An investigation of factors that regulate the size of tracheary elements in plants

In suspension cultured Zinnia mesophyll cells, relationships among cell expansion, auxin, ethylene and tracheary element (TE) differentiation were examined. Over a range of osmotic potential between 220 and 280 mOsm, the rate of cell expansion is inhibited, while the level of TE differentiation increases. Between 360 and 460 mOsm, the percentage of plasmolyzed cells increases, while the level of TE differentiation is reduced. The highest frequency of TE differentiation occurs between 280 and 360 mOsm. These results indicate that the onset of TE differentiation may be correlated with suppression of cell expansion. In addition, this study demonstrates that TE differentiation is correlated with plasmolysis, and that no single extracellular osmotic potential can induce differentiation of all cells in Zinnia cultures. Aloni and Zimmermann\u27s (1983) hypothesis that spatial and developmental gradients in auxin concentration influence vessel element diameter in plants by regulating the rate of TE differentiation was tested. Cells cultured in Fukuda and Komamine (FK) medium supplemented with 0.1 to 5.0 [special characters omitted]M [special characters omitted]-naphthylenacetic acid (NAA) differentiate into TEs. The rate of TE differentiation increases between 0.1 and 0.5 [special characters omitted]M NAA and decreases at high auxin concentrations. Increasing auxin concentration from 0.1 and 0.5 [special characters omitted]M NAA results in smaller TEs; however cell division is also stimulated within this range. The concentration of NAA does not affect the size of TEs that differentiate directly from Zinnia mesophyll cells without dividing, indicating that auxin may regulate TE size through its effects on cell division. Finally, relationships between wound-induced ethylene and TE differentiation were examined. In control cultures, when the time of addition of NAA and N 6-benzyladenine (BA) was delayed from 12 h to 48 h, a gradual reduction in differentiation was observed. In contrast, when ethylene biosynthesis enhancers were added after 48 h of culture, Zinnia cells recovered the capacity to differentiate in response to auxin and cytokinin. When ethylene action or ethylene biosynthesis inhibitors were added at the beginning of cultures, differentiation was reduced. These results indicate that ethylene may be correlated with TE differentiation in xylogenic suspension cultures of Zinnia mesophyll cells