Meiosis requires m6A modification for selection of targets in plants

Abstract

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.