We have shown previously that inositol-1,4,5-trisphosphate (IP3) stimulates an efflux of OCa2+ from fusogenic carrot protoplasts (M Rincon, WF Boss [1987] If stimuli are transduced via polyphosphoinositide turnover in plant cells, then the criteria established for the animal paradigm must be met. Specifically, (a) the polyphosphoinositides must be located in the plasma membrane, (b) the enzymes responsible for the synthesis and degradation of polyphosphoinositides must be associated with the plasma membrane, (c) the polyphosphoinositide levels must transiently change upon cellular stimulation, (d) the products from the polyphosphoinositide hydrolysis must transiently increase upon stimulation, (e) a transient increase of free Ca2" in the cytosol must follow the transient increase of IP3. To date, only the first and the second criteria have been documented for several different plant tissues (15,19,25,29). There are few reports in the literature concerning polyphosphoinositide turnover in plants in response to stimuli. Recently, light-induced polyphosphoinositide turnover in Samanea saman pulvini and auxin-induced polyphosphoinositide turnover in Catharanthus roseus cells have been reported (8,17). However, the connection between the time course of the polyphosphoinositide turnover and the elevation of free Ca2+ in the cytosol in response to light or auxin has not been delineated.We have previously demonstrated that exogenous IP3 stimulates a Ca2' efflux from fusogenic carrot (Daucus carota L.) protoplasts (22