Arabidopsis thaliana lipid phosphate phosphatase 2 is involved in abscisic acid signalling in leaves.

Lipid phosphate phosphatases (LPPs, E.C. 3.1.3.4) catalyse the dephosphorylation of diacylglycerol pyrophosphate (DGPP) and phosphatidic acid (PA), which are secondary messengers in abscisic acid (ABA) signalling. In this study, we investigated the effect of ABA on the expression of AtLPP genes as they encode putative ABA-signalling partners. We observed that AtLPP2 expression was down-regulated by ABA and we performed experiments on Atlpp2-2, an AtLPP2 knockout mutant, to determine whether AtLPP2 was involved in ABA signalling. We observed that Atlpp2-2 plantlets contained about twice as much PA as the wild-type Col-0 and exhibited higher PA kinase (PAK) activity than Col-0 plants. In addition, we showed that ABA stimulated diacylglycerol kinase (DGK) activity independently of AtLPP2 activity but that the ABA-stimulation of PAK activity recorded in Col-0 was dependent on AtLPP2. In order to evaluate the involvement of AtLPP2 activity in guard cell function, we measured the ABA sensitivity of Atlpp2-2 stomata. The inhibition of stomatal opening was less sensitive to ABA in Atlpp2-2 than in Col-0. Watered and water-stressed plants of the two genotypes accumulated ABA to the same extent, thus leading us to consider Atlpp2-2 an ABA-signalling mutant. Taken together our observations show that AtLPP2 is a part of ABA signalling and participate to the regulation of stomatal movements.

Arabidopsis S6 kinase mutants display chromosome instability and altered RBR1-E2F pathway activity.

The 40S ribosomal protein S6 kinase (S6K) is a conserved component of signalling pathways controlling growth in eukaryotes. To study S6K function in plants, we isolated single- and double-knockout mutations and RNA-interference (RNAi)-silencing lines in the linked Arabidopsis S6K1 and S6K2 genes. Hemizygous s6k1s6k2/++ mutant and S6K1 RNAi lines show high phenotypic instability with variation in size, increased trichome branching, produce non-viable pollen and high levels of aborted seeds. Analysis of their DNA content by flow cytometry, as well as chromosome counting using DAPI staining and fluorescence in situ hybridization, revealed an increase in ploidy and aneuploidy. In agreement with this data, we found that S6K1 associates with the Retinoblastoma-related 1 (RBR1)-E2FB complex and this is partly mediated by its N-terminal LVxCxE motif. Moreover, the S6K1-RBR1 association regulates RBR1 nuclear localization, as well as E2F-dependent expression of cell cycle genes. Arabidopsis cells grown under nutrient-limiting conditions require S6K for repression of cell proliferation. The data suggest a new function for plant S6K as a repressor of cell proliferation and required for maintenance of chromosome stability and ploidy levels.

Induction of abscisic acid-regulated gene expression by diacylglycerol pyrophosphate involves Ca2+ and anion currents in Arabidopsis suspension cells.

Diacylglycerol pyrophosphate (DGPP) was recently shown to be a possible intermediate in abscisic acid (ABA) signaling. In this study, reverse transcription-PCR of ABA up-regulated genes was used to evaluate the ability of DGPP to trigger gene expression in Arabidopsis (Arabidopsis thaliana) suspension cells. At5g06760, LTI30, RD29A, and RAB18 were stimulated by ABA and also specifically expressed in DGPP-treated cells. Use of the Ca2+ channel blockers fluspirilene and pimozide and the Ca2+ chelator EGTA showed that Ca2+ was required for ABA induction of DGPP formation. In addition, Ca2+ participated in DGPP induction of gene expression via stimulation of anion currents. Hence, a sequence of Ca2+, DGPP, and anion currents, constituting a core of early ABA-signaling events necessary for gene expression, is proposed.

Diacylglycerol pyrophosphate is a second messenger of abscisic acid signaling in Arabidopsis thaliana suspension cells.

In plants, the importance of phospholipid signaling in responses to environmental stresses is becoming well documented. The involvement of phospholipids in abscisic acid (ABA) responses is also established. In a previous study, we demonstrated that the stimulation of phospholipase D (PLD) activity and plasma membrane anion currents by ABA were both required for RAB18 expression in Arabidopsis thaliana suspension cells. In this study, we show that the total lipids extracted from ABA-treated cells mimic ABA in activating plasmalemma anion currents and induction of RAB18 expression. Moreover, ABA evokes within 5 min a transient 1.7-fold increase in phosphatidic acid (PA) followed by a sevenfold increase in diacylglycerol pyrophosphate (DGPP) at 20 min. PA activated plasmalemma anion currents but was incapable of triggering RAB18 expression. By contrast, DGPP mimicked ABA on anion currents and was also able to stimulate RAB18 expression. Here we show the role of DGPP as phospholipid second messenger in ABA signaling.