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1.  Mechanisms of signal transduction by ethylene: overlapping and non-overlapping signalling roles in a receptor family 
AoB Plants  2013;5:plt010.
The plant hormone ethylene regulates growth and development as well as stress responses. This review focuses on recent discoveries that support a model for ethylene signal transduction that involves overlapping and non-overlapping roles for members of the ethylene receptor family. The roles of ethylene receptors in regulating plant growth, pathogen responses, and development are discussed. Mechanisms are proposed by which receptors can modulate downstream responses together and independently.
The plant hormone ethylene regulates growth and development as well as responses to biotic and abiotic stresses. Over the last few decades, key elements involved in ethylene signal transduction have been identified through genetic approaches, these elements defining a pathway that extends from initial ethylene perception at the endoplasmic reticulum to changes in transcriptional regulation within the nucleus. Here, we present our current understanding of ethylene signal transduction, focusing on recent developments that support a model with overlapping and non-overlapping roles for members of the ethylene receptor family. We consider the evidence supporting this model for sub-functionalization within the receptor family, and then discuss mechanisms by which such a sub-functionalization may occur. To this end, we consider the importance of receptor interactions in modulating their signal output and how such interactions vary in the receptor family. In addition, we consider evidence indicating that ethylene signal output by the receptors involves both phosphorylation-dependent and phosphorylation-independent mechanisms. We conclude with a current model for signalling by the ethylene receptors placed within the overall context of ethylene signal transduction.
doi:10.1093/aobpla/plt010
PMCID: PMC3611092  PMID: 23543258
Arabidopsis; ethylene; ethylene receptors; histidine kinase; hormone signalling; sub-functionalization; two-component system
2.  Nitric oxide in plants: an assessment of the current state of knowledge 
AoB Plants  2013;5:pls052.
Nitric oxide (NO) is a plant signal contributing to plant stress responses and development. We here review some of the key advances in this field but also highlight certain key aspects of plant NO biology that require further attention.
Background and aims
After a series of seminal works during the last decade of the 20th century, nitric oxide (NO) is now firmly placed in the pantheon of plant signals. Nitric oxide acts in plant–microbe interactions, responses to abiotic stress, stomatal regulation and a range of developmental processes. By considering the recent advances in plant NO biology, this review will highlight certain key aspects that require further attention.
Scope and conclusions
The following questions will be considered. While cytosolic nitrate reductase is an important source of NO, the contributions of other mechanisms, including a poorly defined arginine oxidizing activity, need to be characterized at the molecular level. Other oxidative pathways utilizing polyamine and hydroxylamine also need further attention. Nitric oxide action is dependent on its concentration and spatial generation patterns. However, no single technology currently available is able to provide accurate in planta measurements of spatio-temporal patterns of NO production. It is also the case that pharmaceutical NO donors are used in studies, sometimes with little consideration of the kinetics of NO production. We here include in planta assessments of NO production from diethylamine nitric oxide, S-nitrosoglutathione and sodium nitroprusside following infiltration of tobacco leaves, which could aid workers in their experiments. Further, based on current data it is difficult to define a bespoke plant NO signalling pathway, but rather NO appears to act as a modifier of other signalling pathways. Thus, early reports that NO signalling involves cGMP—as in animal systems—require revisiting. Finally, as plants are exposed to NO from a number of external sources, investigations into the control of NO scavenging by such as non-symbiotic haemoglobins and other sinks for NO should feature more highly. By crystallizing these questions the authors encourage their resolution through the concerted efforts of the plant NO community.
doi:10.1093/aobpla/pls052
PMCID: PMC3560241  PMID: 23372921
3.  Mechanisms for calcium sensing receptor-regulated stomatal closure in response to the extracellular calcium signal 
Plant Signaling & Behavior  2012;7(2):289-291.
In Arabidopsis, extracellular calcium (Ca2+o) promotes intracellular calcium (Ca2+i) transients and stomatal closure, which has been found to be regulated by the calcium sensing receptor (CAS). However, the detailed pathways for transducting the Ca2+o signal by CAS are still unclear. We found that nitric oxide (NO) and the hydrogen peroxide (H2O2) accumulated in the guard cell chloroplast were the two elements that act downstream of the CAS signaling and trigger the stomatal closure by prolonging Ca2+i transients.1 Here we provide more commentary on CAS-regulated H2O2 generation from chloroplast and Ca2+i transients in response to Ca2+o, as well as other potential mechanisms that may be involved in the CAS signaling pathway.
doi:10.4161/psb.18882
PMCID: PMC3405689  PMID: 22415046
calcium-sensing receptor; CAS; chloroplast; extracellular calcium signaling; guard cells; hydrogen peroxide; inositol 1,4,5-triphosphate; nitric oxide; phosphatidic acid; stomatal closure
4.  Converting low dose radiation to redox signaling 
Plant Signaling & Behavior  2013;8(2):e23151.
