From Pollen Tube Discharge to Pronuclear Union in Arabidopsis: An Evidence-Ranked View of the Terminal Phase of...

Double fertilization is still commonly discussed as though its terminal phase were a single problem, 'gamete fusion.' Yet the interval between pollen tube discharge and pronuclear union comprises a sequence of transitions with different evidentiary status. This review narrows the field to that terminal window and evaluates each stage using two criteria: directness of evidence in flowering plants and mechanistic completeness of the internal sequence. Direct observation, in planta perturbation, and comparative or heterologous reasoning are treated as sources of evidence used to support or limit those evaluations. Arabidopsis serves as the primary reference system, with selected angiosperm comparisons included only where they test transferability or illuminate unresolved stages. The main stages considered are post-discharge sperm cell exposure and positioning, attachment and biased engagement, competence acquisition, plasmogamy, post-entry activation and developmental progression, closure and recovery, sperm cell nuclear migration, and pronuclear union. Several stages are now directly distinguished even where their internal mechanism remains incomplete. Flowering plant work has yet to resolve the activating step that converts surface-deployed GCS1/HAP2 into a merger-driving fusogen, the decisive arrest state between stable apposition and full cytoplasmic continuity, the sequence of pore opening, expansion, and post-fusion membrane remodeling, or the order of outer- and inner-nuclear-envelope fusion during pronuclear union. The terminal phase is therefore best understood as a sequence of experimentally distinguishable stages, and robust causal interpretation depends on treating recovery, closure, plasmogamy, and karyogamy as analytically distinct processes.