Redox control of protein tyrosine phosphatases and mitogen-activated protein kinases in plants

The Metabolism of Reactive Oxygen Species and Their Effects on Lipid Biosynthesis of Microalgae

Microalgae have outstanding abilities to transform carbon dioxide (CO₂) into useful lipids, which makes them extremely promising as renewable sources for manufacturing beneficial compounds. However, during this process, reactive oxygen species (ROS) can be inevitably formed via electron transfers in basal metabolisms. While the excessive accumulation of ROS can have negative effects, it has been supported that proper accumulation of ROS is essential to these organisms. Recent studies have shown that ROS increases are closely related to total lipid in microalgae under stress conditions. However, the exact mechanism behind this phenomenon remains largely unknown. Therefore, this paper aims to introduce the production and elimination of ROS in microalgae. The roles of ROS in three different signaling pathways for lipid biosynthesis are then reviewed: receptor proteins and phosphatases, as well as redox-sensitive transcription factors. Moreover, the strategies and applications of ROS-induced lipid biosynthesis in microalgae are summarized. Finally, future perspectives in this emerging field are also mentioned, appealing to more researchers to further explore the relative mechanisms. This may contribute to improving lipid accumulation in microalgae.

Potato protein tyrosine phosphatase StPTP1a is activated by StMKK1 to negatively regulate plant immunity

Phytophthora infestans causes severe losses in potato production. The MAPK kinase StMKK1 was previously found to negatively regulate potato immunity to P. infestans. Our results showed that StMKK1 interacts with a protein tyrosine phosphatase, referred to as StPTP1a, and StMKK1 directly phosphorylates StPTP1a at residues Ser-99, Tyr-223 and Thr-290. StPTP1a is a functional phosphatase and the phosphorylation of StPTP1a at these three residues enhances its stability and catalytic activity. StPTP1a negatively regulates potato immunity and represses SA-related gene expression. Furthermore, StPTP1a interacts with, and dephosphorylates, the StMKK1 downstream signalling targets StMPK4 and -7 at their Tyr-203 residue resulting in the repression of salicylic acid (SA)-related immunity. Silencing of NbPTP1a + NbMPK4 or NbPTP1a + NbMPK7 abolished the plant immunity to P. infestans caused by NbPTP1a silencing, indicating that PTP1a functions upstream of NbMPK4 and NbMPK7. StMKK1 requires StPTP1a to negatively regulate SA-related immunity and StPTP1a is phosphorylated and stabilized during immune activation to promote the de-phosphorylation of StMPK4 and -7. Our results reveal that potato StMKK1 activates and stabilizes the tyrosine phosphatase StPTP1a that in its turn de-phosphorylates StMPK4 and -7, thereby repressing plant SA-related immunity.

The role of nitric oxide and hydrogen sulfide in regulation of redox homeostasis at extreme temperatures in plants

Nitric oxide and hydrogen sulfide, as important signaling molecules (gasotransmitters), are involved in many functions of plant organism, including adaptation to stress factors of various natures. As redox-active molecules, NO and H₂S are involved in redox regulation of functional activity of many proteins. They are also involved in maintaining cell redox homeostasis due to their ability to interact directly and indirectly (functionally) with ROS, thiols, and other molecules. The review considers the involvement of nitric oxide and hydrogen sulfide in plant responses to low and high temperatures. Particular attention is paid to the role of gasotransmitters interaction with other signaling mediators (in particular, with Ca²⁺ ions and ROS) in the formation of adaptive responses to extreme temperatures. Pathways of stress-induced enhancement of NO and H₂S synthesis in plants are considered. Mechanisms of the NO and H₂S effect on the activity of some proteins of the signaling system, as well as on the state of antioxidant and osmoprotective systems during adaptation to stress temperatures, were analyzed. Possibilities of practical use of nitric oxide and hydrogen sulfide donors as inductors of plant adaptive responses are discussed.

Reactive oxygen species signalling in plant stress responses

Reactive oxygen species (ROS) are key signalling molecules that enable cells to rapidly respond to different stimuli. In plants, ROS play a crucial role in abiotic and biotic stress sensing, integration of different environmental signals and activation of stress-response networks, thus contributing to the establishment of defence mechanisms and plant resilience. Recent advances in the study of ROS signalling in plants include the identification of ROS receptors and key regulatory hubs that connect ROS signalling with other important stress-response signal transduction pathways and hormones, as well as new roles for ROS in organelle-to-organelle and cell-to-cell signalling. Our understanding of how ROS are regulated in cells by balancing production, scavenging and transport has also increased. In this Review, we discuss these promising developments and how they might be used to increase plant resilience to environmental stress.

S-Nitrosation of Arabidopsis thaliana Protein Tyrosine Phosphatase 1 Prevents Its Irreversible Oxidation by Hydrogen Peroxide

Tyrosine-specific protein tyrosine phosphatases (Tyr-specific PTPases) are key signaling enzymes catalyzing the removal of the phosphate group from phosphorylated tyrosine residues on target proteins. This post-translational modification notably allows the regulation of mitogen-activated protein kinase (MAPK) cascades during defense reactions. Arabidopsis thaliana protein tyrosine phosphatase 1 (AtPTP1), the only Tyr-specific PTPase present in this plant, acts as a repressor of H₂O₂ production and regulates the activity of MPK3/MPK6 MAPKs by direct dephosphorylation. Here, we report that recombinant histidine (His)-AtPTP1 protein activity is directly inhibited by H₂O₂ and nitric oxide (NO) exogenous treatments. The effects of NO are exerted by S-nitrosation, i.e., the formation of a covalent bond between NO and a reduced cysteine residue. This post-translational modification targets the catalytic cysteine C265 and could protect the AtPTP1 protein from its irreversible oxidation by H₂O₂. This mechanism of protection could be a conserved mechanism in plant PTPases.

Salicylic acid: A key regulator of redox signalling and plant immunity

In plants, the reactive oxygen species (ROS) formed during normal conditions are essential in regulating several processes, like stomatal physiology, pathogen immunity and developmental signaling. However, biotic and abiotic stresses can cause ROS over-accumulation leading to oxidative stress. Therefore, a suitable equilibrium is vital for redox homeostasis in plants, and there have been major advances in this research arena. Salicylic acid (SA) is known as a chief regulator of ROS; however, the underlying mechanisms remain largely unexplored. SA plays an important role in establishing the hypersensitive response (HR) and systemic acquired resistance (SAR). This is underpinned by a robust and complex network of SA with Non-Expressor of Pathogenesis Related protein-1 (NPR1), ROS, calcium ions (Ca²⁺), nitric oxide (NO) and mitogen-activated protein kinase (MAPK) cascades. In this review, we summarize the recent advances in the regulation of ROS and antioxidant defense system signalling by SA at the physiological and molecular levels. Understanding the molecular mechanisms of how SA controls redox homeostasis would provide a fundamental framework to develop approaches that will improve plant growth and fitness, in order to meet the increasing global demand for food and bioenergy.