In contrast to the damaging effects of high doses, low dose radiation (UV, gamma) has been reported to provoke constructive changes in plants. However, the mechanisms by which plants recognize and respond to low dose radiation are not understood. We have shown recently that polygalacturonic acid, cell wall polysaccharide, converts the highly reactive product of radiation - hydroxyl radical into superoxide which may be further dismutated to hydrogen peroxide. Superoxide has been proposed to act as a signaling molecule, while hydrogen peroxide is known to be the key species in redox signaling cascades which are involved in the regulation of various physiological processes. Hence we propose that polygalacturonic acid may operate as radiation-signaling convertor. The outlined principles of radiation-sensing could also be valid for mammalian cells, with some other molecules mediating the conversion.
doi:10.4161/psb.23151
PMCID: PMC3657013  PMID: 23299433
radiation; polygalacturonic acid; hydroxyl radical; superoxide; redox signaling
5.  Role of ethylene receptors during senescence and ripening in horticultural crops 
Plant Signaling & Behavior  2012;7(7):827-846.
The past two decades have been rewarding in terms of deciphering the ethylene signal transduction and functional validation of the ethylene receptor and downstream genes involved in the cascade. Our knowledge of ethylene receptors and its signal transduction pathway provides us a robust platform where we can think of manipulating and regulating ethylene sensitivity by the use of genetic engineering and making transgenic. This review focuses on ethylene perception, receptor mediated regulation of ethylene biosynthesis, role of ethylene receptors in flower senescence, fruit ripening and other effects induced by ethylene. The expression behavior of the receptor and downstream molecules in climacteric and non climacteric crops is also elaborated upon. Possible strategies and recent advances in altering the ethylene sensitivity of plants using ethylene receptor genes in an attempt to modulate the regulation and sensitivity to ethylene have also been discussed. Not only will these transgenic plants be a boon to post-harvest physiology and crop improvement but, it will also help us in discovering the mechanism of regulation of ethylene sensitivity.
doi:10.4161/psb.20321
PMCID: PMC3583974  PMID: 22751331
ethylene insensitive; ethylene receptors; ethylene; negative regulation; perception; sensitivity; transgenic
6.  Joining forces 
Plant Signaling & Behavior  2009;4(10):933-941.
In flowering plants, gravity perception appears to involve the sedimentation of starch-filled plastids, called amyloplasts, within specialized cells (the statocytes) of shoots (endodermal cells) and roots (columella cells). Unfortunately, how the physical information derived from amyloplast sedimentation is converted into a biochemical signal that promotes organ gravitropic curvature remains largely unknown. Recent results suggest an involvement of the Translocon of the Outer Envelope of (Chloro) plastids (TOC) in early phases of gravity signal transduction within the statocytes. This review summarizes our current knowledge of the molecular mechanisms that govern gravity signal transduction in flowering plants and summarizes models that attempt to explain the contribution of TOC proteins in this important behavioral plant growth response to its mechanical environment.
PMCID: PMC2801356  PMID: 19826232
gravitropism; root; amyloplast; TOC complex; TOC132; TOC75
7.  Nitric oxide and frataxin: two players contributing to maintain cellular iron homeostasis 
Annals of Botany  2009;105(5):801-810.
Background
Nitric oxide (NO) is a signalling and physiologically active molecule in animals, plants and bacteria. The specificity of the molecular mechanism(s) involved in transducing the NO signal within and between cells and tissues is still poorly understood. NO has been shown to be an emerging and potent signal molecule in plant growth, development and stress physiology. The NO donor S-nitrosoglutathion (GSNO) was shown to be a biologically active compound in plants and a candidate for NO storage and/or mobilization between plant tissues and cells. NO has been implicated as a central component in maintaining iron bioavailavility in plants.
Scope and Conclusions
Iron is an essential nutrient for almost all organisms. This review presents an overview of the functions of NO in iron metabolism in animals and discusses how NO production constitutes a key response in plant iron sensing and availability. In plants, NO drives downstream responses to both iron deficiency and iron overload. NO-mediated improvement of iron nutrition in plants growing under iron-deficient conditions represents a powerful tool to cope with soils displaying low iron availability. An interconversion between different redox forms based on the iron and NO status of the plant cells might be the core of a metabolic process driving plant iron homeostasis. Frataxin, a recently identified protein in plants, plays an important role in mitochondria biogenesis and in maintaining mitochondrial iron homeostasis. Evidence regarding the interaction between frataxin, NO and iron from analysis of frataxin knock-down Arabidopsis thaliana mutants is reviewed and discussed.
doi:10.1093/aob/mcp147
PMCID: PMC2859906  PMID: 19556267
Nitric oxide; iron homeostasis; frataxin; strategy I; strategy II; dinitrosyl iron complexes; oxidative stress
8.  Glutathione Peroxidase-1 in Health and Disease: From Molecular Mechanisms to Therapeutic Opportunities 
Antioxidants & Redox Signaling  2011;15(7):1957-1997.
