Signaling In Higher Plants Research Articles

Herbivore-Triggered Electrophysiological Reactions: Candidates for Systemic Signals in Higher Plants and the Challenge of Their Identification.

In stressed plants, electrophysiological reactions (elRs) are presumed to contribute to long-distance intercellular communication between distant plant parts. Because of the focus on abiotic stress-induced elRs in recent decades, biotic stress-triggered elRs have been widely ignored. It is likely that the challenge to identify the particular elR types (action potential [AP], variation potential, and system potential [SP]) was responsible for this course of action. Thus, this survey focused on insect larva feeding (Spodoptera littoralis and Manduca sexta) that triggers distant APs, variation potentials, and SPs in monocotyledonous and dicotyledonous plant species (Hordeum vulgare, Vicia faba, and Nicotiana tabacum). APs were detected only after feeding on the stem/culm, whereas SPs were observed systemically following damage to both stem/culm and leaves. This was attributed to the unequal vascular innervation of the plant and a selective electrophysiological connectivity of the plant tissue. However, striking variations in voltage patterns were detected for each elR type. Further analyses (also in Brassica napus and Cucurbita maxima) employing complementary electrophysiological approaches in response to different stimuli revealed various reasons for these voltage pattern variations: an intrinsic plasticity of elRs, a plant-specific signature of elRs, a specific influence of the applied (a)biotic trigger, the impact of the technical approach, and/or the experimental setup. As a consequence, voltage pattern variations, which are not irregular but rather common, need to be included in electrophysiological signaling analysis. Due to their widespread occurrence, systemic propagation, and respective triggers, elRs should be considered as candidates for long-distance communication in higher plants.

Cation channels are involved in brassinosteroid signalling in higher plants

Brassinosteroids (BRs) are an important class of plant hormones with a multitude of functions. They have been intensively investigated for their biosynthesis, distribution and physiological functions. The aim of this study was to examine possible effects of BRs on the plant plasma membrane cation conductances and Ca2+ signalling. The wheat root protoplasts (tested by patch-clamping) and excised arabidopsis roots (analysed by Ca2+-aequorin chemiluminometry), were used. In the whole-cell plasma membrane patches, 24-epibrassinolide, 28-homobrassionolide or 24-epicastasterone (1μM) were applied exogenously. 24-Epicastasterone increased the activity of the K+ efflux conductance in 50% of tested protoplasts while 24-epibrassonolide and 28-homobrassionolide did not modify the plasma membrane currents. Addition of 24-epicastasterone at the cytosolic side (to the pipette solution) resulted in dramatic stimulation of a time-dependent K+ efflux current (in 30% of protoplasts) and an activation of Ca2+ influx currents (in 30% of protoplasts). Gadolinium ions, which are blockers of cation channels, inhibited the 24-epicastasterone-induced cation channel activities. In Arabidopsis thaliana plants constitutively expressing aequorin, exogenous 24-epibrassonolide, 28-homobrassionolide and 24-epicastasterone induced a transient elevation of the cytosolic free Ca2+, which was inhibited by Gd3+ and mediated by Ca2+ influx from the bathing solution. In Ca2+-aequorin tests, 10μM of exogenous BRs was the minimal concentration at which statistically significant changes of the cytosolic Ca2+ were observed. In conclusion, the obtained results suggest that the plasma membrane of root cells contains the brassinosteroid-activated cation-permeable channels, which can probably be involved in rapid regulation of the K+ homeostasis and Ca2+ signalling.

A tidal wave of signals: calcium and ROS at the forefront of rapid systemic signaling.