Redox and low-oxygen stress: signal integration and interplay

Plants are aerobic organisms relying on oxygen to serve their energy needs. The amount of oxygen available to sustain plant growth can vary significantly due to environmental constraints or developmental programs. In particular, flooding stress, which negatively impacts crop productivity, is characterized by a decline in oxygen availability. Oxygen fluctuations result in an altered redox balance and the formation of reactive oxygen/nitrogen species (ROS/RNS) during the onset of hypoxia and upon re-oxygenation. In this update, we provide an overview of the current understanding of the impact of redox and ROS/RNS on low-oxygen signaling and adaptation. We first focus on the formation of ROS and RNS during low-oxygen conditions. Following this, we examine the impact of hypoxia on cellular and organellar redox systems. Finally, we describe how redox and ROS/RNS participate in signaling events during hypoxia through potential post-translational modifications (PTMs) of hypoxia-relevant proteins. The aim of this update is to define our current understanding of the field and to provide avenues for future research directions.

Decreased Levels of Thioredoxin o1 Influences Stomatal Development and Aperture but Not Photosynthesis under Non-Stress and Saline Conditions

Salinity has a negative impact on plant growth, with photosynthesis being downregulated partially due to osmotic effect and enhanced cellular oxidation. Redox signaling contributes to the plant response playing thioredoxins (TRXs) a central role. In this work we explore the potential contribution of Arabidopsis TRXo1 to the photosynthetic response under salinity analyzing Arabidopsis wild-type (WT) and two Attrxo1 mutant lines in their growth under short photoperiod and higher light intensity than previous reported works. Stomatal development and apertures and the antioxidant, hormonal and metabolic acclimation are also analyzed. In control conditions mutant plants displayed less and larger developed stomata and higher pore size which could underlie their higher stomatal conductance, without being affected in other photosynthetic parameters. Under salinity, all genotypes displayed a general decrease in photosynthesis and the oxidative status in the Attrxo1 mutant lines was altered, with higher levels of H₂O₂ and NO but also higher ascorbate/glutathione (ASC/GSH) redox states than WT plants. Finally, sugar changes and increases in abscisic acid (ABA) and NO may be involved in the observed higher stomatal response of the TRXo1-altered plants. Therefore, the lack of AtTRXo1 affected stomata development and opening and the mutants modulate their antioxidant, metabolic and hormonal responses to optimize their adaptation to salinity.

Cysteine modifications (oxPTM) and protein sulphenylation-mediated sulfenome expression in plants: evolutionary conserved signaling networks?

Plant resilience to oxidative stress possibly operates through the restoration of intracellular redox milieu and the activity of various posttranslationally modified proteins. Among various modes of redox regulation operative in plants cys oxPTMs are brought about by the activity of reactive oxygen species (ROS), reactive nitrogen species (RNS), and hydrogen peroxide. Cysteine oxPTMs are capable of transducing ROS-mediated long-distance hormone signaling (ABA, JA, SA) in plants. S-sulphenylation is an intermediary modification en route to other oxidative states of cysteine. In silico analysis have revealed evolutionary conservation of certain S-sulphenylated proteins across human and plants. Further analysis of protein sulphenylation in plants should be extended to the functional follow-up studies followed by site-specific characterization and case-by-case validation of protein activity. The repertoire of physiological methods (fluorescent conjugates (dimedone) and yeast AP-1 (YAP1)-based genetic probes) in the recent past has been successful in the detection of sulphenylated proteins and other cysteine-based modifications in plants. In view of a better understanding of the sulfur-based redoxome it is necessary to update our timely progress on the methodological advancements for the detection of cysteine-based oxPTM. This substantiative information can extend our investigations on plant-environment interaction thus improving crop manipulation strategies. The simulation-based computational approach has emerged as a new method to determine the directive mechanism of cysteine oxidation in plants. Thus, sulfenome analysis in various plant systems might reflect as a pinnacle of plant redox biology in the future.

AtMPK6-induced phosphorylation of AtERF72 enhances its DNA binding activity and interaction with TGA4/OBF4 in Arabidopsis

The ethylene-responsive element binding factor (ERF) family is a large family of transcription factors involved in plant development and environmental stress responses. We previously reported the identification of 29 putative substrates of Mitogen-activated Protein Kinase3 (AtMPK3), AtMPK4 and AtMPK6, based on a solid-phase phosphorylation screening using a lambda phage expression library in Arabidopsis thaliana. In this study, a putative MPK substrate, AtERF72 (At3g16770), was strongly phosphorylated by AtMPK6 on the serine residue at position 151 (Ser151). AtERF72 binds to the GCC box (AGCCGCC) in the promoters of several pathogenesis-related (PR) genes and activates their transcription. We also show that the DNA-binding activity of AtERF72 is enhanced upon phosphorylation by AtMPK6 in vitro. In addition, transient co-expression experiments in Arabidopsis protoplasts revealed that effector constructs expressing a mutant variant of AtERF72, AtERF72^S151D (carrying a Ser to aspartic acid [Asp] substitution at amino acid position 151) showed higher expression of the β-glucuronidase (GUS) reporter gene driven by the GCC box element than effector constructs expressing the wild-type AtERF72. Furthermore, yeast two-hybrid assays revealed that the interaction between AtERF72^S151D and TGA4/OBF4 was stronger than that between wild-type AtERF72 and TGA4/OBF4. Since AtERF72^S151D is equivalent to AtERF72 phosphorylated by AtMPK6 at Ser151, these results suggest that the phosphorylation of AtERF72 by AtMPK6 triggers an event of transcriptional regulation from defence signalling in Arabidopsis.

Plant protein phosphatases: What do we know about their mechanism of action?

Protein phosphorylation is a major reversible post-translational modification. Protein phosphatases function as 'critical regulators' in signaling networks through dephosphorylation of proteins, which have been phosphorylated by protein kinases. A large understanding of their working has been sourced from animal systems rather than the plant or the prokaryotic systems. The eukaryotic protein phosphatases include phosphoprotein phosphatases (PPP), metallo-dependent protein phosphatases (PPM), protein tyrosine (Tyr) phosphatases (PTP), and aspartate (Asp)-dependent phosphatases. The PPP and PPM families are serine(Ser)/threonine(Thr)-specific phosphatases (STPs), while PTP family is Tyr specific. Dual-specificity phosphatases (DsPTPs/DSPs) dephosphorylate Ser, Thr, and Tyr residues. PTPs lack sequence homology with STPs, indicating a difference in catalytic mechanisms, while the PPP and PPM families share a similar structural fold indicating a common catalytic mechanism. The catalytic cysteine (Cys) residue in the conserved HCX₅ R active site motif of the PTPs acts as a nucleophile during hydrolysis. The PPP members require metal ions, which coordinate the phosphate group of the substrate, followed by a nucleophilic attack by a water molecule and hydrolysis. The variable holoenzyme assembly of protein phosphatase(s) and the overlap with other post-translational modifications like acetylation and ubiquitination add to their complexity. Though their functional characterization is extensively reported in plants, the mechanistic nature of their action is still being explored by researchers. In this review, we exclusively overview the plant protein phosphatases with an emphasis on their mechanistic action as well as structural characteristics.