Abstract
Reactive oxygen species, such as superoxide and hydrogen peroxide, are generated in all cells by mitochondrial and enzymatic sources. Left unchecked, these reactive species can cause oxidative damage to DNA, proteins, and membrane lipids. Glutathione peroxidase-1 (GPx-1) is an intracellular antioxidant enzyme that enzymatically reduces hydrogen peroxide to water to limit its harmful effects. Certain reactive oxygen species, such as hydrogen peroxide, are also essential for growth factor-mediated signal transduction, mitochondrial function, and maintenance of normal thiol redox-balance. Thus, by limiting hydrogen peroxide accumulation, GPx-1 also modulates these processes. This review explores the molecular mechanisms involved in regulating the expression and function of GPx-1, with an emphasis on the role of GPx-1 in modulating cellular oxidant stress and redox-mediated responses. As a selenocysteine-containing enzyme, GPx-1 expression is subject to unique forms of regulation involving the trace mineral selenium and selenocysteine incorporation during translation. In addition, GPx-1 has been implicated in the development and prevention of many common and complex diseases, including cancer and cardiovascular disease. This review discusses the role of GPx-1 in these diseases and speculates on potential future therapies to harness the beneficial effects of this ubiquitous antioxidant enzyme. Antioxid. Redox Signal. 15, 1957–1997.
I. Introduction
II. GPx‐1 Activity
A. Enzymatic mechanisms of GPx
B. Structure and function: analysis of the active site
C. Inhibitors of GPx
D. Comparison among mammalian GPxs 1–4
III. Regulation of GPx‐1 Expression and Activity
A. Transcriptional regulation
B. Post‐transcriptional and translational regulation
1. Basic mechanisms of Sec incorporation
2. Selenium, nonsense‐mediated decay of GPx‐1 mRNA, and translational repression
3. Post‐transcriptional upregulation of GPx‐1
4. Inhibition of GPx‐1 translation
C. Post‐translational regulation
1. Sec oxidation
2. Stimulation by signal transduction and/or protein–protein interactions
IV. GPx‐1 and Oxidant‐Dependent Cellular Processes
A. Oxidative damage and cell death, apoptosis, and injury
1. Role of oxidants in cell death and apoptosis
2. Role of GPx‐1 in cell death and apoptosis
3. GPx‐1 and response to in vivo ROS
B. Redox‐dependent cell signaling, growth, and survival
V. GPx‐1 and Cancer
A. GPx‐1 and the mechanisms of cancer susceptibility
B. GPx‐1 and genetic polymorphisms
C. GPx‐1: genetic polymorphisms and cancer risk
1. Breast cancer
2. Lung cancer
3. Prostate cancer
4. Bladder cancer
5. Other cancers
VI. GPx‐1, Diabetes, and Cardiovascular Disease
A. GPx‐1 and the mechanisms of susceptibility to diabetes and cardiovascular disease
1. Diabetes mellitus
2. Cardiac dysfunction and toxicity
3. Ischemia/reperfusion injury, angiogenesis, and EPC function
4. Endothelial dysfunction and vascular tone
5. Inflammation and atherogenesis
B. Epidemiologic and genetic studies of GPx‐1 and cardiovascular disease
VII. GPx‐1 and Future Directions for Therapeutic Applications
doi:10.1089/ars.2010.3586
PMCID: PMC3159114  PMID: 21087145
9.  Discovering mechanisms of signaling-mediated cysteine oxidation 
Summary
Accumulating evidence reveals hydrogen peroxide as a key player both as a damaging agent and, from emerging evidence over the last decade, as a second messenger in intracellular signaling. This rather mild oxidant acts upon downstream targets within signaling cascades to modulate the activity of a host of enzymes (e.g. phosphatases and kinases) and transcriptional regulators through chemoselective oxidation of cysteine residues. With the recent development of specific detection reagents for hydrogen peroxide and new chemical tools to detect the generation of the initial oxidation product, sulfenic acid, on reactive cysteines within target proteins, the scene is set to gain a better understanding of the mechanisms through which hydrogen peroxide acts as a second messenger in cell signaling.
doi:10.1016/j.cbpa.2008.01.021
PMCID: PMC2408887  PMID: 18282483
10.  MAPK signaling in plant hormone ethylene signal transduction 
Plant Signaling & Behavior  2008;3(10):848-849.