Systemic signaling pathways enable multicellular organisms to prepare all of their tissues and cells to an upcoming challenge that may initially only be sensed by a few local cells. They are activated in plants in response to different stimuli including mechanical injury, pathogen infection, and abiotic stresses. Key to the mobilization of systemic signals in higher plants are cell-to-cell communication events that have thus far been mostly unstudied. The recent identification of systemically propagating calcium (Ca(2+)) and reactive oxygen species (ROS) waves in plants has unraveled a new and exciting cell-to-cell communication pathway that, together with electric signals, could provide a working model demonstrating how plant cells transmit long-distance signals via cell-to-cell communication mechanisms. Here, we summarize recent findings on the ROS and Ca(2+) waves and outline a possible model for their integration.

Citrus Greening (Huanglongbing): Fast Electrochemical Detection and Phytomonitoring of the Trees Diseases

Citrus greening is probably the most important disease of citrus in the world. Here we show that citrus greening infection induces a very strong decrease in the amplitude of the electrical potential difference in a citrus tree tissue or in a fruit. The non-disruptive method, consisting of a data acquisition system interfaced with computer and identical nonpolarizable reversible electrodes connected to trees, was used for the measuring of electrical potentials and the detection of citrus greening in a tree’s tissue. The amplitude and time dependence of a registered electrical potential difference in a leaf, stem or fruit is sensitive to a tree’s health. Infected trees have a very low DC signal in comparison with healthy citrus trees. This electrophysiological method permits fast detection of citrus greening in citrus trees and can be used for a detection of other diseases in plants and trees. Monitoring the electrical signaling in higher plants represents a promising method to investigate fast electrical communication during environmental changes.

Heat Stress Tolerance and Signaling in Plants

As a consequence of global warming, increasing temperature is a serious threat to crop production worldwide. Studies on plant stress tolerance and signaling are of important significance in both basic research and grain production. In recent years, major advances have been made in better understanding of heat stress signaling in higher plants, including several aspects such as sensory mechanisms of heat stress, regulatory networks involved in heat-responsive activation of <italic>Hsf</italic> and <italic>HSP</italic> genes, functional characterization of HSBP in establishment of heat tolerance in plants. As a harmful effect on cellular homeostasis, heat stress destabilizes membrane systems and structures of RNA and proteins, and leads to a dramatic alteration in enzymatic activities and cytoskeleton systems. Transcriptome analysis revealed that about 2% genes in the whole genome were activated in response to heat stress. Heat stress response represents the first line of inducible defense against imbalances in cellular homeostasis. Heat tolerance in plants is classified into basal thermotolerance and acquired thermotolerance. The acquired thermotolerance reflects the physiological adaptation of plants to heat stress when grown in the field.

Light intensity-dependent retrograde signalling in higher plants

Plants are able to acclimate to highly fluctuating light environment and evolved a short- and long-term light acclimatory responses, that are dependent on chloroplasts retrograde signalling. In this review we summarise recent evidences suggesting that the chloroplasts act as key sensors of light intensity changes in a wide range (low, high and excess light conditions) as well as sensors of darkness. They also participate in transduction and synchronisation of systemic retrograde signalling in response to differential light exposure of distinct leaves. Regulation of intra- and inter-cellular chloroplast retrograde signalling is dependent on the developmental and functional stage of the plastids. Therefore, it is discussed in following subsections: firstly, chloroplast biogenic control of nuclear genes, for example, signals related to photosystems and pigment biogenesis during early plastid development; secondly, signals in the mature chloroplast induced by changes in photosynthetic electron transport, reactive oxygen species, hormones and metabolite biosynthesis; thirdly, chloroplast signalling during leaf senescence. Moreover, with a help of meta-analysis of multiple microarray experiments, we showed that the expression of the same set of genes is regulated specifically in particular types of signals and types of light conditions. Furthermore, we also highlight the alternative scenarios of the chloroplast retrograde signals transduction and coordination linked to the role of photo-electrochemical signalling.

