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Nature Plants：拟南芥春化“记忆”的生成与擦除

已有 5444 次阅读 2019-4-9 13:51 |个人分类:每日摘要|系统分类:论文交流

Embryonic resetting of the parental vernalized state by two B3 domain transcription factors in Arabidopsis

First author: Zeng Tao; Affiliations: CAS Center for Excellence in Molecular Plant Sciences (中科院分子植物科学卓越创新中心): Shanghai, China

Corresponding author: Yuehui He

Some overwintering plants acquire competence to flower, after experiencing prolonged cold in winter, through a process termed vernalization. In the crucifer plant Arabidopsis thaliana, prolonged cold induces chromatin-mediated silencing of the potent floral repressor FLOWERING LOCUS C (FLC) by Polycomb proteins. This vernalized state is epigenetically maintained or ‘memorized’ in warm rendering plants competent to flower in spring, but is reset in the next generation. Here, we show that in early embryogenesis, two homologous B3 domain transcription factors LEAFY COTYLEDON 2 (LEC2) and FUSCA3 (FUS3) compete against two repressive B3-containing epigenome readers and Polycomb partners known as VAL1 and VAL2 for the cis-regulatory cold memory element (CME) of FLC to disrupt Polycomb silencing. Consistently, crystal structures of B3–CME complexes show that B3FUS3, B3LEC2 and B3VAL1 employ a nearly identical binding interface for CME. We further found that LEC2 and FUS3 recruit the scaffold protein FRIGIDA in association with active chromatin modifiers to establish an active chromatin state at FLC, which results in resetting of the silenced FLC to active and erasing the epigenetic parental memory of winter cold in early embryos. Following embryo development, LEC2 and FUS3 are developmentally silenced throughout post-embryonic stages, enabling VALs to bind to the CME again at seedling stages at which plants experience winter cold. Our findings illustrate how overwintering crucifer annuals or biennials in temperate climates employ a subfamily of B3 domain proteins to switch on, off and on again the expression of a key flowering gene in the embryo-to-plant-to-embryo cycle, and thus to synchronize growth and development with seasonal temperature changes in their life cycles.

某些越冬植物能够在经历长时间的低温气候之后获得开花的能力，该过程叫做春化。在十字花科植物拟南芥中，长时间的低温能够通过Polycomb蛋白介导的表观机制沉默主要的开花抑制基因FLC。植物的春化状态是由表观机制所维持的，或者说是表观“记忆”，使得植物能够在温暖的春天开花，但在下一代这种表观“记忆”会被重置。本文，作者的研究显示在胚胎发生的早期，两个同源的B3结构域转录因子LEC2和FUS3会与两个含有B3结构域表观Reader和Polycomb伴侣（即VAL1和VAL2）竞争结合FLC基因的顺式调控低温记忆元件CME，以此打破Polycomb介导的表观沉默。B3–CME复合物的晶体结构显示B3FUS3、B3LEC2以及B3VAL1与CME的结合方式近乎一致。作者进一步发现了LEC2和FUS3会招募与活性染色质修饰相关的支架蛋白FRIGIDA来建立FLC位点的染色质活性状态，从而在胚胎发育早期解除FLC基因的沉默，激活并擦除亲本春化的表观记忆。随着胚胎的发育，LEC2和FUS3逐步在胚胎发育后期沉默，这样使得幼苗在经历低温时VAL又能够重新结合到CME上。本文的发现阐明了温带越冬十字花科一年生或是两年生植物如何通过含B3结构域蛋白来在胚胎，植株，再到胚胎的循环中开启，关闭，然后再开启关键开花基因的表达，从而使得这类植物在其生命周期的生长和发育能够与季节性的温度变化同步。