The signal transduction pathway of the plant stress and defense hormone, ethylene, has been extensively elucidated using the plant genetic model Arabidopsis over the last two decades. Among others, a MAPKKK CTR1 was identified as a negative regulator that has led to the speculation of MAPK involvement in ethylene signaling. However, it remained unclear how the MAPK modules acting downstream of the receptors to mediate ethylene signaling. We have recently presented new evidence that the MKK9-MPK3/6 modules identified by combined functional genomic and genetic screens mediate ethylene signaling, which is negatively regulated by the genetically identified CTR1-dependent cascades. Our genetic studies show consistently that the MKK9-MPK3/MPK6 modules act downstream of the ethylene receptors. Biochemical and transgenic analyses further demonstrated that the positive-acting and negative-acting MAPK activities are integrated and act simultaneously to control the key transcription factor EIN3 through dual phosphorylations to regulate the EIN3 protein stability and downstream transcription cascades. This study has revealed a novel molecular mechanism that defines the specificity of complex MAPK signaling. Comprehensive elucidation of MAPK cascades and the underlying molecular mechanisms would provide more precise explanations for how plant cells utilize MAPK cascades to control specific downstream outputs in response to distinct stimuli.
PMCID: PMC2634393  PMID: 19704518
ethylene; EIN3; CTR1; MKK9; MPK3; MPK6
11.  The Histidine Kinase AHK5 Integrates Endogenous and Environmental Signals in Arabidopsis Guard Cells 
PLoS ONE  2008;3(6):e2491.
Background
Stomatal guard cells monitor and respond to environmental and endogenous signals such that the stomatal aperture is continually optimised for water use efficiency. A key signalling molecule produced in guard cells in response to plant hormones, light, carbon dioxide and pathogen-derived signals is hydrogen peroxide (H2O2). The mechanisms by which H2O2 integrates multiple signals via specific signalling pathways leading to stomatal closure is not known.
Principal Findings
Here, we identify a pathway by which H2O2, derived from endogenous and environmental stimuli, is sensed and transduced to effect stomatal closure. Histidine kinases (HK) are part of two-component signal transduction systems that act to integrate environmental stimuli into a cellular response via a phosphotransfer relay mechanism. There is little known about the function of the HK AHK5 in Arabidopsis thaliana. Here we report that in addition to the predicted cytoplasmic localisation of this protein, AHK5 also appears to co-localise to the plasma membrane. Although AHK5 is expressed at low levels in guard cells, we identify a unique role for AHK5 in stomatal signalling. Arabidopsis mutants lacking AHK5 show reduced stomatal closure in response to H2O2, which is reversed by complementation with the wild type gene. Over-expression of AHK5 results in constitutively less stomatal closure. Abiotic stimuli that generate endogenous H2O2, such as darkness, nitric oxide and the phytohormone ethylene, also show reduced stomatal closure in the ahk5 mutants. However, ABA caused closure, dark adaptation induced H2O2 production and H2O2 induced NO synthesis in mutants. Treatment with the bacterial pathogen associated molecular pattern (PAMP) flagellin, but not elf peptide, also exhibited reduced stomatal closure and H2O2 generation in ahk5 mutants.
Significance
Our findings identify an integral signalling function for AHK5 that acts to integrate multiple signals via H2O2 homeostasis and is independent of ABA signalling in guard cells.
doi:10.1371/journal.pone.0002491
PMCID: PMC2424244  PMID: 18560512
12.  Mitogen-activated protein kinase signaling in plants under abiotic stress 
Plant Signaling & Behavior  2011;6(2):196-203.
Mitogen-activated protein kinase cascade is evolutionarily conserved signal transduction module involved in transducing extracellular signals to the nucleus for appropriate cellular adjustment. This cascade consists essentially of three components, a MAPK kinase kinase (MAPKKK), a MAPK kinase (MAPKK) and a MAPK connected to each other by the event of phosphorylation. These kinases play various roles in intra- and extra-cellular signaling in plants by transferring the information from sensors to responses. Signaling through MAP kinase cascade can lead to cellular responses including cell division, differentiation as well as responses to various stresses. MAPK signaling has also been associated with hormonal responses. In plants, MAP kinases are represented by multigene families and are involved in efficient transmission of specific stimuli and also involved in the regulation of the antioxidant defense system in response to stress signaling. In the current review we summarize and investigate the participation of MAPKs as possible mediators of various abiotic stresses in plants.
