Early Cellular Events During Direct Somatic Embryogenesis in Cotyledon Explants of Solanum aviculare Forst.

A detailed light and electron microscope study of early cellularevents at the onset of somatic embryogenesis in cotyledon explantsof Solanum aviculare Forst., cultured on MS medium supplementedwith 1 mg l^–1 2,4-dichloro-phenoxyacetic acid (2,4-D)for periods of 0–12 d in darkness, is described. Examinationsof longitudinal sections in a plane offset from the centralveins indicated that the earliest embryogenic events in explantstook place within the first 3–4 d of culture in the parenchymacells associated with the vascular traces closest to the cutbasal ends of cotyledons. Thereafter, parenchyma associatedwith more distal vascular traces became active in an apparentlysequential manner such that, by the second week of culture,progressive stages of embryogenesis could be observed alongthe lengths of cotyledon sections. Despite the fact that epidermalcells and palisade tissues were exposed directly to the 2,4-Dmedium, initiation of embryogenic development was never observedin cells other than those directly associated with vasculartraces. None of the embryogenic events characterized at theultrastructural level were observed in cotyledons cultured onMS medium in the absence of 2,4-D with the exception that starchaccumulated in decreasing amounts from the wounded basal endto the distal end of each cotyledon. This system provides a valuable model with which to study earlybiochemical and molecular events occurring in explants duringthe onset of somatic embryogenesis because they occur in a predictablefashion at sequentially situated sites along the explant tissues. Somatic embryogenesis, Solanum aviculare, cotyledon explants, cellular changes