Tobacco zygotic embryogenesisin vitro: the original cell wall of the zygote is essential for maintenance of cell polarity, the apical–basal axis and typical suspensor formation

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We have developed a reliablein vitrozygotic 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 dividein vitrofollowing 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 expansionin 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, asin vivozygotes do before cell division, even when the cell wall was regenerated duringin vitroculture. 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 developmentin vitro.

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