doi:10.4161/psb.6.2.14701
PMCID: PMC3121978  PMID: 21512321
abiotic stress; cross talk; mitogen-activated protein kinases; heat map; MAPK signaling; signal transduction; stress signaling
13.  The beginnings of crop phosphoproteomics: exploring early warning systems of stress 
This review examines why a knowledge of plant protein phosphorylation events is important in devising strategies to protect crops from both biotic and abiotic stresses, and why proteomics should be included when studying stress pathways. Most of the achievements in elucidating phospho-signaling pathways in biotic and abiotic stress are reported from model systems: while these are discussed, this review attempts mainly to focus on work done with crops, with examples of achievements reported from rice, maize, wheat, grape, Brassica, tomato, and soy bean after cold acclimation, hormonal and oxidative hydrogen peroxide treatment, salt stress, mechanical wounding, or pathogen challenge. The challenges that remain to transfer this information into a format that can be used to protect crops against biotic and abiotic stresses are enormous. The tremendous increase in the speed and ease of DNA sequencing is poised to reveal the whole genomes of many crop species in the near future, which will facilitate phosphoproteomics and phosphogenomics research.
doi:10.3389/fpls.2012.00144
PMCID: PMC3387783  PMID: 22783265
abiotic stress; biotic stress; phosphoproteomics; signaling
14.  Jasmonates: An Update on Biosynthesis, Signal Transduction and Action in Plant Stress Response, Growth and Development 
Annals of Botany  2007;100(4):681-697.
Background
Jasmonates are ubiquitously occurring lipid-derived compounds with signal functions in plant responses to abiotic and biotic stresses, as well as in plant growth and development. Jasmonic acid and its various metabolites are members of the oxylipin family. Many of them alter gene expression positively or negatively in a regulatory network with synergistic and antagonistic effects in relation to other plant hormones such as salicylate, auxin, ethylene and abscisic acid.
Scope
This review summarizes biosynthesis and signal transduction of jasmonates with emphasis on new findings in relation to enzymes, their crystal structure, new compounds detected in the oxylipin and jasmonate families, and newly found functions.
Conclusions
Crystal structure of enzymes in jasmonate biosynthesis, increasing number of jasmonate metabolites and newly identified components of the jasmonate signal-transduction pathway, including specifically acting transcription factors, have led to new insights into jasmonate action, but its receptor(s) is/are still missing, in contrast to all other plant hormones.
doi:10.1093/aob/mcm079
PMCID: PMC2749622  PMID: 17513307
Oxylipins; jasmonic acid; jasmonate metabolites; enzymes in biosynthesis and metabolism; signal function
15.  Shedding Some Light over the Floral Metabolism by Arum Lily (Zantedeschia aethiopica) Spathe De Novo Transcriptome Assembly 
PLoS ONE  2014;9(3):e90487.
Zantedeschia aethiopica is an evergreen perennial plant cultivated worldwide and commonly used for ornamental and medicinal purposes including the treatment of bacterial infections. However, the current understanding of molecular and physiological mechanisms in this plant is limited, in comparison to other non-model plants. In order to improve understanding of the biology of this botanical species, RNA-Seq technology was used for transcriptome assembly and characterization. Following Z. aethiopica spathe tissue RNA extraction, high-throughput RNA sequencing was performed with the aim of obtaining both abundant and rare transcript data. Functional profiling based on KEGG Orthology (KO) analysis highlighted contigs that were involved predominantly in genetic information (37%) and metabolism (34%) processes. Predicted proteins involved in the plant circadian system, hormone signal transduction, secondary metabolism and basal immunity are described here. In silico screening of the transcriptome data set for antimicrobial peptide (AMP) –encoding sequences was also carried out and three lipid transfer proteins (LTP) were identified as potential AMPs involved in plant defense. Spathe predicted protein maps were drawn, and suggested that major plant efforts are expended in guaranteeing the maintenance of cell homeostasis, characterized by high investment in carbohydrate, amino acid and energy metabolism as well as in genetic information.
doi:10.1371/journal.pone.0090487
PMCID: PMC3948674  PMID: 24614014
16.  Ubiquitin, Hormones and Biotic Stress in Plants 
Annals of Botany  2007;99(5):787-822.
Background
The covalent attachment of ubiquitin to a substrate protein changes its fate. Notably, proteins typically tagged with a lysine48-linked polyubiquitin chain become substrates for degradation by the 26S proteasome. In recent years many experiments have been performed to characterize the proteins involved in the ubiquitylation process and to identify their substrates, in order to understand better the mechanisms that link specific protein degradation events to regulation of plant growth and development.
