). † Both authors contributed equally to this work and should jointly be considered second authors. SUMMARYThe onset and progression of senescence are under genetic and environmental control. The Arabidopsis thaliana NAC transcription factor ANAC092 (also called AtNAC2 and ORE1) has recently been shown to control age-dependent senescence, but its mode of action has not been analysed yet. To explore the regulatory network administered by ANAC092 we performed microarray-based expression profiling using estradiolinducible ANAC092 overexpression lines. Approximately 46% of the 170 genes up-regulated upon ANAC092 induction are known senescence-associated genes, suggesting that the NAC factor exerts its role in senescence through a regulatory network that includes many of the genes previously reported to be senescence regulated. We selected 39 candidate genes and confirmed their time-dependent response to enhanced ANAC092 expression by quantitative RT-PCR. We also found that the majority of them (24 genes) are up-regulated by salt stress, a major promoter of plant senescence, in a manner similar to that of ANAC092, which itself is salt responsive. Furthermore, 24 genes like ANAC092 turned out to be stage-dependently expressed during seed growth with low expression at early and elevated expression at late stages of seed development. Disruption of ANAC092 increased the rate of seed germination under saline conditions, whereas the opposite occurred in respective overexpression plants. We also detected a delay of salinity-induced chlorophyll loss in detached anac092-1 mutant leaves. Promoter-reporter (GUS) studies revealed transcriptional control of ANAC092 expression during leaf and flower ageing and in response to salt stress. We conclude that ANAC092 exerts its functions during senescence and seed germination through partly overlapping target gene sets.

In stomatal guard cells of higher-plant leaves, abscisic acid (ABA) evokes increases in cytosolic free Ca 2؉ concentration ( (5), and nodulation (6) and are central to hormonal physiology (7-9). Changes in [Ca 2ϩ ] i influence ion channel gating (1, 9, 10), light-mediated gene expression (11), cell differentiation, elongation, and tip growth (3).In stomatal guard cells, one of the best-characterized plant cell models, increasing [Ca 2ϩ ] i is known to inactivate inwardrectifying K ϩ channels and to activate Cl Ϫ channels, biasing the plasma membrane for solute efflux, which drives stomatal closure (9). Changes in [Ca 2ϩ ] i have been associated with stimuli that lead to stomatal closure, notably abscisic acid (ABA) and CO 2 (9, 12). These changes in [Ca 2ϩ ] i depend on Ca 2ϩ release from intracellular stores (13-16) and on Ca 2ϩ entry across the plasma membrane (17, 18). Nonetheless, direct evidence for channels that could mediate Ca 2ϩ influx has been lacking. Indeed, little evidence has come forth for Ca 2ϩ channels at the plasma membrane of higher-plant cells until recently (19)(20)(21)(22)(23).One clue to a major pathway for Ca 2ϩ entry into guard cells has come from measurements of [Ca 2ϩ ] i and its elevation by ABA under voltage clamp (17). These studies indicated a voltage dependence to the [Ca 2ϩ ] i rise, suggesting that ABA stimulated a Ca 2ϩ channel, but that its activity also required membrane hyperpolarization. We have recorded single-channel currents from Vicia guard cell protoplasts under conditions that eliminate the background of current through K ϩ and Cl Ϫ channels. The results reported here demonstrate the presence of Ca 2ϩ channels at the plasma membrane that open on membrane hyperpolarization and are activated by ABA. Materials and MethodsPlant Material. Epidermal strips of Vicia faba L., cv. Bunyard Exhibition, were obtained and protoplasts were prepared as described (24,25). All operations were carried out on a Zeiss Axiovert microscope with 40ϫ LWD Nomarski DIC optics at 20-22°C. Solution was added (Ӎ20 chamber vol͞min) by gravity feed and removed by aspiration.Electrophysiology. Pipettes were pulled with a Narishige (Tokyo) PP-81 puller modified for three-stage pulls (input resistances, 30-50 M⍀) to reduce the number of channels under a patch. Pipettes were coated with Sigmacote (Sigma) to reduce capacitance. Connections to amplifier and bath were by a 0.1 M KCl͞Ag-AgCl liquid junctions, and junction potentials were taken into account (26). Single-channel currents were recorded with an Axopatch 200B patch amplifier (Axon Instruments, Foster City, CA) after filtering at 5 kHz and sampled at 44 kHz for analysis. Data were filtered at 1 kHz (Kemo, Beckenham, U.K.) offline and analyzed with N-PRO (Wye Science, Wye, Kent, U.K.), P/V CLAMP V. 6 (CED, Cambridge, U.K.) software. Channel amplitudes were calculated from point-amplitude histograms estimated from open events Ն5-ms duration (Fig. 1) beyond closed levels determined from periods of no channel activity (27). Channel numbers w...

