Plant mitochondrial genomes undergo frequent homologous recombination (HR). Ectopic HR activity is inhibited by the HR surveillance pathway, but the underlying regulatory mechanism is unclear. Here, we show that the mitochondrial RNase H1 AtRNH1B impairs the formation of RNA:DNA hybrids (R-loops) and participates in the HR surveillance pathway in Arabidopsis thaliana. AtRNH1B suppresses ectopic HR at intermediate-sized repeats (IRs) and thus maintains mitochondrial DNA (mtDNA) replication. The RNase H1 AtRNH1C is restricted to the chloroplast; however, when cells lack AtRNH1B, transport of chloroplast AtRNH1C into the mitochondria secures HR surveillance, thus ensuring the integrity of the mitochondrial genome and allowing embryogenesis to proceed. HR surveillance is further regulated by the single-stranded DNA-binding protein ORGANELLAR SINGLE-STRANDED DNA BINDING PROTEIN1 (OSB1), which decreases the formation of R-loops. This study uncovers a facultative dual targeting mechanism between organelles and sheds light on the roles of RNase H1 in organellar genome maintenance and embryogenesis.

中文翻译：

---

线粒体 RNase H1 活性调节 R-loop 稳态以维持基因组完整性并使拟南芥中的早期胚胎发生成为可能。

植物线粒体基因组经历频繁的同源重组 (HR)。异位 HR 活动受到 HR 监测途径的抑制，但潜在的调控机制尚不清楚。在这里，我们表明线粒体 RNase H1 AtRNH1B 会损害 RNA:DNA 杂交体（R 环）的形成，并参与拟南芥中的 HR 监测途径。AtRNH1B 在中等大小的重复序列 (IR) 处抑制异位 HR，从而维持线粒体 DNA (mtDNA) 复制。RNase H1 AtRNH1C 仅限于叶绿体；然而，当细胞缺乏 AtRNH1B 时，叶绿体 AtRNH1C 转运到线粒体中可确保 HR 监测，从而确保线粒体基因组的完整性并允许胚胎发生。HR 监测进一步受到单链 DNA 结合蛋白 ORGANELLAR SINGLE-STRANDED DNA BINDING PROTEIN1 (OSB1) 的调控，这会减少 R 环的形成。这项研究揭示了细胞器之间的兼性双重靶向机制，并阐明了 RNase H1 在细胞器基因组维持和胚胎发生中的作用。