Scope
This review focuses on the role that ubiquitin plays in hormone synthesis, hormonal signalling cascades and plant defence mechanisms. Several examples are given of how targeted degradation of proteins affects downstream transcriptional regulation of hormone-responsive genes in the auxin, gibberellin, abscisic acid, ethylene and jasmonate signalling pathways. Additional experiments suggest that ubiquitin-mediated proteolysis may also act upstream of the hormonal signalling cascades by regulating hormone biosynthesis, transport and perception. Moreover, several experiments demonstrate that hormonal cross-talk can occur at the level of proteolysis. The more recently established role of the ubiquitin/proteasome system (UPS) in defence against biotic threats is also reviewed.
Conclusions
The UPS has been implicated in the regulation of almost every developmental process in plants, from embryogenesis to floral organ production probably through its central role in many hormone pathways. More recent evidence provides molecular mechanisms for hormonal cross-talk and links the UPS system to biotic defence responses.
doi:10.1093/aob/mcl255
PMCID: PMC2802907  PMID: 17220175
Ubiquitin; E3 ligase; RING; U-Box; SCF; CRL; ubiquitylation; regulated proteolysis; plant defence; hormonal signalling; biotic stress; pathogen response
17.  New insights into the mechanisms of polyphenols beyond antioxidant properties; lessons from the green tea polyphenol, epigallocatechin 3-gallate☆ 
Redox Biology  2014;2:187-195.
Green tea is rich in polyphenol flavonoids including catechins. Epigallocatechin 3-gallate (EGCG) is the most abundant and potent green tea catechin. EGCG has been extensively studied for its beneficial health effects as a nutriceutical agent. Based upon its chemical structure, EGCG is often classified as an antioxidant. However, treatment of cells with EGCG results in production of hydrogen peroxide and hydroxyl radicals in the presence of Fe (III). Thus, EGCG functions as a pro-oxidant in some cellular contexts. Recent investigations have revealed many other direct actions of EGCG that are independent from anti-oxidative mechanisms. In this review, we discuss these novel molecular mechanisms of action for EGCG. In particular, EGCG directly interacts with proteins and phospholipids in the plasma membrane and regulates signal transduction pathways, transcription factors, DNA methylation, mitochondrial function, and autophagy to exert many of its beneficial biological actions.
Graphical abstract
Molecular mechanisms for beneficial health effects of EGCG. Low concentrations of EGCG have beneficial effects on cardiovascular and metabolic functions in normal physiology and pathophysiology. Low concentrations of EGCG directly and indirectly stimulate cellular events including intracellular signaling, nuclear and mitochondrial functions, and lysosomal autophagy. By contrast, high concentrations of EGCG may cause severe stress that damages cellular integrity and disrupts nuclear and mitochondrial function and lysosomal autophagy. These actions can be applied to cancer therapy by inducing cell death. Green oval () indicates EGCG that directly binds or is transported into the indicated organelles.
Highlights
•Many biological actions of EGCG are mediated by specific mechanisms other than its well-known anti-oxidant properties.•EGCG is a pro-oxidant per se in some biological contexts.•EGCG directly interacts with cell surface membrane proteins and specific known receptors.•Treatment of cells with EGCG regulates specific intracellular signaling pathways and transcription.•Specific biological actions of EGCG are regulated in a concentration-dependent manner.
doi:10.1016/j.redox.2013.12.022
PMCID: PMC3909779  PMID: 24494192
EGCG, epigallocatechin 3-gallate; ECG, epicatechin gallate; EGC, epigallocatechin; EC, epicatechin; Polyphenol; EGCG; Anti-oxidant; Pro-oxidant
18.  A common response to common danger? Comparison of animal and plant signaling pathways involved in cadmium sensing 
Exposure to cadmium results in disturbances in cell homeostasis in all living organisms. The first response to stress factors, including cadmium, is activation of signal transduction pathways that mobilize cell defense mechanisms. The aim of this review is a comparison between the signaling network triggered by Cd in plants and animals. Despite differences in the structure and physiology of plant and animal cells, their cadmium signal transduction pathways share many common elements. These elements include signaling molecules such as ROS, Ca2+ and NO, the involvement of phospholipase C, mitogen-activated protein kinase cascades, and activation of transcription factors. Undoubtedly, both animals and plants also possess specific signaling pathways. In case of animals, Wnt/β-catenin, sonic hedgehog and oestorgen signaling are engaged in the transduction of cadmium signal. Plant specific signal transduction pathways include signaling mediated by plant hormones. The role of ethylene and jasmonic, salicylic and abscisic acid in plant response to cadmium is also discussed.
doi:10.1007/s12079-012-0173-3
PMCID: PMC3497896  PMID: 22865263
Calcium ions; Cadmium; Nitric oxide; Mitogen-activated protein kinases; Reactive oxygen species; Transcription factors
19.  Induction of alternative respiratory pathway involves nitric oxide, hydrogen peroxide and ethylene under salt stress 
Plant Signaling & Behavior  2010;5(12):1636-1637.
