The use of dihaplold plants for obtaining new varieties has been widely reported in different plant species. The regeneration of these plants is carried out by in vitro induction of embryogenesis in microspores and pollen grains. This process is switched by the application of stress treatments and hormones, but the efficiency is still very low in many crops. The molecular and cellular processes responsible of the change in the developmental program of the microspore are still under investigation. Defined ultrastructural and expression changes have been reported to accompany the reprogramming of the microspore to embryogenesis in dicot systems (Testillano et at., 2000), but less is known on the cellular characterization of the process in monocots.