Hypersensitivity of Abscisic Acid–Induced Cytosolic Calcium Increases in the Arabidopsis Farnesyltransferase Mutant era1-2

Cytosolic calcium increases were analyzed in guard cells of the Arabidopsis farnesyltransferase deletion mutant era1-2 (enhanced response to abscisic acid). At low abscisic acid (ABA) concentrations (0.1 μM), increases of guard cell cytosolic calcium and stomatal closure were activated to a greater extent in the era1-2 mutant compared with the wild type. Patch clamping of era1-2 guard cells showed enhanced ABA sensitivity of plasma membrane calcium channel currents. These data indicate that the ERA1 farnesyltransferase targets a negative regulator of ABA signaling that acts between the points of ABA perception and the activation of plasma membrane calcium influx channels. Experimental increases of cytosolic calcium showed that the activation of S-type anion currents downstream of cytosolic calcium and extracellular calcium-induced stomatal closure were unaffected in era1-2, further supporting the positioning of era1-2 upstream of cytosolic calcium in the guard cell ABA signaling cascade. Moreover, the suppression of ABA-induced calcium increases in guard cells by the dominant protein phosphatase 2C mutant abi2-1 was rescued partially in era1-2 abi2-1 double mutant guard cells, further reinforcing the notion that ERA1 functions upstream of cytosolic calcium and indicating the genetic interaction of these two mutations upstream of ABA-induced calcium increases

Integration of ion channel activity in calcium signalling pathways

Perception of a wide range of developmental and stress signals by plants results in rapid elevation of cytosolic free calcium ([Ca2+](c): Bush, 1995). The change in [Ca2+](c) is widely accepted to comprise an early step in signal transduction, with downstream targets of the Ca2+ signal including activation of calmodulin-dependent enzymes, calmodulin-domain protein kinases (CDPKs: Roberts & Harmon, 1992), or activation of ion channels (Schroeder & Hagiwara, 1989). Yet this simple notion gives rise to a number of equally elementary questions: From which compartment is Ca2+ mobilised? Which membrane pathways (ion channels) facilitate passive Ca2+ flow into the cytosol in response to the primary signal? How do these Ca2+-permeable channels interact with other cellular response elements (including other ion channels, ligands and phosphorylation cascades) to evoke the Ca2+ signal? How is stimulus specificity encoded in the Ca2+ signal? Answers to all four questions require as a first step an understanding of the properties of Ca2+-permeable channels in plant cells: which membranes the channels are located in, what activates (gates) them, and how their activities might be integrated with other signalling pathways in the cell. The remainder of this chapter highlights areas of achievement and of ignorance in our attempts to address these questions

Abscisic acid-induced stomatal closure mediated by cyclic ADP-ribose

Abscisic acid (ABA) is a plant hormone involved in the response of plants to reduced water availability. Reduction of guard cell turgor by ABA diminishes the aperture of the stomatal pore and thereby contributes to the ability of the plant to conserve water during periods of drought. Previous work has demonstrated that cytosolic Ca(2+) is involved in the signal transduction pathway that mediates the reduction in guard cell turgor elicited by ABA. Here we report that ABA uses a Ca(2+)-mobilization pathway that involves cyclic adenosine 5′-diphosphoribose (cADPR). Microinjection of cADPR into guard cells caused reductions in turgor that were preceded by increases in the concentration of free Ca(2+) in the cytosol. Patch clamp measurements of isolated guard cell vacuoles revealed the presence of a cADPR-elicited Ca(2+)-selective current that was inhibited at cytosolic Ca(2+) ≥ 600 nM. Furthermore, microinjection of the cADPR antagonist 8-NH(2)-cADPR caused a reduction in the rate of turgor loss in response to ABA in 54% of cells tested, and nicotinamide, an antagonist of cADPR production, elicited a dose-dependent block of ABA-induced stomatal closure. Our data provide definitive evidence for a physiological role for cADPR and illustrate one mechanism of stimulus-specific Ca(2+) mobilization in higher plants. Taken together with other recent data [Wu, Y., Kuzma, J., Marechal, E., Graeff, R., Lee, H. C., Foster, R. & Chua, N.-H. (1997) Science 278, 2126–2130], these results establish cADPR as a key player in ABA signal transduction pathways in plants

Abscisic acid-mediates stomatal closure mediated by cyclic ADP-ribose.

Abscisic acid (ABA) is a plant hormone involved in the response of plants to reduced water availability. Reduction of guard cell turgor by ABA diminishes the aperture of the stomatal pore and thereby contributes to the ability of the plant to conserve water during periods of drought. Previous work has demonstrated that cytosolic Ca2+ is involved in the signal transduction pathway that mediates the reduction in guard cell turgor elicited by ABA. Here we report that ABA uses a Ca2+-mobilization pathway that involves cyclic adenosine 5′-diphosphoribose (cADPR). Microinjection of cADPR into guard cells caused reductions in turgor that were preceded by increases in the concentration of free Ca2+ in the cytosol. Patch clamp measurements of isolated guard cell vacuoles revealed the presence of a cADPR-elicited Ca2+-selective current that was inhibited at cytosolic Ca2+ ≥ 600 nM. Furthermore, microinjection of the cADPR antagonist 8-NH2-cADPR caused a reduction in the rate of turgor loss in response to ABA in 54% of cells tested, and nicotinamide, an antagonist of cADPR production, elicited a dose-dependent block of ABA-induced stomatal closure. Our data provide definitive evidence for a physiological role for cADPR and illustrate one mechanism of stimulus-specific Ca2+ mobilization in higher plants. Taken together with other recent data [Wu, Y., Kuzma, J., Marechal, E., Graeff, R., Lee, H. C., Foster, R. & Chua, N.-H. (1997) Science 278, 2126–2130], these results establish cADPR as a key player in ABA signal transduction pathways in plants