CHANGES IN THE ULTRASTRUCTURE OF MAIZE COLEOPTILE CELLS RELATED TO THE STIMULATION OF CELL ELONGATION BY EXOGENOUS AUXIN

Elongating coleoptile segments of maize (Zea mays L.) were examined by electron microscopy to describe ultrastructural changes associated wi th the stimulation of growth by exogenous auxin. The rate of elongatio n was assessed both directly and indirectly from visual observations a nd acidification of the incubation medium, respectively. Auxin treatme nt evoked transient changes in cell structure: a higher frequency of G olgi elements, a smaller volume of periplasmic space, tight junctions between the plasmalemma and cell wall, and alterations in the plasmale mma staining pattern. The ultrastructure of coleoptile cells was alter ed by stress factors (a considerable increase in the osmolarity of the incubation medium or supplementing the medium with cycloheximide or m onensin), but these alterations were largely reversed in the presence of auxin. It was concluded that coleoptile cells exposed to auxin unde rgo substantial structural rearrangements indicative of the increasing secretory activity required for cell elongation.