Intracellular Ca2+ stores could participate to abscisic acid-induced depolarization and stomatal closure in Arabidopsis thaliana

A clade of receptor-like cytoplasmic kinases and 14-3-3 proteins coordinate inositol hexaphosphate accumulation

Inositol hexaphosphate (InsP6) is the major storage form of phosphorus in seeds. Reducing seed InsP6 content is a breeding objective in agriculture, as InsP6 negatively impacts animal nutrition and the environment. Nevertheless, how InsP6 accumulation is regulated remains largely unknown. Here, we identify a clade of receptor-like cytoplasmic kinases (RLCKs), named Inositol Polyphosphate-related Cytoplasmic Kinases 1-6 (IPCK1-IPCK6), deeply involved in InsP6 accumulation. The InsP6 concentration is dramatically reduced in seeds of ipck quadruple (T-4m/C-4m) and quintuple (C-5m) mutants, accompanied with the obviously increase of phosphate (Pi) concentration. The plasma membrane-localized IPCKs recruit IPK1 involved in InsP6 synthesis, and facilitate its binding and activity via phosphorylation of GRF 14-3-3 proteins. IPCKs also recruit IPK2s and PI-PLCs required for InsP4/InsP5 and InsP3 biosynthesis respectively, to form a potential IPCK-GRF-PLC-IPK2-IPK1 complex. Our findings therefore uncover a regulatory mechanism of InsP6 accumulation governed by IPCKs, shedding light on the mechanisms of InsP biosynthesis in eukaryotes.

Calmodulin Is the Fundamental Regulator of NADK-Mediated NAD Signaling in Plants

Calcium (Ca2+) signaling and nicotinamide adenine dinucleotide (NAD) signaling are two basic signal regulation pathways in organisms, playing crucial roles in signal transduction, energy metabolism, stress tolerance, and various developmental processes. Notably, calmodulins (CaMs) and NAD kinases (NADKs) are important hubs for connecting these two types of signaling networks, where CaMs are the unique activators of NADKs. NADK is a key enzyme for NADP (including NADP+ and NADPH) biosynthesis by phosphorylating NAD (including NAD+ and NADH) and therefore, maintains the balance between NAD pool and NADP pool through an allosteric regulation mode. In addition, the two respective derivatives from NAD+ (substrate of NADK) and NADP+ (product of NADK), cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP), have been considered to be the important messengers for intracellular Ca2+ homeostasis which could finally influence the combination between CaM and NADK, forming a feedback regulation mechanism. In this review article, we briefly summarized the major research advances related to the feedback regulation pathway, which is activated by the interaction of CaM and NADK during plant development and signaling. The theories and fact will lay a solid foundation for further studies related to CaM and NADK and their regulatory mechanisms as well as the NADK-mediated NAD signaling behavior in plant development and response to stress.

Methanol induces cytosolic calcium variations, membrane depolarization and ethylene production in arabidopsis and tobacco

Background and Aims

Methanol is a volatile organic compound released from plants through the action of pectin methylesterases (PMEs), which demethylesterify cell wall pectins. Plant PMEs play a role in developmental processes but also in responses to herbivory and infection by fungal or bacterial pathogens. However, molecular mechanisms that explain how methanol could affect plant defences remain poorly understood.

Methods

Using cultured cells and seedlings from Arabidopsis thaliana and tobacco BY2 expressing the apoaequorin gene, allowing quantification of cytosolic Ca2+, a reactive oxygen species (ROS) probe (CLA, Cypridina luciferin analogue) and electrophysiological techniques, we followed early plant cell responses to exogenously supplied methanol applied as a liquid or as volatile.

Key Results

Methanol induces cytosolic Ca2+ variations that involve Ca2+ influx through the plasma membrane and Ca2+ release from internal stores. Our data further suggest that these Ca2+ variations could interact with different ROS and support a signalling pathway leading to well known plant responses to pathogens such as plasma membrane depolarization through anion channel regulation and ethylene synthesis.