Kinesin-3 motor UNC-104/KIF1A is essential for transporting synaptic precursors to synapses. Although the mechanism of cargo binding is well understood, little is known how motor activity is regulated. We mapped functional interaction domains between SYD-2 and UNC-104 by using yeast 2-hybrid and pull-down assays and by using FRET/ fluorescence lifetime imaging microscopy to image the binding of SYD-2 to UNC-104 in living Caenorhabditis elegans. We found that UNC-104 forms SYD-2-dependent axonal clusters (appearing during the transition from L2 to L3 larval stages), which behave in FRAP experiments as dynamic aggregates. High-resolution microscopy reveals that these clusters contain UNC-104 and synaptic precursors (synaptobrevin-1). Analysis of motor motility indicates bi-directional movement of UNC-104, whereas in syd-2 mutants, loss of SYD-2 binding reduces net anterograde movement and velocity (similar after deleting UNC-104's liprin-binding domain), switching to retrograde transport characteristics when no role of SYD-2 on dynein and conventional kinesin UNC-116 motility was found. These data present a kinesin scaffolding protein that controls both motor clustering along axons and motor motility, resulting in reduced cargo transport efficiency upon loss of interaction.motor regulation ͉ synaptic vesicle transport ͉ active zone protein ͉ axonal transport ͉ dynein

To explore possible pathways for anions to enter the xylem in the root during the transport of salts to the shoot, we used the patchclamp method on protoplasts prepared from the xylem parenchyma of barley (Hordeum vulgare L.) plants. K ؉ currents were suppressed by tetraethylammonium or N-methylglucamine in the solutions in the pipette and the bath, and the permeating anions were Cl ؊ or NO 3 ؊ . We recorded the activities of three distinct anion conductances: (a) an inwardly rectifying anion channel (X-IRAC), characterized by activation at hyperpolarization and open times of up to several seconds; (b) a quickly activating anion conductance (X-QUAC), important for anion efflux at voltages between ؊50 mV and the equilibrium potential of the prevailing anion; and (c) a slowly activating anion conductance (X-SLAC), activating above ؊100 mV. Both X-IRAC and X-QUAC were permeable for Cl ؊ and NO 3 ؊ ; X-QUAC was also permeable for malate. The occurrence of X-IRAC became more frequent with an increase in cytoplasmic Ca 2؉ , while the occurrence of X-QUAC decreased. Anion currents through X-SLAC, and particularly through X-QUAC, were estimated to be large enough to account for reported rates of xylem loading, which is in accordance with the notion that xylem loading is a passive process.

