Observations on the structural development of immature maize embryos (Zea mays L.) during in vitro culture in the presence or absence of 2,4-D

Immature embryos of two inbred lines, the Black Mexican Sweet corn (BMS) and A188, were excised from developing caryopses and cultured on nutrient media in the absence or presence of 2,4-dichlorophenoxy acetic acid (2,4-D). The morphological and cytological development was investigated by light and electron microscopy in vitro. Immature BMS embryos, cultured on solidified nutrient medium without 2,4-D, exhibited premature germination and normal plant development, provided the excised embryos were older than 9 days after pollination (DAP), the scutellum was in contact with the nutrient medium and the endosperm was removed completely. Embryos failed to germinate and exhibited deformations in roots and shoots when excised before 9 DAP and when the axis side was placed onto the medium or when the embryos were still surrounded by endosperm. The culture of immature BMS and A188 embryos on solidified media in the presence of 2 mg 2,4-D/l induced reactions in both embryo axis and scutellum such as cell degeneration, differentiation of chlorenchyma, collenchyma and vascular tissue and cell multiplication. Callus formation and the development of somatic embryos or adventitious shoots were observed at the basal end of the scutellum, provided the axis side of the embryo was in contact with the medium. The regeneration capacity of the inbred line A188 was highest. Nearly all embryos regenerated up to 15 adventitious shoots or somatic embryos which developed from cell clusters with a presumable single cell origin. Suspension cultures were established from both inbred lines. Cell divisions were, however, exclusively found in root tip-like aggregates. Somatic embryogenesis and organogenesis were not observed in liquid media.