THE SIGNIFICANCE OF MICROSPORE DIVISION AND DIVISION SYMMETRY FOR VEGETATIVE CELL-SPECIFIC TRANSCRIPTION AND GENERATIVE CELL-DIFFERENTIATION

The significance of the onset and symmetry of pollen mitosis I (PMI) f or the subsequent differentiation of the vegetative and generative cel ls was investigated by the in vitro maturation of isolated microspores of transgenic tobacco. Free uninucleate microspores of transgenic pla nts harboring the vegetative cell (VC)-specific late anther tomato lat 52 promoter fused to the beta-glucuronidase (gus) gene showed normal a symmetric cell division at PMI and activated the lat52 promoter specif ically in the nascent VC during in vitro maturation. In vitro maturati on in the presence of high levels of colchicine effectively blocked PM I, resulting in the formation of uninucleate pollen grains in which th e lat52 promoter was activated. Furthermore, matured uninucleate polle n grains were capable of germination and pollen tube growth despite th e absence of a functional generative cell (GC). Lower levels of colchi cine induced symmetric division at PMI, producing two similar daughter cells in which typical GC chromatin condensation was prevented, Simil ar cultures of transgenic microspores harboring the lat52 promoter dri ving the expression of a nuclear-targeted GUS fusion protein showed th at lat52 promoter activation occurred in both symmetric daughter cells . These results directly demonstrate that division asymmetry at PMI is essential for correct GC differentiation and that activation of VC-sp ecific transcription and functional VC maturation may be uncoupled fro m cytokinesis at PMI. These results are discussed in relation to model s proposed to account for the role and distribution of factors control ling the differing fates of the vegetative and generative cells.