The ability of the amphiphilic peptides, melittin and mastoparan, to modulate the production of inositol(1,4,5)trisphosphate in cultured plant (Daucus carota L.) cells was investigated. When added to intact cells melittin and mastoparan caused a rapid and dose-dependent increase in inositol(1,4,5)trisphosphate concentrations. In isolated protoplasts, inositol(1,4,5)trisphosphate levels were 12- to 16-fold higher than in the corresponding cells and neither melittin nor mastoparan was able to significantly affect inositol(1,4,5)trisphosphate production. Melittin and mastoparan had a strong inhibitory effect (IC50: 20 microM) on the activity of polyphosphoinositide-specific phospholipase C in purified plasma membranes. These results demonstrate that the plant phosphoinositide system can be activated by amphiphilic peptides in a manner analogous to that observed in specialized mammalian cells but that important functional components are altered, or lost, by the disruption of the intact cell state.