Comparative Development of Zygotic and Microspore-Derived Embryos in Brassica napus L. CV Topas. I. Histodifferentiation

The structural pattern of tissue and meristem formation during zygotic and microspore embryo development in canola was examined. A defined sequence of cell divisions could be found during zygotic embryogenesis. The tissue pattern was delineated at the globular stage. The shoot apical meristem began to initiate at the late heart stage of development and took on a dome shape by the torpedo stage. The root meristem was derived from hypophyseal derivatives and adjoining cells from the embryo proper. In the microspore embryo, a defined pattern of cell division was not observed during early development. A suspensor was not found in all microspore embryos. Tissue differentiation began with the formation of the protoderm after a globular mass of cells was formed. The organization of the root meristem was not as well defined as in its zygotic counterpart. The shoot meristem was derived from a group of cells located at the terminal end of the embryo. At the cotyledon stage, the shoot meristem organization was similar between the zygotic and microspore embryos. Upon prolonged culture, the tissue pattern began to deteriorate in the microspore embryo. This change corresponded to the lowering of conversion frequency upon transferring the embryos onto germination medium. Large intercellular spaces developed within the shoot pole. The cells of the procambium and the meristems differentiated into parenchyma cells with prominent starch grains. As a result, the procambium and the meristem organization were drastically altered. These observations indicated that the tissues of the microspore embryo were not fully determined