Exogenous Cytokinins Cause Cell Separation and Cell Expansion in the Root Tip Cortex of Zea mays

Root tips, entire roots, and entire seedlings of Zea mays were cultured in a modified B5 medium containing kinetin, zeatin, or 6-benzylaminopurine at either 0.46 or 4.65 μM. After 6 days, the diameter of the root tips in all three cytokinin-containing media was ca. fourfold that of the controls. Fluorescence microscopic analyses of the cytokinin-treated roots showed expansion and partial isolation of cortical cells. Files of cells were separated by intercellular spaces parallel to the root axis. Electron microscopic observations indicated that the cell isolation resulted from dissolution of the middle lamellae. Ethylene was not involved in cell expansion and cell isolation. Cytokinin-induced separation of the cortical cells probably resulted from the intensification of localized cell wall loosening in the growing root.