Cellular changes during the acquisition of embryogenic potential in isolated pollen grains ofNicotiana tabacum

Summary We used electron microscopical techniques to study ultrastructural changes during the acquisition of embryogenic competence in immature pollen grains ofNicotiana tabacum, isolated at the early- or mid-bicellular stage and cultured in vitro under starvation conditions. Cytoplasmic and nuclear changes during the starvation treatment are reported. Dedifferentiation of plastids, dilation of the wall of the generative cell, the appearance of a large vacuole, loss of nuclear pores in the vegetative nucleus, changes in chromatin and nucleolar structure, and a decrease in the size of the nucleolus were observed. We suggest that these events are the first step in the switch from generative to vegetative generation during pollen embryogenesis.