Calcium signalling in early embryos

The onset of development in most species studied is triggered by one of the largest and longest calcium transients known to us. It is the most studied and best understood aspect of the calcium signals that accompany and control development. Its properties and mechanisms demonstrate what embryos are capable of and thus how the less-understood calcium signals later in development may be generated. The downstream targets of the fertilization calcium signal have also been identified, providing some pointers to the probable targets of calcium signals further on in the process of development.In one species or another, the fertilization calcium signal involves all the known calcium-releasing second messengers and many of the known calcium-signalling mechanisms. These calcium signals also usually take the form of a propagating calcium wave or waves. Fertilization causes the cell cycle to resume, and therefore fertilization signals are cell-cycle signals. In some early embryonic cell cycles, calcium signals also control the progress through each cell cycle, controlling mitosis.Studies of these early embryonic calcium-signalling mechanisms provide a background to the calcium-signalling events discussed in the articles in this issue.     

Modulation of environmental responses of plants by circadian clocks

Circadian clocks are signalling networks that enhance an organism's relationship with the rhythmic environment. The plant circadian clock modulates a wide range of physiological and biochemical events, such as stomatal and organ movements, photosynthesis and induction of flowering. Environmental signals regulate the phase and period of the plant circadian clock, which results in an approximate synchronization of clock outputs with external events. One of the consequences of circadian control is that stimuli of the same strength applied at different times of the day can result in responses of different intensities. This is known as 'gating'. Gating of a signal may allow plants to better process and react to the wide range and intensities of environmental signals to which they are constantly subjected. Light signalling, stomatal movements and low-temperature responses are examples of signalling pathways that are gated by the circadian clock. In this review, we describe the many levels at which the circadian clock interacts with responses to the environment. We discuss how environmental rhythms of temperature and light intensity entrain the circadian clock, how photoperiodism may be regulated by the relationship between environmental rhythms and the phasing of clock outputs, and how gating modulates the sensitivity of the clock and other responses to environmental and physiological signals. Finally, we describe evidence that the circadian clock can increase plant fitness.     

Modulation of cell calcium signals by mitochondria

It is now clearer and clearer that mitochondria play a role, and perhaps an active role, in cell calcium signalling. The fact that mitochondria can exhibit a Ca^2+>-induced Ca^2+> release (mCICR, Ichas et al. [37]) reinforces this concept and makes the mitochondria an essential element in the relay of Ca^2+> wave propagation. It must be emphasized that the modulation of cell Ca^2+> signals by mitochondria depends upon their energetic status, thus making mitochondria an essential link between energy metabolism and calcium signalling inside the cell.     

Cell‐wall invertases,key enzymes in the modulation of plant metabolism during defence responses

Most plant–pathogen interactions do not result in pathogenesis because of pre‐formed defensive plant barriers or pathogen‐triggered activation of effective plant immune responses. The mounting of defence reactions is accompanied by a profound modulation of plant metabolism. Common metabolic changes are the repression of photosynthesis, the increase in heterotrophic metabolism and the synthesis of secondary metabolites. This enhanced metabolic activity is accompanied by the reduced export of sucrose or enhanced import of hexoses at the site of infection, which is mediated by an induced activity of cell‐wall invertase (Cw‐Inv). Cw‐Inv cleaves sucrose, the major transport sugar in plants, irreversibly yielding glucose and fructose, which can be taken up by plant cells via hexose transporters. These hexose sugars not only function in metabolism, but also act as signalling molecules. The picture of Cw‐Inv regulation in plant–pathogen interactions has recently been broadened and is discussed in this review. An interesting emerging feature is the link between Cw‐Inv and the circadian clock and new modes of Cw‐Inv regulation at the post‐translational level.     

Opinion: Stomatal responses to light and CO2 depend on the mesophyll

The mechanisms by which stomata respond to red light and CO₂ are unknown, but much of the current literature assumes that these mechanisms reside wholly within the guard cells. However, responses of guard cells in isolated epidermes are typically much smaller than those in leaves, and there are several lines of evidence in the literature suggesting that the mesophyll is necessary for these responses in leaves. This paper advances the opinion that although guard cells may have small direct responses to red light and CO₂, most of the stomatal response to these factors in leaves is caused by an unknown signal that originates in the mesophyll.     

Sustained calcium signalling and caspase-3 activation involve NMDA receptors in thymocytes in contact with dendritic cells

L-glutamate, the major excitatory neurotransmitter, also has a role in non-neuronal tissues and modulates immune responses. Whether NMDA receptor (NMDAR) signalling is involved in T-cell development is unknown. In this study, we show that mouse thymocytes expressed an array of glutamate receptors, including NMDARs subunits. Sustained calcium (Ca²⁺) signals and caspase-3 activation in thymocytes were induced by interaction with antigen-pulsed dendritic cells (DCs) and were inhibited by NMDAR antagonists MK801 and memantine. NMDARs were transiently activated, triggered the sustained Ca²⁺ signal and were corecruited with the PDZ-domain adaptor postsynaptic density (PSD)-95 to thymocyte-DC contact zones. Although T-cell receptor (TCR) activation was sufficient for relocalization of NMDAR and PSD-95 at the contact zone, NMDAR could be activated only in a synaptic context. In these T-DC contacts, thymocyte activation occurred in the absence of exogenous glutamate, indicating that DCs could be a physiological source of glutamate. DCs expressed glutamate, glutamate-specific vesicular glutamate transporters and were capable of fast glutamate release through a Ca²⁺-dependent mechanism. We suggest that glutamate released by DCs could elicit focal responses through NMDAR-signalling in T cells undergoing apoptosis. Thus, synapses between T and DCs could provide a functional platform for coupling TCR activation and NMDAR signalling, which might reflect on T-cell development and modulation of the immune response.     

Reciprocal Control of the Circadian Clock and Cellular Redox State - a Critical Appraisal

Redox signalling comprises the biology of molecular signal transduction mediated by reactive oxygen (or nitrogen) species. By specific and reversible oxidation of redox-sensitive cysteines, many biological processes sense and respond to signals from the intracellular redox environment. Redox signals are therefore important regulators of cellular homeostasis. Recently, it has become apparent that the cellular redox state oscillates in vivo and in vitro, with a period of about one day (circadian). Circadian time-keeping allows cells and organisms to adapt their biology to resonate with the 24-hour cycle of day/night. The importance of this innate biological time-keeping is illustrated by the association of clock disruption with the early onset of several diseases (e.g. type II diabetes, stroke and several forms of cancer). Circadian regulation of cellular redox balance suggests potentially two distinct roles for redox signalling in relation to the cellular clock: one where it is regulated by the clock, and one where it regulates the clock. Here, we introduce the concepts of redox signalling and cellular timekeeping, and then critically appraise the evidence for the reciprocal regulation between cellular redox state and the circadian clock. We conclude there is a substantial body of evidence supporting circadian regulation of cellular redox state, but that it would be premature to conclude that the converse is also true. We therefore propose some approaches that might yield more insight into redox control of cellular timekeeping.     

Calcium and rhythms in plant cells

Plants cells, like any other living organism, experience the daily rotation of the Earth. They also depend strikingly on light, as a result of which much of the plant's biochemistry, physiology, and behaviour are temporally organised with respect to the environmental oscillation of day and night. Here we review the most recent findings on plants rhythms and how they seem to be so tightly connected to calcium-signalling aspects. We also try to establish parallels between different cell types, such as pollen tubes and fungal hyphae, where the existence and function of rhythms and oscillations is not obvious. Additionally, we discuss new methodologies and how these are shaping our current working hypothesis to study Ca²⁺ rhythms in plant cells.