A Smart Advanced Chemiluminescence-Sensing Platform for Determination and Imaging of the Tissue Distribution of Natural Antioxidants

Antioxidants have gained marked attention owing to their ability to prevent the oxidation of biological components and to protect the body from reactive oxygen species, thereby maintaining human health. Thus, antioxidant-rich dietary supplements and natural foods can be effective against oxidative stress and can even act as chemopreventive agents. Therefore, a simple and rapid assay for evaluation of antioxidant capacity and assessment of their distribution profile in natural sources is vital. Herein, we report a rapid, innovative chemiluminescence (CL) platform for evaluation and visualization of antioxidant capacity. We found that intense and long-lasting CL was formed upon the redox reaction of quinones, e.g., menadione, with antioxidants, e.g., l-ascorbic acid, in the presence of luminol. The produced CL intensities were proportional to the antioxidants' concentrations with a detection limit of 0.18 μM for the model antioxidant, l-ascorbic acid. As the formed CL was long-lasting, it could be easily captured and detected with a charge-coupled device (CCD) camera. To evaluate the quantification ability of the CCD camera, we developed a smart and fast microplate-based assay based on photographing the generated CL with a cooled CCD camera. The photographed CL intensities were linearly proportional with the antioxidant concentrations, and then the method was applied for photographing multiple food sample extracts. Ultimately, we utilized our method for the distribution profiling of antioxidant capacity in food cut sections. Samples were dipped in luminol and then in quinone, followed by CCD camera photography, without the need for any pulverization/extraction procedure, giving precise antioxidant distribution information.

Poplar PdPTP1 Gene Negatively Regulates Salt Tolerance by Affecting Ion and ROS Homeostasis in Populus

High concentrations of Na⁺ in saline soil impair plant growth and agricultural production. Protein tyrosine phosphorylation is crucial in many cellular regulatory mechanisms. However, regulatory mechanisms of plant protein tyrosine phosphatases (PTPs) in controlling responses to abiotic stress remain limited. We report here the identification of a Tyrosine (Tyr)-specific phosphatase, PdPTP1, from NE19 (Populus nigra × (P. deltoides × P. nigra). Transcript levels of PdPTP1 were upregulated significantly by NaCl treatment and oxidative stress. PdPTP1 was found both in the nucleus and cytoplasm. Under NaCl treatment, transgenic plants overexpressing PdPTP1 (OxPdPTP1) accumulated more Na⁺ and less K⁺. In addition, OxPdPTP1 poplars accumulated more H₂O₂ and O₂·^-, which is consistent with the downregulation of enzymatic ROS-scavengers activity. Furthermore, PdPTP1 interacted with PdMAPK3/6 in vivo and in vitro. In conclusion, our findings demonstrate that PdPTP1 functions as a negative regulator of salt tolerance via a mechanism of affecting Na⁺/K⁺ and ROS homeostasis.

MKP1 acts as a key modulator of stomatal development

The MAPK signaling cascade is universal among eukaryotes and mediates a variety of environmental and developmental responses. Two Arabidopsis MAPKs, MPK3 and MPK6, have been shown to be activated by various stimuli and suggested as a convergence point of different signaling pathways. It is known that these MAPKs, MPK3/MPK6, control the discrete stages of stomatal development in Arabidopsis, but how they are regulated and how the same MAPK components can achieve signaling specificity is largely unknown. We recently demonstrated that MAP Kinase Phosphatase 1 (MKP1) promotes stomatal differentiation by suppressing activation of MPK3/MPK6 in the stomatal lineage. By expressing MKP1 in discrete stomatal precursor cell types, we further identified that MKP1 plays an important role at the early stage of stomatal development for the cell fate transition leading to stomatal differentiation. While MKP1 was previously known as a key regulator of environmental stress responses, our data illustrate a novel role of MKP1 in plant development: it acts as one of the specificity-determining regulators of MAPK signaling to enforce proper stomatal development in Arabidopsis.

MAP KINASE PHOSPHATASE1 Controls Cell Fate Transition during Stomatal Development

Stomata on the plant epidermis control gas and water exchange and are formed by MAPK-dependent processes. Although the contribution of MAP KINASE3 (MPK3) and MPK6 (MPK3/MPK6) to the control of stomatal patterning and differentiation in Arabidopsis (Arabidopsis thaliana) has been examined extensively, how they are inactivated and regulate distinct stages of stomatal development is unknown. Here, we identify a dual-specificity phosphatase, MAP KINASE PHOSPHATASE1 (MKP1), which promotes stomatal cell fate transition by controlling MAPK activation at the early stage of stomatal development. Loss of function of MKP1 creates clusters of small cells that fail to differentiate into stomata, resulting in the formation of patches of pavement cells. We show that MKP1 acts downstream of YODA (a MAPK kinase kinase) but upstream of MPK3/MPK6 in the stomatal signaling pathway and that MKP1 deficiency causes stomatal signal-induced MAPK hyperactivation in vivo. By expressing MKP1 in the three discrete cell types of stomatal lineage, we further identified that MKP1-mediated deactivation of MAPKs in early stomatal precursor cells directs cell fate transition leading to stomatal differentiation. Together, our data reveal the important role of MKP1 in controlling MAPK signaling specificity and cell fate decision during stomatal development.

Characterization of thiol-based redox modifications of Brassica napusSNF1-related protein kinase 2.6-2C

Sucrose nonfermenting 1‐related protein kinase 2.6 (SnRK2.6), also known as Open Stomata 1 (OST1) in Arabidopsis thaliana, plays a pivotal role in abscisic acid (ABA)‐mediated stomatal closure. Four SnRK2.6 paralogs were identified in the Brassica napus genome in our previous work. Here we studied one of the paralogs, BnSnRK2.6‐2C, which was transcriptionally induced by ABA in guard cells. Recombinant BnSnRK2.6‐2C exhibited autophosphorylation activity and its phosphorylation sites were mapped. The autophosphorylation activity was inhibited by S‐nitrosoglutathione (GSNO) and by oxidized glutathione (GSSG), and the inhibition was reversed by reductants. Using monobromobimane (mBBr) labeling, we demonstrated a dose‐dependent modification of BnSnRK2.6‐2C by GSNO. Furthermore, mass spectrometry analysis revealed previously uncharacterized thiol‐based modifications including glutathionylation and sulfonic acid formation. Of the six cysteine residues in BnSnRK2.6‐2C, C159 was found to have different types of thiol modifications, suggesting its high redox sensitivity and versatility. In addition, mBBr labeling on tyrosine residues was identified. Collectively, these data provide detailed biochemical characterization of redox‐induced modifications and changes of the BnSnRK2.6‐2C activity.

Mitogen activated protein kinase 6 and MAP kinase phosphatase 1 are involved in the response of Arabidopsis roots to L-glutamate

The function and components of L-glutamate signaling pathways in plants have just begun to be elucidated. Here, using a combination of genetic and biochemical strategies, we demonstrated that a MAPK module is involved in the control of root developmental responses to this amino acid. Root system architecture plays an essential role in plant adaptation to biotic and abiotic factors via adjusting signal transduction and gene expression. L-Glutamate (L-Glu), an amino acid with neurotransmitter functions in animals, inhibits root growth, but the underlying genetic mechanisms are poorly understood. Through a combination of genetic analysis, in-gel kinase assays, detailed cell elongation and division measurements and confocal analysis of expression of auxin, quiescent center and stem cell niche related genes, the critical roles of L-Glu in primary root growth acting through the mitogen-activated protein kinase 6 (MPK6) and the dual specificity serine-threonine-tyrosine phosphatase MKP1 could be revealed. In-gel phosphorylation assays revealed a rapid and dose-dependent induction of MPK6 and MPK3 activities in wild-type Arabidopsis seedlings in response to L-Glu. Mutations in MPK6 or MKP1 reduced or increased root cell division and elongation in response to L-Glu, possibly modulating auxin transport and/or response, but in a PLETHORA1 and 2 independent manner. Our data highlight MPK6 and MKP1 as components of an L-Glu pathway linking the auxin response, and cell division for primary root growth.

