Multiple gating steps in pollination lock the species specificity

In flowering plants, pollen grain must undergo a series of critical processes, including adhesion, hydration, and germination, which are dependent on the stigma, to develop a pollen tube. This pollen tube then penetrates the stigma to reach the internal tissues of pistil, facilitating the transport of non-motile sperm cells to the embryo sac for fertilization. However, the dry stigma, characterized by the absence of an exudate that typically envelops the wet stigma, functions as a multi-layered filter in adhesion, hydration, germination and penetration that permits the acceptance of compatible pollen or tubes while rejecting incompatible ones, thereby protecting the embryo sac from ineffective fertilization and maintaining species specificity. Given the significance of these selective events, related research has consistently been at the forefront of reproductive studies, with notable advancements being made in recent times. In this review, we systematically synthesize the selective events and provide comprehensive, up-to-date summaries of occurrences on dry stigmas with a particular focus on the Brassicaceae family, following the chronological sequence of these events. Our objective is to update and elucidate the critical checkpoints within the pollination, identify unresolved questions, and propose potential avenues for future research in other plant families.