Focus Issue on Calcium Signaling: Calcium and Reactive Oxygen Species Rule the Waves of Signaling

Calcium signaling and reactive oxygen species signaling are directly connected, and both contribute to cell-to-cell signal propagation in plants.Calcium (Ca²⁺) is an important second messenger with diverse functions not only in mammals but also in plants. It is released in response to a variety of stimuli like biotic and abiotic stresses and facilitates a tight regulation of response reactions as well as of developmental processes (Sanders et al., 2002; Steinhorst and Kudla, 2012). Ca²⁺ accumulation events are characterized by distinct temporal and spatial features, and they can vary in terms of amplitude, frequency, and duration (Webb et al., 1996; Scrase-Field and Knight, 2003; Dodd et al., 2010; Kudla et al., 2010). Spatially defined Ca²⁺ signals can be generated due to the especially slow diffusion rate of the Ca²⁺ ion in the cytoplasm in combination with tightly regulated release and uptake from and into different intracellular stores and the apoplast. Together, these characteristics encode information about particular stimuli, for example, drought stress that is presented to the cell as so-called Ca²⁺ signatures (Webb et al., 1996). This information has to be decoded and transmitted by a signaling machinery in order to initiate adequate response reactions, for example, stomatal closure (Allen et al., 2000, 2001; Sanders et al., 2002). Ca²⁺ signatures can be sensed by proteins that bind Ca²⁺ via helix-loop-helix EF-hand motifs. Arabidopsis (Arabidopsis thaliana) possesses at least 250 putative EF-hand proteins, 100 of which have been classified as Ca²⁺ sensor proteins (Day et al., 2002; Hashimoto and Kudla, 2011). Given that each member of this intricate set of Ca²⁺ sensor proteins can exhibit characteristic expression and subcellular localization profiles as well as distinct Ca²⁺ affinities, plants are equipped with a complex signal-decoding machinery to process a wide range of different Ca²⁺ signals (Batistič and Kudla, 2004; Batistič and Kudla, 2010). Ca²⁺ functions in concert with other important second messengers like reactive oxygen species (ROS). ROS can be generated in a controlled manner by several types of enzymes, such as NADPH oxidases, in order to contribute to pathogen defense and cell signaling. Recent findings point to direct connections between ROS and Ca²⁺ signaling pathways that enable cell-to-cell communication and thereby long-distance transmission of signals in plants. In this Update, we focus on new findings in the field of plant Ca²⁺ signaling during the past 3 years since the status of the field was discussed in comprehensive reviews (Dodd et al., 2010; Kudla et al., 2010; Mazars et al., 2011; Reddy et al., 2011) and put special emphasis on the contribution of a plant-specific Ca²⁺ signaling network to deciphering defined Ca²⁺ signals and its integration with ROS signaling.     

Effects of Calcium Ions and of Calcium Channel Blockers on Galvanotaxis of Chlamydomonas reinhardtii

The effects of calcium ions and of the calcium channel blockers verapamil, diltiazem and nifedipine on galvanotaxis in Chlamydomonas have been investigated using a fully automated and computerized population system. Galvanotaxis is a function of the voltage applied to the cell population. However, the galvanotactic orientation also depends on the external calcium concentration. In a calcium-deprived nutrient medium which still contains 6 × 10^?7M calcium, galvanotactic orientation is about 20% of orientation at optimal calcium concentration of 10^?4 M at 9 V. The higher the external calcium concentration is, the lower is the voltage necessary for optimal galvanotactic orientation. The calcium channel blockers diltiazem and nifedipine likewise inhibit galvanotaxis of Chlamydomonas very specifically without impairing motility. Verapamil is effective, but also inhibits motility by causing detachment or shortening of the flagella. Nevertheless, inhibition of galvanotaxis by verapamil is not the only result of decreased motility, because the galvanotactic orientation is impaired to a greater extent than motility. The effectiveness of the three blockers tested in inhibiting galvanotaxis depends on the concentration and on the voltage applied. At 10^?5 M, verapamil causes maximal inhibition of galvanotaxis at 9 V. At increasing concentrations up to 10^?4 M, diltiazem inhibits galvanotaxis more strongly than the other blockers. If the voltage is varied at a constant blocker concentration of 2 × 10^?5 M, nifedipine causes maximal inhibition at 3 V–6 V, diltiazem at 9 V and verapamil above 12 V.     

