From the MMC Specification to Endosperm Cellularization in Arabidopsis: A Developmental-Handover Framework for Seed Initiation.

Seed initiation in Arabidopsis depends on regulatory transitions that begin before fertilization, yet these events are often treated as separate developmental episodes rather than as a connected sequence. Here, we synthesize evidence from megaspore mother cell (MMC) specification to endosperm cellularization and ask whether particular stage boundaries meet a narrow definition of developmental handover: a shift between dominant control logics, with detectable first-order consequences in the ensuing interval and acknowledged overlap across the boundary. This framework goes beyond canonical staging by distinguishing chronological succession from shifts in regulatory control, thereby clarifying where earlier states are expected to constrain later outcomes, which developmental boundaries are mechanistically well supported, and where further mechanistic resolution is most needed. We first examine how MMC singleness (restriction to a single reproductive founder cell per ovule primordium) emerges through coupled sporophytic restriction and local competence. We then consider how meiosis and female gametophyte maturation establish regulatory poise (an actively restrained and asymmetric mature female-gametophytic state), including cell-cycle restraint, companion-cell-restricted demethylation, and unequal gametic chromatin states that condition subsequent embryo and endosperm behavior. After fertilization, release of central-cell restraint, activation of an endosperm auxin program, and recruitment of maternal tissues together mark the onset of seed initiation. In this view, syncytial endosperm is an actively maintained developmental state shaped by parental dosage, epigenetic control, hormone signaling, and maternal interaction, whereas endosperm cellularization represents a regulated switch with seed-wide consequences. In Arabidopsis, the clearest handover is the mature female gametophyte-to-fertilization boundary, whereas the boundaries linking MMC specification to female gametophyte maturation and syncytial endosperm to cellularization remain provisional.