Ethylene signaling modulates tomato pollen tube growth through modifications of cell wall remodeling and calcium gradient.

Ethylene modulates plant developmental processes including flower development. Previous studies have suggested ethylene participates in pollen tube (PT) elongation, and both ethylene production and perception seem critical at the time of fertilization. The full gene set regulated by ethylene during PT growth is unknown. To study this, we used various EThylene Receptor (ETR) tomato (Solanum lycopersicum) mutants: etr3-ko, a loss-of-function (LOF) mutant; and NR (NEVER RIPE), a gain-of-function (GOF) mutant. The etr3-ko PTs grew faster than wild-type (WT) PTs. Oppositely, NR PT elongation was slower than in WT, and PTs displayed larger diameters. ETR mutations result in feedback control of ethylene production. Furthermore, ethylene treatment of germinating pollen grains increased PT length in etr-ko mutants and WT, but not in NR. Treatment with the ethylene perception inhibitor 1-methylcyclopropene decreased PT length in etr-ko mutants and WT, but had no effect on NR. This confirmed that ethylene regulates PT growth. The comparison of PT transcriptomes in LOF and GOF mutants, etr3-ko and NR, both harboring mutations of the ETR3 gene, revealed that ethylene perception has major impacts on cell wall- and calcium-related genes as confirmed by microscopic observations showing a modified distribution of the methylesterified homogalacturonan pectic motif and of calcium load. Our results establish links between PT growth, ethylene, calcium, and cell wall metabolism, and also constitute a transcriptomic resource.

Identification of the Arabidopsis Calmodulin-Dependent NAD+ Kinase That Sustains the Elicitor-Induced Oxidative Burst.

NADP(H) is an essential cofactor of multiple metabolic processes in all living organisms, and in plants, NADP(H) is required as the substrate of Ca2+-dependent NADPH oxidases, which catalyze a reactive oxygen species burst in response to various stimuli. While NADP+ production in plants has long been known to involve a calmodulin (CaM)/Ca2+-dependent NAD+ kinase, the nature of the enzyme catalyzing this activity has remained enigmatic, as has its role in plant physiology. Here, we used proteomic, biochemical, molecular, and in vivo analyses to identify an Arabidopsis (Arabidopsis thaliana) protein that catalyzes NADP+ production exclusively in the presence of CaM/Ca2+ This enzyme, which we named NAD kinase-CaM dependent (NADKc), has a CaM-binding peptide located in its N-terminal region and displays peculiar biochemical properties as well as different domain organization compared with known plant NAD+ kinases. In response to a pathogen elicitor, the activity of NADKc, which is associated with the mitochondrial periphery, contributes to an increase in the cellular NADP+ concentration and to the amplification of the elicitor-induced oxidative burst. Based on a phylogenetic analysis and enzymatic assays, we propose that the CaM/Ca2+-dependent NAD+ kinase activity found in photosynthetic organisms is carried out by NADKc-related proteins. Thus, NADKc represents the missing link between Ca2+ signaling, metabolism, and the oxidative burst.

Sphingolipid-induced cell death in Arabidopsis is negatively regulated by the papain-like cysteine protease RD21.

It is now well established that sphingoid Long Chain Bases (LCBs) are crucial mediators of programmed cell death. In plants, the mycotoxin fumonisin B1 (FB1) produced by the necrotrophic fungus Fusarium moniliforme disrupts the sphingolipid biosynthesis pathway by inhibiting the ceramide synthase leading to an increase in the amount of phytosphingosine (PHS) and dihydrosphingosine (DHS), the two major LCBs in Arabidopsis thaliana. To date, the signaling pathway involved in FB1-induced cell death remains largely uncharacterized. It is also well acknowledged that plant proteases such as papain-like cysteine protease are largely involved in plant immunity. Here, we show that the papain-like cysteine protease RD21 (responsive-to-desiccation-21) is activated in response to PHS and FB1 in Arabidopsis cultured cells and leaves, respectively. Using two allelic null mutants of RD21, and two different PCD bioassays, we demonstrate that the protein acts as a negative regulator of FB1-induced cell death in Arabidopsis.

Enhanced elevations of hypo-osmotic shock-induced cytosolic and nucleic calcium concentrations in tobacco cells by pretreatment with dimethyl sulfoxide.

Dimethyl sulfoxide (DMSO) is a dipolar aprotic solvent widely used in biological assays. Here, we observed that DMSO enhanced the hypo-osmotically induced increases in the concentration of Ca2+ in cytosolic and nucleic compartments in the transgenic cell-lines of tobacco (BY-2) expressing aequorin.

Calcium Signalling in Plant Biotic Interactions.

