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Tobacco zygotic embryogenesis in vitro: the original cell wall of the zygote is essential for maintenance of cell polarity, the apical-basal axis and typical suspensor formation.

We have developed a reliable in vitro zygotic embryogenesis system in tobacco. A single zygote of a dicotyledonous plant was able to develop into a fertile plant via direct embryogenesis with the aid of a co-culture system in which fertilized ovules were employed as feeders. The results confirmed that a tobacco zygote could divide in vitro following the basic embryogenic pattern of the Solanad type. The zygote cell wall and directional expansion are two critical points in maintaining apical-basal polarity and determining the developmental fate of the zygote. Only those isolated zygotes with an almost intact original cell wall could continue limited directional expansion in vitro, and only these directionally expanded zygotes could divide into typical apical and basal cells and finally develop into a typical embryo with a suspensor. In contrast, isolated zygote protoplasts deprived of cell walls could enlarge but could not directionally elongate, as in vivo zygotes do before cell division, even when the cell wall was regenerated during in vitro culture. The zygote protoplasts could also undergo asymmetrical division to form one smaller and one larger daughter cell, which could develop into an embryonic callus or a globular embryo without a suspensor. Even cell walls that hung loosely around the protoplasts appeared to function, and were closely correlated with the orientation of the first zygotic division and the apical-basal axis, further indicating the essential role of the original zygotic cell wall in maintaining apical-basal polarity and cell-division orientation, as well as subsequent cell differentiation during early embryo development in vitro.