Ultrastructure of Early Secondary Embryogenesis by Multicellular and Unicellular Pathways in Cork Oak ( Quercus suber L.)

Early cellular events during secondary embryogenesis were studied in a cork oak recurrent embryogenic system in which embryos arise either in a multicellular budding pathway from a compact mass of proliferation or from isolated single cells in friable callus. The compact mass of proliferation originated from the epidermal cells at the hypocotyl whose growth and convolution was characterized by a decrease in the nucleus/cytoplasm ratio and a marked increase in storage products. The transition from the compact mass to meristematic primordia occurred at the periphery and was accompanied by cell dedifferentiation and a drastic reduction of storage products. Meristematic primordia evolved to globular embryos by the organization of a protodermis and two internal centres. Microscope analysis of friable callus showed an hypothetical sequence from single cells to aggregates of a few cells, meristematic cell clusters and globular embryos. Single cells showed typical features of embryogenic cells such as rich cytoplasm and a large number of starch grains and lipid bodies. A progressive cell dedifferentiation and a drastic reduction of storage products was observed when aggregates of a few cells and meristematic cell clusters were compared. Progressive bipolarization in large meristematic cell clusters initiated globular embryo formation. The comparison of both embryogenic pathways at the ultrastructural level showed that subcellular changes follow a similar sequential pattern, especially with regard to the storage products. The possible role of plastid extrusions and multivesicular bodies in the changing pattern of starch metabolism during embryogenesis is discussed.