Histone H2B monoubiquitination regulates salt stress-induced microtubule depolymerization in Arabidopsis

Histone H2B monoubiquitination (H2Bub1) is recognized as a regulatory mechanism that controls a range of cellular processes. We previously showed that H2Bub1 was involved in responses to biotic stress in Arabidopsis. However, the molecular regulatory mechanisms of H2Bub1 in controlling responses to abiotic stress remain limited. Here, we report that HISTONE MONOUBIQUITINATION1 (HUB1) and HUB2 played important regulatory roles in response to salt stress. Phenotypic analysis revealed that H2Bub1 mutants confer decreased tolerance to salt stress. Further analysis showed that H2Bub1 regulated the depolymerization of microtubules (MTs), the expression of PROTEIN TYROSINE PHOSPHATASE1 (PTP1) and MAP KINASE PHOSPHATASE (MKP) genes - DsPTP1, MKP1, IBR5, PHS1, and was required for the activation of mitogen-activated protein kinase3 (MAP kinase3, MPK3) and MPK6 in response to salt stress. Moreover, both tyrosine phosphorylation and the activation of MPK3 and MPK6 affected MT stability in salt stress response. Thus, the results indicate that H2Bub1 regulates salt stress-induced MT depolymerization, and the PTP-MPK3/6 signalling module is responsible for integrating signalling pathways that regulate MT stability, which is critical for plant salt stress tolerance.

A review of redox signaling and the control of MAP kinase pathway in plants

Mitogen-activated protein kinase (MAPK) cascades are evolutionarily conserved modules among eukaryotic species that range from yeast, plants, flies to mammals. In eukaryotic cells, reactive oxygen species (ROS) has both physiological and toxic effects. Both MAPK cascades and ROS signaling are involved in plant response to various biotic and abiotic stresses. It has been observed that not only can ROS induce MAPK activation, but also that disturbing MAPK cascades can modulate ROS production and responses. This review will discuss the potential mechanisms by which ROS may activate and/or regulate MAPK cascades in plants. The role of MAPK cascades and ROS signaling in regulating gene expression, stomatal function, and programmed cell death (PCD) is also discussed. In addition, the relationship between Rboh-dependent ROS production and MAPK activation in PAMP-triggered immunity will be reviewed.

Toxicological proteomic responses of halophyte Suaeda salsa to lead and zinc

The long term (30 days) toxicological effects of environmentally relevant concentrations of Pb²⁺ (20μg/L) and Zn²⁺ (100μg/L) were characterized in Suaeda salsa using proteomics techniques. The responsive proteins were related to metabolism (Krebs cycle and Calvin cycle), protein biosynthesis, stress and defense, energy, signaling pathway and photosynthesis in Pb²⁺, Zn²⁺ and Pb²⁺+ Zn²⁺ exposed groups in S. salsa after exposures for 30 days. The proteomic profiles also showed differential responses in S. salsa to metal exposures. In Pb²⁺-treated group, the proteins were categorized into cystein metabolism and pentose phosphate pathway. The responsive proteins were basically involved in glutathione metabolism, glycolysis, cystein and methane metabolism, and voltage-dependent anion channel in Zn²⁺-treated group. In Pb²⁺+ Zn²⁺-treated group, the proecular mechanism at protein level remtein responses were devided into tyrosine metabolism and glycolysis. Our results showed that the two typical heavy metals, lead and zinc, could induce toxicological effects in halophyte S. salsa at protein level.

A low-temperature-responsive element involved in the regulation of the Arabidopsis thaliana At1g71850/At1g71860 divergent gene pair

The bidirectional promoter of the Arabidopsis thaliana gene pair At1g71850/At1g71860 harbors low-temperature-responsive elements, which participate in anti-correlated transcription regulation of the driving genes in response to environmental low temperature. A divergent gene pair is defined as two adjacent genes organized head to head in opposite orientation, sharing a common promoter region. Divergent gene pairs are mainly coexpressed, but some display opposite regulation. The mechanistic basis of such anti-correlated regulation is not well understood. Here, the regulation of the Arabidopsis thaliana gene pair At1g71850/At1g71860 was investigated. Semi-quantitative RT-PCR and Genevestigator analyses showed that while one of the pair was upregulated by exposure to low temperature, the same treatment downregulated the other. Promoter::GUS fusion transgenes were used to show that this behavior was driven by a bidirectional promoter, which harbored an as-1 motif, associated with the low-temperature response; mutation of this sequence produced a significant decrease in cold-responsive expression. With regard to the as-1 motif in the native orientation repressing the promoter's low-temperature responsiveness, the same as-1 motif introduced in the reverse direction showed a slight enhancement in the promoter's responsiveness to low-temperature exposure, indicating that the orientation of the motif was important for the promoter's activity. These findings provide new insights into the complex transcriptional regulation of bidirectional gene pairs as well as plant stress response.

The LIKE SEX FOUR2 regulates root development by modulating reactive oxygen species homeostasis in Arabidopsis

Maintaining reactive oxygen species (ROS) homeostasis plays a central role in plants, and is also critical for plant root development. Threshold levels of ROS act as signals for elongation and differentiation of root cells. The protein phosphatase LIKE SEX FOUR2 (LSF2) has been reported to regulate starch metabolism in Arabidopsis, but little is known about the mechanism how LSF2 affect ROS homeostasis. Here, we identified that LSF2 function as a component modulating ROS homeostasis in response to oxidative stress and, thus regulate root development. Compared with wild type Arabidopsis, lsf2-1 mutant exhibited reduced rates of superoxide generation and higher levels of hydrogen peroxide upon oxidative stress treatments. The activities of several antioxidant enzymes, including superoxide dismutase, catalase, and ascorbate peroxidase, were also affected in lsf2-1 mutant under these oxidative stress conditions. Consequently, lsf2-1 mutant exhibited the reduced root growth but less inhibition of root hair formation compared to wild type Arabidopsis plants. Importantly, protein phosphatase LSF2 interacted with mitogen-activated protein kinase 8 (MPK8), a known component of ROS homeostasis pathways in the cytoplasm. These findings indicated the novel function of LSF2 that controls ROS homeostasis to regulate root development.

Quantitative phosphoproteomics of protein kinase SnRK1 regulated protein phosphorylation in Arabidopsis under submergence

SNF1 RELATED PROTEIN KINASE 1 (SnRK1) is proposed to be a central integrator of the plant stress and energy starvation signalling pathways. We observed that the Arabidopsis SnRK1.1 dominant negative mutant (SnRK1.1 (K48M) ) had lower tolerance to submergence than the wild type, suggesting that SnRK1.1-dependent phosphorylation of target proteins is important in signalling pathways triggered by submergence. We conducted quantitative phosphoproteomics and found that the phosphorylation levels of 57 proteins increased and the levels of 27 proteins decreased in Col-0 within 0.5-3h of submergence. Among the 57 proteins with increased phosphorylation in Col-0, 38 did not show increased phosphorylation levels in SnRK1.1 (K48M) under submergence. These proteins are involved mainly in sugar and protein synthesis. In particular, the phosphorylation of MPK6, which is involved in regulating ROS responses under abiotic stresses, was disrupted in the SnRK1.1 (K48M) mutant. In addition, PTP1, a negative regulator of MPK6 activity that directly dephosphorylates MPK6, was also regulated by SnRK1.1. We also showed that energy conservation was disrupted in SnRK1.1 (K48M) , mpk6, and PTP1 (S7AS8A) under submergence. These results reveal insights into the function of SnRK1 and the downstream signalling factors related to submergence.

