Plant Cell Signaling Research Articles

Regulation of Nicotiana tabacum osmotic stress-activated protein kinase and its cellular partner GAPDH by nitric oxide in response to salinity

Several studies focusing on elucidating the mechanism of NO (nitric oxide) signalling in plant cells have highlighted that its biological effects are partly mediated by protein kinases. The identity of these kinases and details of how NO modulates their activities, however, remain poorly investigated. In the present study, we have attempted to clarify the mechanisms underlying NO action in the regulation of NtOSAK (Nicotiana tabacum osmotic stress-activated protein kinase), a member of the SNF1 (sucrose non-fermenting 1)-related protein kinase 2 family. We found that in tobacco BY-2 (bright-yellow 2) cells exposed to salt stress, NtOSAK is rapidly activated, partly through a NO-dependent process. This activation, as well as the one observed following treatment of BY-2 cells with the NO donor DEA/NO (diethylamine-NONOate), involved the phosphorylation of two residues located in the kinase activation loop, one being identified as Ser158. Our results indicate that NtOSAK does not undergo the direct chemical modifications of its cysteine residues by S-nitrosylation. Using a co-immunoprecipitation-based strategy, we identified several proteins present in immunocomplex with NtOSAK in salt-treated cells including the glycolytic enzyme GAPDH (glyceraldehyde-3-phosphate dehydrogenase). Our results indicate that NtOSAK directly interacts with GAPDH in planta. Furthermore, in response to salt, GAPDH showed a transient increase in its S-nitrosylation level which was correlated with the time course of NtOSAK activation. However, GADPH S-nitrosylation did not influence its interaction with NtOSAK and did not have an impact on the activity of the protein kinase. Taken together, the results support the hypothesis that NtOSAK and GAPDH form a cellular complex and that both proteins are regulated directly or indirectly by NO.

Sugar signalling and antioxidant network connections in plant cells

Sugars play important roles as both nutrients and regulatory molecules throughout plant life. Sugar metabolism and signalling function in an intricate network with numerous hormones and reactive oxygen species (ROS) production, signalling and scavenging systems. Although hexokinase is well known to fulfil a crucial role in glucose sensing processes, a scenario is emerging in which the catalytic activity of mitochondria-associated hexokinase regulates glucose-6-phosphate and ROS levels, stimulating antioxidant defence mechanisms and the synthesis of phenolic compounds. As a new concept, it can be hypothesized that the synergistic interaction of sugars (or sugar-like compounds) and phenolic compounds forms part of an integrated redox system, quenching ROS and contributing to stress tolerance, especially in tissues or organelles with high soluble sugar concentrations.

Signaling in plants

This book provides detailed coverage of a wide range of aspects of signalling and emphasizes the critical importance of such signalling to plants, which as sessile organisms are at the mercy of their biotic and abiotic environment. The book provides insight into how plants can respond and overcome such challenges. The dynamism of plant signalling pathways are clearly demonstrated, for example in cell wall composition, in which mutants are described that result in different wall compositions and relative amounts of wall components can be altered, indicating a sensing-and-response network to this pathway. The book breaks plant cells down into their varying components and provides individual chapters describing how different aspects of the cell relate to whole-plant physiology. This provides the reader with the opportunity to focus on specific aspects of signalling, or to get to grips with various aspects in manageable pieces. Given that the book fragments the different signalling mechanisms into ‘bite-sized pieces’ it also achieves good integration of these pieces. The authors link very well between the different chapters with citations of relevant sections from other chapters covering related topics. This makes following links between different responses and stimuli easier. Chapters are included that provide detailed analysis of specific signalling molecules, with particular focus paid to auxin and calcium, and they also cover situations where perception of changing environments are critical, such as tropic responses and stress responses. The fundamental role of auxin is described in a number of chapters. This is particularly evident in the chapter focusing on the specific role of auxin signalling, for example the role of auxin associated with modifications to the cell wall and gravitropism. The book provides a valuable perspective to the dynamic nature of plants. It clearly demonstrates the complexity of the response networks and shows how plants are able to respond to the changing environments they face, both on a daily and seasonal basis, but also under conditions of stress, including pathogen attack, oxygen deprivation and also in symbiotic associations, such as with rhizobial bacteria. The book also provides a good perspective on how different aspects of cells interact with each other, and the dynamism of the actin cytoskeleton in accommodating growth and development is an excellent example of this. This chapter illustrates how the actin cytoskeleton serves as a signalling hub that regulates numerous activities, such as subcellular compartmentation, organelle and vesicle mobility and structural reinforcement. The book provides an excellent historical perspective to signalling in plants; this is put into context with recent developments and the use of molecular genetic analysis to confirm hypotheses that were made by physiological observations. In addition, most of the chapters also discuss recent developments and provide observations on where the key targets for future research currently are. This provides the reader with a focused outlook on how the fields are developing and what are the major questions that can be addressed with the new post-genomics tools. The illustrations are of high quality; although some chapters have only a limited number of figures and pathways included, these provide valuable information and aid the reader in understanding the complex interactions and pathways that are being discussed. Overall this volume provides a very focused discussion of key topics of plant signalling and provides the reader with good opportunities to follow the specific areas in more detail. It will serve as an excellent resource for advanced graduate students and post-graduates who wish to obtain a detailed insight into plant cell signalling.