Alternative respiratory pathway (AP) plays an important role in plant thermogenesis, fruit ripening and responses to environmental stresses. AP may participate in the adaptation to salt stress since salt stress increased the activity of the AP. Recently, new evidence revealed that ethylene and hydrogen peroxide (H2O2) are involved in the salt-induced increase of the AP, which plays an important role in salt tolerance in Arabidopsis callus, and ethylene may be acting downstream of H2O2. Recent observations also indicated both ethylene and nitric oxide (NO) act as signaling molecules in responses to salt stress, and ethylene may be a part of the downstream signal molecular in NO action. In this addendum, a hypothetical model for NO function in regulation of H2O2- and ethylene-mediated induction of AP under salt stress is presented.
doi:10.4161/psb.5.12.13775
PMCID: PMC3115120  PMID: 21139431
alternative respiratory pathway; ethylene; hydrogen peroxide; nitric oxide; salt stress; signaling molecule
20.  Plant Stress Surveillance Monitored by ABA and Disease Signaling Interactions 
Molecules and Cells  2012;33(1):1-7.
Abiotic and biotic stresses are the major factors that negatively impact plant growth. In response to abiotic environmental stresses such as drought, plants generate resistance responses through abscisic acid (ABA) signal transduction. In addition to the major role of ABA in abiotic stress signaling, ABA signaling was reported to downregulate biotic stress signaling. Conversely recent findings provide evidence that initial activation of plant immune signaling inhibits subsequent ABA signal transduction. Stimulation of effector-triggered disease response can interfere with ABA signal transduction via modulation of internal calcium-dependent signaling pathways. This review overviews the interactions of abiotic and biotic stress signal transduction and the mechanism through which stress surveillance system operates to generate the most efficient resistant traits against various stress condition.
doi:10.1007/s10059-012-2299-9
PMCID: PMC3887741  PMID: 22314325
abscisic acid; Ca2+; guard cell; pathogen; R genes
21.  Hydrogen Peroxide Sensing and Signaling by Protein Kinases in the Cardiovascular System 
Antioxidants & Redox Signaling  2013;18(9):1042-1052.
Abstract
Significance: Oxidants were once principally considered perpetrators of injury and disease. However, this has become an antiquated view, with cumulative evidence showing that the oxidant hydrogen peroxide serves as a signaling molecule. Hydrogen peroxide carries vital information about the redox state of the cell and is crucial for homeostatic regulation during health and adaptation to stress. Recent Advances: In this review, we examine the contemporary concepts for how hydrogen peroxide is sensed and transduced into a biological response by introducing post-translational oxidative modifications on select proteins. Oxidant sensing and signaling by kinases are of particular importance as they integrate oxidant signals into phospho-regulated pathways. We focus on CAMKII, PKA, and PKG, kinases whose redox regulation has notable impact on cardiovascular function. Critical Issues: In addition, we examine the mechanism for regulating intracellular hydrogen peroxide, considering the net concentrations that may accumulate. The effects of endogenously generated oxidants are often modeled by applying exogenous hydrogen peroxide to cells or tissues. Here we consider whether model systems exposed to exogenous hydrogen peroxide have relevance to systems where the oxidant is generated endogenously, and if so, what concentration can be justified in terms of relevance to health and disease. Future Directions: Improving our understanding of hydrogen peroxide signaling and the sensor proteins that it can modify will help us develop new strategies to regulate intracellular signaling to prevent disease. Antioxid. Redox Signal. 18, 1042–1052.
doi:10.1089/ars.2012.4817
PMCID: PMC3567777  PMID: 22867279
22.  Receptor-like kinases as surface regulators for RAC/ROP-mediated pollen tube growth and interaction with the pistil 
AoB Plants  2011;2011:plr017.
Pollen tube growth is regulated by female tissue-produced factors that facilitate growth and provide directional guidance. We discuss here signal perception and transduction molecules on the male and the female cell surfaces mediate male-female interactions that underlie successful reproduction.
Background
RAC/ROPs are RHO-type GTPases and are known to play diverse signalling roles in plants. Cytoplasmic RAC/ROPs are recruited to the cell membrane and activated in response to extracellular signals perceived and mediated by cell surface-located signalling assemblies, transducing the signals to regulate cellular processes. More than any other cell types in plants, pollen tubes depend on continuous interactions with an extracellular environment produced by their surrounding tissues as they grow within the female organ pistil to deliver sperm to the female gametophyte for fertilization.
