T he challenge for today's scientists is to maintain a broad perspective of interactive signaling pathways, while dissecting their parts sufficiently to render them understandable. Our goal is to stimulate the reader to consider the phosphoinositide (PI) pathway as a functional component of a complex growth response and to move beyond the reductionist's perspective of confining PI signaling to the generation of a transient, Ins(1,4,5)P 3 -induced Ca 2ϩ oscillation. (For additional coverage of plant PIs and the enzymes involved in their metabolism see Refs 1,2.) Inositol phospholipids as regulators of growth Most plant responses to external stimuli involve a change in growth and therefore membrane biogenesis. Membrane trafficking and signaling are inexorably linked in regulating cellular metabolism and controlling growth. To coordinate these processes, evolution appears to have capitalized on the stereospecificity of the PIs. During membrane trafficking, individual inositol phospholipids on the vesicle surface specify functional information like cogs on a wheel. As vesicles traffic from the endoplasmic reticulum (ER) to the plasma membrane, and from the ER to the vacuole and retrograde pathways, inositol phospholipids attract specific proteins necessary for budding, docking and fusion Biosynthesis and function of PtdIns(3) P The first evidence for the exacting requirements for stereospecific isomers of phosphatidylinositol (PtdIns) phosphate in membrane trafficking was revealed when the gene for yeast PtdIns-3-kinase, VPS34, was shown to be essential for trafficking of hydrolytic enzymes to the vacuole 5 . The plant PtdIns-3-kinase is related to the yeast Vps34p, which uses only PtdIns as a substrate. In plants, PtdIns-3-kinase activity is associated with nodule formation during symbiotic nitrogen fixation Exploiting yeast genetics to demonstrate the necessity of PtdIns(4)P in membrane trafficking and to elucidate the physiological roles of the distinct PtdIns-4-kinase isoforms has corroborated these findings. In an elegant mutant screen, yeast mutants were identified that were compromised in various steps of intracellular lipid transpor