Influence of Phosvitin and Calcium Gluconate Concentration on Permeation and Intestinal Absorption of Calcium Ions

The effect of egg yolk phosvitin on the permeation and absorption of calcium was investigated in vitro in relation to calcium gluconate concentration. Obtained results indicate that phosvitin significantly reduces the intestinal calcium absorption from 1 and 10 mM of calcium gluconate solution. It is associated with the formation of the complex of Ca (II) ions with phosvitin. The process of calcium permeation increases under phosvitin influence when calcium gluconate concentrations rise up to 10 mM. At a higher concentration of calcium gluconate (20 mM), no effect of phosvitin was seen on permeation of calcium ions.     

环境钙对鲤鱼血浆钙含量及斯坦尼氏小体结构的影响

将鲤鱼①饲养在富钙淡水(天然淡水中添加0．5％CaCl2)中,②在富钙淡水饲养8d后分别腹腔注射维生素D3(VD3,5000U／100g体重,以提高细胞外钙浓度)和鲤鱼斯坦尼氏小体(CS)匀浆液(0．5mg/100g体重,以降低细胞外钙浓度),③富钙淡水饲养28d后转入天然淡水。单纯的高钙环境能够诱导鲤鱼出现高血钙;注射VD3和CS匀浆液则分别引起血钙水平升高和降低。血钙升高时,CS囊泡萎缩,分泌细胞的核增大、粗面内质网增多、细胞脱颗粒化,提示CS分泌细胞在Ca^2 的作用下合成与分泌活动旺盛,但过度刺激将导致分泌细胞的衰竭与破裂;当血钙降低时,上述症状有所缓解,分泌细胞内的分泌颗粒积累,提示CS分泌细胞的分泌活动减弱,合成活动正常;从高钙环境转入天然淡水后,CS分泌细胞的结构与功能亦可逐渐恢复正常。鲤鱼CS的状态受细胞外和体外钙水平变动的影响,而且这种影响是可逆的。     

Calcium and the cell wall

Abstract. From this brief review it appears that the interactions between calcium ions and cell walls play a key role in plant physiology. Calcium ions are involved in many mechnisms: for example, stabilization of cell wall structures, acidic growth, ion exchange properties, control of the activities of wall enzymes. All these properties originate from the tight binding of calcium ions to the pectins present in the cell walls. The factor most important for controlling wall behaviour is the density of non-diffusible charges and, due to its high affinity, calcium can significantly affect this factor. We also discuss the theoretical ion exchange models in relation to the specific role of calcium ions.     

Calcium and the plant cytoskeleton

Abstract. The regulation of the plant cytoskeleton by the concentration of free ionized Ca²⁺ in the cytosol is discussed. The effect is illustrated by considering the role of Ca²⁺ in regulating cytoplasmic streaming in Physarum and characean algae, and flagellar beating in Chlamydomonas . Examples are given of cytoskeletal processes in other plant cells that may be Ca²⁺ -regulated, together with an appraisal of some technical difficulties that currently limit their study.     

Calcium gradients and the Golgi

Changes in intracellular free calcium regulate many intracellular processes. With respect to the secretory pathway and the Golgi apparatus, changes in calcium concentration occurring either in the adjacent cytosol or within the lumen of the Golgi act to regulate Golgi function. Conversely, the Golgi sequesters calcium to shape cytosolic calcium signals as well as initiate them by releasing calcium via inositol-1,4,5-triphosphate (IP(3)) receptors, located on Golgi membranes. Local calcium transients juxtaposed to the Golgi (arising from release by the Golgi or other organelles) can activate calcium dependent signalling molecules located on or around the Golgi. This review focuses on the reciprocal relationship between the cell biology of the Golgi apparatus and intracellular calcium homeostasis.     

