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Key message: The miR822 together with of AGO9 protein, modulates monosporic development in Arabidopsis thaliana through the regulation of target genes encoding Cysteine/Histidine-Rich C1 domain proteins, revealing a new role of miRNAs in the control of megaspore formation in flowering plants. In the ovule of flowering plants, the establishment of the haploid generation occurs when a somatic cell differentiates into a megaspore mother cell (MMC) and initiates meiosis. As most flowering plants, Arabidopsis thaliana (Arabidopsis) undergoes a monosporic type of gametogenesis as three meiotically derived cells degenerate, and a single one—the functional megaspore (FM), divides mitotically to form the female gametophyte. The genetic basis and molecular mechanisms that control monosporic gametophyte development remain largely unknown. Here, we show that Arabidopsis plants carrying loss-of-function mutations in the miR822, give rise to extranumerary surviving megaspores that acquire a FM identity and divides without giving rise to differentiated female gametophytes. The overexpression of three miR822 putative target genes encoding cysteine/histidine-rich C1 (DC1) domain proteins, At5g02350, At5g02330 and At2g13900 results in defects equivalent to those found in mutant mir822 plants. The three miR822 targets genes are overexpressed in ago9 mutant ovules, suggesting that miR822 acts through an AGO9-dependent pathway to negatively regulate DC1 domain proteins and restricts the survival of meiotically derived cells to a single megaspore. Our results identify a mechanism mediated by the AGO9-miR822 complex that modulates monosporic female gametogenesis in Arabidopsis thaliana. [ABSTRACT FROM AUTHOR]

Summary: Methylation of DNA is an epigenetic mechanism for the control of gene expression. Alterations in the regulatory pathways involved in the establishment, perpetuation and removal of DNA methylation can lead to severe developmental alterations. Our understanding of the mechanistic aspects and relevance of DNA methylation comes from remarkable studies in well‐established angiosperm plant models including maize and Arabidopsis. The study of plant models positioned at basal lineages opens exciting opportunities to expand our knowledge on the function and evolution of the components of DNA methylation. In this Tansley Insight, we summarize current progress in our understanding of the molecular basis and relevance of DNA methylation in the liverwort Marchantia polymorpha. [ABSTRACT FROM AUTHOR]

Apomictic plants reproduce asexually through seeds by avoiding both meiosis and fertilization. While apomixis is genetically controlled, individual loci contributing to its expression have yet to be identified. Here, we review recent results indicating that RNA-dependent DNA methylation pathways acting during female reproduction are essential for proper reproductive development in plants, and may represent key regulators of the differentiation between apomictic and sexual reproduction. [ABSTRACT FROM AUTHOR]

Some angiosperms reproduce by apomixis, a natural way of cloning through seeds. Apomictic plants bypass both meiosis and egg cell fertilization, producing progeny that are genetic replicas of the mother plant. In this report, we analyze reproductive development in Tripsacum dactyloides, an apomictic relative of maize, and in experimental apomictic hybrids between maize and Tripsacum. We show that apomictic reproduction is characterized by an alteration of developmental timing of both sporogenesis and early embryo development. The absence of female meiosis in apomictic Tripsacum results from an early termination of female meiosis. Similarly, parthenogenetic development of a maternal embryo in apomicts results from precocious induction of early embryogenesis events. We also show that male meiosis in apomicts is characterized by comparable asynchronous expression of developmental stages. Apomixis thus results in an array of possible phenotypes, including wild-type sexual development. Overall, our observations suggest that apomixis in Tripsacum is a heterochronic phenotype; i.e., it relies on a deregulation of the timing of reproductive events, rather than on the alteration of a specific component of the reproductive pathway. [ABSTRACT FROM AUTHOR]