Inositol phospholipids as plant second messengers.

Two plasma membrane lipids, phosphatidylinositol monophosphate (PtdIns4P) and phosphatidylinositol bisphosphate (PtdIns(4,5)P2), have been shown to be key intermediates in stimulus response pathways in many animal cells. PtdIns4P and PtdIns(4,5)P2 act as sources of second messengers and they directly alter the activity of membrane enzymes. These lipids and the enzymes involved in their metabolism are found in the plasma membranes of plant cells. A clear role for the inositol phospholipids in plant signal transduction, however, has not emerged. In this chapter we present some of the questions raised by current work in the area and propose an alternative focus for phosphoinositide metabolism in plants: a role for PtdIns4P and PtdIns(4,5)P2 as direct effectors of membrane structure and function.