Calcium as an extracellular signalling molecule: perspectives on the Calcium Sensing Receptor in the brain

Calcium acts as a universal signal that is responsible for controlling a spectrum of cellular processes ranging from fertilization to apoptosis. For a long time, calcium was regarded solely as an intracellular second messenger. However, the discovery that calcium can also act as an external ligand together with the molecular cloning of its cell surface receptor, the Calcium Sensing Receptor (CaSR), demonstrated that calcium also acts as an important extracellular or first messenger. Here, we give an overview of the main structural, pharmacological and physiological features of the CaSR and provide an assessment of its functions and cellular and molecular mechanisms of action. In addition, we propose possible avenues for future research into the trafficking of CaSR and the role(s) of this receptor in the central nervous system.     

饲料钙磷含量和钙磷比对生长期大鼠体重增长与骨骼发育的影响

目的研究饲料中不同的钙磷含量和钙磷比对生长期实验大鼠体重及骨骼发育的影响。方法采用完全随机化设计方案,取1月龄（100±10）g清洁级Wistar雄性大鼠70只,随机分成7组,每组10只,饲喂7种不同钙磷含量的饲料。自由采食,饮用去离子水,试验期42 d。各组于实验开始和结束时称重。试验结束后,处死动物并分离一侧胫骨和股骨用于实验指标的检测。结果当保持饲料中钙或者磷中的一种含量不变的情况下,调整另一种的含量即改变钙磷比例时,1.2∶1的正常钙磷比例组生长期实验大鼠增重及骨骼发育要明显好于0.4∶1的低钙磷比例组和4∶1的高钙磷比例组（P〈0.05）,并且当保持饲料中钙磷比例不变时,即使在低钙低磷和高钙高磷进食水平下,正常钙磷比例组大鼠增重及骨骼发育都较好,但低钙磷比例组和高钙磷比例组大鼠增重及骨骼发育则由于饲料中钙磷含量的变化波动较大（P〈0.05）。结论饲料中钙和磷需按比例添加,如果两者绝对含量相差过大就会影响到动物的增重及骨骼发育,本实验证明饲料钙磷比为1.2∶1时生长期实验大鼠增重及骨骼发育较好。     

Effect of Some Treatments of Milk on the Exchangeability of Colloidal Calcium in Milk with Soluble Calcium

Effect of heating, freeze-drying, frozen storage and coagulation by chymosin on the exchangeability of colloidal calcium with soluble calcium in milk was investigated by the radioisotopic method previously reported. Heating at 100°C for 30 min caused a slight decrease of the exchangeability. Frozen storage at ?20°C for 120 days resulted in a marked decrease of the exchangeability accompanying a decrease of soluble calcium. Coagulation by chymosin gave no significant effect on the amount of soluble calcium or on the exchangeability of colloidal calcium.     

Calcium and the mechanism of axoplasmic transport

Using desheathed cat peroneal nerves in in vitro studies, Ca2+ was recently shown to be required to maintain axoplasmic transport. Calmodulin was also shown to be present in nerve and to participate in transport. These findings open up new possibilities for a better understanding of the underlying mechanism of transport. In the transport filament model, the materials transported are bound to a common carrier, the transport filaments, which are moved along the microtubules by means of an interaction with the side arms of the microtubules. This is an energy-requiring process that depends on a supply of ATP, which is utilized by the Ca2+,Mg2+-ATPase associated with the side arms of the microtubules. The Ca2+,Mg2+-ATPase is activated by calmodulin at the low micromolar levels of free Ca2+ present in the axon. The level is kept low by calcium-regulatory mechanisms that include mitochondria, endoplasmic reticulum, and calcium-binding proteins. Nerves exposed to higher-than-normal concentrations of Ca2+ in the medium show an increased number of particles in these organelles as expected of their Ca2+-regulatory role. The nature of the calmodulin-Ca,Mg-ATPase complex associated with the side arms is discussed on the basis of the transport model. Also discussed is slow transport, which is explained on the basis of the model as a differential binding affinity to the transport filaments.     

