Up-Regulation of Phosphoinositide Metabolism in Tobacco Cells Constitutively Expressing the Human Type I Inositol Polyphosphate 5-Phosphatase

Abstract

Abstract To evaluate the impact of suppressing inositol 1,4,5-trisphosphate (InsP3) in plants, tobacco (Nicotiana tabacum) cells were transformed with the human type I inositol polyphosphate 5-phosphatase (InsP 5-ptase), an enzyme which specifically hydrolyzes InsP3. The transgenic cell lines showed a 12- to 25-fold increase in InsP 5-ptase activity in vitro and a 60% to 80% reduction in basal InsP3 compared with wild-type cells. Stimulation with Mas-7, a synthetic analog of the wasp venom peptide mastoparan, resulted in an approximately 2-fold increase in InsP3 in both wild-type and transgenic cells. However, even with stimulation, InsP3 levels in the transgenic cells did not reach wild-type basal values, suggesting that InsP3 signaling is compromised. Analysis of whole-cell lipids indicated that phosphatidylinositol 4,5-bisphosphate (PtdInsP2), the lipid precursor of InsP3, was greatly reduced in the transgenic cells. In vitro assays of enzymes involved in PtdInsP2 metabolism showed that the activity of the PtdInsP2-hydrolyzing enzyme phospholipase C was not significantly altered in the transgenic cells. In contrast, the activity of the plasma membrane PtdInsP 5 kinase was increased by approximately 3-fold in the transgenic cells. In vivo labeling studies revealed a greater incorporation of 32P into PtdInsP2 in the transgenic cells compared with the wild type, indicating that the rate of PtdInsP2 synthesis was increased. These studies show that the constitutive expression of the human type I InsP 5-ptase in tobacco cells leads to an up-regulation of the phosphoinositide pathway and highlight the importance of PtdInsP2 synthesis as a regulatory step in this system.