Conclusions

Methanol is not only a by-product of PME activities, and our data suggest that [Ca2+]cyt variations could participate in signalling processes induced by methanol upstream of plant defence responses.

Root phonotropism: Early signalling events following sound perception in Arabidopsis roots

Sound is a fundamental form of energy and it has been suggested that plants can make use of acoustic cues to obtain information regarding their environments and alter and fine-tune their growth and development. Despite an increasing body of evidence indicating that it can influence plant growth and physiology, many questions concerning the effect of sound waves on plant growth and the underlying signalling mechanisms remains unknown. Here we show that in Arabidopsis thaliana, exposure to sound waves (200Hz) for 2 weeks induced positive phonotropism in roots, which grew towards to sound source. We found that sound waves triggered very quickly (within  minutes) an increase in cytosolic Ca²⁺, possibly mediated by an influx through plasma membrane and a release from internal stock. Sound waves likewise elicited rapid reactive oxygen species (ROS) production and K⁺ efflux. Taken together these results suggest that changes in ion fluxes (Ca²⁺ and K⁺) and an increase in superoxide production are involved in sound perception in plants, as previously established in animals.

Deciphering the dual effect of lipopolysaccharides from plant pathogenic Pectobacterium

Lipopolysaccharides (LPS) are a component of the outer cell surface of almost all Gram-negative bacteria and play an essential role for bacterial growth and survival. Lipopolysaccharides represent typical microbe-associated molecular pattern (MAMP) molecules and have been reported to induce defense-related responses, including the expression of defense genes and the suppression of the hypersensitive response in plants. However, depending on their origin and the challenged plant, LPS were shown to have complex and different roles. In this study we showed that LPS from plant pathogens Pectobacterium atrosepticum and Pectobacterium carotovorum subsp. carotovorum induce common and different responses in A. thaliana cells when compared to those induced by LPS from non-phytopathogens Escherichia coli and Pseudomonas aeruginosa. Among common responses to both types of LPS are the transcription of defense genes and their ability to limit of cell death induced by Pectobacterium carotovorum subsp carotovorum. However, the differential kinetics and amplitude in reactive oxygen species (ROS) generation seemed to regulate defense gene transcription and be determinant to induce programmed cell death in response to LPS from the plant pathogenic Pectobacterium. These data suggest that different signaling pathways could be activated by LPS in A. thaliana cells.

Could FaRP-Like Peptides Participate in Regulation of Hyperosmotic Stress Responses in Plants?

The ability to respond to hyperosmotic stress is one of the numerous conserved cellular processes that most of the organisms have to face during their life. In metazoans, some peptides belonging to the FMRFamide-like peptide (FLP) family were shown to participate in osmoregulation via regulation of ion channels; this is, a well-known response to hyperosmotic stress in plants. Thus, we explored whether FLPs exist and regulate osmotic stress in plants. First, we demonstrated the response of Arabidopsis thaliana cultured cells to a metazoan FLP (FLRF). We found that A. thaliana express genes that display typical FLP repeated sequences, which end in RF and are surrounded by K or R, which is typical of cleavage sites and suggests bioactivity; however, the terminal G, allowing an amidation process in metazoan, seems to be replaced by W. Using synthetic peptides, we showed that amidation appears unnecessary to bioactivity in A. thaliana, and we provide evidence that these putative FLPs could be involved in physiological processes related to hyperosmotic stress responses in plants, urging further studies on this topic.