The EVERSHED receptor-like kinase modulates floral organ shedding inArabidopsis

Plant cell signaling triggers the abscission of entire organs, such as fruit, leaves and flowers. Previously, we characterized an ADP-ribosylation factor GTPase-activating protein, NEVERSHED (NEV), that regulates membrane trafficking and is essential for floral organ shedding in Arabidopsis. Through a screen for mutations that restore organ separation in nev flowers, we have identified a leucine-rich repeat receptor-like kinase, EVERSHED (EVR), that functions as an inhibitor of abscission. Defects in the Golgi structure and location of the trans-Golgi network in nev abscission zone cells are rescued by a mutation in EVR, suggesting that EVR might regulate membrane trafficking during abscission. In addition to shedding their floral organs prematurely, nev evr flowers show enlarged abscission zones. A similar phenotype was reported for plants ectopically expressing INFLORESCENCE DEFICIENT IN ABSCISSION, a predicted signaling ligand for the HAESA/HAESA-LIKE2 receptor-like kinases, indicating that this signaling pathway may be constitutively active in nev evr flowers. We present a model in which EVR modulates the timing and region of abscission by promoting the internalization of other receptor-like kinases from the plasma membrane.

Nuclear protein kinases: still enigmatic components in plant cell signalling

Plants constantly face changing conditions in their environment. Unravelling the transduction mechanisms from signal perception at the plasma membrane level down to gene expression in the nucleus is a fascinating challenge. Protein phosphorylation, catalysed by protein kinases, is one of the major posttranslational modifications involved in the specificity, kinetic(s) and intensity of a signal transduction pathway. Although commonly assumed, the involvement of nuclear protein kinases in signal transduction is often poorly characterized. In particular, both their regulation and mode of action remain to be elucidated and may lead to the unveiling of new original mechanisms. For example, unlike animal cells, plant cells contain only a few strictly nucleus-localized protein kinases, which calls into question the role of this cellular distribution between the cytosol and the nucleus in their activation and functions. The control of their nucleocytoplasmic trafficking appears to play a major role in their regulation, probably through promoting interactions with their substrates under specific cellular conditions. However, recent findings showing that the nucleus can generate complex networks of second messengers (e.g. Ca(2+)or diacyglycerol) suggest that nuclear protein kinases could play an active role in the decoding of such signals.

Actin Dynamics Regulates Voltage‐Dependent Calcium‐Permeable Channels of the Vicia faba Guard Cell Plasma Membrane

Free cytosolic Ca(2+) ([Ca(2+)](cyt)) is an ubiquitous second messenger in plant cell signaling, and [Ca(2+)](cyt) elevation is associated with Ca(2+)-permeable channels in the plasma membrane and endomembranes regulated by a wide range of stimuli. However, knowledge regarding Ca(2+) channels and their regulation remains limited in planta. A type of voltage-dependent Ca(2+)-permeable channel was identified and characterized for the Vicia faba L. guard cell plasma membrane by using patch-clamp techniques. These channels are permeable to both Ba(2+) and Ca(2+), and their activities can be inhibited by micromolar Gd(3+). The unitary conductance and the reversal potential of the channels depend on the Ca(2+) or Ba(2+) gradients across the plasma membrane. The inward whole-cell Ca(2+) (Ba(2+)) current, as well as the unitary current amplitude and NP(o) of the single Ca(2+) channel, increase along with the membrane hyperpolarization. Pharmacological experiments suggest that actin dynamics may serve as an upstream regulator of this type of calcium channel of the guard cell plasma membrane. Cytochalasin D, an actin polymerization blocker, activated the NPo of these channels at the single channel level and increased the current amplitude at the whole-cell level. But these channel activations and current increments could be restrained by pretreatment with an F-actin stabilizer, phalloidin. The potential physiological significance of this regulatory mechanism is also discussed.

