Salinity and Hyperosmotic Stress Induce Rapid Increases in Phosphatidylinositol 4,5-Bisphosphate, Diacylglycerol Pyrophosphate, and Phosphatidylcholine in Arabidopsis thaliana Cells

Abstract

In animal cells, phosphoinositides are key components of the inositol 1,4,5-trisphosphate/diacylglycerolbased signaling pathway, but also have many other cellular functions. These lipids are also believed to fulfill similar functions in plant cells, although many details concerning the components of a plant phosphoinositide system, and their regulation are still missing. Only recently have the different phosphoinositide isomers been unambiguously identified in plant cells. Another problem that hinders the study of the function of phosphoinositides and their derivatives, as well as the regulation of their metabolism, in plant cells is the need for a homogenous, easily obtainable material, from which the extraction and purification of phospholipids is relatively easy and quantitatively reproducible. We present here a thorough characterization of the phospholipids purified from [ 32 P]orthophosphate-and myo-[2-3 H]inositol-radiolabeled Arabidopsis thaliana suspension-cultured cells. We then show that NaCl treatment induces dramatic increases in the levels of phosphatidylinositol 4,5-bisphosphate and diacylglycerol pyrophosphate and also affects the turnover of phosphatidylcholine. The increase in phosphatidylinositol 4,5-bisphosphate was also observed with a non-ionic hyperosmotic shock. In contrast, the increase in diacylglycerol pyrophosphate and the turnover of phosphatidylcholine were relatively specific to salt treatments as only minor changes in the metabolism of these two phospholipids were detected when the cells were treated with sorbitol instead of NaCl.Phosphoinositides are quantitatively minor phospholipids that play an important role in the transduction of physiological signals, such as hormones, growth factors, and neurotransmitters in animal cells (1). One of the key early events triggered by these physiological stimuli is the activation of phosphoinositide-specific phospholipase C (PI-PLC), 1 resulting in the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P 2 ) to the two second messengers, inositol 1,4,5-trisphosphate (Ins(1,4,5)P 3 ) and diacylglycerol, which induce Ca 2ϩ release from internal stores and stimulate protein kinase C, respectively (1, 2). During the last decade, it has become evident that in addition to serving as precursors to Ins(1,4,5)P 3 and diacylglycerol, phosphoinositides actively participate in other cellular functions: they have been shown to regulate the dynamics of the actin cytoskeleton through the interaction with actin-binding proteins (3, 4), and to potentiate the activation of protein kinase C (5) and PI-PLC (6, 7). In addition, phosphoinositides phosphorylated at the D 3 -hydroxy group of the inositol headgroup are required for specific vesicle trafficking steps (8, 9) and are able to activate the recently identified novel protein kinases Akt/PKB and phosphoinositide-dependent kinases (10). Recently, a new 3-phosphorylated phosphoinositide, phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P 2 ), was identified and shown to accumulate in yeast cells ...

Rapid Accumulation of Phosphatidylinositol 4,5-Bisphosphate and Inositol 1,4,5-Trisphosphate Correlates with Calcium Mobilization in Salt-Stressed Arabidopsis

“…This assumption is not fully accurate because a recent report indicated that Arabidopsis suspension culture cells accumulate primarily PtdIns(4,5)P 2 instead of PtdIns(3,5)P 2 following osmotic shock . Our analyses confirm the findings of Pical et al (1999) and support one potential mechanism for PtdIns(4,5)P 2 -signaling during stress.…”

“…An amalgam of experimental data support the model that plants respond to salinity and osmotic stress by utilizing phosphoinositides or phosphoinositide-derived second messengers (Cho et al, 1993;Hirayama et al, 1995;Brearley et al, 1997;Mikami et al, 1998;Munnik et al, 1998;Meijer et al, 1999;Pical et al, 1999) and the aforementioned observation that algal species such as C. moewusii and D. salina respond in a fashion similar to yeast by producing PtdIns(3,5)P 2 following osmotic stress would suggest that higher plants might respond similarly. This assumption is not fully accurate because a recent report indicated that Arabidopsis suspension culture cells accumulate primarily PtdIns(4,5)P 2 instead of PtdIns(3,5)P 2 following osmotic shock .…”

“…Various reports suggest that abiotic cues such as salinity, hyper-and hypoosmotic stress, and gravity effects (Einspahr et al, 1988;Perera et al, 1999;Pical et al, 1999), as well as biotic cues such as hormones may activate phosphoinositide-signaling systems (Staxén et al, 1999). It is interesting that IP 3 -binding channels have been identified in plants (Allen et al, 1995), and delivery of caged IP 3 to plant cells has been demonstrated to cause release of calcium from intracellular stores (Alexandre et al, 1990;Franklin-Tong et al, 1996).…”

Regulation of phosphatidylcholine biosynthesis under salt stress involves choline kinases in Arabidopsis thaliana

“…Freezing tolerance is correlated with the amount and the degree of polyunsaturation of PC in planta [8,9] and high salt stress induces a rapid increase in the PC turnover in suspension-cultured cells [10]. From these data it seems that the pool of PC might be tightly regulated and is critical to maintain cell structure and function under stress.…”

Inositol(1,4,5)trisphosphate production in plant cells: an early response to salinity and hyperosmotic stress

“…These ¢ndings suggest that a link might exist between the activation of the plant phosphoinositide system [6] and the elevation of cytosolic Ca 2 . Although it has been known for some time that induction of genes encoding enzymes of the plant phosphoinositide cycle can be triggered by hyperosmotic stress [7,8] little detailed information is currently available about the immediate e¡ects of salt stress and hyperosmotic stress on phosphoinositide-metabolising enzymes and inositol (1,4,5)trisphosphate (Ins(1,4,5)P 3 ) generation. We now describe the e¡ect of salt and osmotic stress on Ins(1,4,5)P 3 production in suspension-cultured carrot (Daucus carota L.) cells.…”