Abstract
Since the discovery of the phosphoinositide/phospholipase C (PI/PLC) system in animal systems, we know that phospholipids are much more then just structural components of biological membranes. In the beginning, this idea was fairly straightforward. Receptor stimulation activates PLC, which hydrolyses phosphatidylinositol4,5-bisphosphate [PtdIns(4,5)P2] into two second messengers: inositol 1,4,5-trisphosphate (InsP3) and diacylglycerol (DG). While InsP3 difuses into the cytosol and triggers the release of calcium from an internal store via ligand-gated calcium channels, DG remains in the membrane where it recruits and activates members of the PKC family. The increase in calcium, together with the change in phosphorylation status, (in)activates a variety of protein targets, leading to a massive reprogramming, allowing the cell to appropriately respond to the extracellular stimulus. Later, it became obvious that not just PLC, but a variety of other phospholipid-metabolizing enzymes were activated, including phospholipase A, phospholipase D, and PI 3-kinase. More recently, it has become apparent that PtdIns4P and PtdIns(4,5)P2 are not just signal precursors but can also function as signaling molecules themselves. While plants contain most of the components described above, and evidence for their role in cell signaling is progressively increasing, major differences between plants and the mammalian paradigms exist. Below, these are described "in a nutshell."
Highlights
Since the discovery of the phosphoinositide/ phospholipase C (PI/PLC) system in animal systems, we know that phospholipids are much more just structural components of biological membranes
calmodulindependent protein kinase (CDPK) and CBLinteracting protein kinases (CIPK) respond to calcium, while some members of the CDPK and AGC family may be regulated by phosphoinositides and/or phosphatidic acid (PA; see below)
A convincing role for DG as a plant signaling molecule has never really been reported. Though it cannot be excluded, the lack of evidence over the last 20 years, together with the absence of its primary target PKC, leaves very little ground to put it into the plant PI/PLC model today (Fig. 1)
Summary
A convincing role for DG as a plant signaling molecule has never really been reported Though it cannot be excluded, the lack of evidence over the last 20 years, together with the absence of its primary target PKC, leaves very little ground to put it into the plant PI/PLC model today (Fig. 1). Apart from being signaling precursors, it is evident that PtdIns4P and PtdIns[4,5]P2 can function as signaling molecules themselves Both lipids are involved in polar growth of pollen tubes and root hairs and during cell plate formation, as shown by lipid biosensors, and judged from the recent phenotypes of PI 4-kinase, PtdInsP 5-kinase and PtdIns4P phosphatase mutants (Table 1) [8, 20,21,22,23,24,25]. The latter requires inositol as a precursor and functions to protect cellular structures during desiccation and as carbon reserves for early germination [27]
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