG3途径调控。DOG1能与AHG1/AHG3结合,通过结合ABA信号传递的负调控因子和增加对ABA的敏感性而引起种子休眠。最后,提出了该领域需要进一步研究的科学问题,包括ABA代谢中ABA 8'-羟化酶、ABA葡糖基转移酶和β-葡糖苷酶及其基因怎样响应发育和环境的变化以维持正常的ABA水平。ABA的重要调控因子例如Ca ²⁺或者活性氧对核心ABA信号传递途径的影响,核心ABA信号传递途径与DOG1-AHG1/AHG3途径的下游重叠组分PP2C在整合生理条件或者环境信号时优先响应哪一条途径、这两条途径怎样被协调、以及PP2C有哪些新的靶组分。本文将为深入研究ABA调控种子休眠与萌发的分子机理提供参考。

关键词: 脱落酸, 休眠, 休眠基因DOG1, 萌发, 代谢, 信号传递

Abstract:

Seed dormancy is an adaptive characteristic to environmental changes acquired by many plants during long-term phylogenetic development, and is an effective way regulating the optimal spatiotemporal distribution of seed germination and seedling formation, and is also a selective strategy for the successful reproduction and propagation in species. Phytohormonal regulation of seed dormancy and germination may be a highly conserved mechanism, of which abscisic acid (ABA) plays a master role in dormancy release and germination, and gibberellin (GA) functions as stimulating seed germination after dormancy is released. The role of ABA in seed dormancy and germination is mainly regulated by its metabolism (biosynthesis and catabolism) and signaling pathways. Therefore, in this paper, we mainly summarize the research progresses of ABA metabolism and signaling, the effects of ABA on seed development, dormancy and germination as well as the relationships between DOG1 (DELAY OF GERMINATION1, a specific gene involved in seed dormancy) and ABA signaling components. The researches showed that C40 epoxycarotenoid is a precursor, and zeaxanthin epoxidase and 9-cis-epoxycarotenoid dioxygenase are the principal regulatory enzymes in ABA biosynthesis. The ABA catabolism includes hydroxylation and conjugation with glucose. The hydroxylation of ABA at C-8' position is catalyzed by the CYP707A, which is an important step for ABA catabolism. In the core ABA signaling pathway, ABA binds to PYR/PYL/RCAR receptors and triggers a conformational change that allows receptor-ABA complex to bind to and inhibit type 2C protein phosphatase (PP2C) activity, which results in de-repression and activation of kinases such as sucrose non-fermenting1-related protein kinase 2 (SnRK2). These kinases then phosphorylate and activate transcription factors (TF), which bind to the target promoters and induce the expression of ABA response gene downstream. ABA accumulates in seeds during mid- and late-maturation stages, and ABA synthesized in zygotic tissues induces primary dormancy and promotes seed maturation. ABA content accumulated during development and preserved in dry seeds declines at the early stage of seed imbibition. ABA is a positive regulator of seed dormancy induction and maintenance, and is a negative regulator of seed germination. DOG1 expresses and functions during seed maturation, and its expression is regulated by alternative splicing and alternative polyadenylation. Antisense DOG1 is a repressor of seed dormancy, which negatively regulates DOG1 expression and seed dormancy by causing transcriptional interference and affecting transcription extension. Seed dormancy and germination are regulated not only by core ABA signaling pathway, but also by DOG1-AHG1 (ABA HYPERSENSITIVE GERMINATION1)/AHG3 pathway. DOG1 can bind to AHG1/AHG3 and cause seed dormancy by sequestrating those negative regulators of ABA signaling and increasing ABA sensitivity in seeds. Finally, we propose some scientific issues required for investigation further in the future. How do ABA 8'-hydroxylase, ABA glucosyltransferase and β-glucosidase and their genes respond to developmental and environmental changes to maintain the normal ABA levels in ABA catabolism? How do the important regulators in ABA physiology such as Ca ²⁺ or reactive oxygen species influence the core ABA signaling pathway? Which pathway is preferentially responded by PP2C, a downstream overlapping component of core ABA signaling pathway and DOG1-AHG1/AHG3 pathway, when it integrates physiological conditions or environmental signals, and how are these two pathways coordinated, and what new target components does PP2C have? This paper will provide a basis to further investigate the molecular mechanism regulating seed dormancy and germination by ABA.

Key words: abscisic acid, dormancy, dormancy gene DOG1, germination, metabolism, signaling

宋松泉,刘军,徐恒恒,刘旭,黄荟. 脱落酸代谢与信号传递及其调控种子休眠与萌发的分子机制[J]. 中国农业科学, 2020, 53(5): 857-873.

SONG SongQuan,LIU Jun,XU HengHeng,LIU Xu,HUANG Hui. ABA Metabolism and Signaling and Their Molecular Mechanism Regulating Seed Dormancy and Germination[J]. Scientia Agricultura Sinica, 2020, 53(5): 857-873.

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