Plant inositol monophosphatase is a lithium-sensitive enzyme encoded by a multigene family

myo-lnositol monophosphatase (IMP) is a soluble, Lif-sensitive protein that catalyzes the removal of a phosphate from myo-inositol phosphate substrates. IMP is required for de novo inositol synthesis from glucose 6-phosphate and for breakdown of inositol trisphosphate, a second messenger generated by the phosphatidylinositol signaling pathway, We cloned the IMP gene from tomato (LelMP) and show that the plant enzyme is encoded by a small gene family. Three different LelMP cDNAs encode distinct but highly conserved IMP enzymes that are catalytically active in vitro. Similar to the single IMP from animals, the activities of all three LelMPs are inhibited by low concentrations of LiCl, LelMP mRNA levels are developmentally regulated in seedlings and fruit and in response to light. Immunoblot analysis detected three proteins of distinct molecular masses (30, 29, and 28 kD) in tomato; these correspond to the predicted molecular masses of the LelMPs encoded by the genes. Immunoreactive proteins in the same size range are also present in several other plants, Immunolocalization studies indicated that many cell types within seedlings accumulate LelMP proteins, In particular, cells associated with the vasculature express high levels of LelMP protein; this may indicate a coordinate regulation between phloem transport and synthesis of inositol. The presence of three distinct enzymes in tomato most likely reflects the complexity of inositol utilization in higher plants.