Calcium-calmodulin signalling is involved in light-induced acidification by epidermal leaf cells of pea, Pisum sativum L.

Pathways of signal transduction of red and blue light-dependentacidification by leaf epidermal cells were studied using epidermalstrips of the Argenteum mutant of Pisum sativum. In these preparationsthe contribution of guard cells to the acidification is minimal.The hydroxypyridine nifedipine, a Ca²⁺-channel blocker, partlyinhibited the response to both blue and red light, while thephenylalkylamine, verapamil, a Ca²⁺-channel blocker that hasbeen shown in plant cells also to block K⁺-channels, causednearly complete inhibition. The Ca²⁺-channel activator S(–)BayK 8644 induced acidification when added in the dark and diminishedthe light-induced lowering of the extracellular pH. The Ca²⁺-ionophores,ionomycin and A23187 [GenBank] , also reduced the light response. Furthermore,the light-induced acidification was inhibited by the calmodulinantagonists W-7 and trifluoperazine, but not by W-5. These calmodulininhibitors completely inhibited the red light-induced acidification,but inhibited the response to blue light by only 60–70%.In general, inhibition by compounds affecting Ca-calmodulinsignalling was always stronger on the red light response thanthat on the blue light response (with the exception of verapamilthat blocked both the red and blue light responses equally well).This differential effect on red and blue light-induced responsesindicates a role for Ca²⁺-CaM signalling in both the red andblue light responses, while a second process, independent ofCa²⁺ is activated by blue light. Key words: Signal transduction, light-induced acidification, epidermal cells, pea     

Calcineurin is part of a negative feedback loop in the InsP3/Ca signalling pathway in blowfly salivary glands

The ubiquitous InsP₃/Ca²⁺ signalling pathway is modulated by diverse mechanisms, i.e. feedback of Ca²⁺ and interactions with other signalling pathways. In the salivary glands of the blowfly Calliphora vicina, the hormone serotonin (5-HT) causes a parallel rise in intracellular [Ca²⁺] and [cAMP] via two types of 5-HT receptors. We have shown recently that cAMP/protein kinase A (PKA) sensitizes InsP₃-induced Ca²⁺ release. We have now identified the protein phosphatase that counteracts the effect of PKA on 5-HT-induced InsP₃/Ca²⁺ signalling. We demonstrate that (1) tautomycin and okadaic acid, inhibitors of protein phosphatases PP1 and PP2A, have no effect on 5-HT-induced Ca²⁺ signals; (2) cyclosporin A and FK506, inhibitors of Ca²⁺/calmodulin-activated protein phosphatase calcineurin, cause an increase in the frequency of 5-HT-induced Ca²⁺ oscillations; (3) the sensitizing effect of cyclosporin A on 5-HT-induced Ca²⁺ responses does not involve Ca²⁺ entry into the cells; (4) cyclosporin A increases InsP₃-dependent Ca²⁺ release; (5) inhibition of PKA abolishes the effect of cyclosporin A on the 5-HT-induced Ca²⁺ responses, indicating that PKA and calcineurin act antagonistically on the InsP₃/Ca²⁺ signalling pathway. These findings suggest that calcineurin provides a negative feedback on InsP₃/Ca²⁺ signalling in blowfly salivary glands, counteracting the effect of PKA and desensitizing the signalling cascade at higher 5-HT concentrations.     

Ca uptake and plasma membrane depolarization associated with blue light-sensitive exogenous NADH oxidation by Cuscuta protoplasts

Protoplasts isolated from the apical segments of Cuscuta reflexa exhibited blue light-sensitive PM-linked NADH oxidase activity and increased rate of Ca²⁺-uptake in presence of NADH in dark, which was also stimulated by blue light. Contrary to marginal inhibition by Con A treatment, the ATPase inhibitors significantly inhibited the Ca²⁺ uptake by the protoplasts both in dark and under blue light. The Ca²⁺-calmodulin antagonists, W-7 and calmidazolium, also inhibited Ca²⁺-uptake by protoplasts under similar conditions. The state of PM polarization was monitored by the fluorescent dye 9-amino acridine. It was observed that PM-linked NADH oxidation caused hyperpolarization of the membrane, the exposure of which to blue light resulted in membrane depolarization. The presence of Ca²⁺-calmodulin antagonists or Con A treatment completely abolished the blue light-induced membrane depolarization. It is argued that these actities at the PM, having some glycoproteic components, are functionally closely involved in blue light-induced signal transduction in Cuscuta     

Enhancement of sensitivity in photoreceptors of the honey bee drone by light and by Ca2+

Summary Deeply dark adapted (1 h) photoreceptor cells of the honey bee drone show a light-induced enhancement of sensitivity (facilitation) as an aftereffect of illumination or in the presence of dim backgrounds.The Ca²⁺-dependency of this effect was studied: Reduction of extracellular Ca²⁺ to 0.1 mM decreases the sensitivity of a dark adapted cell, and the light-induced increase in sensitivity due to repetitive, dim, 20 ms test flashes is slower than in normal saline. After a sensitizing conditioning light, the sensitivity drops faster in low-calcium saline. The light-induced enhancement of sensitivity is mimicked by pressure injections of low amounts of Ca²⁺ (Ca²⁺/EGTA-buffers; 0.15 M free Ca²⁺) into a dark adapted cell. Injection of EGTA alone decreases the sensitivity. Injection of a solution containing ca1 mM free Ca²⁺ sequentially decreases and later increases the sensitivity transiently.These results suggest a model in which a progressive increase in intracellular Ca²⁺ concentration by light first increases (facilitates), and, at higher concentrations, decreases (light adapts) the sensitivity of the cells. One possible site of action for this positive and negative feedback control of cell sensitivity by Ca²⁺ is the endoplasmic reticulum.     

Stromal free calcium concentration and light-mediated activation of chloroplast fructose-1,6-bisphosphatase

Light-mediated activation of fructose-1,6-bisphosphatase (EC 3.1.3.11) in intact spinach chloroplasts (Spinacia oleracea L.) is enhanced in the presence of 10⁻⁵ molar external free Ca²⁺. The most pronounced effect is observed during the first minutes of illumination. Ruthenium red, an inhibitor of light-induced Ca²⁺ influx, inhibits this Ca²⁺ stimulated activation. In isolated stromal preparations, the activation of fructose-1,6-bisphosphatase is already enhanced by 2 minutes of exposure to elevated Ca²⁺ concentrations in the presence of physiological concentrations of Mg²⁺ and fructose-1,6-bisphosphate. Maximal activation of the enzyme is achieved between 0.34 and 0.51 millimolar Ca²⁺. The Ca²⁺ mediated activation decreases with increasing fructose-1,6-bisphosphate concentration and with increasing pH. The data are consistent with the proposal that the illumination of chloroplasts leads to a transient increase of free stromal Ca²⁺. In dark-kept chloroplasts the steady-state concentration of free stromal Ca²⁺ is 2.4 to 6.3 micromolar as determined by null point titration. These observations support our previous proposal that light-induced Ca²⁺ influx into chloroplasts does not only influence the cytosolic concentration of free Ca²⁺ but also regulates enzymatic processes inside the chloroplast.