钙对水稻幼苗抗冷性的影响

ＣａＣｌ２浸种提高水稻幼苗叶片中结合态钙、内源抗氧化剂（ＧＳＨ、ＡｓＡ）含量和膜保护酶（ＣＡＴ、ＳＯＤ和ＰＯＤ）活性,也增加可溶性蛋白质中煮沸稳定蛋白质（ｂｏｉｌｉｎｇ－ｓｔａｂｌｅｐｒｏｔｅｉｎ）的含量。冷胁迫期间,ＣａＣｌ２并能减少因冷胁迫引起的ＧＳＨ、ＡｓＡ含量,ＣＡＴ、ＳＯＤ和ＰＯＤ活性以及煮沸稳定蛋白质下降的程度。在恢复期间,经ＣａＣｌ２处理的幼苗其ＧＳＨ、ＡＳＡ、ＣＡＴ、ＳＯＤ和ＰＯＤ以及煮沸稳定蛋白质水平均有回升。     

Calcium and the photopolarization ofPelvetia zygotes

The initially apolar zygotes of the brown algae,Fucus andPelvetia, form their main axes during the hours following fertilization and each cell expresses its axis by germinating at one location. The germinating region is destined to become the rhizoid and the rest of the zygote gives rise to the thallus. In response to unilateral blue light, the zygotes organize their developmental axes so that the rhizoids emerge on the shaded side, away from the light source. In the research reported here, the signaltransduction elements involved in the photopolarization ofPelvetia fastigiata De Toni zygotes have been investigated. It was found that exposure of zygotes to 90or 150-min pulses of unilateral light in the absence of extracellular Ca²⁺ completely eliminated photopolarization; that is, the cells formed their rhizoid-thallus axes randomly with respect to the light direction, while controls similarly exposed to light in normal (10 mM) Ca²⁺ were well polarized. When the cells were incubated in Ca²⁺-free sea water for an hour before being given the light pulse (while still in Ca²⁺-free sea water), they exhibited an unusual negative polarization: they formed their rhizoids on the hemisphere nearer the light source. Organic and inorganic calcium-channel blockers reduced or abolished photopolarization when present during light pulses. Reducing external Ca²⁺ to one-tenth of normal has the paradoxical effect of increasing calcium influx intoPelvetia zygotes. When zygotes were given light pulses in reduced extracellular calcium, the degree of photopolarization was increased substantially. These data are consistent with the idea that the formation of an intracellular gradient of [Ca²⁺] is an essential part of the polarization process. The fungus-derived calmodulin antagonist, ophiobolin A, blocked or greatly delayed germination when present continuously at a concentration of 100–300 nM. However, when present at 300 nM during a brief light pulse, it markedly increased the sensitivity of the cells to light. These results suggest that calmodulin may be the mediator of intracellular [Ca²⁺] gradients in the photopolarization process.     

Influence of Calcium and Magnesium on Manganese Absorption

Mutual effects between Mn, Ca, and Mg were studied during steady-state absorption experiments with excised barley roots. Calcium appeared to enhance the rate of Mn absorption; whereas, Mg had a highly depressive effect. The combination of both Ca and Mg was even more inhibitory to Mn absorption than Mg alone. Manganese had no effect on the usual negligible Ca absorption by this tissue, but effectively inhibited the absorption of Mg. Although divalent cation absorption from the Ca-Mg-Mn system was essentially nil, K absorption was greatly stimulated in the presence of these cations.These mutual effects and others reported in the literature are explained by the hypothesis that selectivity in ion absorption results from cation-induced conformational changes in the structure of the carrier molecule.