Potassium Transport and Signaling in Higher Plants

As one of the most important mineral nutrient elements, potassium (K(+)) participates in many plant physiological processes and determines the yield and quality of crop production. In this review, we summarize K(+) signaling processes and K(+) transport regulation in higher plants, especially in plant responses to K(+)-deficiency stress. Plants perceive external K(+) fluctuations and generate the initial K(+) signal in root cells. This signal is transduced into the cytoplasm and encoded as Ca(2+) and reactive oxygen species signaling. K(+)-deficiency-induced signals are subsequently decoded by cytoplasmic sensors, which regulate the downstream transcriptional and posttranslational responses. Eventually, plants produce a series of adaptive events in both physiological and morphological alterations that help them survive K(+) deficiency.

Xenobiotic sensing and signalling in higher plants

Anthropogenic changes and chemical pollution confront plant communities with various xenobiotic compounds or combinations of xenobiotics, involving chemical structures that are at least partially novel for plant species. Plant responses to chemical challenges and stimuli are usually characterized by the approaches of toxicology, ecotoxicology, and stress physiology. Development of transcriptomics and proteomics analysis has demonstrated the importance of modifications to gene expression in plant responses to xenobiotics. It has emerged that xenobiotic effects could involve not only biochemical and physiological disruption, but also the disruption of signalling pathways. Moreover, mutations affecting sensing and signalling pathways result in modifications of responses to xenobiotics, thus confirming interference or crosstalk between xenobiotic effects and signalling pathways. Some of these changes at gene expression, regulation and signalling levels suggest various mechanisms of xenobiotic sensing in higher plants, in accordance with xenobiotic-sensing mechanisms that have been characterized in other phyla (yeast, invertebrates, vertebrates). In higher plants, such sensing systems are difficult to identify, even though different lines of evidence, involving mutant studies, transcription factor analysis, or comparative studies, point to their existence. It remains difficult to distinguish between the hypothesis of direct xenobiotic sensing and indirect sensing of xenobiotic-related modifications. However, future characterization of xenobiotic sensing and signalling in higher plants is likely to be a key element for determining the tolerance and remediation capacities of plant species. This characterization will also be of interest for understanding evolutionary dynamics of stress adaptation and mechanisms of adaptation to novel stressors.

Hydrogen sulfide donor sodium hydrosulfide-induced heat tolerance in tobacco (Nicotiana tabacum L) suspension cultured cells and involvement of Ca2+ and calmodulin

Hydrogen sulfide (H2S) is considered as a new emerging cell signal in higher plants. Hydrogen sulfide donor, sodium hydrosulfide, pretreatment significantly increased survival percentage of tobacco suspension cultured cells under heat stress and regrowth ability after heat stress, and alleviated decrease in vitality of cells, increase in electrolyte leakage and accumulation of malondialdehyde (MDA). In addition, sodium hydrosulfide-induced heat tolerance was markedly strengthened by application of exogenous Ca2+ and its ionophore A23187, respectively, while this heat tolerance was weakened by addition of Ca2+ chelator ethylene glycol-bis(b-aminoethylether)-N,N,N′,N′-tetraacetic acid (EGTA), plasma membrane channel blocker La3+, as well as calmodulin (CaM) antagonists chlorpromazine (CPZ) and trifluoperazine (TFP), respectively, but intracellular channel blocker ruthenium red (RR) did not. These results suggested that sodium hydrosulfide pretreatment could improve heat tolerance in tobacco suspension cultured cells and the acquisition of this heat tolerance requires the entry of extracellular Ca2+ into cells across the plasma membrane and the mediation of intracellular CaM.