Control of Nuclear and Nucleolar Localization of Nuclear Inclusion Protein a of Picorna-LikePotato virus AinNicotianaSpecies

The multifunctional nuclear inclusion protein a (NIa) of potyviruses (genus Potyvirus; Potyviridae) accumulates in the nucleus of virus-infected cells for unknown reasons. In this study, two regions in the viral genome-linked protein (VPg) domain of NIa in Potato virus A (PVA) were found to constitute nuclear and nucleolar localization signals (NLS) in plant cells (Nicotiana spp). Amino acid substitutions in both NLS I (residues 4 to 9) and NLS II (residues 41 to 50) prevented nuclear localization, whereas mutations in either single NLS did not. Mutations in either NLS, however, prevented nucleolar localization and prevented or diminished virus replication in protoplasts, accumulation in infected plant tissues, and/or systemic movement in plants. One NLS mutant was partially complemented by the wild-type VPg expressed in transgenic plants. Furthermore, NLS I controlled NIa accumulation in Cajal bodies. The VPg domain interacted with fibrillarin, a nucleolar protein, and depletion of fibrillarin reduced PVA accumulation. Overexpression of VPg in leaf tissues interfered with cosuppression of gene expression (i.e., RNA silencing), whereas NLS I and NLS II mutants, which exhibited reduced nuclear and nucleolar localization, showed no such activity. These results demonstrate that some of the most essential viral functions required for completion of the infection cycle are tightly linked to regulation of the NIa nuclear and nucleolar localization.

Microtubule reorganization as a response to implementation of NO signals in plant cells

The effects of an exogeneous NO donor, sodium nitroprusside, on the orientation and organization of cortical microtubules in Arabidopsis thaliana root cells expressing GFP-MAP4 were studied in vivo. It was found that sodium nitroprusside treatment (10–500 μM, 24 h) caused the acceleration of primary root growth and enhanced initiation of root hairs in the differentiation zone. The influence of sodium nitroprusside revealed in changes in the orientation and organization of cortical microtubules in different types of cells of A. thaliana root. The most sensitive to sodium nitroprusside exposure were microtubules in epidermal cells of the elongation zone, where native transverse orientation of cortical microtubules turned into random, oblique, or longitudinal relative to the primary root axis. We suggest that NO, as one of the intracellular secondary messengers, triggers cell differentiation by reorientation of cortical microtubules, possibly via tubulin nitrotyrosination.

A systems approach uncovers restrictions for signal interactions regulating genome-wide responses to nutritional cues in Arabidopsis.

As sessile organisms, plants must cope with multiple and combined variations of signals in their environment. However, very few reports have studied the genome-wide effects of systematic signal combinations on gene expression. Here, we evaluate a high level of signal integration, by modeling genome-wide expression patterns under a factorial combination of carbon (C), light (L), and nitrogen (N) as binary factors in two organs (O), roots and leaves. Signal management is different between C, N, and L and in shoots and roots. For example, L is the major factor controlling gene expression in leaves. However, in roots there is no obvious prominent signal, and signal interaction is stronger. The major signal interaction events detected genome wide in Arabidopsis roots are deciphered and summarized in a comprehensive conceptual model. Surprisingly, global analysis of gene expression in response to C, N, L, and O revealed that the number of genes controlled by a signal is proportional to the magnitude of the gene expression changes elicited by the signal. These results uncovered a strong constraining structure in plant cell signaling pathways, which prompted us to propose the existence of a “code” of signal integration.

Plant ion channels: gene families, physiology, and functional genomics analyses.

Distinct potassium, anion, and calcium channels in the plasma membrane and vacuolar membrane of plant cells have been identified and characterized by patch clamping. Primarily owing to advances in Arabidopsis genetics and genomics, and yeast functional complementation, many of the corresponding genes have been identified. Recent advances in our understanding of ion channel genes that mediate signal transduction and ion transport are discussed here. Some plant ion channels, for example, ALMT and SLAC anion channel subunits, are unique. The majority of plant ion channel families exhibit homology to animal genes; such families include both hyperpolarization- and depolarization-activated Shaker-type potassium channels, CLC chloride transporters/channels, cyclic nucleotide-gated channels, and ionotropic glutamate receptor homologs. These plant ion channels offer unique opportunities to analyze the structural mechanisms and functions of ion channels. Here we review gene families of selected plant ion channel classes and discuss unique structure-function aspects and their physiological roles in plant cell signaling and transport.

Phosphatidylinositol (4,5)bisphosphate inhibits K+-efflux channel activity in NT1 tobacco cultured cells.

