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Calcium (Ca 2+ ) is a universal second messenger involved in various cellular processes, leading to plant development and to biotic and abiotic stress responses. Intracellular variation in free Ca 2+ concentration is among the earliest events following the plant perception of environmental change. These Ca 2+ variations differ in their spatio-temporal properties according to the nature, strength and duration of the stimulus. However, their conversion into biological responses requires Ca 2+ sensors for decoding and relaying. The occurrence in plants of calmodulin (CaM) but also of other sets of plant-specific Ca 2+ sensors such as calmodulin-like proteins (CMLs), Ca 2+ -dependent protein kinases (CDPKs) and calcineurin B-like proteins (CBLs) indicate that plants possess specific tools and machineries to convert Ca 2+ signals into appropriate responses. Here, we focus on recent progress made in monitoring the generation of Ca 2+ signals at the whole plant or cell level and their long distance propagation during biotic interactions. The contribution of CaM/CMLs and CDPKs in plant immune responses mounted against bacteria, fungi, viruses and insects are also presented., Competing Interests: The authors declare no conflicts of interest.

An increase in cellular calcium ion (Ca(2+)) concentration is now acknowledged to be one of the earliest events occurring during the induction of plant defence responses to a wide variety of pathogens. Sphingoid long-chain bases (LCBs) have also been recently demonstrated to be important mediators of defence-related programmed cell death during pathogen attack. Here, we present recent data highlighting how Ca(2+) and LCBs may be interconnected to regulate cellular processes which lead either to plant susceptibility or to resistance mechanisms. This article is part of a Special Issue entitled: 12th European Symposium on Calcium., (Copyright © 2012 Elsevier B.V. All rights reserved.)

Calcium is a key second messenger in signaling pathways associated with developmental and adaptive processes in plants. Stimulus-specific calcium signals, considered as calcium signatures, are translated into appropriate cellular responses through the action of various calcium-binding proteins and downstream effectors. We review here recent progress made in calcium signaling in the nucleus of plant cell. Experimental evidences show that nuclei can generate calcium signals on their own and point out the importance of calcium in the regulation of gene transcription. Future directions are given concerning the need to elucidate the mechanisms involved in the regulation of nuclear calcium homeostasis, the conversion of calcium signals into transcriptional responses or other fundamental downstream nuclear functions. Overall, a better understanding of nuclear signaling will be useful to get an integrated picture of the signaling network of the plant cell.

Calcium (Ca(2+)) ions act as intracellular second messengers in many different signalling processes in plant cells and thus contribute to the amplification step of the signalling pathway and the specificity of the adaptative response. Dynamics of calcium described as spatial and temporal changes of the Ca(2+) concentrations either in the cytosol and/or in other compartments of the plant cell are now accepted to generate "calcium signatures", which might be responsible for the initiation of specific downstream events leading to the mounting of an appropriate response. To identify and elucidate the properties of such signatures, highly sensitive and specific methods have been developed and are used to measure and monitor variations in intracellular Ca(2+) concentrations. Two of these methods, namely bio-luminescence and fluorescence in combination with confocal laser scanning microscopy, are presented.

Increases in the concentration of free calcium in the cytosol are one of the general events that relay an external stimulus to the internal cellular machinery and allow eukaryotic organisms, including plants, to mount a specific biological response. Different lines of evidence have shown that other intracellular organelles contribute to the regulation of free calcium homeostasis in the cytosol. The vacuoles, the endoplasmic reticulum and the cell wall constitute storage compartments for mobilizable calcium. In contrast, the role of organelles surrounded by a double membrane (e.g. mitochondria, chloroplasts and nuclei) is more complex. Here, we review experimental data showing that these organelles harbor calcium-dependent biological processes. Mitochondria, chloroplasts as well as nuclei are equipped to generate calcium signal on their own. Changes in free calcium in a given organelle may also favor the relocalization of proteins and regulatory components and therefore have a profound influence on the integrated functioning of the cell. Studying, in time and space, the dynamics of different components of calcium signaling pathway will certainly give clues to understand the extraordinary flexibility of plants to respond to stimuli and mount adaptive responses. The availability of technical and biological resources should allow breaking new grounds by unveiling the contribution of signaling networks in integrative plant biology.

The Ca2+ cation is fully recognized as an important intracellular second messenger coupling a wide range of extracellular stimuli to characteristic responses in plant cells. Such a pleiotropic effect raises questions regarding the mechanisms by which the signalling pathways, all of then involving an increase in intracellular calcium concentration, can be specific to a given stimulus. Here, we present recent results which shed light into different concepts which may explain the response specificity in signalling processes, such as "the cross-talk between signalling pathways", "the Ca2+ signatures" and "the compartmentation of Ca(2+)-signalling".

Space experiments provide a unique opportunity to advance our knowledge of how plants respond to the space environment, and specifically to the absence of gravity. The European Modular Cultivation System ( EMCS) has been designed as a dedicated facility to improve and standardise plant growth in the International Space Station ( ISS). The EMCS is equipped with two centrifuges to perform experiments in microgravity and with variable gravity levels up to 2.0 g. Seven experiments have been performed since the EMCS was operational on the ISS. The objectives of these experiments aimed to elucidate phototropic responses (experiments TROPI-1 and -2), root gravitropic sensing ( GRAVI-1), circumnutation ( MULTIGEN-1), cell wall dynamics and gravity resistance ( Cell wall/ Resist wall), proteomic identification of signalling players ( GENARA- A) and mechanism of Ins P3 signalling ( Plant signalling). The role of light in cell proliferation and plant development in the absence of gravity is being analysed in an on-going experiment ( Seedling growth). Based on the lessons learned from the acquired experience, three preselected ISS experiments have been merged and implemented as a single project ( Plant development) to study early phases of seedling development. A Topical Team initiated by European Space Agency ( ESA), involving experienced scientists on Arabidopsis space research experiments, aims at establishing a coordinated, long-term scientific strategy to understand the role of gravity in Arabidopsis growth and development using already existing or planned new hardware. [ABSTRACT FROM AUTHOR]

In eukaryotic cells, sphingoid long chain bases (LCBs) such as sphingosine or phytosphingosine (PHS) behave as second messengers involved in various processes including programmed cell death (PCD). In plants, induction of PCD by LCBs has now been described, but the signalling pathway is still enigmatic. Using Arabidopsis, we identify new key steps in this pathway. We demonstrate that PHS induces activation of the calcium-dependent kinase CPK3, which phosphorylates its binding partners, the 14-3-3 proteins. This phosphorylation leads to the disruption of the complex and to CPK3 degradation. Using cpk3 knockout lines, we demonstrate that CPK3 is a positive regulator of LCB-mediated PCD. These findings establish 14-3-3-regulated CPK3 as a key component of the LCB pathway leading to PCD in plants. [ABSTRACT FROM AUTHOR]