Stress as the major signal controlling the developmental fate of tobacco microspores: towards a unified model of induction of microspore/pollen embryogenesis

Specific stress treatments (sucrose starvation, alone or combined with a heat shock) applied to isolated tobacco (Nicotiana tabacum L.) microspores irreversibly blocked normal gametophytic development and induced the formation of embryogenic cells, which developed subsequnetly into pollen-derived embryos by culture at 25°C in a sugar-containing medium. A cold shock at 4°C did not inhibit microspore maturation in vitro and did not induce cell division activity, even when combined with a starvation treatment. In the absence of sucrose, microspores isolated in the G1 phase of the cell cycle replicated their DNA and accumulated in G2. Late microspores underwent miotosis during the first day of culture which resulted in a mixed population of bicellular pollen grains and uninucleate microspores, both embryogenic. After the inductive stress treatments the origin of the first multicellular structures, formed in the sugar-containing medium, could be traced to divisions of the microspore cell or divisions of the vegetative cell of bicellular pollen, indicating that the symmetry of microspore mitosis in vitro is not important for embryogenic induction. These results represent a step forward towards a unified model of induction of embryogenesis from microspores/pollen which, within a relatively wide developmental window, are competent to deviate from normal gametophytic development and initiate the alternative sporophytic programme, in response to specific stress signals.