Origin of somatic embryos from cultured shoot tips of Sorghum bicolor (L.) moench.

Histologic examination of shoot-tip explants, 1 wk after culture initiation on Murashige and Skoog medium with 2.5 mg/liter 2,4-dichlorophenoxyacetic acid (2,4-D) and 0.05 mg/liter kinetin, reveals active meristematic centers inside cultured tissue. Clusters of cells in these meristematic centers exhibit remarkable resemblance to the initial three divisions in the zygotic embryo. Several such meristematic groups of cells are observed in the cultured explant at this stage. Embryogenesis is obviously initiated very early in this tissue in the presence of 2,4-D. A well-defined, white globular embryogenic callus develops in culture in about 4 wk, and it consists of clusters of embryoids with large cells characterized by thick cell walls, numerous lipoidal vesicles, and localized areas of carbohydrate storage. These cells resemble the scutellar tissue of the embryo. However, there are cells within this tissue that themselves appear embryogenic. They undergo cell division giving rise to small clusters of cells. As long as 2,4-D is present in the medium, the cells apparently retain the capacity to proliferate and to produce more cells capable of embryogenesis. Embryogenesis seems to occur via two processes, initiation of somatic embryos early in culture and secondary embryogensis from the scutellar tissue that forms in vitro. [ABSTRACT FROM AUTHOR]