ROS-Mediated Inhibition of S-nitrosoglutathione Reductase Contributes to the Activation of Anti-oxidative Mechanisms

Nitric oxide (NO) has emerged as a signaling molecule in plants being involved in diverse physiological processes like germination, root growth, stomata closing and response to biotic and abiotic stress. S-nitrosoglutathione (GSNO) as a biological NO donor has a very important function in NO signaling since it can transfer its NO moiety to other proteins (trans-nitrosylation). Such trans-nitrosylation reactions are equilibrium reactions and depend on GSNO level. The breakdown of GSNO and thus the level of S-nitrosylated proteins are regulated by GSNO-reductase (GSNOR). In this way, this enzyme controls S-nitrosothiol levels and regulates NO signaling. Here we report that Arabidopsis thaliana GSNOR activity is reversibly inhibited by H₂O₂in vitro and by paraquat-induced oxidative stress in vivo. Light scattering analyses of reduced and oxidized recombinant GSNOR demonstrated that GSNOR proteins form dimers under both reducing and oxidizing conditions. Moreover, mass spectrometric analyses revealed that H₂O₂-treatment increased the amount of oxidative modifications on Zn²⁺-coordinating Cys47 and Cys177. Inhibition of GSNOR results in enhanced levels of S-nitrosothiols followed by accumulation of glutathione. Moreover, transcript levels of redox-regulated genes and activities of glutathione-dependent enzymes are increased in gsnor-ko plants, which may contribute to the enhanced resistance against oxidative stress. In sum, our results demonstrate that reactive oxygen species (ROS)-dependent inhibition of GSNOR is playing an important role in activation of anti-oxidative mechanisms to damping oxidative damage and imply a direct crosstalk between ROS- and NO-signaling.

Abiotic Stresses Antagonize the Rice Defence Pathway through the Tyrosine-Dephosphorylation of OsMPK6

Plants, as sessile organisms, survive environmental changes by prioritizing their responses to the most life-threatening stress by allocating limited resources. Previous studies showed that pathogen resistance was suppressed under abiotic stresses. Here, we show the mechanism underlying this phenomenon. Phosphorylation of WRKY45, the central transcription factor in salicylic-acid (SA)-signalling-dependent pathogen defence in rice, via the OsMKK10-2-OsMPK6 cascade, was required to fully activate WRKY45. The activation of WRKY45 by benzothiadiazole (BTH) was reduced under low temperature and high salinity, probably through abscisic acid (ABA) signalling. An ABA treatment dephosphorylated/inactivated OsMPK6 via protein tyrosine phosphatases, OsPTP1/2, leading to the impaired activation of WRKY45 and a reduction in Magnaporthe oryzae resistance, even after BTH treatment. BTH induced a strong M. oryzae resistance in OsPTP1/2 knockdown rice, even under cold and high salinity, indicating that OsPTP1/2 is the node of SA-ABA signalling crosstalk and its down-regulation makes rice disease resistant, even under abiotic stresses. These results points to one of the directions to further improve crops by managing the tradeoffs between different stress responses of plants.

Role of Protein Tyrosine Phosphatases in Plants

Reversible protein phosphorylation is a crucial regulatory mechanism that controls many biological processes in eukaryotes. In plants, phosphorylation events primarily occur on serine (Ser) and threonine (Thr) residues, while in certain cases, it was also discovered on tyrosine (Tyr) residues. In contrary to plants, extensive reports on Tyr phosphorylation regulating a large numbers of biological processes exist in animals. Despite of such prodigious function in animals, Tyr phosphorylation is a least studied mechanism of protein regulation in plants. Recently, various chemical analytical procedures have strengthened the view that Tyr phosphorylation is equally prevalent in plants as in animals. However, regardless of Tyr phosphorylation events occuring in plants, no evidence could be found for the existence of gene encoding for Tyr phosphorylation i.e. the typical Tyr kinases. Various methodologies have suggested that plant responses to stress signals and developmental processes involved modifications in protein Tyr phosphorylation. Correspondingly, various reports have established the role of PTPs (Protein Tyrosine Phosphatases) in the dephosphorylation and inactivation of mitogen activated protein kinases (MAPKs) hence, in the regulation of MAPK signaling cascade. Besides this, many dual specificity protein phosphatases (DSPs) are also known to bind starch and regulate starch metabolism through reversible phosphorylation. Here, we are emphasizing the significant progress on protein Tyr phosphatases to understand the role of these enzymes in the regulation of post-translational modification in plant physiology and development.

Oxidative post-translational modifications of cysteine residues in plant signal transduction

In plants, fluctuation of the redox balance by altered levels of reactive oxygen species (ROS) can affect many aspects of cellular physiology. ROS homeostasis is governed by a diversified set of antioxidant systems. Perturbation of this homeostasis leads to transient or permanent changes in the redox status and is exploited by plants in different stress signalling mechanisms. Understanding how plants sense ROS and transduce these stimuli into downstream biological responses is still a major challenge. ROS can provoke reversible and irreversible modifications to proteins that act in diverse signalling pathways. These oxidative post-translational modifications (Ox-PTMs) lead to oxidative damage and/or trigger structural alterations in these target proteins. Characterization of the effect of individual Ox-PTMs on individual proteins is the key to a better understanding of how cells interpret the oxidative signals that arise from developmental cues and stress conditions. This review focuses on ROS-mediated Ox-PTMs on cysteine (Cys) residues. The Cys side chain, with its high nucleophilic capacity, appears to be the principle target of ROS. Ox-PTMs on Cys residues participate in various signalling cascades initiated by plant stress hormones. We review the mechanistic aspects and functional consequences of Cys Ox-PTMs on specific target proteins in view of stress signalling events.

The thioredoxin/peroxiredoxin/sulfiredoxin system: current overview on its redox function in plants and regulation by reactive oxygen and nitrogen species

In plants, the presence of thioredoxin (Trx), peroxiredoxin (Prx), and sulfiredoxin (Srx) has been reported as a component of a redox system involved in the control of dithiol-disulfide exchanges of target proteins, which modulate redox signalling during development and stress adaptation. Plant thiols, and specifically redox state and regulation of thiol groups of cysteinyl residues in proteins and transcription factors, are emerging as key components in the plant response to almost all stress conditions. They function in both redox sensing and signal transduction pathways. Scarce information exists on the transcriptional regulation of genes encoding Trx/Prx and on the transcriptional and post-transcriptional control exercised by these proteins on their putative targets. As another point of control, post-translational regulation of the proteins, such as S-nitrosylation and S-oxidation, is of increasing interest for its effect on protein structure and function. Special attention is given to the involvement of the Trx/Prx/Srx system and its redox state in plant signalling under stress, more specifically under abiotic stress conditions, as an important cue that influences plant yield and growth. This review focuses on the regulation of Trx and Prx through cysteine S-oxidation and/or S-nitrosylation, which affects their functionality. Some examples of redox regulation of transcription factors and Trx- and Prx-related genes are also presented.

