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 multic ellular budding pathway from a compact mass of proliferation or from isolat ed single cells in friable callus. The compact mass of proliferation origin ated from the epidermal cells at the hypocotyl whose growth and convolution was characterized by a decrease in the nucleus/cytoplasm ratio and a marke d increase in storage products. The transition from the compact mass to mer istematic primordia occurred at the periphery and was accompanied by cell d edifferentiation 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 h ypothetical sequence from single cells to aggregates of a few cells, merist ematic cell clusters and globular embryos. Single cells showed typical feat ures of embryogenic cells such as rich cytoplasm and a large number of star ch grains and lipid bodies. A progressive cell dedifferentiation and a dras tic reduction of storage products was observed when aggregates of a few cel ls and meristematic cell clusters were compared. Progressive bipolarization in large meristematic cell clusters initiated globular embryo formation. T he comparison of both embryogenic pathways at the ultrastructural level sho wed that subcellular changes follow a similar sequential pattern, especiall y with regard to the storage products. The possible role of plastid extrusi ons and multivesicular bodies in the changing pattern of starch metabolism during embryogenesis is discussed. (C) 2001 Annals of Botany Company.