Pollen exine spontaneously shedding in the angiosperm Annona montana (Annonaceae)

Background: It is generally considered that exine is vital for interactions with stigmas, and pollen exine shedding during pollination has never been reported in angiosperms. However, Annona montana , located at Annonaceae family in basal angiosperms, was observed spontaneously shedding exine in stigma exudates and any watery solution. Results We demonstrated the entire process of A. montana pollen exine shedding in vivo and in vitro , verified that water is the only necessary medium for A. montana pollen shedding, and exine shedding is an unavoidable step before pollen germination. Since pollen volume of A. montana did not change remarkably before and after hydration, it is undoubtedly that exine shedding in this species might be related with exine, rather than the intine-absorbing water. Conclusions Pollen shedding exine in A. montana exhibits a special and rare reproductive characteristic in angiosperms, which will update what we have previously recognized that exine is essential in reproductive processes.