SummaryProtein phosphorylation and cytosolic-free [Ca 2þ ] ([Ca 2þ ] i ) contribute to signalling cascades evoked by the water-stress hormone abscisic acid (ABA) that lead to stomatal closure in higher-plant leaves. ABA activates an inward-rectifying Ca 2þ channel at the plasma membrane of stomatal guard cells, promoting Ca 2þ entry by shifting the voltage-sensitivity of the channels. Because many of these effects could be mediated by kinase/phosphatase action at the membrane, we examined a role for protein (de-)phosphorylation in plasma membrane patches from Vicia guard cells. Ca 2þ channel activity decayed rapidly in excised patches, and recovered on adding ATP (K 1/2 , 1.3 AE 0.7 mM) but not the non-hydrolyzable analog ATPcS. ABA activation of the channel required the presence of ATP and like ABA, the 1/2 A-type protein phosphatase antagonists okadaic acid (OA) and calyculin A (CA) enhanced Ca 2þ channel activity by increasing the open probability and number of active channels. Neither ATP nor the antagonists affected the mean open lifetime of the channel, suggesting an action through changes in closed lifetime distributions. Like ABA, OA and CA shifted the voltage-sensitivities of the Ca 2þ current and [Ca 2þ ] i increases in intact guard cells towards positive voltages. OA and CA also augmented the [Ca 2þ ] i rise evoked by hyperpolarization and delayed its recovery. These results demonstrate a membrane-delimited interaction between 1/2 A-type protein phosphatase(s) and the Ca 2þ channel or associated proteins, and they are consistent with a role for protein (de-)phosphorylation in ABA signalling mediated directly through Ca 2þ channel gating that leads to [Ca 2þ ] i increases in the guard cells.

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Employing fluorescence resonance energy transfer (FRET)imaging, we previously demonstrated that effector caspase activation is often an all-or-none response independent of drug choice or dose administered. We here investigated the signaling dynamics during apoptosis initiation via the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) receptor pathway to investigate how variability in drug exposure can be translated into largely kinetically invariant cell death execution pathways. FRET-based microscopy demonstrated dosedependent responses of caspase-8 activation and activity within individual living HeLa cells. Caspase-8 on average was activated 45-600 min after TRAIL/cycloheximide addition. Caspase-8-like activities persisted for 15-60 min before eventually inducing mitochondrial outer membrane permeabilization. Independent of the TRAIL concentrations used or the resulting caspase-8-like activities, mitochondrial outer membrane permeabilization was induced when 10% of the FRET substrate was cleaved. In contrast, in Bid-depleted cells, caspase-8-like activity persisted for hours without causing immediate cell death. Our findings provide detailed insight into the intracellular signaling kinetics during apoptosis initiation and describe a threshold mechanism controlling the induction of apoptosis execution. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)2 is a potent cytotoxic ligand inducing apoptosis preferentially in tumor cells (1). New TRAIL-based treatment regimes for adjuvant chemotherapies therefore are currently being studied in phase I and II clinical trials (2). TRAIL binding to its cognate death receptors TRAIL-R1 and -R2 induces receptor trimerization. At their cytoplasmic domains, TRAIL-R1 and -R2 recruit the adaptor protein Fas-associated death domain into the so-called death inducing signaling complex (DISC). Via interaction of their death effector domains, Fas-associated death domain recruits procaspase-8 and -10 to the DISC, resulting in activation and processing of these initiator proteases (3). Although in some cell lines caspase-8/-10 can directly activate effector caspase-3 (type I signaling) (4), the majority of cells require caspase-8/-10 to initiate apoptosis by cleaving the BH-3-only protein Bid (type II signaling). Truncated Bid (tBid) then translocates to mitochondria and induces Bax/Bak-dependent mitochondrial outer membrane permeabilization (MOMP) (5-8). MOMP results in the release of mitochondrial intermembrane space proteins, such as cytochrome c and Smac, from the mitochondria into the cytosol, mitochondrial depolarization, and subsequent apoptosis execution by effector caspases, such as caspase-3, -7, and -6 (9 -11).Independent of the choice of stimulus or the dose applied, the induction of MOMP and the subsequent execution of apoptosis by effector caspases were shown to be kinetically invariant all-or-none signaling processes that guarantee cell death over a wide range of key protein concentrations (9, 12, 13). In the case of effector caspase activati...