Embryo CHH hypermethylation is mediated by RdDM and is autonomously directed in Brassica rapa

大背景：RNA-directed DNA methylation (RdDM) initiates cytosine methylation in all contexts and maintains asymmetric CHH methylation. 

小背景：Mature plant embryos show one of the highest levels of CHH methylation, and it has been suggested that RdDM is responsible for this hypermethylation. 

现有知识与推测：Because loss of RdDM in Brassica rapa causes seed abortion, embryo methylation might play a role in seed development. RdDM is required in the maternal sporophyte, suggesting that small RNAs from the maternal sporophyte might translocate to the developing embryo, triggering DNA methylation that prevents seed abortion. 

提出问题：This raises the question of whether embryo hypermethylation is autonomously regulated by the embryo itself or influenced by the maternal sporophyte. 

结果1-白菜胚胎超甲基化需要RdDM：Here, we demonstrate that B. rapa embryos are hypermethylated in both euchromatin and heterochromatin and that this process requires RdDM. 

结果2-母本孢子体不影响胚胎的超甲基化：Contrary to the current models, B. rapa embryo hypermethylation is not correlated with demethylation of the endosperm. We also show that maternal somatic RdDM is not sufficient for global embryo hypermethylation, and we find no compelling evidence for maternal somatic influence over embryo methylation at any locus. 

结果3-胚胎超甲基化并不影响胚胎发育：Decoupling of maternal and zygotic RdDM leads to successful seed development despite the loss of embryo CHH hypermethylation. We conclude that embryo CHH hypermethylation is conserved, autonomously controlled, and not required for embryo development. 

结论-母本RdDM不影响胚胎超甲基化，但是对于胚胎发育是必需的：Furthermore, maternal somatic RdDM, while required for seed development, does not directly influence embryo methylation patterns.

 摘 要 

RNA指导的DNA甲基化（RdDM）起始所有的胞嘧啶甲基化，以及维持CHH甲基化的不对称性。成熟的植物胚胎表现出最高水平的CHH甲基化，已有的研究显示RdDM作用于该超甲基化。由于白菜（Brassica rapa）RdDM的缺失会导致种子败育，所以胚胎甲基化可能在种子发育过程中发挥重要作用。RdDM对于母本孢子体来说是必需的，因此来自母本孢子体的小RNA可能会移动到正在发育的胚胎中，从而诱导DNA甲基化，防止种子败育。这就带了一个问题，即胚胎的超甲基化是由胚胎自身自主调控还是受母本孢子体的影响。本文中，作者发现白菜胚胎的超甲基化同时发生在常染色质和异染色质区，并且该过程需要RdDM。与现有的模型相反，白菜胚胎的超甲基化与胚乳的去甲基化无关。作者还发现母本体细胞的RdDM不足以诱导胚胎整体的超甲基化，并且作者没有发现任何有力的证据能够表明母本体细胞对任何位点的胚胎甲基化有影响。尽管胚胎CHH甲基化在母本和合子RdDM分离时缺失，但是种子成功发育。因此，作者认为胚胎CHH超甲基化是保守的、受到胚胎自主控制的，但不是胚胎发育所必需的。此外，母本体细胞RdDM虽然是种子发育所必需的，但并不直接影响胚胎的甲基化模式。