Exine dehiscing induces rape microspore polarity, which results in different daughter cell fate and fixes the apical–basal axis of the embryo

The roles of cell polarity and the first asymmetric cell division during early embryogenesis in apical–basal cell fate determination remain unclear. Previously, a novel Brassica napus microspore embryogenesis system was established, by which rape exine-dehisced microspores were induced by physical stress. Unlike traditional microspore culture, cell polarity and subsequent asymmetric division appeared in the exine-dehisced microspore, which finally developed into a typical embryo with a suspensor. Further studies indicated that polarity is critical for apical–basal cell fate determination and suspensor formation. However, the pattern of the first division was not only determined by cell polarity but was also regulated by the position of the ruptured exine. The first division could be equal or unequal, with its orientation essentially perpendicular to the polar axis. In both types of cell division, the two daughter cells could have different cell fates and give rise to an embryo with a suspensor, similar to zygotic apical–basal cell differentiation. The alignment of the two daughter cells is consistent with the orientation of the apical–basal axis of future embryonic development. Thus, the results revealed that exine dehiscing induces rape microspore polarization, and this polarity results in a different cell fate and fixes the apical–basal axis of embryogenesis, but is uncoupled from cell asymmetric division. The present study demonstrated the relationships among cell polarity, asymmetric cell division, and cell fate determination in early embryogenesis.