Unique properties of plant phosphoinositide metabolism: A characteristic of plant phosphoinositide metabolism is that PtdIns(4)P levels are typically 20 times higher than those of PtdIns(4,5)P₂, suggesting that PtdIns(4)P may be the key regulator of cellular metabolism and a signaling molecule in its own right, which however has yet to be characterized.

Major differences between plants, yeasts and animals are evident in the plasma membrane-associated phospholipase C (PLC), which hydrolyzes PtdIns(4,5)P₂ to produce Ins(1,4,5)P₃. In animal cells, there are three major families of PLCs: PLCb, which is regulated by G-proteins, PLCg, which is regulated by phosphotyrosine receptors, and PLCd, which is regulated by Ca²⁺ and possibly other mechanisms. Plants and yeast contain only the d-family of PLCs. In addition, unlike the animal and yeast enzymes, none of the plant d family of PLCs contain a PH domain. The PLC-PH domain is thought to facilitate enzyme substrate interaction as well as feedback regulation. These differences in PLCs and the fact that the plant PtdIns(4,5)P₂-PLC is primarily membrane associated led to the hypothesis that in plant membranes PtdIns(4,5)P₂ hydrolysis would occur continuously, unless the lipid was bound and protected by proteins such as profilin.

Metabolization of the intermediate Ins(1,4,5)P₃ in plants also differs from animals. While in animal cells Ins(1,4,5)P₃ is dephosphorylated by a 5'phosphatase (ptase), in plants it can alternatively be dephosphorylated by a 5'ptase or a 1'ptase.

Although many aspects of the phosphoinositide pathway have been evolutionarily conserved between plants, yeast and animals, plasma membrane signaling appears to have evolved to the specification of the organism. For example, plants lack the class I PtdIns 3-kinase that is involved in signal transduction via phosphotyrosine-coupled plasma membrane receptors in animal cells. Class I PtdIns 3- kinases phosphorylate PtdIns 4P, PtdIns 5P, and PtdIns(4,5)P₂ and produce PtdIns(3,4)P₂, PtdIns(3,5)P₂ and PtdIns(3,4,5)P₃. The plant PtdIns 3-kinase only phosphorylates PtdIns and produces PtdIns(3)P. PtdIns 3-kinase appears to be essential for plant growth and has been reported to be involved in vacuolar trafficking and transcription initiation; however, there is no evidence that it is directly involved in plasma membrane signaling.

Cellular functions of Phosphoinositides

The inositol headgroup

Cellular localization of phosphoinositides

Phosphoinositides as central mediators of plant growth responses

Unique properties of plant phosphoinositide metabolism

Inositol 1,4,5-trisphosphate regulates intracellular Ca²⁺ levels