AtDsPTP1 acts as a negative regulator in osmotic stress signalling during Arabidopsis seed germination and seedling establishment

Dual-specificity protein phosphatases (DsPTPs) target both tyrosine and serine/threonine residues and play roles in plant growth and development. We have characterized an Arabidopsis mutant, dsptp1, which shows a higher seed germination rate and better root elongation under osmotic stress than the wild type. By contrast, its overexpression line, DsPTP1-OE, shows inhibited seed germination and root elongation; and its complemented line, DsPTP1-Com, resembles the wild type and rescues DsPTP1-OE under osmotic stress. Expression of AtDsPTP1 is enhanced by osmotic stress in seed coats, bases of rosette leaves, and roots. Compared with the wild type, the dsptp1 mutant shows increased proline accumulation, reduced malondialdehyde (MDA) content and ion leakage, and enhanced antioxidant enzyme activity in response to osmotic stress. AtDsPTP1 regulates the transcript levels of various dehydration-responsive genes under osmotic stress. Abscisic acid (ABA) accumulation in dsptp1 under osmotic stress is reduced with reduced expression of the ABA-biosynthesis gene NCED3 and increased expression of the ABA-catabolism gene CYP707A4. AtDsPTP1 also regulates the expression of key components in the ABA-signalling pathway. In conclusion, AtDsPTP1 regulates ABA accumulation, and acts as a negative regulator in osmotic stress signalling during Arabidospsis seed germination and seedling establishment.

Phosphatases in plants

Reversible protein phosphorylation is an essential posttranslational modification mechanism executed by opposing actions of protein phosphatases and protein kinases. About 1,000 predicted kinases in Arabidopsis thaliana kinome predominate the number of protein phosphatases, of which there are only ~150 members in Arabidopsis. Protein phosphatases were often referred to as "housekeeping" enzymes, which act to keep eukaryotic systems in balance by counteracting the activity of protein kinases. However, recent investigations reveal the crucial and specific regulatory functions of phosphatases in cell signaling. Phosphatases operate in a coordinated manner with the protein kinases, to execute their important function in determining the cellular response to a physiological stimulus. Closer examination has established high specificity of phosphatases in substrate recognition and important roles in plant signaling pathways, such as pathogen defense and stress regulation, light and hormonal signaling, cell cycle and differentiation, metabolism, and plant growth. In this minireview we provide a compact overview about Arabidopsis protein phosphatase families, as well as members of phosphoglucan and lipid phosphatases, and highlight the recent discoveries in phosphatase research.

ROS mediated MAPK signaling in abiotic and biotic stress- striking similarities and differences

Plants encounter a number of environmental stresses throughout their life cycles, most of which activate mitogen activated protein kinase (MAPK) pathway. The MAPKs show crosstalks at several points but the activation and the final response is known to be specific for particular stimuli that in-turn activates specific set of downstream targets. Interestingly, reactive oxygen species (ROS) is an important and common messenger produced in various environmental stresses and is known to activate many of the MAPKs. ROS activates a similar MAPK in different environmental stimuli, showing different downstream targets with different and specific responses. In animals and yeast, the mechanism behind the specific activation of MAPK by different concentration and species of ROS is elaborated, but in plants this aspect is still unclear. This review mainly focuses on the aspect of specificity of ROS mediated MAPK activation. Attempts have been made to review the involvement of ROS in abiotic stress mediated MAPK signaling and how it differentiates with that of biotic stress.

ROS-mediated redox signaling during cell differentiation in plants

BACKGROUND

Reactive oxygen species (ROS) have emerged in recent years as important regulators of cell division and differentiation.

SCOPE OF REVIEW

The cellular redox state has a major impact on cell fate and multicellular organism development. However, the exact molecular mechanisms through which ROS manifest their regulation over cellular development are only starting to be understood in plants. ROS levels are constantly monitored and any change in the redox pool is rapidly sensed and responded upon. Different types of ROS cause specific oxidative modifications, providing the basic characteristics of a signaling molecule. Here we provide an overview of ROS sensors and signaling cascades that regulate transcriptional responses in plants to guide cellular differentiation and organ development.

MAJOR CONCLUSIONS

Although several redox sensors and cascades have been identified, they represent only a first glimpse on the impact that redox signaling has on plant development and growth.

GENERAL SIGNIFICANCE

We provide an initial evaluation of ROS signaling cascades involved in cell differentiation in plants and identify potential avenues for future studies. This article is part of a Special Issue entitled Redox regulation of differentiation and de-differentiation.

Xanthomonas campestris lipooligosaccharides trigger innate immunity and oxidative burst in Arabidopsis

Plants lack the adaptive immunity mechanisms of jawed vertebrates, so they rely on innate immune responses to defense themselves from pathogens. The plant immune system perceives the presence of pathogens by recognition of molecules known as pathogen-associated molecular patterns (PAMPs). PAMPs have several common characteristics, including highly conserved structures, essential for the microorganism but absent in host organisms. Plants can specifically recognize PAMPs using a large set of receptors and can respond with appropriate defenses by activating a multicomponent and multilayered response. Lipopolysaccharides (LPSs) and lipooligosaccharides (LOSs) are major components of the cell surface of Gram-negative bacteria with diverse roles in bacterial pathogenesis of animals and plants that include elicitation of host defenses. Little is known on the mechanisms of perception of these molecules by plants and the associated signal transduction pathways that trigger plant immunity.Here we addressed the question whether the defense signaling pathway in Arabidopsis thaliana was triggered by LOS from Xanthomonas campestris pv. campestris (Xcc), using proteomic and transcriptomic approaches. By using affinity capture strategies with immobilized LOS and LC-MS/MS analyses, we identified 8 putative LOS protein ligands. Further investigation of these interactors led to the definition that LOS challenge is able to activate a signal transduction pathway that uses nodal regulators in common with salicylic acid-mediated pathway. Moreover, we proved evidence that Xcc LOS are responsible for oxidative burst in Arabidopsis either in infiltrated or systemic leaves. In addition, gene expression studies highlighted the presence of gene network involved in reactive oxygen species transduction pathway.

How plants cope with heavy metals

Heavy metals are naturally occurring in the earth's crust but anthropogenic and industrial activities have led to drastic environmental pollutions in distinct areas. Plants are able to colonize such sites due to several mechanisms of heavy metal tolerance. Understanding of these pathways enables different fruitful approaches like phytoremediation and biofortification.Therefore, this review addresses mechanisms of heavy metal tolerance and toxicity in plants possessing a sophisticated network for maintenance of metal homeostasis. Key elements of this are chelation and sequestration which result either in removal of toxic metal from sensitive sites or conduct essential metal to their specific cellular destination. This implies shared pathways which can result in toxic symptoms especially in an excess of metal. These overlaps go on with signal transduction pathways induced by heavy metals which include common elements of other signal cascades. Nevertheless, there are specific reactions some of them will be discussed with special focus on the cellular level.

Redox regulation in plant immune function

SIGNIFICANCE

Production of reactive oxygen species (ROS) and reactive nitrogen species (RNS) occurs rapidly in response to attempted pathogen invasion of potential host plants. Such reduction-oxidation (redox) changes are sensed and transmitted to engage immune function, including the hypersensitive response, a programmed execution of challenged plant cells.

RECENT ADVANCES

Pathogen elicitors trigger changes in calcium that are sensed by calmodulin, calmodulin-like proteins, and calcium-dependent protein kinases, which activate ROS and RNS production. The ROS and RNS production is compartmentalized within the cell and occurs through multiple routes. Mitogen-activated protein kinase (MAPK) cascades are engaged upstream and downstream of ROS and nitric oxide (NO) production. NO is increasingly recognized as a key signaling molecule, regulating downstream protein function through S-nitrosylation, the addition of an NO moiety to a reactive cysteine thiol.

