In Arabidopsis (Arabidopsis thaliana), DNA-dependent RNA polymerase IV (Pol IV) is required for the formation of transposable element (TE)-derived small RNA transcripts. These transcripts are processed by DICER-LIKE3 into 24-nucleotide small interfering RNAs (siRNAs) that guide RNA-directed DNA methylation. In the pollen grain, Pol IV is also required for the accumulation of 21/22-nucleotide epigenetically activated siRNAs, which likely silence TEs via post-transcriptional mechanisms. Despite this proposed role of Pol IV, its loss of function in Arabidopsis does not cause a discernible pollen defect. Here, we show that the knockout of NRPD1, encoding the largest subunit of Pol IV, in the Brassicaceae species Capsella (Capsella rubella), caused postmeiotic arrest of pollen development at the microspore stage. As in Arabidopsis, all TE-derived siRNAs were depleted in Capsella nrpd1 microspores. In the wild-type background, the same TEs produced 21/22-nucleotide and 24-nucleotide siRNAs; these processes required Pol IV activity. Arrest of Capsella nrpd1 microspores was accompanied by the deregulation of genes targeted by Pol IV-dependent siRNAs. TEs were much closer to genes in Capsella compared with Arabidopsis, perhaps explaining the essential role of Pol IV in pollen development in Capsella. Our discovery that Pol IV is functionally required in Capsella microspores emphasizes the relevance of investigating different plant models.

在拟南芥（Arabidopsis thaliana）中，需要DNA依赖性RNA聚合酶IV（Pol IV）来形成转座因子（TE）衍生的小RNA转录本。这些转录物由DICER-LIKE3处理成24个核苷酸的小干扰RNA（siRNA），该RNA指导RNA定向的DNA甲基化。在花粉粒中，21/22核苷酸表观遗传激活的siRNA的积累也需要Pol IV，这可能通过转录后机制使TE沉默。尽管提出了Pol IV的建议功能，但其在拟南芥中的功能丧失并未引起明显的花粉缺陷。在这里，我们显示了在十字花科科植物Capsella（Capsella rubella）中，编码Pol IV最大亚基的NRPD1的敲除），导致小孢子期花粉发育减数分裂后停止。与拟南芥一样，Cap素nrpd1小孢子中所有TE衍生的siRNA都被耗尽。在野生型背景中，相同的TE产生21/22个核苷酸和24个核苷酸的siRNA。这些过程需要Pol IV活性。Capsella nrpd1小孢子的逮捕伴随着Pol IV依赖性siRNA靶向基因的失控。与拟南芥相比，TEs更接近于s菜中的基因，这也许可以解释Pol IV在Cap菜花粉发育中的重要作用。我们发现Capsella小孢子在功能上需要Pol IV，这强调了研究不同植物模型的相关性。