Determining Germ Cells in Flowering Plants: Common Concepts and Molecular Mechanisms Steering Meiocyte Specification and Mode of Cell Division

The specification of meiocyte, a specialized cell type capable of entering meiosis, is a hallmark of the initiation of the reproductive process in eukaryotic organisms. Unlike animals that set aside the progenitor of germinal lineages during early embryogenesis, flowering plants exploit a unique route to specify meiocytes de novo late in post-embryonic development, which involves the acquisition of meiocyte fate by a defined set of somatic cells in the female and male flower organs. Following the earliest discoveries from model species, such as Arabidopsis thaliana, recent updates have broadened and detailed insights into the mechanisms controlling the specification of meiocyte in diverse plant species. As we review here, the findings emphasize the evolutionarily conserved common concepts and molecular systems, involving gene regulation, cell-to-cell communication, and interconnection between developmental and cell-cycle controllers. The trajectories of meiocyte lineage bifurcation have been also dissected at the single-cell levels, and the reconstructed pathway illustrates the complex journey of a cell destined to execute meiosis and to instigate plant reproduction. Furthermore, we highlight gaps and potentials of currently available data and prospect future directions to understand these remarkable events in the life of plants that are biologically important but also tightly associated with agronomic applications.