Scope
We review studies on pollen tube growth that provide compelling evidence indicating that RAC/ROPs are crucial for regulating the cellular processes that underlie the polarized cell growth process. Efforts to identify cell surface regulators that mediate extracellular signals also point to RAC/ROPs being the molecular switches targeted by growth-regulating female factors for modulation to mediate pollination and fertilization. We discuss a large volume of work spanning more than two decades on a family of pollen-specific receptor kinases and some recent studies on members of the FERONIA family of receptor-like kinases (RLKs).
Significance
The research described shows the crucial roles that two RLK families play in transducing signals from growth regulatory factors to the RAC/ROP switch at the pollen tube apex to mediate and target pollen tube growth to the female gametophyte and signal its disintegration to achieve fertilization once inside the female chamber.
doi:10.1093/aobpla/plr017
PMCID: PMC3158858  PMID: 22476487
23.  Molecular Characterization of a Strawberry FaASR Gene in Relation to Fruit Ripening 
PLoS ONE  2011;6(9):e24649.
Background
ABA-, stress- and ripening-induced (ASR) proteins have been reported to act as a downstream component involved in ABA signal transduction. Although much attention has been paid to the roles of ASR in plant development and stress responses, the mechanisms by which ABA regulate fruit ripening at the molecular level are not fully understood. In the present work, a strawberry ASR gene was isolated and characterized (FaASR), and a polyclonal antibody against FaASR protein was prepared. Furthermore, the effects of ABA, applied to two different developmental stages of strawberry, on fruit ripening and the expression of FaASR at transcriptional and translational levels were investigated.
Methodology/Principal Findings
FaASR, localized in the cytoplasm and nucleus, contained 193 amino acids and shared common features with other plant ASRs. It also functioned as a transcriptional activator in yeast with trans-activation activity in the N-terminus. During strawberry fruit development, endogenous ABA content, levels of FaASR mRNA and protein increased significantly at the initiation of ripening at a white (W) fruit developmental stage. More importantly, application of exogenous ABA to large green (LG) fruit and W fruit markedly increased endogenous ABA content, accelerated fruit ripening, and greatly enhanced the expression of FaASR transcripts and the accumulation of FaASR protein simultaneously.
Conclusions
These results indicate that FaASR may be involved in strawberry fruit ripening. The observed increase in endogenous ABA content, and enhanced FaASR expression at transcriptional and translational levels in response to ABA treatment might partially contribute to the acceleration of strawberry fruit ripening.
doi:10.1371/journal.pone.0024649
PMCID: PMC3167850  PMID: 21915355
24.  Evolution of Catalases from Bacteria to Humans 
Antioxidants & redox signaling  2008;10(9):1527-1548.
Excessive hydrogen peroxide is harmful for almost all cell components, so its rapid and efficient removal is of essential importance for aerobically living organisms. Conversely, hydrogen peroxide acts as a second messenger in signal-transduction pathways. H2O2 is degraded by peroxidases and catalases, the latter being able both to reduce H2O2 to water and to oxidize it to molecular oxygen. Nature has evolved three protein families that are able to catalyze this dismutation at reasonable rates. Two of the protein families are heme enzymes: typical catalases and catalase–peroxidases. Typical catalases comprise the most abundant group found in Eubacteria, Archaeabacteria, Protista, Fungi, Plantae, and Animalia, whereas catalase–peroxidases are not found in plants and animals and exhibit both catalatic and peroxidatic activities. The third group is a minor bacterial protein family with a dimanganese active site called manganese catalases. Although catalyzing the same reaction (2 H2O2 → 2 H2O + O2), the three groups differ significantly in their overall and active-site architecture and the mechanism of reaction. Here, we present an overview of the distribution, phylogeny, structure, and function of these enzymes. Additionally, we report about their physiologic role, response to oxidative stress, and about diseases related to catalase deficiency in humans.
doi:10.1089/ars.2008.2046
PMCID: PMC2959186  PMID: 18498226
25.  Signal Transduction by Vascular Endothelial Growth Factor Receptors 
Vascular endothelial growth factors (VEGFs) are master regulators of vascular development and of blood and lymphatic vessel function during health and disease in the adult. It is therefore important to understand the mechanism of action of this family of five mammalian ligands, which act through three receptor tyrosine kinases (RTKs). In addition, coreceptors like neuropilins (NRPs) and integrins associate with the ligand/receptor signaling complex and modulate the output. Therapeutics to block several of the VEGF signaling components have been developed with the aim to halt blood vessel formation, angiogenesis, in diseases that involve tissue growth and inflammation, such as cancer. In this review, we outline the current information on VEGF signal transduction in relation to blood and lymphatic vessel biology.
In mammals, five vascular endothelial growth factors (VEGFs) act through three receptor tyrosine kinases to control vascular development and function.
doi:10.1101/cshperspect.a006502
PMCID: PMC3385940  PMID: 22762016

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