拟南芥PcG蛋白EMF1介导的RNA甲基化调控幼苗叶绿体发育的分子机理研究

Plant development progresses through distinct phases: germination, seedling and vegetative growth, followed by a reproductive phase and eventually seed set and senescence. The transitions between these phases are controlled by distinct genetic circuits that integrate endogenous and environmental cues. Extended vegetative growth allows plants to accumulate sufficient reserves needed for energy-intensive flowering and seed production. Thus, the transition from seed germination to seedling phase must be fine regulated. However, how to maintain a vigorous vegetative growth phase after germination is so far unknown. Polycomb group (PcG) protein-mediated gene silencing is a major mechanism that regulates gene expression at the transcriptional level in higher eukaryotes. In Arabidopsis, PcG proteins are required for the transition from seedling to reproductive phase through the repression of flower organ and flowering time genes. However, little is known about the regulatory function of PcG proteins in seedling growth phase after germination. Our previous studies showed that a plant specific PcG protein, EMBRYONIC FLOWER1 (EMF1) participates in the repression of seed developmental genes, but activation of photosynthesis genes during early seedling stage. Disrupted chloroplast structures were observed in the emf1 mutant seedlings. EMF1 maintains silencing state of seed developmental genes through trimethylation of histone 3 Lys-27 (H3K27me3), but photosynthesis genes are not marked by H3K27me3. We found that EMF1 may regulate photosynthesis genes via a new epigenetic modification, 5-Methylcytosine RNA methylation (m5C), to promote vegetative development. Contrary to the conventional view that EMF1 inhibits the expression of downstream genes through the PcG pathway, these results suggest EMF1 may play a new role as a activator to regulate photosynthesis genes and chloroplast development. In this project, we plan to characterize the molecular mechanism underlying the new role for EMF1 in regulation of chloroplast development during seedling development. Our findings will shed light on the novel function of plant-specific chromatin proteins on regulation of seedling establishment and development in Arabidopsis and also has very important implications for improvement of crop yield and quality.

高等植物中，外源和内源因素共同调控种子萌发到幼苗生长期的转换，旺盛的苗期不仅是植物繁育后代的基础，还直接影响植物尤其是农作物的产量。已知拟南芥中一类重要的表观遗传蛋白PcG通过抑制苗期开花基因调控幼苗到生殖期的转换。但是关于调控植物萌发到幼苗生长发育的模式及机制还不完善。我们前期工作发现一个植物特异PcG基因EMF1，突变体矮小黄化、叶绿体发育不正常。突变体幼苗中种子发育基因上调，光合基因下调；EMF1通过组蛋白甲基化抑制种子发育基因，但是光合基因并没有该修饰，我们推测作为抑制子的EMF1可能以一种新表观修饰激活光合基因表达促进叶绿体发育从而维持幼苗正常的生长模式。但是具体的转录激活机理、如何调控叶绿体发育等仍不清楚。本项目在此研究基础上，深入探索EMF1介导的新表观修饰在叶绿体发育中的作用，阐明植物萌发到幼苗生长发育的模式及调控机制，同时为改良作物提高产量提供重要理论依据。

植物的营养生长受多重发育程序的调控，会直接影响种子发育和作物产量，但植物响应环境、平衡复杂的发育程序以维持营养生长阶段能量积累和快速生长的分子机制目前却并不清楚。RNA甲基化是近年来揭示的表观遗传新修饰，在调控前体RNA剪接、RNA编辑、RNA翻译、RNA稳定性等方面发挥重要作用。RNA 5-甲基胞嘧啶（m5C）和6-甲基腺嘌呤(m6A)是目前研究的最为常见的两种RNA转录后修饰，广泛存在于真核生物中。然而，相比于研究深入的RNA m6A，RNA m5C的研究进展较慢，尤其是在动态染色质修饰上的直接作用仍然很大程度上是未知的。研究发现植物特异Polycomb（PcG）蛋白EMBRYONIC FLOWER1 （EMF1）介导RNA甲基化m5C和组蛋白甲基化H3K27me3在全基因组水平上的协作关系，在植物营养生长阶段可同时作为抑制子和激活子调控特定的开花、种子发育以及叶绿体发育和光合作用等关键下游靶基因转录。一方面，EMF1通过组蛋白甲基化（H3K4me3）调控RNA甲基转移酶TRM4B的表达，影响RNA甲基化m5C修饰在叶绿体发育和光合基因上的富集，进而激活这些基因的表达（EMF1-TRM4B-m5C）。另一方面，EMF1又可以通过PcG途径影响组蛋白甲基化H3K27me3修饰在花器官、种子和淀粉等发育基因上的富集，进而抑制这些基因的表达（EMF1-PcG- H3K27me3）。两个不同的作用方式精确调控了植物正常的营养生长，促进了植物的光合作用，从而为种子的形成积累了能量。这些研究结果不仅为植物表观遗传/基因组学相关研究提供了重要数据资源，而且深入揭示了RNA甲基化在染色质表观修饰可塑性的贡献，并为复杂的表观调控网络在作物高光效、高产等基因转录调控中的作用提供了新认知。