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A detailed electron microscopy study of early cellular events during somatic embryogenesis in leaves of Cichorium is described. Leaves on in-vitro grown plantlets were sectioned and put at 35°C, in darkness, in an agitated liquid induction medium. No sign of embryogenic predetermination, such as thick cell wall, dense cytoplasm and enlarged nucleus, could be seen in any cell before treatment. Perivascular cells were the first to react. Addition of glycerol (330 mM) allowed the arrest of embryogenic cells at an activated stage. The main events were a thickening of the wall, with extracellular secretion and an accumulation of Ca<sup>2+</sup> in the vacuole, demonstrated by an antimonate procedure. After 5 d, leaves were transferred to glycerol-free medium where multicellular proembryos could be observed. They showed reduced vacuoles, cortical microtubules, numerous multivesicular bodies and lipid globules. The embryoid cells were lined along the mesophyll lacunae by an extracellular secretion with a tubular structure; histochemical tests proved its complex lipo-glyco-proteic nature.Copyright 1993, 1999 Academic Press |
