Showing papers by "Thomas Martin published in 2001"

Synaptotagmin modulation of fusion pore kinetics in regulated exocytosis of dense-core vesicles.

Abstract: In the exocytosis of neurotransmitter, fusion pore opening represents the first instant of fluid contact between the vesicle lumen and extracellular space The existence of the fusion pore has been established by electrical measurements, but its molecular composition is unknown The possibility that synaptotagmin regulates fusion pores was investigated with amperometry to monitor exocytosis of single dense-core vesicles Overexpression of synaptotagmin I prolonged the time from fusion pore opening to dilation, whereas synaptotagmin IV shortened this time Both synaptotagmin isoforms reduced norepinephrine flux through open fusion pores Thus, synaptotagmin interacts with fusion pores, possibly by associating with a core complex of membrane proteins and/or lipid

The effect of nitrogen and phosphorus deficiency on flavonol accumulation in plant tissues

Abstract: The flavonol content of Arabidopsis thaliana and tomato seedlings was assessed in conditions of reduced nitrogen or phosphorus availability. In both systems, a significant inverse relationship was observed between nutrient availability and flavonol accumulation, with nitrogen limitation promoting the greatest increase in flavonols. A trial was established to determine the effects of decreased nitrogen and phosphorus availability on the flavonol content of leaf and fruit tissues of tomato plants (Lycopersicon esculentum cv. Chaser) in a commercial situation. Nutrients were supplied by a hydroponic system with nutrient regimes designed to provide the highest and lowest nitrogen and phosphorus levels with which it is possible to support plant growth and fruit set. Fruiting was abundant and tomato fruits were harvested at mature green, breaker and red stages of ripening; leaves were also harvested from the tops of the plants. All tissues were analysed for flavonol content using reversed-phase high-performance liquid chromatography. Flavonol accumulation in the leaves of mature tomato plants was found to increase significantly in response to nitrogen stress, whereas phosphorus deficiency did not elicit this response. Reduced nitrogen availability had no consistent effect on the flavonol content of tomato fruits. Phosphorus deficiency elicited an increase in flavonol content in early stages of ripening. Effects of nutrient stress on the flavonol content of tomato fruits were lost as ripening progressed. The findings suggest that nutrient status may be employed to manipulate the flavonol content of vegetative tissues but cannot be used to elevate the flavonol content of tomato fruit.

G Protein βγ Subunit-Mediated Presynaptic Inhibition: Regulation of Exocytotic Fusion Downstream of Ca2+ Entry

Abstract: The nervous system can modulate neurotransmitter release by neurotransmitter activation of heterotrimeric GTP–binding protein (G protein)–coupled receptors. We found that microinjection of G protein βγ subunits (Gβγ) mimics serotonin9s inhibitory effect on neurotransmission. Release of free Gβγ was critical for this effect because a Gβγ scavenger blocked serotonin9s effect. Gβγ had no effect on fast, action potential–evoked intracellular Ca 2+ release that triggered neurotransmission. Inhibition of neurotransmitter release by serotonin was still seen after blockade of all classical Gβγ effector pathways. Thus, Gβγ blocked neurotransmitter release downstream of Ca 2+ entry and may directly target the exocytotic fusion machinery at the presynaptic terminal.

Plastid redox state and sugars: Interactive regulators of nuclear-encoded photosynthetic gene expression

Abstract: Feedback regulation of photosynthesis by carbon metabolites has long been recognized, but the underlying cellular mechanisms that control this process remain unclear. By using an Arabidopsis cell culture, we show that a block in photosynthetic electron flux prevents the increase in transcript levels of chlorophyll a/b-binding protein and the small subunit of Rubisco that typically occurs when intracellular sugar levels are depleted. In contrast, the expression of the nitrate reductase gene, which is induced by sugars, is not affected. These findings were confirmed in planta by using Arabidopsis carrying the firefly luciferase reporter gene fused to the plastocyanin and chlorophyll a/b-binding protein 2 gene promoters. Transcription from both promoters increases on carbohydrate depletion. Blocking photosynthetic electron transport with 3-(3′, 4′-dichlorophenyl)-1,1′-dimethylurea prevents this increase in transcription. We conclude that plastid-derived redox signaling can override the sugar-regulated expression of nuclear-encoded photosynthetic genes. In the sugar-response mutant, sucrose uncoupled 6 (sun6), plastocyanin-firefly luciferase transcription actually increases in response to exogenous sucrose rather than decreasing as in the wild type. Interestingly, plastid-derived redox signals do not influence this defective pattern of sugar-regulated gene expression in the sun6 mutant. A model, which invokes a positive inducer originating from the photosynthetic electron transport chain, is proposed to explain the nature of the plastid-derived signal.