摘要:植物花的发育依赖于花分生组织（floral meristem，FM）活性的维持与分化。当FM完成各轮花器官原基的起始后，其活性会程序性地终止（termination），这个过程就是FM的终止发育过程（FM determinacy）。FM的终止发育是一个复杂精细且多步骤的调控过程。WUSCHEL（WUS）是一个具有同源异型结构域（homeodomain）的转录因子，其对FM的活性维持及终止发育发挥着重要作用。越来越多的研究表明许多转录因子以及环境信号、激素信号和表观遗传相关因子通过对WUS及其调节基因的调控来影响FM终止发育过程。本研究组首次在组织水平阐明了生长素和细胞分裂素调控FM活性的分子机制：AUXIN RESPONSE FACTOR3（ARF3）能够整合AG、APETALA2（AP2）和生长素（auxin）的信号通过抑制细胞分裂素（cytokinin）信号系统调控FM终止发育；还首次证实光信号对FM活性调控的分子机制：FAR-RED ELONGATED HYPOCOTYL3（FHY3）通过PHYTOCHROME A（PHY A）信号途径介导光信号对WUS表达的调控；在基因水平上首次揭示了染色质构象变化对FM终止发育的调控作用：AGAMOUS（AG）可以直接结合到WUS的调控区并招募PcG蛋白对WUS5'-TSS和WUS3'-CRE的结合，以介导WUS染色质环状结构（chromatin loop）的形成，进而抑制WUS的转录，而DNA TOPOISOMERASE 1 TOP1α（TOP1α）对染色质的高级结构的重塑作用也是该染色质环形成的基础。随着3D基因组（three dimensional genomics）时代的到来为我们进一步理解FM终止发育机制提供了新的窗口，而FM终止发育机制在农业生产上的运用也体现了其巨大的应用价值。本文首先简述了拟南芥（Arabidopsis thaliana）花发育研究的发展史，介绍了花发育相关领域所关心的3个科学问题，并着重阐述了FM终止发育调控的研究进展与前景。
关键词:花分生组织/
终止发育/
WUS/
环境信号/
激素信号/
表观遗传
Abstract:In higher plants, plant tissues and organs are generated from meristems. Shoot apical meristem (SAM) gives rise to all of the aboveground parts for the entire life of plant through continuous production of new organ primordial, including floral meristem (FM) which finally develops as flowers. Floral development is based on the balance between FM meristem maintenance and termination. At the initial stage, floral stem cells proliferate and produce defined number of floral organs based on the "ABC model" rules. At this stage, FM activity is maintained mainly by CLV (CLAVATA)-WUSCHEL (WUS) feedback loop. WUS encodes a homeodomain containing protein. It promotes stem cell marker gene CLV3 expression when WUS expression is low. It also inhibits CLV3 expression when WUS expression is high. Thus FM activity is maintained and can promote initiation of floral organs. However, after two carpels primordia initiation, FM activity is terminated in a process called FM determinacy. FM determinacy is a dynamic and multi-step process in which WUS plays a central role. WUS expression is regulated by many transcription factors related to floral organ identity[AGAMOUS (AG), APETALA2 (AP2) and SUPERMAN], environmental signals (light, temperature, etc.), plant hormones (auxin, cytokinin, gibberellin, etc.) and epigenetic-related factors (histone modification, chromatin remodeling, non-coding RNA, DNA methylation, etc.). Using model plant Arabidopsis, our study noted that AG terminates FM maintenance by directly repressing WUS through chromatin higher structure (chromatin loop), formed by AG and one of Polycomb Group components TERMINAL FLOWER2/LIKE HETEROCHROMATIN PROTEIN1 (TFL2/LHP1); binding to WUS5'-TSS (transcription start site) and WUS3'-CRE (cis-regulatory element). DNA TOPOISOMERASE 1 (TOP1α) inhibited WUS expression by modulating WUS nucleosome density to inhibit DNA accessibility, which also participated in the progress. AUXIN RESPONSE FACTOR3 (ARF3) induced by auxin regulated FM determinacy by repressing cytokinin biosynthesis[inhibiting cytokinin synthesis genes ISOPENTENYLTRANSFERASE (IPTs) and LONELY GUY (LOGs)] and signaling[inhibiting cytokinin receptor gene ARABIDOPSIS HISTIDINE KINASE4 (AHK4)], which clarified how auxin and cytokinin integrated to regulate FM activity; FAR-RED ELONGATED HYPOCOTYL 3(FHY3) activated SEPALLATA2, but inhibited CLAVATA3 to regulate meristem determinacy and maintenance, which shed light on how light affected meristem activity. As 3D (3-dimentional) genome organization technology developed, the importance of the impact of chromatin structure on gene expression was realized and more techniques were developed and improved. Using the newly reported methods, FM determinacy mechanism required further in-depth studies. What was more was that since plant FM determinacy was regulated precisely and accurately, any defects in FM determinacy affected seed development. Exploitation of FM determinacy mechanism had the potential to importantly contribute to agricultural production, which was helpful for ensuring reproductive success, seed development and yield of agricultural crops (maize, tomato, etc.). In this review, we gave a short introduction on floral organ identity in Arabidopsis thaliana and the mechanism of meristem maintenance and differentiation. Then we mainly focused on FM determinacy, including some recent studies by our group. Finally, we advanced the application of fundamental studies in crop yields and further prospects for research.
Key words:Floral meristem/
Determinacy/
WUS/
Environmental signal/
Plant hormone/
Epigenetics
? Equal contributors
注释:
1) ? 同等贡献者
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