Inositol phosphates and their metabolism in plants.

Abstract

The discovery that plasma membrane phosphoinositides mediate cellular responses to external signals has led to tremendous interest in the structure and metabolism of phosphoinositides and inositol phosphates. The role of inositol1,4,5-trisphosphate [Ins(1,4,5)P3] as a mediator of receptor-initiated changes in intracellular calcium is well characterized (Berridge, 1993; Berridge and Irvine, 1989). The role of Ins(1,3,4,5)P4 (the abbreviations used are defined in Section 2) in regulating cellular calcium entry at the plasma membrane (Berridge, 1993; Berridge and Irvine, 1989) is evolving rapidly. Neurotransmitter functions for Ins(1,3,4,5,6)P5 and InsP6 (Vallejo et al., 1988) have been proposed. The ability of Ins(1,3,4,5,6,)P5 to modulate the affinity of hemoglobin for oxygen is widely accepted (Isaacks and Harkness, 1980). The possibility that other inositol phosphates found in cells may perform cellular functions has heightened interest in the structure and metabolism of inositol phosphates (Menniti et al., 1993b). In vivo, inositol phosphates are interrelated by a complex web of reactions (Majerus et al., 1988), and a rapid dynamic equilibrium exists among the different inositol phosphates. A number of recent reviews (Drobak, 1992, 1993; Cote and Crain, 1993; Hetherington and Drobak, 1992; Rincon and Boss, 1990) and other chapters in this book contain information on the structure and metabolism of phosphoinositides in plant cells. This review describes the pathways and enzymes involved in the metabolism of inositol phosphates in plant cells.