Deciphering early events involved in hyperosmotic stress-induced programmed cell death in tobacco BY-2 cells

Hyperosmotic stresses represent one of the major constraints that adversely affect plants growth, development, and productivity. In this study, the focus was on early responses to hyperosmotic stress- (NaCl and sorbitol) induced reactive oxygen species (ROS) generation, cytosolic Ca(2+) concentration ([Ca(2+)]cyt) increase, ion fluxes, and mitochondrial potential variations, and on their links in pathways leading to programmed cell death (PCD). By using BY-2 tobacco cells, it was shown that both NaCl- and sorbitol-induced PCD seemed to be dependent on superoxide anion (O2·(-)) generation by NADPH-oxidase. In the case of NaCl, an early influx of sodium through non-selective cation channels participates in the development of PCD through mitochondrial dysfunction and NADPH-oxidase-dependent O2·(-) generation. This supports the hypothesis of different pathways in NaCl- and sorbitol-induced cell death. Surprisingly, other shared early responses, such as [Ca(2+)]cyt increase and singlet oxygen production, do not seem to be involved in PCD.

Diverse functional interactions between nitric oxide and abscisic acid in plant development and responses to stress

The extensive support for abscisic acid (ABA) involvement in the complex regulatory networks controlling stress responses and development in plants contrasts with the relatively recent role assigned to nitric oxide (NO). Because treatment with exogenous ABA leads to enhanced production of NO, it has been widely considered that NO participates downstream of ABA in controlling processes such as stomata movement, seed dormancy, and germination. However, data on leaf senescence and responses to stress suggest that the functional interaction between ABA and NO is more complex than previously thought, including not only cooperation but also antagonism. The functional relationship is probably determined by several factors including the time- and place-dependent pattern of accumulation of both molecules, the threshold levels, and the regulatory factors important for perception. These factors will determine the actions exerted by each regulator. Here, several examples of well-documented functional interactions between NO and ABA are analysed in light of the most recent reported data on seed dormancy and germination, stomata movements, leaf senescence, and responses to abiotic and biotic stresses.

A role for oxalic acid generation in ozone-induced signallization in Arabidopis cells

Ozone (O(3) ) is an air pollutant with an impact increasingly important in our industrialized world. It affects human health and productivity in various crops. We provide the evidences that treatment of Arabidopsis thaliana with O(3) results in ascorbate-derived oxalic acid production. Using cultured cells of A. thaliana as a model, here we further showed that oxalic acid induces activation of anion channels that trigger depolarization of the cell, increase in cytosolic Ca(2+) concentration, generation of reactive oxygen species and cell death. We confirmed that O(3) reacts with ascorbate in the culture, thus resulting in production of oxalic acid and this could be part of the O(3) -induced signalling pathways that trigger programmed cell death.

Ryanodine receptor calcium release channels: an evolutionary perspective

Ryanodine receptors (RyRs), along with the related inositol 1,4,5-trisphosphate receptors (IP(3)Rs), mediate the release of Ca(2+) from intracellular organelles of eukaryotes. As discussed in other chapters, such increases in intracellular Ca(2+) levels act a fundamental second messenger, regulating a diverse array of cellular processes. For over two decades, it has been reported that vertebrates express multiple RYR genes, whereas non-vertebrate multicellular organisms possess a single homologue within their genomes. Recently, the existence of RyR-like channels in unicellular organisms has also been reported. This chapter exploits recent expansions in available genome data to generate an overview of the expression of RyR-like genes in organisms representing a broad range of viral, archaeal, bacterial and eukaryotic taxa. Analyses of the multidomain structures and phylogenetic relationships of these proteins has lead to a model in which, early during eukaryotic evolution, IP(3)R-like ancestral Ca(2+) release channels were converted to RyR proteins via the addition of promiscuous protein domains, possibly via horizontal gene transfer mechanisms.

