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Phosphatidic acid (PA) functions as a lipid signaling molecule in plants. Physiological analysis showed that PA triggers early signal transduction events that lead to responses to abscisic acid (ABA) during seed germination. We measured PA production during seed germination and found increased PA levels during early germination. To investigate the role of PA during seed germination, we focused on the PA catabolic enzyme lipid phosphate phosphatase (LPP). LPP catalyzes the conversion of PA to diacylglycerol (DAG). There are 4 LPP genes in the Arabidopsis genome. Among them, AtLPP2 and AtLPP3 are expressed during seed germination. Two AtLPP2 T-DNA insertional mutants ( lpp2-1 and lpp2-2) showed hypersensitivity to ABA and significant PA accumulation during germination. Furthermore, double-mutant analysis showed that ABA-insensitive 4 ( ABI4) is epistatic to AtLPP2 but ABA-insensitive 3 ( ABI3) is not. These results suggest that PA is involved in ABA signaling and that AtLPP2 functions as a negative regulator upstream of ABI4, which encodes an AP2-type transcription factor, in ABA signaling during germination. [ABSTRACT FROM AUTHOR]

Summary Phospholipid metabolism is involved in plant responses to drought and salinity stress. To investigate the role of phospholipase D (PLD) and its product phosphatidic acid (PtdOH) in stress signalling, we isolated a novel PLD cDNA, designated AtPLDδ, by screening a cDNA library prepared from dehydrated Arabidopsis thaliana. The AtPLDδ protein, of 868 amino acids, has a putative catalytic domain and a C2 domain that is involved in Ca2+/phospholipid binding. The AtPLDδ mRNA accumulated in response to dehydration and high salt stress. Histochemical analysis showed that the AtPLDδ gene is strongly expressed in the vascular tissues of cotyledons and leaves under dehydration stress conditions. Under normal growth conditions, AtPLDδ was expressed in roots, leaves, stems and flowers but not in siliques. We showed that dehydration stimulates the accumulation of PtdOH. The accumulation of PtdOH in response to dehydration was significantly suppressed in AtPLDδ antisense transgenic plants. These results suggest that AtPLDδ may be involved in PtdOH accumulation in the dehydration stress response. [ABSTRACT FROM AUTHOR]

SummaryPhosphatidylinositol-4-phosphate 5-kinase (PIP5K) phosphorylates phosphatidylinositol-4-phosphate to produce phosphatidylinositol-4,5-bisphosphate as a precursor of two second messengers, inositol-1,4,5-triphosphate and diacylglycerol, and as a regulator of many cellular proteins involved in signal transduction and cytoskeletal organization. Despite PIP5K playing such an essential role in a number of physiological processes, much still remains to be made clear about its association with plants. Searching the Arabidopsis expression sequence tag database against already known yeast and mammalian PIP5K cDNAs, we identified two clones which partly encode the same Arabidopsis PIP5K and isolated a corresponding full-length cDNA encoding a protein that we designated AtPIP5K1. Recombinant AtPIP5K1 expressed in Escherichia coli possessed a PIP5K activity in vitro. Due to some structural and biochemical differences, AtPIP5K1 was not categorized as either a type I or type II PIP5K. The expression of the AtPIP5K1 mRNA was induced rapidly by treating Arabidopsis plants with drought, salt and abscisic acid, which suggests that AtPIP5K1 is involved in water-stress signal transduction. These data give evidence for a close link between phosphoinositide signaling cascades and water-stress responses in plants. [ABSTRACT FROM AUTHOR]