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