Calcium (Ca2+) 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 Ca2+ concentration is among the earliest events following the plant perception of environmental change. These Ca2+ variations differ in their spatio-temporal properties according to the nature, strength and duration of the stimulus. However, their conversion into biological responses requires Ca2+ sensors for decoding and relaying. The occurrence in plants of calmodulin (CaM) but also of other sets of plant-specific Ca2+ sensors such as calmodulin-like proteins (CMLs), Ca2+-dependent protein kinases (CDPKs) and calcineurin B-like proteins (CBLs) indicate that plants possess specific tools and machineries to convert Ca2+ signals into appropriate responses. Here, we focus on recent progress made in monitoring the generation of Ca2+ 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.

CDPKs and 14-3-3 Proteins: Emerging Duo in Signaling.

Calcium-dependent protein kinases (CDPKs) are Ca2+-sensors that play pivotal roles in plant development and stress responses. They have the unique ability to directly translate intracellular Ca2+ signals into reversible phosphorylation events of diverse substrates which can mediate interactions with 14-3-3 proteins to modulate protein functions. Recent studies have revealed roles for the coordinated action of CDPKs and 14-3-3s in regulating diverse aspects of plant biology including metabolism, development, and stress responses. We review here the underlying interaction and cross-regulation of the two signaling proteins, and we discuss how this insight has led to the emerging concept of CDPK/14-3-3 signaling modules that could contribute to response specificity.

CML8, an Arabidopsis Calmodulin-Like Protein, Plays a Role in Pseudomonas syringae Plant Immunity.

Calcium is a universal second messenger involved in various cellular processes including plant development and stress responses. Its conversion into biological responses requires the presence of calcium sensor relays such as calmodulin (CaM) and calmodulin-like (CML) proteins. While the role of CaM is well described, the functions CML proteins remain largely uncharacterized. Here, we show that Arabidopsis CML8 expression is strongly and transiently induced by Pseudomonas syringae, and reverse genetic approaches indicated that the overexpression of CML8 confers on plants a better resistance to pathogenic bacteria compared with wild-type, knock-down and knock-out lines, indicating that CML8 participates as a positive regulator in plant immunity. However, this difference disappeared when inoculations were performed using bacteria unable to inject effectors into a plant host cell or deficient for some effectors known to target the salicylic acid (SA) signaling pathway. SA content and PR1 protein accumulation were altered in CML8 transgenic lines, supporting a role for CML8 in SA-dependent processes. Pathogen-associated molecular pattern (PAMP) treatments with flagellin and elf18 peptides have no effects on CML8 gene expression and do not modify root growth of CML8 knock-down and overexpressing lines compared with wild-type plants. Collectively, our results support a role for CML8 in plant immunity against P. syringae.

Calcium- and Nitric Oxide-Dependent Nuclear Accumulation of Cytosolic Glyceraldehyde-3-Phosphate Dehydrogenase in Response to Long Chain Bases in Tobacco BY-2 Cells.

Sphinganine or dihydrosphingosine (d18:0, DHS), one of the most abundant free sphingoid long chain bases (LCBs) in plants, is known to induce a calcium-dependent programmed cell death (PCD) in plants. In addition, in tobacco BY-2 cells, it has been shown that DHS triggers a rapid production of H2O2 and nitric oxide (NO). Recently, in analogy to what is known in the animal field, plant cytosolic glyceraldehyde-3-phosphate dehydrogenase (GAPC), a ubiquitous enzyme involved in glycolysis, has been suggested to fulfill other functions associated with its oxidative post-translational modifications such as S-nitrosylation on cysteine residues. In particular, in mammals, stress signals inducing NO production promote S-nitrosylation of GAPC and its subsequent translocation into the nucleus where the protein participates in the establishment of apoptosis. In the present study, we investigated the behavior of GAPC in tobacco BY-2 cells treated with DHS. We found that upon DHS treatment, an S-nitrosylated form of GAPC accumulated in the nucleus. This accumulation was dependent on NO production. Two genes encoding GAPCs, namely Nt(BY-2)GAPC1 and Nt(BY-2)GAPC2, were cloned. Transient overexpression of Nt(BY-2)GAPC-green fluorescent protein (GFP) chimeric constructs indicated that both proteins localized in the cytoplasm as well as in the nucleus. Mutating into serine the two cysteine residues thought to be S-nitrosylated in response to DHS did not modify the localization of the proteins, suggesting that S-nitrosylation of GAPCs was probably not necessary for their nuclear relocalization. Interestingly, using Förster resonance energy transfer experiments, we showed that Nt(BY-2)GAPCs interact with nucleic acids in the nucleus. When GAPCs were mutated on their cysteine residues, their interaction with nucleic acids was abolished, suggesting a role for GAPCs in the protection of nucleic acids against oxidative stress.

