Department of Molecular and Cellular Biology, The University of Arizona, Tucson, AZ 85721.
Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720.
Proc Natl Acad Sci U S A. 2020 Jun 30;117(26):15305-15315. doi: 10.1073/pnas.2001332117. Epub 2020 Jun 15.
Small RNAs are abundant in plant reproductive tissues, especially 24-nucleotide (nt) small interfering RNAs (siRNAs). Most 24-nt siRNAs are dependent on RNA Pol IV and RNA-DEPENDENT RNA POLYMERASE 2 (RDR2) and establish DNA methylation at thousands of genomic loci in a process called RNA-directed DNA methylation (RdDM). In , RdDM is required in the maternal sporophyte for successful seed development. Here, we demonstrate that a small number of siRNA loci account for over 90% of siRNA expression during seed development. These loci exhibit unique characteristics with regard to their copy number and association with genomic features, but they resemble canonical 24-nt siRNA loci in their dependence on RNA Pol IV/RDR2 and role in RdDM. These loci are expressed in ovules before fertilization and in the seed coat, embryo, and endosperm following fertilization. We observed a similar pattern of 24-nt siRNA expression in diverse angiosperms despite rapid sequence evolution at siren loci. In the endosperm, siren siRNAs show a marked maternal bias, and siren expression in maternal sporophytic tissues is required for siren siRNA accumulation. Together, these results demonstrate that seed development occurs under the influence of abundant maternal siRNAs that might be transported to, and function in, filial tissues.
小 RNA 在植物生殖组织中大量存在，尤其是 24 核苷酸（nt）的小干扰 RNA（siRNA）。大多数 24-nt siRNA 依赖于 RNA 聚合酶 IV 和 RNA 依赖性 RNA 聚合酶 2（RDR2），并在称为 RNA 指导的 DNA 甲基化（RdDM）的过程中在数千个基因组位点建立 DNA 甲基化。在 中，RdDM 在母本孢子体中对于成功的种子发育是必需的。在这里，我们证明，少数几个 siRNA 基因座占种子发育过程中超过 90%的 siRNA 表达。这些基因座在其拷贝数及其与基因组特征的关联方面表现出独特的特征，但它们在依赖 RNA 聚合酶 IV/RDR2 和在 RdDM 中的作用方面与典型的 24-nt siRNA 基因座相似。这些基因座在受精前的胚珠中和受精后的种皮、胚和胚乳中表达。尽管在 siren 基因座的快速序列进化，但我们在不同的被子植物中观察到了类似的 24-nt siRNA 表达模式。在内胚乳中，siren siRNA 表现出明显的母性偏倚，母本孢子体组织中 siren 的表达对于 siren siRNA 的积累是必需的。总之，这些结果表明，种子发育是在大量母性 siRNA 的影响下发生的，这些 siRNA 可能被运输到并在后代组织中发挥作用。
