The plant hormone gibberellin is involved in the regulation ofa number ofdiverse processes, including germination, shoot elongation, and fruit formation. Gibberellin-induced a-amylase synthesis and secretion during germination has been extensively studied and the involvement of Ca2" in this process is well established (4,12,14). However, the molecular mechanism of hormone perception, message transduction, and biochemical regulation is unknown.Membrane-bound phosphoinositides mediate signal transduction across cell membranes in a wide variety of animal cells (3,6). In these systems, ligand-receptor interaction on the extracellular face of the membrane leads to activation of a membrane-bound phospholipase-C which cleaves PtdInsP22 to diacylglycerol and Ins(1,4,5)P3. Ins(1,4,5)P3 releases calcium from endoplasmic reticulum, thereby increasing intracellular free calcium levels. At these elevated levels, calcium modifies enzyme activity by binding directly or as a Ca2+_ calmodulin complex. In addition, the released diglyceride activates protein kinase-C. 'Supported by a grant from the National Science Foundation (DCB-8609038).2Abbreviations: PtdInsP2, phosphatidylinositol bisphosphate; Ins( 1, 4, 5)P3, myo-inositol-1,4,5-trisphosphate; PtdInsP, phosphatidylinositol phosphate; PtdCh, phosphatidylcholine; LPtdCH, lysophosphatidylcholine; LPtdIns, lysophosphatidylinositol; PtdIns, phosphatidylinositol; GroPIns, glycerophosphoinositol; GroPInsP, glycerophosphoinositol phosphate; GroPInsP2, glycerophosphoinositol bisphosphate. MATERIALS AND METHODS Labeling with Radioactive PrecursorsBarley seeds (Hordeum vulgare L. cv Himalaya, 1979) were deembryonated, surface-sterilized (for 30 min in 1% [w/v] NaOCl, made by 5-fold dilution of commercial bleach), washed with sterile water, and allowed to imbibe on wet sterile sand for 4 d at room temperature. To label the myo-inositolderived compounds with tritium, aleurone layers were removed from imbibed seeds as described by Chrispeels and Varner (7)