Meiosis, a specialized form of cell division, halves diploid chromosome numbers to generate haploid gametophytes, which is essential for sexual reproduction in most eukaryotes. Meiotic recombination not only facilitates the exchange of genetic information between homologous chromosomes (homologs), but also assures their subsequent proper segregation, which has a great impact on the genetic diversity and genomic integrity of progenies. Meiotic recombination is initiated from the programmed formation of DSBs, catalyzed by the evolutionarily conserved type-II DNA topoisomerase SPO11-1 and MTOPVIB complex. The repair of DSBs can result in exchange of DNA between homologs known as crossovers (COs) and noncrossovers (NCOs) (Wang & Copenhaver, 2018; Zickler & Kleckner, 2023). In the past 30 years, molecular genetic studies have identified dozens of genes involved in regulating the formation and repair of meiotic DSBs in plants, including some epigenetic factors (Wang & Copenhaver, 2018). However, the function of RNA modification related to this process is largely unknown.

Xue et al. have made a first step towards understanding the mechanism of the m⁶A eraser ALKBH5 in regulating DSB formation and repair in rice meiosis. They started this project by identifying a male sterile mutant from a gamma-irradiated rice mutant library. The author further cloned the target OsALKBH5 gene and demonstrated that its mutations affected DSB formation and repair during meiosis I, thus leading to male sterility. OsALKBH5 preferentially localizes in the cytoplasm and nucleoplasm of pollen mother cells (PMCs), microspores, and tapetal cells. They confirmed the m⁶A demethylase activity of OsALKBH5 in vitro and in vivo, and that the mutation of OsALKBH5 impacts on m⁶A modifications and stability of mRNA during meiosis. Further observations showed that the Osalkbh5 mutant displays upmethylated m⁶A modification on associated downregulated genes, such as PAIR2, PAIR3, OsCOM1, OsZIP4, and HEIP1, which are required for meiotic recombination. These results suggest that OsALKBH5 participates in meiotic recombination by mediating m⁶A modification on targeted genes to maintain their mRNA stability (Fig. 1). Similarly, deficiency of the mice Alkbh5 homolog resulted in increased m⁶A modifications and infertility of spermatocytes (Zheng et al., 2013). Further study demonstrated that ALKBH5-mediated m⁶A erasure from mRNAs is required for correct splicing and the selective degradation of long 3′UTR transcripts in spermatocytes and spermatids (Tang et al., 2018). Therefore, the molecular mechanism of ALKBH5-mediated m⁶A may serve as a fundamental basis shared between plants and animals, while the target genes involved in meiosis appear to be diversified across different species (Zheng et al., 2013; Tang et al., 2018). As expected, a very recent study revealed comprehensive maps of m⁶A at single-base precision in different tissues throughout the life cycle of both rice and Arabidopsis. Comparative analysis with mammals uncovered the existence of comparable distribution patterns and modification sites across species and tissues (Wang et al., 2024).

It has been observed that the expression of numerous meiotic genes occurs before the initiation of maize meiosis, and certain genes exhibit varying levels of transcript abundance during different stages of meiosis (Nelms & Walbot, 2019). In mammals, both the m⁶A ‘writer’ Mettl3 and the ‘eraser’ Alkbh5 have been reported to regulate mRNA splicing and stability (Xu et al., 2017; Tang et al., 2018). By contrast, Mettl3 regulates spermatogonia differentiation and meiosis initiation, whereas Alkbh5 functions in meiotic spermatogenic cells and round spermatids. Consistently, the inactivation of METTL3 in mice causes a much earlier meiotic arrest than Alkbh5-knockout mice (Zheng et al., 2013; Xu et al., 2017). These observations provide evidence that posttranscriptional m⁶A modification is required for maintaining the activity of meiotic genes, and the stage-specific deposition and removal of m⁶A ensures the orderly progression of meiosis. This conclusion is supported by the finding of Xue et al. that multiple meiosis-specific genes related to DSB formation and repair are regulated by OsALKBH5-induced m⁶A erasure. The identification of the ‘writer’ responsible for m⁶A modification during meiosis in plants, as well as the relationship between this ‘writer’ and the ‘eraser’, including their special targets, needs to be further investigated.

Xue et al.'s study also raises other interesting questions. For example, during meiosis, OsALKBH5 is expressed not only in meiocytes but also in the tapetum, indicating a potential role in mediating genes for either tapetum or pollen development. They also pointed out that, a large number of meiosis-specific genes with downregulated expression showed unexpected decreased m⁶A methylation in the Osalkbh5 mutant, including OsMTOPVIB required for meiotic DSB formation. This presents the question: how does the knockout of a demethylase reduce the m⁶A of certain mRNA transcripts. Recent studies revealed that RNA m⁶A methylation is closely correlated with other epigenetic factors, including DNA methylation, histone modifications, microRNAs, long noncoding RNAs, and chromatin remodeling (Hu et al., 2024), which regulates gene expression in a more complex manner. A study in tomatoes showed that RNA demethylase SlALKBH2 and DNA demethylase SlDML2 form a feedback loop to regulate fruit ripening (Zhou et al., 2019). In mammalian cells, histone H3K36me3 is recognized and bound directly by m⁶A writer complex, which subsequently facilitates the deposition of m⁶A mark onto actively transcribed RNAs (Huang et al., 2019). The bidirectional interactions between mRNA m⁶A methylation and other epigenetic regulators commonly exist among species (Hu et al., 2024), and deserves further exploration in meiosis.

