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.

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