Apparent coordination of isocitrate dehydrogenase and glutamate decarboxylase expression in early stages of tree development

The biosynthesis of 2-oxoglutarate and glutamate are key steps in the biosynthesis of nitrogen compounds and plant development. The reaction catalyzed by cytosolic isoenzyme of NADP+-linked isocitrate dehydrogenase (IDH) is also considered as the main route in the production of 2-oxoglutarate. According to its expression pattern during development, IDH is also involved in other, yet unknown, processes [1,2]. In addition to the importance of glutamate in the biosynthesis of nitrogen compounds, glutamate also serves as precursor of GABA, a molecule that is currently considered as a signal in higher plants. GABA is produced by the action of glutamate decarboxylase (GAD), a cytosolic enzyme that is regulated by Ca2+/calmodulin and pH. In contrast to IDH, that it is encoded by just one gene in most of plant genomes [2], GAD is encoded by a small family of nuclear genes [3]. The expression of IDH and GAD has been investigated during the differentiation of hypocotyl and stem in tree species. Our results indicate a coordination of the expression of IDH and GAD in developmental processes suggesting a role for 2-oxoglutarate supply and GABA synthesis during early stages of organ differentiation in trees.

Effect of Ca2+ and cAMP on protein phosphorylation in mitochondria of maize seedlings

The effect of common intracellular signals (Ca2+ and cAMP) on the activity of protein phosphorylation in mitochondria was investigated in coleoptiles of maize (Zea mays L.). Treatment of isolated mitochondria with 2 mM CaCl2 brought about an increase in the level of phosphorylation of proteins with mol ws of 74, 60, and 33 kD but considerably reduced phosphorylation of the protein with a mol wt of 51.5 kD. In the presence of Ca2+, phosphorylation of polypeptides with mol wts of 59 and 66 kD was also detected. cAMP considerably reduced phosphorylation of essentially all the investigated proteins in isolated mitochondria, which could be explained by activation of their dephosphorylation. Phosphorylation of mitochondrial proteins involves a polypeptide of about 94 kD showing kinase activity, which may be proper protein kinase or one of the subunits of a compound structure. In maize mitochondria, PP1A phosphatases were found. A hypothesis was advanced that redox-dependent phosphorylation/dephosphorylation of mitochondrial proteins plays an important role in mitochondrial signaling in higher plants.

Study of Electrophysiology of Thermal Shock in Higher Plants using High Speed Data Acquisition

Vascular plants such as the soybean plant, the Aloe vera plant, and the Mimosa pudica plant have developed mechanisms in order to respond quickly to external stimuli. Throughout the history of plant electrophysiology research, several scientists have attempted to measure the speeds of electrical signal propagation in higher plants in response to thermal stress and other stressors. Several earlier researchers produced signals which were erroneously reported as 0.1 mm/s to 20 cm/s, much slower than actual propagation speeds. These incorrect signaling data resulted from aliasing effects of antiquated data acquisition systems. In this research study, new high-speed data acquisition systems were used to obtain accurate speeds for electrical signals in higher plants. Our results show solitary waves in response to localized thermal stress, with speeds of propagation measured from a few meters per second to approximately 105 m/s [1,2]. In this study, possible mechanisms for electrical signal propagation in response to heat shock are also introduced.[1] Lang, R.D., A.G. Volkov (2008). Solitary waves in soybean induced by localized thermal stress. Plant Signal Behav 3, 224-228.[2] Volkov, A.G., R.D. Lang, M.I. Volkova-Gugeshashvili. (2007). Electrical signaling in Aloe vera induced by localized thermal stress. Bioelectrochem 71, 192-197.

System Potentials, a Novel Electrical Long-Distance Apoplastic Signal in Plants, Induced by Wounding

Systemic signaling was investigated in both a dicot (Vicia faba) and a monocot (Hordeum vulgare) plant. Stimuli were applied to one leaf (S-leaf), and apoplastic responses were monitored on a distant leaf (target; T-leaf) with microelectrodes positioned in substomatal cavities of open stomata. Leaves that had been injured by cutting and to which a variety of cations were subsequently added caused voltage transients at the T-leaf, which are neither action potentials nor variation potentials: with respect to the cell interior, the initial polarity of these voltage transients is hyperpolarizing; they do not obey the all-or-none rule but depend on both the concentration and the type of substance added and propagate at 5 to 10 cm min(-1). This response is thought to be due to the stimulation of the plasma membrane H(+)-ATPase, a notion supported by the action of fusicoccin, which also causes such voltage transients to appear on the T-leaf, whereas orthovanadate prevents their propagation. Moreover, apoplastic ion flux analysis reveals that, in contrast to action or variation potentials, all of the investigated ion movements (Ca(2+), K(+), H(+), and Cl(-)) occur after the voltage change begins. We suggest that these wound-induced "system potentials" represent a new type of electrical long-distance signaling in higher plants.