Calcium Sensors as Key Hubs in Plant Responses to Biotic and Abiotic Stresses.

The Ca(2+) ion is recognized as a crucial second messenger in signaling pathways coupling the perception of environmental stimuli to plant adaptive responses. Indeed, one of the earliest events following the perception of environmental changes (temperature, salt stress, drought, pathogen, or herbivore attack) is intracellular variation of free calcium concentrations. These calcium variations differ in their spatio-temporal characteristics (subcellular location, amplitude, kinetics) with the nature and strength of the stimulus and, for this reason, they are considered as signatures encrypting information from the initial stimulus. This information is believed to drive a specific response by decoding via calcium-binding proteins. Based on recent examples, we illustrate how individual calcium sensors from the calcium-dependent protein kinase and calmodulin-like protein families can integrate inputs from various environmental changes. Focusing on members of these two families, shown to be involved in plant responses to both abiotic and biotic stimuli, we discuss their role as key hubs and we put forward hypotheses explaining how they can drive the signaling pathways toward the appropriate plant responses.

The activity of the wheat MAP kinase phosphatase 1 is regulated by manganese and by calmodulin.

MAPK phosphatases (MKPs) are negative regulators of MAPKs in eukaryotes and play key roles in the regulation of different cellular processes. However in plants, little is known about the regulation of these Dual Specific Phosphatases (DSPs) by Ca(2+) and calmodulin (CaM). Here, we showed that the wheat MKP (TMKP1) harboring a calmodulin (CaM) binding domain, binds to CaM in a Ca(2+)-dependent manner. In addition, TMKP1 exhibited a phosphatase activity in vitro that is specifically enhanced by Mn(2+) and to a lesser extent by Mg(2+), but without any synergistic effect between the two bivalent cations. Most interestingly, CaM/Ca(2+) complex inhibits the catalytic activity of TMKP1 in a CaM-dose dependent manner. However, in the presence of Mn(2+) this activity is enhanced by CaM/Ca(2+) complex. These dual regulatory effects seem to be mediated via interaction of CaM/Ca(2+) to the CaM binding domain in the C-terminal part of TMKP1. Such effects were not reported so far, and raise a possible role for CaM and Mn(2+) in the regulation of plant MKPs during cellular response to external signals.

Microgravity induces changes in microsome-associated proteins of Arabidopsis seedlings grown on board the international space station.

The "GENARA A" experiment was designed to monitor global changes in the proteome of membranes of Arabidopsis thaliana seedlings subjected to microgravity on board the International Space Station (ISS). For this purpose, 12-day-old seedlings were grown either in space, in the European Modular Cultivation System (EMCS) under microgravity or on a 1 g centrifuge, or on the ground. Proteins associated to membranes were selectively extracted from microsomes and identified and quantified through LC-MS-MS using a label-free method. Among the 1484 proteins identified and quantified in the 3 conditions mentioned above, 80 membrane-associated proteins were significantly more abundant in seedlings grown under microgravity in space than under 1 g (space and ground) and 69 were less abundant. Clustering of these proteins according to their predicted function indicates that proteins associated to auxin metabolism and trafficking were depleted in the microsomal fraction in µg space conditions, whereas proteins associated to stress responses, defence and metabolism were more abundant in µg than in 1 g indicating that microgravity is perceived by plants as a stressful environment. These results clearly indicate that a global membrane proteomics approach gives a snapshot of the cell status and its signaling activity in response to microgravity and highlight the major processes affected.

Imaging long distance propagating calcium signals in intact plant leaves with the BRET-based GFP-aequorin reporter.

Calcium (Ca(2+)) is a second messenger involved in many plant signaling processes. Biotic and abiotic stimuli induce Ca(2+) signals within plant cells, which, when decoded, enable these cells to adapt in response to environmental stresses. Multiple examples of Ca(2+) signals from plants containing the fluorescent yellow cameleon sensor (YC) have contributed to the definition of the Ca(2+) signature in some cell types such as root hairs, pollen tubes and guard cells. YC is, however, of limited use in highly autofluorescent plant tissues, in particular mesophyll cells. Alternatively, the bioluminescent reporter aequorin enables Ca(2+) imaging in the whole plant, including mesophyll cells, but this requires specific devices capable of detecting the low amounts of emitted light. Another type of Ca(2+) sensor, referred to as GFP-aequorin (G5A), has been engineered as a chimeric protein, which combines the two photoactive proteins from the jellyfish Aequorea victoria, the green fluorescent protein (GFP) and the bioluminescent protein aequorin. The Ca(2+)-dependent light-emitting property of G5A is based on a bioluminescence resonance energy transfer (BRET) between aequorin and GFP. G5A has been used for over 10 years for enhanced in vivo detection of Ca(2+) signals in animal tissues. Here, we apply G5A in Arabidopsis and show that G5A greatly improves the imaging of Ca(2+) dynamics in intact plants. We describe a simple method to image Ca(2+) signals in autofluorescent leaves of plants with a cooled charge-coupled device (cooled CCD) camera. We present data demonstrating how plants expressing the G5A probe can be powerful tools for imaging of Ca(2+) signals. It is shown that Ca(2+) signals propagating over long distances can be visualized in intact plant leaves and are visible mainly in the veins.

