Phosphoinositide signal transduction in guard cells.

All living cells are continually bombarded with signals from the surrounding environment, which are perceived and translated to give an appropriate cellular response Signals are detected by protein receptors at the cell surface giving activation of signal transduction pathways which then stimulate the appropriate response pathway [ 11. The phosphoinositide (PI) signalling chain in animal cells is activated following stimulation of cell surface receptors by a wide range of extracellular signals including hormones, growth factors and neurotransmitters Once an external stimulus has been received phosphoinositide-specific phospholipase-C(s) (PIPLC) are activated to hydrolyse phosphatidylinositol 4 3 bisphosphate, (PIP2), to produce the two second messengers inositol 1.4,s-trisphosphate (IP;) and diacylglycerol (DAG). These second messengers indirectly affect a wide variety of cellular responses, in particular DAG activates protein kinase-C (PKC) which modulates the activity of a great number of proteins, and IPz modulates Ca” ion channel activity [ 1,2]. There is now good evidence that similar pathways operate in plant cells. For example, PI-PLC activity has been detected in many tissues and other components of the animal pathway have been identified [2-71. Several genes from this pathway have been characterised, for example a putative phosphatidylinositol4-phosphate 5-kinase from A. thuliutiu, but their role in plant cells has yet to be defined.