Solitary waves in soybean induced by localized thermal stress

Action potentials in higher plants are believed to be the information carriers in intercellular and intracellular communication in the presence of an environmental stressor. Plant electrophysiologists have recorded long distance electrical signaling in higher plants during the last two hundred years. Reproducing the duration, speed of propagation, and the shape of the action potential is challenging. Early measurements revealed that the speed of action potential propagation in plants is extremely slow – from 0.1 mm/s to 20 cm/s, although many faster plant responses to stress have been recorded as well. We hypothesized that this discrepancy is most likely due to the artifacts of aliasing from slow registration systems. In this study, we employ real time measurements using modern data acquisition techniques to detect ultra fast action potentials in green plants induced by localized heat stress. Thermal shock or heat stress is the most common environmental stress. Based on more sophisticated measuring techniques, we show that plants transmit solitary waves and that the speed of action potential propagation in green plants is similar to the speed of action potentials in mammalians, varying from a few meters per second up to 105 m/s. Possible pathways for electrical signal propagation in vascular plants are discussed.

Domesticated Mobile Elements Modulate Light Responses

Plants constantly gauge their environments and adjust their growth and development in order to survive. Plants can "see" their surroundings using a battery of photoreceptors to determine the color, intensity, direction, and duration of light around them. Phytochrome A (phyA) mediates various far-red light-induced responses through a cascade of signaling events. Lin et al. report that two proteins co-opted from ancestral Mutator -like transposable element(s) act as transcription factors essential for phyA-controlled gene expression. These results provide insight into the mechanisms and evolutionary history of phytochrome signaling in higher plants and lend support for a role of mobile elements as driving forces for host evolution. R. Lin, L. Ding, C. Casola, D. R. Ripoll, C. Feschotte, H. Wang, Transposase-derived transcription factors regulate light signaling in Arabidopsis . Science 318 , 1302-1305 (2007). [Abstract] [Full Text]

Involvement of phospholipid signaling in plant growth and hormone effects

Biochemical and genetics studies demonstrated the critical roles of phospholipid signaling and relevant molecules in regulating multiple processes of plant growth and development, signal transduction, mediating hormone effects and cell responses to environmental stimuli, through modulating protein subcellular localization, cross-talking with other signaling or metabolic pathways, or interacting with signaling molecules. The updated achievements of physiological effects and functional mechanism of phospholipid signaling in higher plants were reviewed.

Heterotrimeric G protein α subunit is involved in rice brassinosteroid response

Heterotrimeric G proteins are known to function as messengers in numerous signal transduction pathways. The null mutation of RGA (rice heterotrimeric G protein alpha subunit), which encodes the alpha subunit of heterotrimeric G protein in rice, causes severe dwarfism and reduced responsiveness to gibberellic acid in rice. However, less is known about heterotrimeric G protein in brassinosteroid (BR) signaling, one of the well-understood phytohormone pathways. In the present study, we used root elongation inhibition assay, lamina inclination assay and coleoptile elongation analysis to demonstrated reduced sensitivity of d1 mutant plants (caused by the null mutation of RGA) to 24-epibrassinolide (24-epiBL), which belongs to brassinosteroids and plays a wide variety of roles in plant growth and development. Moreover, RGA transcript level was decreased in 24-epiBL-treated seedlings in a dose-dependent manner. Our results show that RGA is involved in rice brassinosteroid response, which may be beneficial to elucidate the molecular mechanisms of G protein signaling and provide a novel perspective to understand BR signaling in higher plants.