减数分裂是细胞分裂的一种特殊形式，它将二倍体染色体数量减半以产生单倍体配子体，这对于大多数真核生物的有性繁殖至关重要。减数分裂重组不仅促进同源染色体（同系物）之间遗传信息的交换，而且保证其随后的正确分离，这对后代的遗传多样性和基因组完整性有很大影响。减数分裂重组是从 DSB 的程序化形成开始的，由进化上保守的 II 型 DNA 拓扑异构酶 SPO11-1 和 MTOPVIB 复合物催化。 DSB 的修复可能导致同系物之间发生 DNA 交换，称为交叉 (CO) 和非交叉 (NCO)（Wang & Copenhaver， 2018 ；Zickler & Kleckner， 2023 ）。在过去的30年里，分子遗传学研究已经鉴定出数十个参与调节植物减数分裂DSB形成和修复的基因，其中包括一些表观遗传因子（Wang & Copenhaver， 2018 ）。然而，与此过程相关的RNA修饰的功能很大程度上未知。

薛等人。在了解 m ⁶ A 擦除器 ALKBH5 在调节水稻减数分裂中 DSB 形成和修复的机制方面迈出了第一步。他们通过从伽马射线照射的水稻突变体库中鉴定出雄性不育突变体开始了这个项目。作者进一步克隆了目标OsALKBH5基因，并证明其突变影响减数分裂I期间DSB的形成和修复，从而导致雄性不育。 OsALKBH5 优先定位于花粉母细胞 (PMC)、小孢子和绒毡层细胞的细胞质和核质中。他们在体外和体内证实了OsALKBH5的m ⁶ A去甲基化酶活性，并且OsALKBH5的突变影响减数分裂过程中m ⁶ A修饰和mRNA的稳定性。进一步的观察表明， Osalkbh5突变体对相关下调基因（例如减数分裂重组所需的PAIR2 、 PAIR3 、 OsCOM1 、 OsZIP4和HEIP1 ）表现出上甲基化的 m ⁶ A 修饰。这些结果表明，OsALKBH5 通过介导靶基因上的 m ⁶ A 修饰以维持其 mRNA 稳定性来参与减数分裂重组（图 1）。同样，小鼠Alkbh5同源物的缺乏导致 m ⁶ A 修饰增加和精母细胞不育（Zheng等人， 2013 ）。进一步的研究表明，精母细胞和精子细胞中长 3'UTR 转录物的正确剪接和选择性降解需要 ALKBH5 介导的 m ⁶ A 从 mRNA 中的擦除（Tang等人，2015 ）。， 2018 ）。因此，ALKBH5介导的m ⁶ A的分子机制可能是植物和动物之间共有的基础，而参与减数分裂的靶基因似乎在不同物种中具有多样性(Zheng等， 2013 ；Tang等，2013 )。 ， 2018 ）。正如预期的那样，最近的一项研究揭示了水稻和拟南芥整个生命周期中不同组织中单碱基精度的 m ⁶ A 综合图谱。与哺乳动物的比较分析揭示了跨物种和组织之间存在可比的分布模式和修饰位点（Wang等人， 2024 ）。

据观察，许多减数分裂基因的表达发生在玉米减数分裂开始之前，并且某些基因在减数分裂的不同阶段表现出不同水平的转录本丰度（Nelms＆Walbot， 2019 ）。据报道，在哺乳动物中，m ⁶ A“写入器”Mettl3 和“擦除器”Alkbh5 都可以调节 mRNA 剪接和稳定性（Xu等人， 2017 ；Tang等人， 2018 ）。相比之下， Mettl3调节精原细胞分化和减数分裂启动，而Alkbh5在减数分裂生精细胞和圆形精子细胞中发挥作用。一致地，小鼠中 METTL3 的失活导致比Alkbh5敲除小鼠更早的减数分裂停滞（Zheng等人， 2013 ；Xu等人， 2017 ）。这些观察结果证明，转录后 m ⁶ A 修饰是维持减数分裂基因活性所必需的，并且 m ⁶ A 的阶段特异性沉积和去除确保了减数分裂的有序进行。薛等人的发现支持了这一结论。与 DSB 形成和修复相关的多个减数分裂特异性基因受到 OsALKBH5 诱导的 m ⁶ A 擦除的调节。植物减数分裂过程中负责 m ⁶ A 修饰的“书写者”的识别，以及该“书写者”和“擦除器”之间的关系，包括它们的特殊目标，需要进一步研究。

薛等人的研究还提出了其他有趣的问题。例如，在减数分裂期间，OsALKBH5 不仅在性母细胞中表达，而且在绒毡层中表达，表明在介导绒毡层或花粉发育的基因中具有潜在作用。他们还指出， Osalkbh5突变体中大量表达下调的减数分裂特异性基因显示出意想不到的m ⁶ A甲基化降低，其中包括减数分裂DSB形成所需的OsMTOPVIB 。这就提出了一个问题：去甲基化酶的敲除如何减少某些 mRNA 转录本的 m ⁶ A。最近的研究表明，RNA m ⁶ A 甲基化与其他表观遗传因素密切相关，包括 DNA 甲基化、组蛋白修饰、microRNA、长链非编码 RNA 和染色质重塑（Hu et al ., 2024 ），以更复杂的方式调节基因表达。方式。一项针对番茄的研究表明，RNA 去甲基酶 SlALKBH2 和 DNA 去甲基酶 SlDML2 形成反馈环来调节果实成熟（Zhou et al ., 2019 ）。在哺乳动物细胞中，组蛋白 H3K36me3 被 m ⁶ A writer 复合物直接识别和结合，随后促进 m ⁶ A 标记沉积到活跃转录的 RNA 上（Huang et al ., 2019 ）。 mRNA m ⁶ A 甲基化与其他表观遗传调节因子之间的双向相互作用通常存在于物种之间（Hu et al ., 2024 ），值得在减数分裂中进一步探索。