INVOLVEMENT OF CORK CELLS IN THE SECRETION OF EPICUTICULAR WAX FILAMENTS ON SORGHUM-BICOLOR (L) MOENCH

Tubular epicuticular wax (EW) filaments on Sorghum bicolor were shown to be secreted from smooth conical papillae within the apical walls of epidermal cork cells. Ultrastructural changes during light-induced EW secretion were examined in wild-type plants and near-isogenic mutants with reduced total EW deposition. Our results indicated that cork cell ER membranes were involved in the production of epicuticular wax precursors (EWPs). The density of ER increased during light exposure and preceded EW synthesis. The increase in ER was directly related to total EW deposition on wild-type and mutant abaxial sheaths. The orientation of ER membranes toward papillae secretion sites indicated that EWP may undergo ER-mediated directional transport. The high vesicle density in cytoplasmic extensions under papillae indicated that EWPs were vesiculated for exocytosis at the papillar secretion sites. Osmiophilic globules did not appear to be direct EWPs as previously reported. Osmiophilic globules in cork cells were never present in cell walls, cuticles, vesicles, or preferentially associated with ER; globules were randomly dispersed in the cytoplasm and rarely present during the EW-induction period. Distinct microchannels or pores were not evident in the cell wall or cuticle layers, indicating that EWPs diffused to the surface. Wall swellings near the base of papillae where a dense-staining wall modification first contacts the cuticle and where EW filaments emerge indicate a potential preferred pathway for EWP transport. An osmiophilic layer within apical cork cell walls appears to function in EW secretion; however, its exact role is yet unclear.