[beta]-Furfuryl-[beta]-glucoside (FG) has been shown to be a specific endogenous activator of higher plant callose synthase (P. Ohana, D.P. Delmer, G. Volman, J.C. Steffens, D.E. Matthews, M. Benziman [1992] Plant Physiol 98: 708-715). Because glycosides such as FG are usually sequestered in vacuoles, we have proposed that activation of callose synthesis in vivo may involve a change in the compartmentation of FG and Ca2+, resulting in a synergistic activation of callose synthase. The use of suspension-cultured barley (Hordeum bulbosum L.) cells provides evidence that FG is largely sequestered in the vacuole. Furthermore, conditions that lead to induction of callose synthesis in vivo correspondingly lead to elevation of the cytoplasmic concentration of FG. These conditions include the lowering of cytoplasmic pH or elevation of cytoplasmic Ca2+. Oligogalacturonide elicitors have also been reported to cause similar changes in cytoplasmic pH and Ca2+ concentration (Y. Mathieu, A. Kurkdjian, H. Xia, J. Guern, A. Koller, M.D. Spiro, M. O'Neill, P. Albersheim, A. Darvill [1991] The Plant Journal 1: 333-343), and such an elicitor also causes an elevation in cytoplasmic FG coupled with stimulation of callose synthesis. These results support the concept that a relative redistribution of FG between cytoplasm and vacuole may be one of the components of the signal transduction pathway for elicitation of callose synthase in vivo.