CRITICAL ISSUES

How multiple sources of ROS and RNS are coordinated is unclear. The putative protein sensors that detect and translate fluxes in ROS and RNS into differential gene expression are obscure. Protein tyrosine nitration following reaction of peroxynitrite with tyrosine residues has been proposed as another signaling mechanism or as a marker leading to protein degradation, but the reversibility remains to be established.

FUTURE DIRECTIONS

Research is needed to identify the full spectrum of NO-modified proteins with special emphasis on redox-activated transcription factors and their cognate target genes. A systems approach will be required to uncover the complexities integral to redox regulation of MAPK cascades, transcription factors, and defense genes through the combined effects of calcium, phosphorylation, S-nitrosylation, and protein tyrosine nitration.

Redox regulation of transcription factors in plant stress acclimation and development

SIGNIFICANCE

The redox regulatory signaling network of the plant cell controls and co-regulates transcriptional activities, thereby enabling adjustment of metabolism and development in response to environmental cues, including abiotic stress.

RECENT ADVANCES

Our rapidly expanding knowledge on redox regulation of plant transcription is driven by methodological advancements such as sensitive redox proteomics and in silico predictions in combination with classical targeted genetic and molecular approaches, often in Arabidopsis thaliana. Thus, transcription factors (TFs) are both direct and indirect targets of redox-dependent activity modulation. Redox control of TF activity involves conformational switching, nucleo-cytosolic partitioning, assembly with coregulators, metal-S-cluster regulation, redox control of upstream signaling elements, and proteolysis.

CRITICAL ISSUES

While the significance of redox regulation of transcription is well established for prokaryotes and non-plant eukaryotes, the momentousness of redox-dependent control of transcription in plants still receives insufficient awareness and, therefore, is discussed in detail in this review.

FUTURE DIRECTIONS

Improved proteome sensitivity will enable characterization of low abundant proteins and to simultaneously address the various post-translational modifications such as nitrosylation, hydroxylation, and glutathionylation. Combining such approaches by gradually increasing biotic and abiotic stress strength is expected to result in a systematic understanding of redox regulation. In the end, only the combination of in vivo, ex vivo, and in vitro results will provide conclusive pictures on the rather complex mechanism of redox regulation of transcription.

Sulfenome mining in Arabidopsis thaliana

Reactive oxygen species (ROS) have been shown to be potent signaling molecules. Today, oxidation of cysteine residues is a well-recognized posttranslational protein modification, but the signaling processes steered by such oxidations are poorly understood. To gain insight into the cysteine thiol-dependent ROS signaling in Arabidopsis thaliana, we identified the hydrogen peroxide (H2O2)-dependent sulfenome: that is, proteins with at least one cysteine thiol oxidized to a sulfenic acid. By means of a genetic construct consisting of a fusion between the C-terminal domain of the yeast (Saccharomyces cerevisiae) AP-1-like (YAP1) transcription factor and a tandem affinity purification tag, we detected ∼ 100 sulfenylated proteins in Arabidopsis cell suspensions exposed to H2O2 stress. The in vivo YAP1-based trapping of sulfenylated proteins was validated by a targeted in vitro analysis of dehydroascorbate reductase2 (DHAR2). In DHAR2, the active site nucleophilic cysteine is regulated through a sulfenic acid-dependent switch, leading to S-glutathionylation, a protein modification that protects the protein against oxidative damage.

Phosphoglucan phosphatase function sheds light on starch degradation

Phosphoglucan phosphatases are novel enzymes that remove phosphates from complex carbohydrates. In plants, these proteins are vital components in the remobilization of leaf starch at night. Breakdown of starch is initiated through reversible glucan phosphorylation to disrupt the semi-crystalline starch structure at the granule surface. The phosphoglucan phosphatases starch excess 4 (SEX4) and like-SEX4 2 (LSF2) dephosphorylate glucans to provide access for amylases that release maltose and glucose from starch. Another phosphatase, LSF1, is a putative inactive scaffold protein that may act as regulator of starch degradative enzymes at the granule surface. Absence of these phosphatases disrupts starch breakdown, resulting in plants accumulating excess starch. Here, we describe recent advances in understanding the biochemical and structural properties of each of these starch phosphatases.

Arabidopsis protein phosphatase DBP1 nucleates a protein network with a role in regulating plant defense

Arabidopsis thaliana DBP1 belongs to the plant-specific family of DNA-binding protein phosphatases. Although recently identified as a novel host factor mediating susceptibility to potyvirus, little is known about DBP1 targets and partners and the molecular mechanisms underlying its function. Analyzing changes in the phosphoproteome of a loss-of-function dbp1 mutant enabled the identification of 14-3-3λ isoform (GRF6), a previously reported DBP1 interactor, and MAP kinase (MAPK) MPK11 as components of a small protein network nucleated by DBP1, in which GRF6 stability is modulated by MPK11 through phosphorylation, while DBP1 in turn negatively regulates MPK11 activity. Interestingly, grf6 and mpk11 loss-of-function mutants showed altered response to infection by the potyvirus Plum pox virus (PPV), and the described molecular mechanism controlling GRF6 stability was recapitulated upon PPV infection. These results not only contribute to a better knowledge of the biology of DBP factors, but also of MAPK signalling in plants, with the identification of GRF6 as a likely MPK11 substrate and of DBP1 as a protein phosphatase regulating MPK11 activity, and unveils the implication of this protein module in the response to PPV infection in Arabidopsis.

Behind the scenes: the roles of reactive oxygen species in guard cells

Guard cells regulate stomatal pore size through integration of both endogenous and environmental signals; they are widely recognized as providing a key switching mechanism that maximizes both the efficient use of water and rates of CO₂ exchange for photosynthesis; this is essential for the adaptation of plants to water stress. Reactive oxygen species (ROS) are widely considered to be an important player in guard cell signalling. In this review, we focus on recent progress concerning the role of ROS as signal molecules in controlling stomatal movement, the interaction between ROS and intrinsic and environmental response pathways, the specificity of ROS signalling, and how ROS signals are sensed and relayed. However, the picture of ROS-mediated signalling is still fragmented and the issues of ROS sensing and the specificity of ROS signalling remain unclear. Here, we review some recent advances in our understanding of ROS signalling in guard cells, with an emphasis on the main players known to interact with abscisic acid signalling.

Redox control of plant growth and development

Redox changes determined by genetic and environmental factors display well-organized interactions in the control of plant growth and development. Diurnal and seasonal changes in the environmental conditions are important for the normal course of these physiological processes and, similarly to their mild irregular alterations, for stress adaptation. However, fast or large-scale environmental changes may lead to damage or death of sensitive plants. The spatial and temporal redox changes influence growth and development due to the reprogramming of metabolism. In this process reactive oxygen and nitrogen species and antioxidants are involved as components of signalling networks. The control of growth, development and flowering by reactive oxygen and nitrogen species and antioxidants in interaction with hormones at organ, tissue, cellular and subcellular level will be discussed in the present review. Unsolved problems of the field, among others the need for identification of new components and interactions in the redox regulatory network at various organization levels using systems biology approaches will be also indicated.

