PI-PLC: Phosphoinositide-Phospholipase C in Plant Signaling

Historically, phosphoinositide-specific phospholipase C (PI-PLC) catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) to generate the second messengers, inositol 1,4,5-trisphosphate (InsP3) and diacylglycerol (DAG), which release Ca2+ from intracellular stores and activate members of the protein kinase C (PKC) family, respectively. While this signaling system is well understood in animal cells, for plants this is still far from clear, as they lack homologs for the InsP3 receptor and PKC, and display extremely low PIP2 levels in their membranes under normal conditions. Plant genomes do contain numerous genes coding for phosphatidylinositol kinase (PIK) to make phosphatidylinositol 4-phosphate (PIP), PIP kinase (PIPK) to synthesize PIP2, and PI-PLC to hydrolyze these lipids. Data is also emerging that not InsP3 or DAG but their phosphorylated products, i.e., inositolpolyphosphates (IPPs) such as InsP5 and InsP6, and phosphatidic acid (PA), are functioning as plant signaling molecules. The goal of this chapter is to provide a critical overview of what is currently known about plant PI-PLC signaling and to indicate directions for future research