After transient exposure to the gaseous hormone ethylene, dark-grown cucumber (Cucumis sativus) hypocotyls developed unusual features. Upon ethylene's removal, the developing epidermis showed significant increases in cell division rates, producing an abundance of guard cells and trichomes. These responses to ethylene depended on the stage of development at the time of ethylene exposure. In the upper region of the hypocotyl, where cells were least differentiated at the onset of ethylene treatment, complex, multicellular protuberances formed. Further down the hypocotyl, where stomata and trichomes were beginning to develop at the onset of ethylene exposure, an increase in the number of stomata and trichomes was observed. Stomatal complexes developing after the ethylene treatment had a significant increase in the number of stomatal subsidiary cells and the number of cells per trichome increased. Analysis of division patterns in stomatal complexes indicated that exposure to ethylene either suspended or altered cell fate. Ethylene also altered cell division polarity, resulting in aberrant stomatal complexes and branched trichomes. To our knowledge, the results of this study demonstrate for the first time that transient treatment with physiological concentrations of ethylene can alter cell fate and increase the propensity of cells to divide.Ethylene regulates a variety of physiological and biochemical processes in plants, such as fruit ripening, senescence, abscission, sex determination, root initiation, and cell elongation (Abeles et al., 1992). Ethylene also regulates the size and stature of plants in response to various environmental cues (Abeles, 1973;Kieber, 1997). Its effects on dark-grown seedling development, described initially as the triple response, include increased radial growth of the root and stem, reduced root and stem elongation, and abnormal horizontal stem growth (Neljubow, 1901;Knight et al., 1910). The term triple response, as it is now commonly used, includes exaggeration in the curvature of the apical hook instead of abnormal horizontal growth (Bleecker et al., 1988;Guzman and Ecker, 1990 and references therein). The triple response of Arabidopsis has been exploited for characterizing the ethylene signal transduction system by identifying numerous mutants that either phenocopy the triple response in the absence of exogenous ethylene or fail to respond properly to ethylene (Guzman and Ecker, 1990).The dramatic stimulation of radial swelling of stems and roots by ethylene has led to physiological studies and cellular analyses of this phenomenon (Burg and Burg, 1966;Lang et al., 1982;Bleecker et al., 1988;Abeles et al., 1992;Baskin and Williamson, 1992;Kieber, 1997). Depending on the timing of ethylene's application, the physiological condition, seedling age, and the plant species concerned, either promotive (Ku et al., 1970;Lehman et al., 1996;Smalle et al., 1997) or inhibitory (Kieber et al., 1993) effects on organ elongation have been reported. For young developing dicot seedlings, ethylene has ...

The effects of ethylene on cell division are generally considered inhibitory. In this study, we demonstrate that transient ethylene exposure, while suppressing cytokinesis, stimulates DNA synthesis. We monitored DNA synthesis and cytokinesis in the epidermis of cucumber (Cucumis sativus) hypocotyls, an organ whose post-germination development involves strictly limited cell division. During exposure to ethylene, DNA synthesis, assessed by the incorporation of the thymidine homolog 5-bromo-2Ј-deoxyuridine, was detected in 20% of the epidermal cells, whereas DNA synthesis was nearly undetectable in normal air. Cytofluorometric analysis of nuclei in affected cells showed an up to 8-fold increase in DNA content. During this time, new cell plate formation was not detected. However, shortly after ethylene was removed, DNA content was rapidly restored to 2C (diploid) levels in all cells, and new cell plate formation dramatically increased. These results demonstrate that ethylene promotes DNA synthesis and its endoreduplication but inhibits cytokinesis, thereby maintaining some cells in G 2 phase.