In the animal world, the regulation of ion channels by phosphoinositides (PIs) has been investigated extensively, demonstrating a wide range of channels controlled by phosphatidylinositol (4,5)bisphosphate (PtdInsP2). To understand PI regulation of plant ion channels, we examined the in planta effect of PtdInsP2 on the K+-efflux channel of tobacco (Nicotiana tabacum), NtORK (outward-rectifying K channel). We applied a patch clamp in the whole-cell configuration (with fixed "cytosolic" Ca2+ concentration and pH) to protoplasts isolated from cultured tobacco cells with genetically manipulated plasma membrane levels of PtdInsP2 and cellular inositol (1,4,5)trisphosphate: "Low PIs" had depressed levels of these PIs, and "High PIs" had elevated levels relative to controls. In all of these cells, K channel activity, reflected in the net, steady-state outward K+ currents (IK), was inversely related to the plasma membrane PtdInsP2 level. Consistent with this, short-term manipulations decreasing PtdInsP2 levels in the High PIs, such as pretreatment with the phytohormone abscisic acid (25 microM) or neutralizing the bath solution from pH 5.6 to pH 7, increased IK (i.e. NtORK activity). Moreover, increasing PtdInsP2 levels in controls or in abscisic acid-treated high-PI cells, using the specific PI-phospholipase C inhibitor U73122 (2.5-4 microM), decreased NtORK activity. In all cases, IK decreases stemmed largely from decreased maximum attainable NtORK channel conductance and partly from shifted voltage dependence of channel gating to more positive potentials, making it more difficult to activate the channels. These results are consistent with NtORK inhibition by the negatively charged PtdInsP2 in the internal plasma membrane leaflet. Such effects are likely to underlie PI signaling in intact plant cells.

MEK1/2 and p38-like MAP kinase successively mediate H2O2 signaling in Vicia guard cell

As a second messenger, H2O2 generation and signal transduction is subtly controlled and involves various signal elements, among which the members of MAP kinase family. The increasing evdences indicated that both MEK1/2 and p38-like MAP protein kinase have been shown to mediate ABA-induced H2O2 signaling in plant cells. Here we analyze the mechanisms of similarity and difference between MEK1/2 and p38-like MAP protein kinase in mediating ABA-induced H2O2 generation, inhibition of inward K+ currents, and stomatal closure, suggesting that activation of MEK1/2 is prior to p38-like protein kinase in Vicia guard cells.Addendum to: Jing J, Peng-Tao W, Guo-Yong A, Peng-Cheng W, Chun-Peng S. The involvement of a P38-like MAP kinase in ABA-induced and H2O2-mediated stomatal closure in Vicia faba L. Plant Cell Rep 2008; 27:377-85, DOI: 10.1007/s00299-007-0449-x.

Pollen generates nitric oxide and nitrite: a possible link to pollen-induced allergic responses

Reactive nitrogen species (RNS), such as nitric oxide (NO), are ubiquitous and diverse signalling molecules involved in a wide range of physiological and pathophysiological processes in both animals and plants. Nitrite, a metabolite of NO turnover, has also been recently characterised as an important mediator of fundamental physiological mechanisms in mammalian cells, and is a substrate for NO production in several plant cell signalling processes. A previous study demonstrated that during plant reproductive processes, intracellular NO is produced by pollen, and that such NO could be important in signalling interactions between pollen and stigma. The aim of this study was to establish whether pollen releases NO and nitrite, using a wide range of plant species. Using a fluorimetric assay in conjunction with electron paramagnetic resonance (EPR) spectroscopy, the present study demonstrated that all hydrating pollen examined released NO, although some appeared to have more activity than others. Additionally, gas phase ozone-based chemiluminescence data showed that nitrite is also released from hydrating pollen. Given that pollen has interactions with other cells, for example in allergenic rhinitis (hay fever) in humans, it suggests that NO might be involved in mediating the responses of both plant and animal cells to pollen. These findings may have important implications for future allergy research, as it is possible that pollen-derived NO and nitrite may impact on mammalian cells during pollen-induced allergic responses.