Microsome-associated proteome modifications of Arabidopsis seedlings grown on board the International Space Station reveal the possible effect on plants of space stresses other than microgravity.

Growing plants in space for using them in bioregenerative life support systems during long-term human spaceflights needs improvement of our knowledge in how plants can adapt to space growth conditions. In a previous study performed on board the International Space Station (GENARA A experiment STS-132) we evaluate the global changes that microgravity can exert on the membrane proteome of Arabidopsis seedlings. Here we report additional data from this space experiment, taking advantage of the availability in the EMCS of a centrifuge to evaluate the effects of cues other than microgravity on the relative distribution of membrane proteins. Among the 1484 membrane proteins quantified, 227 proteins displayed no abundance differences between µ g and 1 g in space, while their abundances significantly differed between 1 g in space and 1 g on ground. A majority of these proteins (176) were over-represented in space samples and mainly belong to families corresponding to protein synthesis, degradation, transport, lipid metabolism, or ribosomal proteins. In the remaining set of 51 proteins that were under-represented in membranes, aquaporins and chloroplastic proteins are majority. These sets of proteins clearly appear as indicators of plant physiological processes affected in space by stressful factors others than microgravity.

Relationships between calcium and sphingolipid-dependent signalling pathways during the early steps of plant-pathogen interactions.

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.

Nuclear calcium signaling and its involvement in transcriptional regulation in plants.

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.

Transcriptome analysis of tobacco BY-2 cells elicited by cryptogein reveals new potential actors of calcium-dependent and calcium-independent plant defense pathways.

Cryptogein is a proteinaceous elicitor secreted by the oomycete Phytophthora cryptogea, which induces a hypersensitive response in tobacco plants. We have previously reported that in tobacco BY-2 cells treated with cryptogein, most of the genes of the phenylpropanoid pathway were upregulated and cell wall-bound phenolics accumulated. Both events were Ca(2+) dependent. In this study, we designed a microarray covering a large proportion of the tobacco genome and monitored gene expression in cryptogein-elicited BY-2 cells to get a more complete view of the transcriptome changes and to assess their Ca(2+) dependence. The predominant functional gene categories affected by cryptogein included stress- and disease-related proteins, phenylpropanoid pathway, signaling components, transcription factors and cell wall reinforcement. Among the 3819 unigenes whose expression changed more than fourfold, 90% were Ca(2+) dependent, as determined by their sensitivity to lanthanum chloride. The most Ca(2+)-dependent transcripts upregulated by cryptogein were involved in defense responses or the oxylipin pathway. This genome-wide study strongly supports the importance of Ca(2+)-dependent transcriptional regulation of regulatory and defense-related genes contributing to cryptogein responses in tobacco.

Nuclear calcium signaling: an emerging topic in plants.

The calcium ion is probably one of the most studied second messenger both in plant and animal fields. A large number of reviews have browsed the diversity of cytosolic calcium signatures and evaluated their pleiotropic roles in plant and animal cells. In the recent years, an increasing number of reviews has focused on nuclear calcium, especially on the possible roles of nuclear calcium concentration variations on nuclear activities. Experiments initially performed on animal cells gave conflicting results that brought about a controversy about the ability of the nucleus to generate its own calcium signals and to regulate its calcium level. But in plant cells, several converging scientific pieces of evidence support the hypothesis of nucleus autonomy. The present review briefly summarizes data supporting this hypothesis and tries to put forward some possible roles for these nucleus-generated calcium signals in controlling nuclear activity.

Nitric oxide production is not required for dihydrosphingosine-induced cell death in tobacco BY-2 cells.

Sphinganine or dihydrosphingosine (d18:0, DHS), one of the most abundant free sphingoid Long Chain Base (LCB) in plants, is known to induce a calcium dependent programmed cell death (PCD) in tobacco BY-2 cells. In addition, we have recently shown that DHS triggers a production of H2O2, via the activation of NADPH oxidase(s). However, this production of H2O2 is not correlated with the DHS-induced cell death but would rather be associated with basal cell defense mechanisms. In the present study, we extend our current knowledge of the DHS signaling pathway, by demonstrating that DHS also promotes a production of nitric oxide (NO) in tobacco BY-2 cells. As for H2O2, this NO production is not necessary for cell death induction.