The GAOLAOZHUANGREN2 gene is required for normal glucose response and development of Arabidopsis

Recent studies of glucose (Glc) sensing and signaling have revealed that Glc acts as a critical signaling molecule in higher plants. Several Glc sensing-defective Arabidopsis mutants have been characterized in detail, and the corresponding genes encoding Glc-signaling proteins have been isolated. However, the full complexity of Glc signaling in higher plants is not yet fully understood. Here, we report the identification and characterization of a new Glc-insensitive mutant, gaolaozhuangren2 (glz2), which was isolated from transposon mutagenesis experiments in Arabidopsis. In addition to its insensitivity to Glc, the glz2 plant exhibits several developmental defects such as short stature with reduced apical dominance, short roots, small and dark-green leaves, late flowering and female sterility. Treatment with 4% Glc blocked expression of the OE33 gene in wild-type plants, whereas expression of this gene was unchanged in the glz2 mutant plants. Taken together, our results suggest that the GLZ2 gene is required for normal glucose response and development of Arabidopsis.

ABA-activated SnRK2 protein kinase is required for dehydration stress signaling in Arabidopsis.

Protein phosphorylation has pivotal roles in ABA and osmotic stress signaling in higher plants. Two protein phosphatase genes, ABI1 and ABI2, are known to regulate these signaling pathways in Arabidopsis: The identity of ABA-activated protein kinases required for the ABA signaling, however, remains to be elucidated. Here we demonstrate that two protein kinases, p44 and p42, were activated by ABA in Arabidopsis T87 cultured cells, and at least one protein kinase, p44, was activated not only by ABA but also by low humidity in Arabidopsis plants. Analysis of T-DNA knockout mutants and biochemical analysis using a specific antibody revealed that the p44 is encoded by a SnRK2-type protein kinase gene, SRK2E. The srk2e mutation resulted in a wilty phenotype mainly due to loss of stomatal closure in response to a rapid humidity decrease. ABA-inducible gene expression of rd22 and rd29B was suppressed in srk2e. These results show that SRK2E plays an important role in ABA signaling in response to water stress.

Nuclear and cytosolic events of light-induced, phytochrome-regulated signaling in higher plants.

Plants as sessile organisms cannot walk away but have to adapt to changes in their natural environments. Light is obviously the most important external factor, being the energy source. To regulate their fitness, the photosynthetic capacity of plants must be adapted to changes in the ambient light environment. Therefore, plants have evolved a large set of photoreceptors to monitor light quality and quantity, ranging from the UV to the infrared part of the spectrum. At least three different photoreceptor classes have been identified and analyzed: (i) UV‐B receptors, primarily characterized by action spectroscopy (Wellmann, 1983); (ii) the blue UV‐A photoreceptors, cry1 and cry2 and phototropine (Ahmad and Cashmore, 1993; Christie et al ., 1998; Lin et al ., 1998); and (iii) the red/far‐red reversible phytochromes. These photoreceptors control different aspects of plant growth and development, and in this review we focus on the physiological and molecular events regulated by the most characterized plant photoreceptors, the phytochromes. In higher plants, phytochromes are encoded by a small multigene family; five genes ( PHYA–PHYE , Sharrock and Quail, 1989; Clack et al ., 1994) have been identified in Arabidopsis . Phytochromes exist as dimers composed of two 125 kDa polypeptides, each carrying a covalently linked tetrapyrrol chromophore in the N‐terminal domain and dimerization domains in the C‐terminal domain. The photosensory function of the molecule is based on its capacity for reversible interconversion between the red light‐absorbing Pr form and the far‐red light absorbing Pfr form following sequential absorption of red and far‐red light. Photosignal perception by the receptor is followed by conformational changes, and activates, through an as yet poorly understood mechanism, signaling pathways leading to changes in the expression of genes that underlie developmental responses to light (Quail et al ., 1995). In order to finely tune their …