Phosphatidylinositol (4,5)Bisphosphate Inhibits K+-Efflux Channel Activity in NT1 Tobacco Cultured Cells

In the animal world, the regulation of ion channels by phosphoinositides (PIs) has been investigated extensively, demonstrating a wide range of channels controlled by phosphatidylinositol (4,5)bisphosphate (PtdInsP(2)). To understand PI regulation of plant ion channels, we examined the in planta effect of PtdInsP(2) on the K+-efflux channel of tobacco (Nicotiana tabacum), NtORK (outward-rectifying K channel). We applied a patch clamp in the whole-cell configuration (with fixed "cytosolic" Ca2+ concentration and pH) to protoplasts isolated from cultured tobacco cells with genetically manipulated plasma membrane levels of PtdInsP(2) and cellular inositol (1,4,5)trisphosphate: "Low PIs" had depressed levels of these PIs, and "High PIs" had elevated levels relative to controls. In all of these cells, K channel activity, reflected in the net, steady-state outward K+ currents (I-K), was inversely related to the plasma membrane PtdInsP(2) level. Consistent with this, short-term manipulations decreasing PtdInsP(2) levels in the High PIs, such as pretreatment with the phytohormone abscisic acid (25 mu M) or neutralizing the bath solution from pH 5.6 to pH 7, increased I-K (i. e. NtORK activity). Moreover, increasing PtdInsP(2) levels in controls or in abscisic acid-treated high-PI cells, using the specific PI-phospholipase C inhibitor U73122 (2.5-4 mu M), decreased NtORK activity. In all cases, I-K decreases stemmed largely from decreased maximum attainable NtORK channel conductance and partly from shifted voltage dependence of channel gating to more positive potentials, making it more difficult to activate the channels. These results are consistent with NtORK inhibition by the negatively charged PtdInsP(2) in the internal plasma membrane leaflet. Such effects are likely to underlie PI signaling in intact plant cells.