Phosphoinositide-specific phospholipase C (PI-PLC) cleaves, in a Ca(2+)-dependent manner, phosphatidylinositol-4,5-bisphosphate (PI-4,5-P2) into diacylglycerol (DAG) and inositol triphosphate (IP3). PI-PLCs are multidomain proteins that are structurally related to the PI-PLCζs, the simplest animal PI-PLCs. Like these animal counterparts, they are only composed of EF-hand, X/Y and C2 domains. However, plant PI-PLCs do not have a conventional EF-hand domain since they are often truncated, while some PI-PLCs have no EF-hand domain at all. Despite this simple structure, plant PI-PLCs are involved in many essential plant processes, either associated with development or in response to environmental stresses. The action of PI-PLCs relies on the mediators they produce. In plants, IP3 does not seem to be the sole active soluble molecule. Inositol pentakisphosphate (IP5) and inositol hexakisphosphate (IP6) also transmit signals, thus highlighting the importance of coupling PI-PLC action with inositol-phosphate kinases and phosphatases. PI-PLCs also produce a lipid molecule, but plant PI-PLC pathways show a peculiarity in that the active lipid does not appear to be DAG but its phosphorylated form, phosphatidic acid (PA). Besides, PI-PLCs can also act by altering their substrate levels. Taken together, plant PI-PLCs show functional differences when compared to their animal counterparts. However, they act on similar general signalling pathways including calcium homeostasis and cell phosphoproteome. Several important questions remain unanswered. The cross-talk between the soluble and lipid mediators generated by plant PI-PLCs is not understood and how the coupling between PI-PLCs and inositol-kinases or DAG-kinases is carried out remains to be established.

Abbreviations: Gα, α-subunit of heterotrimeric G-protein; PKC, protein kinase C; PLD, phospholipase D; PtdIns(4,5)P 2 , phosphatidylinositol-4,5-bisphosphate. * To whom correspondence should be addressed.Abstract⎯Phospholipase D (PLD) catalyzes hydrolysis of phospholipids with production of phosphatidic acids, which often act as secondary messengers on transmission of intracellular signals. This review summarizes data of various leading laboratories on specific features of organization and regulation of PLD activity in plant and animal cells. The main structural domains of PLD (C2, PX, PH), the active site, and other functionally important parts of the enzyme are considered. Regulatory mechanisms of PLD activity are characterized in detail. Studies associated with molecular design, analysis, and synthesis of new nontoxic substances capable of inhibiting different PLD isoenzymes in vivo are shown to be promising for biotechnology and medicine.

Gravity is one of the environmental cues that direct plant growth and development. Recent investigations of different gravity signalling pathways have added complexity to how we think gravity is perceived. Particular cells within specific organs or tissues perceive gravity stimulus. Many downstream signalling events transmit the perceived information into subcellular, biochemical, and genomic responses. They are rapid, non-genomic, regulatory, and cell-specific. The chain of events may pass by signalling lipids, the cytoskeleton, intracellular calcium levels, protein phosphorylation-dependent pathways, proteome changes, membrane transport, vacuolar biogenesis mechanisms, or nuclear events. These events culminate in changes in gene expression and auxin lateral redistribution in gravity response sites. The possible integration of these signalling events with amyloplast movements or with other perception mechanisms is discussed. Further investigation is needed to understand how plants coordinate mechanisms and signals to sense this important physical factor.

The suggested link between intracellular cytokinin signaling and phospholipase D (PLD, EC 3.1.4.4.) activity (Romanov et al. 2000, 2002) was investigated. The activity of PLD in the early period of cytokinin action was studied in vivo in derooted Amaranthus caudatus seedlings, using the level of phosphatidylbutanol production as a measure of PLD activity. Rapid activation of phosphatidylbutanol synthesis was demonstrated as early as within 5 min of cytokinin administration. Neomycin, a known phosphatidylinositol-4,5-bisphosphate (PIP(2)) antagonist, strongly repressed both physiological cytokinin effect and cytokinin-dependent PLD activation. N-acylethanolamine (NAE 12), an inhibitor of alpha-class PLD, did not influence significantly cytokinin effect on Amaranthus seedlings. Together, results suggest the involvement of PIP(2)-dependent non-class alpha-PLD in the molecular mechanism of cytokinin action.