Redox modulation of plant developmental regulators from the class I TCP transcription factor family

TEOSINTE BRANCHED1-CYCLOIDEA-PROLIFERATING CELL FACTOR1 (TCP) transcription factors participate in plant developmental processes associated with cell proliferation and growth. Most members of class I, one of the two classes that compose the family, have a conserved cysteine at position 20 (Cys-20) of the TCP DNA-binding and dimerization domain. We show that Arabidopsis (Arabidopsis thaliana) class I proteins with Cys-20 are sensitive to redox conditions, since their DNA-binding activity is inhibited after incubation with the oxidants diamide, oxidized glutathione, or hydrogen peroxide or with nitric oxide-producing agents. Inhibition can be reversed by treatment with the reductants dithiothreitol or reduced glutathione or by incubation with the thioredoxin/thioredoxin reductase system. Mutation of Cys-20 in the class I protein TCP15 abolished its redox sensitivity. Under oxidizing conditions, covalently linked dimers were formed, suggesting that inactivation is associated with the formation of intermolecular disulfide bonds. Inhibition of class I TCP protein activity was also observed in vivo, in yeast (Saccharomyces cerevisiae) cells expressing TCP proteins and in plants after treatment with redox agents. This inhibition was correlated with modifications in the expression of the downstream CUC1 gene in plants. Modeling studies indicated that Cys-20 is located at the dimer interface near the DNA-binding surface. This places this residue in the correct orientation for intermolecular disulfide bond formation and explains the sensitivity of DNA binding to the oxidation of Cys-20. The redox properties of Cys-20 and the observed effects of cellular redox agents both in vitro and in vivo suggest that class I TCP protein action is under redox control in plants.

Analysis of tyrosine phosphorylation and phosphotyrosine-binding proteins in germinating seeds from Scots pine

Protein tyrosine phosphorylation in angiosperms has been implicated in various physiological processes, including seed development and germination. In conifers, the role of tyrosine phosphorylation and the mechanisms of its regulation are yet to be investigated. In this study, we examined the profile of protein tyrosine phosphorylation in Scots pine seeds at different stages of germination. We detected extensive protein tyrosine phosphorylation in extracts from Scots pine (Pinus sylvestris L.) dormant seeds. In addition, the pattern of tyrosine phosphorylation was found to change significantly during seed germination, especially at earlier stages of post-imbibition which coincides with the initiation of cell division, and during the period of intensive elongation of hypocotyls. To better understand the molecular mechanisms of phosphotyrosine signaling, we employed affinity purification and mass spectrometry for the identification of pTyr-binding proteins from the extracts of Scots pine seedlings. Using this approach, we purified two proteins of 10 and 43 kDa, which interacted specifically with pTyr-Sepharose and were identified by mass spectrometry as P. sylvestris defensin 1 (PsDef1) and aldose 1-epimerase (EC:5.1.3.3), respectively. Additionally, we demonstrated that both endogenous and recombinant PsDef1 specifically interact with pTyr-Sepharose, but not Tyr-beads. As the affinity purification approach did not reveal the presence of proteins with known pTyr binding domains (SH2, PTB and C2), we suggest that plants may have evolved a different mode of pTyr recognition, which yet remains to be uncovered.

Nuclear accumulation of cytosolic glyceraldehyde-3-phosphate dehydrogenase in cadmium-stressed Arabidopsis roots

NAD-dependent glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a ubiquitous enzyme involved in the glycolytic pathway. It has been widely demonstrated that mammalian GAPDH, in addition to its role in glycolysis, fulfills alternative functions mainly linked to its susceptibility to oxidative posttranslational modifications. Here, we investigated the responses of Arabidopsis (Arabidopsis thaliana) cytosolic GAPDH isoenzymes GAPC1 and GAPC2 to cadmium-induced stress in seedlings roots. GAPC1 was more responsive to cadmium than GAPC2 at the transcriptional level. In vivo, cadmium treatments induced different concomitant effects, including (1) nitric oxide accumulation, (2) cytosolic oxidation (e.g. oxidation of the redox-sensitive Green fluorescent protein2 probe), (3) activation of the GAPC1 promoter, (4) GAPC1 protein accumulation in enzymatically inactive form, and (5) strong relocalization of GAPC1 to the nucleus. All these effects were detected in the same zone of the root tip. In vitro, GAPC1 was inactivated by either nitric oxide donors or hydrogen peroxide, but no inhibition was directly provided by cadmium. Interestingly, nuclear relocalization of GAPC1 under cadmium-induced oxidative stress was stimulated, rather than inhibited, by mutating into serine the catalytic cysteine of GAPC1 (C155S), excluding an essential role of GAPC1 nitrosylation in the mechanism of nuclear relocalization, as found in mammalian cells. Although the function of GAPC1 in the nucleus is unknown, our results suggest that glycolytic GAPC1, through its high sensitivity to the cellular redox state, may play a role in oxidative stress signaling or protection in plants.

Low oxygen response mechanisms in green organisms

Low oxygen stress often occurs during the life of green organisms, mostly due to the environmental conditions affecting oxygen availability. Both plants and algae respond to low oxygen by resetting their metabolism. The shift from mitochondrial respiration to fermentation is the hallmark of anaerobic metabolism in most organisms. This involves a modified carbohydrate metabolism coupled with glycolysis and fermentation. For a coordinated response to low oxygen, plants exploit various molecular mechanisms to sense when oxygen is either absent or in limited amounts. In Arabidopsis thaliana, a direct oxygen sensing system has recently been discovered, where a conserved N-terminal motif on some ethylene responsive factors (ERFs), targets the fate of the protein under normoxia/hypoxia. In Oryza sativa, this same group of ERFs drives physiological and anatomical modifications that vary in relation to the genotype studied. The microalga Chlamydomonas reinhardtii responses to low oxygen seem to have evolved independently of higher plants, posing questions on how the fermentative metabolism is modulated. In this review, we summarize the most recent findings related to these topics, highlighting promising developments for the future.

Reactive oxygen species generation and signaling in plants

The introduction of molecular oxygen into the atmosphere was accompanied by the generation of reactive oxygen species (ROS) as side products of many biochemical reactions. ROS are permanently generated in plastids, peroxisomes, mitochiondria, the cytosol and the apoplast. Imbalance between ROS generation and safe detoxification generates oxidative stress and the accumulating ROS are harmful for the plants. On the other hand, specific ROS function as signaling molecules and activate signal transduction processes in response to various stresses. Here, we summarize the generation of ROS in the different cellular compartments and the signaling processes which are induced by ROS.

OsPFA-DSP1, a rice protein tyrosine phosphatase, negatively regulates drought stress responses in transgenic tobacco and rice plants

Dephosphorylation plays a pivotal role in regulating plant growth, development and abiotic/biotic stress responses. Here, we characterized a plant and fungi atypical dual-specificity phosphatase (PFA-DSP) subfamily member, OsPFA-DSP1, from rice. OsPFA-DSP1 was determined to be a functional protein tyrosine phosphatase (PTP) in vitro using phosphatase activity assays. Quantitative real-time PCR and GENEVESTIGATOR analysis showed that OsPFA-DSP1 mRNA was induced by drought stress. Transfection of rice protoplasts showed that OsPFA-DSP1 accumulated in both the cytoplasm and nucleus. Ectopic overexpression of OsPFA-DSP1 in tobacco increased sensitivity to drought stress and insensitivity to ABA-induced stomatal closure and inhibition of stomatal opening. Furthermore, overexpression of OsPFA-DSP1 in rice also increased sensitivity to drought stress. These results indicated that OsPFA-DSP1 is a functional PTP and may act as a negative regulator in drought stress responses.