The cellular language of myo-inositol signaling

The simple polyol, myo-inositol, is used as a building block of a cellular language that plays various roles in signal transduction. This review describes the terminology used to denote myo-inositol-containing molecules, with an emphasis on how phosphate and fatty acids are added to create second messengers used in signaling. Work in model systems has delineated the genes and enzymes required for synthesis and metabolism of many myo-inositol-containing molecules, with genetic mutants and measurement of second messengers playing key roles in developing our understanding. There is increasing evidence that molecules such as myo- inositol(1,4,5)trisphosphate and phosphatidylinositol(4,5)bisphosphate are synthesized in response to various signals plants encounter. In particular, the controversial role of myo-inositol(1,4,5)trisphosphate is addressed, accompanied by a discussion of the multiple enzymes that act to regulate this molecule. We are also beginning to understand new connections of myo-inositol signaling in plants. These recent discoveries include the novel roles of inositol phosphates in binding to plant hormone receptors and that of phosphatidylinositol(3)phosphate binding to pathogen effectors.

Abscisic acid deficiency of developing pea embryos achieved by immunomodulation attenuates developmental phase transition and storage metabolism

The transition of pea embryos from pre-storage to maturation is partially controlled by abscisic acid (ABA). Immunomodulation in pea embryos specifically reduces free ABA levels during transition stages. Such seeds are, therefore, suitable models for studying ABA deficiency by global transcript and metabolite analysis. Compared with the wild type, anti-ABA seeds are smaller, contain fewer globulins and show lower dry matter accumulation and delayed differentiation. Free sugars are decreased, indicating lower uptake and/or elevated mobilisation. Lower levels of trans-zeatins suggest that ABA reduction influences rates of cytokinin synthesis and/or its level of accumulation. Abscisic acid deficiency leads to a general downregulation of gene expression related to transcription and translation. At the transcriptional level, anti-ABA embryos reveal a wide-range repression of carbohydrate oxidation, downregulated sucrose mobilisation, glycolysis and the tricarboxylic acid cycle/Krebs cycle (TCA cycle). Genes related to starch, amino acid and storage protein biosynthesis are downregulated, indicating a general decrease in metabolic fluxes. We conclude that during embryo differentiation ABA triggers broad upregulation of gene activity and genetic reprogramming, involving regulated protein degradation via the ubiquitin/proteasome system. Abscisic acid deficiency affects gene expression associated with transport processes and stimulation of membrane energisation. Our study identified mediators and downstream signalling elements of ABA during embryo differentiation, such as the transcription factor FUSCA3, SnRK1 kinase and Ca(2+) signalling processes. This suggests that ABA interacts with SnRK1 complexes, thus connecting SnRK1, sugar and stress signalling with ABA. Certain protein kinases/phosphatases known to negatively respond to ABA are upregulated in the modulated line, whilst those which respond positively are downregulated, pointing to a highly coordinated response of the gene network to ABA levels.

Increased anion channel activity is an unavoidable event in ozone-induced programmed cell death

BACKGROUND

Ozone is a major secondary air pollutant often reaching high concentrations in urban areas under strong daylight, high temperature and stagnant high-pressure systems. Ozone in the troposphere is a pollutant that is harmful to the plant.

PRINCIPAL FINDINGS

By exposing cells to a strong pulse of ozonized air, an acute cell death was observed in suspension cells of Arabidopsis thaliana used as a model. We demonstrated that O(3) treatment induced the activation of a plasma membrane anion channel that is an early prerequisite of O(3)-induced cell death in A. thaliana. Our data further suggest interplay of anion channel activation with well known plant responses to O(3), Ca(2+) influx and NADPH-oxidase generated reactive oxygen species (ROS) in mediating the oxidative cell death. This interplay might be fuelled by several mechanisms in addition to the direct ROS generation by O(3); namely, H(2)O(2) generation by salicylic and abscisic acids. Anion channel activation was also shown to promote the accumulation of transcripts encoding vacuolar processing enzymes, a family of proteases previously reported to contribute to the disruption of vacuole integrity observed during programmed cell death.

SIGNIFICANCE

Collectively, our data indicate that anion efflux is an early key component of morphological and biochemical events leading to O(3)-induced programmed cell death. Because ion channels and more specifically anion channels assume a crucial position in cells, an understanding about the underlying role(s) for ion channels in the signalling pathway leading to programmed cell death is a subject that warrants future investigation.