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.) micr ospores irreversibly blocked normal gametophytic development and induc ed the formation of embryogenic cells, which developed subsequently in to pollen-derived embryos by culture at 25 degrees C in a sugar-contai ning medium. A cold shock at 4 degrees C did not inhibit microspore ma turation in vitro and did not induce cell division activity even when combined with a starvation treatment. In the absence of sucrose, micro spores isolated in the G1 phase of the cell cycle replicated their DNA and accumulated in G2. Late microspores underwent miotosis during the first clay of culture which resulted in a mixed population of bicellu lar 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 mitos is in vitro is not important for embryogenic induction. These results represent a step forward tow ards a unified model of induction of embr yogenesis from microspores/pollen which, within a relatively wide deve lopmental window, are competent to deviate from normal gametophytic de velopment and initiate the alternative sporophytic programme, in respo nse to specific stress signals.