Revisiting the Female Germline and Its Expanding Toolbox

In the plant ovule, a single cell of somatic origin initiates a developmental program distinct from that of adjoining cells. This cell, typically known as the megaspore mother cell (MMC), is also referred to as the primary female germline cell. Rapid advances in cell type-specific profiling technologies, deep-tissue microscopy, and marker gene resources have reinvigorated the study of female germline development. Key results show that epigenetic pathways, cell-cycle regulators, and mobile signals act to establish and restrict female germline identity. Recent reports detail how several of these pathways are interconnected. The Arabidopsis thaliana ovule arises as a female reproductive organ composed solely of somatic diploid cells. Among them, one cell will acquire a unique identity and initiate female germline development. In this review we explore the complex network that facilitates differentiation of this single cell, and consider how it becomes committed to a distinct developmental program. We highlight recent progress towards understanding the role of intercellular communication, cell competency, and cell-cycle regulation in the ovule primordium, and we discuss the possibility that distinct pathways restrict germline development at different stages. Importantly, these recent findings suggest a renaissance in plant ovule research, restoring the female germline as an attractive model to study cell communication and cell fate establishment in multicellular organs. The Arabidopsis thaliana ovule arises as a female reproductive organ composed solely of somatic diploid cells. Among them, one cell will acquire a unique identity and initiate female germline development. In this review we explore the complex network that facilitates differentiation of this single cell, and consider how it becomes committed to a distinct developmental program. We highlight recent progress towards understanding the role of intercellular communication, cell competency, and cell-cycle regulation in the ovule primordium, and we discuss the possibility that distinct pathways restrict germline development at different stages. Importantly, these recent findings suggest a renaissance in plant ovule research, restoring the female germline as an attractive model to study cell communication and cell fate establishment in multicellular organs. a form of asexual reproduction that results in clonal maternal progeny through seed. a mode of apomictic reproduction where at least one unreduced female gametophyte (FG) is produced from a somatic cell adjoining the megaspore mother cell (MMC) or functional megaspore (FM). precursor cell of the female germline and progenitor of the MMC. a mechanism that occurs in a cell and produces effects on that cell. a mode of apomictic reproduction where an unreduced female gametophyte is produced directly from an MMC that avoids meiosis. the egg cell, which is located in the female gametophyte. a haploid structure produced by divisions of the FM, also known as the embryo sac. The mature gametophyte contains four different cell types, one of which is the egg cell that functions as the female gamete. the cellular lineage originating from the diploid MMC and that gives rise to a haploid FM, ending with the formation of a mature female gamete. the only megaspore out of four meiotic products that survives, enters megagametogenesis, and forms the FG. refers to cells that adopt a germline developmental program. cells including the AC, MMC, FM, and FG, as well as abnormal nucellar cells from mutants where cellular identity has been compromised in the ovule. formation of an AC and its differentiation into the primary germline cell, the MMC. defines the stages during which the FM undergoes a series of mitotic divisions and cellularization to form the FG. describes the processes occurring from FM expansion until the production of a mature FG. one of four haploid cells resulting from meiosis of the MMC. the most distal cell within the L2 of the nucellus that acts as the primary germline cell. a term describing the processes occurring from MMC formation until the production of a FM. a mechanism that occurs in one cell but produces effects on another cell. distal ovule tissue composed of diploid cells organized into two layers (L), including the L1 epidermal cells and L2 hypodermal cells, where the germline cell lineage will develop.