Involvement of sphingosine kinase in plant cell signalling

In mammalian cells sphingosine-1-phosphate (S1P) is a well-established messenger molecule that participates in a wide range of signalling pathways. The objective of the work reported here was to investigate the extent to which phosphorylated long-chain sphingoid bases, such as sphingosine-1-phosphate and phytosphingosine-1-phosphate (phytoS1P) are used in plant cell signalling. To do this, we manipulated Arabidopsis genes capable of metabolizing these messenger molecules. We show that Sphingosine kinase1 (SPHK1) encodes an enzyme that phosphorylates sphingosine, phytosphingosine and other sphingoid long-chain bases. The stomata of SPHK1-KD Arabidopsis plants were less sensitive, whereas the stomata of SPHK1-OE plants were more sensitive, than wild type to ABA. The rate of germination of SPHK1-KD was enhanced, whereas the converse was true for SPHK1-OE seed. Reducing expression of either the putative Arabidopsis S1P phosphatase (SPPASE) or the DPL1 gene, which encodes an enzyme with S1P lyase activity, individually, had no effect on guard-cell ABA signalling; however, stomatal responses to ABA in SPPASEDPL1 RNAi plants were compromised. Reducing the expression of DPL1 had no effect on germination; however, germination of SPPASE RNAi seeds was more sensitive to applied ABA. We also found evidence that expression of SPHK1 and SPPASE were coordinately regulated, and discuss how this might contribute to robustness in guard-cell signalling. In summary, our data establish SPHK1 as a component in two separate plant signalling systems, opening the possibility that phosphorylated long-chain sphingoid bases such as S1P and phytoS1P are ubiquitous messengers in plants.

Unwrapping the Laccaria genome

Unwrapping the <i>Laccaria</i> genome

Lateral root emergence: A paradigm for cell signaling in plants

Lateral root emergence: A paradigm for cell signaling in plants

The role of sphingosine kinase in plant cell signalling

The role of sphingosine kinase in plant cell signalling

Reactive oxygen species, antioxidants and signaling in plants

Several reactive oxygen species (ROS) are continuously produced in plants as byproducts of many metabolic reactions, such as photosynthesis, photo respiration and respiration, Depending on the nature of the ROS species, some are highly toxic and rapidly detoxified by various cellular enzymatic and nonenzymatic mechanisms. Oxidative stress occurs when there is a serious imbalance between the production of ROS and antioxidative defence. ROS participate in signal transduction, but also modify cellular components and cause damage. ROS is highly reactive molecules and can oxidize all types of cellular components. Various enzymes involved in ROS-scavenging have been manipulated and over expressed or down regulated. An overview of the literature is presented in terms of primary antioxidant free radical scavenging and redox signaling in plant cells. Special attention is given to ROS and ROS-anioxidant interaction as a metabolic interface for different types of signals derived from metabolisms and from the changing environment.

Higher plant antioxidants and redox signaling under environmental stresses

Main antioxidants in higher plants include glutathione, ascorbate, tocopherol, proline, betaine, and others, which are also information-rich redox buffers and important redox signaling components that interact with biomembrane-related compartments. As an evolutionary consequence of aerobic life for higher plants, reactive oxygen species (ROS) are formed by partial reduction of molecular oxygen. The above enzymatic and non-enzymatic antioxidants in higher plants can protect their cells from oxidative damage by scavenging ROS. In addition to crucial roles in defense system and as enzyme cofactors, antioxidants influence higher plant growth and development by modifying processes from mitosis and cell elongation to senescence and death. Most importantly, they provide essential information on cellular redox state, and regulate gene expression associated with biotic and abiotic stress responses to optimize defense and survival. An overview of the literature is presented in terms of main antioxidants and redox signaling in plant cells. Special attention is given to ROS and ROS-antioxidant interaction as a metabolic interface for different types of signals derived from metabolism and from the changing environment, which regulates the appropriate induction of acclimation processes or, execution of cell death programs, which are the two essential directions for higher plants. To cite this article: H.-B. Shao et al., C. R. Biologies 331 (2008).

Primary antioxidant free radical scavenging and redox signaling pathways in higher plant cells.

Antioxidants in plant cells mainly include glutathione, ascorbate, tocopherol, proline, betaine and others, which are also information-rich redox buffers and important redox signaling components that interact with cellular compartments. As an unfortunate consequence of aerobic life for higher plants, reactive oxygen species (ROS) are formed by partial reduction of molecular oxygen. The above enzymatic and non-enzymatic antioxidants in higher plant cells can protect their cells from oxidative damage by scavenging ROS. In addition to crucial roles in defense system and as enzyme cofactors, antioxidants influence higher plant growth and development by modifying processes from miotosis and cell elongation to senescence and death. Most importantly, they provide essential information on cellular redox state, and regulate gene expression associated with biotic and abiotic stress responses to optimize defense and survival. An overview of the literature is presented in terms of primary antioxidant free radical scavenging and redox signaling in plant cells. Special attention is given to ROS and ROS-anioxidant interaction as a metabolic interface for different types of signals derived from metabolisms and from the changing environment. This interaction regulates the appropriate induction of acclimation processes or execution of cell death programs, which are the two essential directions for higher plant cells.