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 cel l ER membranes were involved in the production of epicuticular wax pre cursors (EWPs). The density of ER increased during light exposure and preceded EW synthesis. The increase in ER was directly related to tota l EW deposition on wild-type and mutant abaxial sheaths. The orientati on of ER membranes toward papillae secretion sites indicated that EWP may undergo ER-mediated directional transport. The high vesicle densit y in cytoplasmic extensions under papillae indicated that EWPs were ve siculated for exocytosis at the papillar secretion sites. Osmiophilic globules did not appear to be direct EWPs as previously reported. Osmi ophilic globules in cork cells were never present in cell walls, cutic les, vesicles, or preferentially associated with ER; globules were ran domly dispersed in the cytoplasm and rarely present during the EW-indu ction period. Distinct microchannels or pores were not evident in the cell wall or cuticle layers, indicating that EWPs diffused to the surf ace. Wall swellings near the base of papillae where a dense-staining w all modification first contacts the cuticle and where EW filaments eme rge indicate a potential preferred pathway for EWP transport. An osmio philic layer within apical cork cell walls appears to function in EW s ecretion; however, its exact role is yet unclear.