cGMP regulates hydrogen peroxide accumulation in calcium-dependent salt resistance pathway in Arabidopsis thaliana roots

Wheat TaPYL9-involved signalling pathway impacts plant drought response through regulating distinct osmotic stress-associated physiological indices

The abscisic acid (ABA) signalling pathway plays a crucial role in plants' response to drought stress. In this study, we aimed to characterize the impact of an ABA signalling module, which consisted of TaPYL9 and its downstream partners in Triticum aestivum, on plant drought adaptation. Our results showed that TaPYL9 protein contains conserved motifs and targets plasma membrane and nucleus after being sorted by the endoplasmic reticulum. In addition, TaPYL9 transcripts in both roots and leaves were significantly upregulated in response to drought stress. We conducted glucuronidase (GUS) histochemical staining analysis for transgenic plants carrying a truncated TaPYL9 promoter, which suggested that cis-elements associate with ABA and drought response, such as ABRE, DRE and recognition sites MYB and MYC, regulating the gene transcription under drought conditions. Using protein interaction assays (i.e., yeast two-hybrid, bimolecular fluorescence complementation (BiFC), co-immunoprecipitation (Co-IP) and in vitro pull-down), we demonstrated interactions between the intermediate segment of TaPYL9, the intermediate segment of TaPP2C6, the N-terminus of TaSnRK2.8 and the C-terminus of the transcription factor TabZIP1 in wheat, indicating the involvement of TaPYL9 in the constitution of an ABA signalling module, namely TaPYL9/TaPP2C6/TaSnRK2.8/TabZIP1. Transgene analysis revealed that TaPYL9, TaSnRK2.8 and TabZIP1 positively regulated drought response, while TaPP2C6 negatively regulated it, and that these genes were closely associated with the regulation of stomata movement, osmolyte accumulation and ROS homeostasis. Electrophoretic mobility shift (EMSA) and transcriptioal activation assays indicated that TabZIP1 interacted promoters of TaP5CS2, TaSLAC1-1 and TaCAT2 and activated transcription of these genes, which regulated proline biosynthesis, stomata movement and ROS scavenging upon drought signalling, respectively. Furthermore, we found that the transcripts of TaPYL9 and stress-responsive genes were positively correlated with yields in wheat cultivars under field drought conditions. Altogether, our findings suggest that the TaPYL9-involved signalling pathway significantly regulates drought response by modulating osmotic stress-associated physiological processes in T. aestivum.

Cyclic guanosine monophosphate improves salt tolerance in Solanum lycopersicum

The cyclic nucleotide cyclic guanosine monophosphate (cGMP) is a powerful cell signaling molecule involved in biotic and abiotic stress perception and signal transduction. In the model plant Arabidopsis thaliana, salt and osmotic stress rapidly induce increase in cGMP which plays role by modulating the activity of monovalent cation transporters, possibly by direct binding to these proteins and by altering the expression of many abiotic stress responsive genes. In a recent study, a membrane permeable analogue of cGMP (8-bromo-cGMP) was found to have a promotive effect on soluble sugar, flavonoids and lignin content, and membrane integrity in Solanum lycopersicum seedlings under salt stress. However, it remains to be elucidated how salt stress affects the endogenous cGMP level in S. lycopersicum and if Br-cGMP-induced improvement in salt tolerance in S. lycopersicum involves altered cation fluxes. The current study was conducted to answer these questions. A rapid increase (within 30 s) in endogenous cGMP level was determined in S. lycopersicum roots after treatment with 100 mM NaCl. Addition of membrane permeable Br-cGMP in growth medium remarkably ameliorated the inhibitory effects of NaCl on seedlings' growth parameters, chlorophyll content and net photosynthesis rate. In salt stressed plants, Br-cGMP significantly decreased Na+ content by reducing its influx and increasing efflux while it improved plants K+ content by reducing its efflux and enhancing influx. Furthermore, supplementation with Br-cGMP improved plant's proline content and total antioxidant capacity, resulting in markedly decreased electrolyte leakage under salt stress. Br-cGMP increased the expression of Na+/H+ antiporter genes in roots and shoots of S. lycopersicum growing under salt stress, potentially enhancing plant's ability to sequester Na+ into the vacuole. The findings of this study provide insights into the mechanism of cGMP-induced salt stress tolerance in S. lycopersicum.

Genome-wide analysis of respiratory burst oxidase homolog gene family in pea (Pisum sativum L.)

Plant respiratory burst oxidase homologs (RBOHs) are key enzymes regulating superoxide production, which is important for plant development and responses to biotic and abiotic stresses. This study aimed to characterize the RBOH gene family in pea (Pisum sativum L.). Seven PsRBOH genes were identified in the pea genome and were phylogenetically clustered into five groups. Collinearity analyses of the RBOHs identified four pairs of orthologs between pea and soybean. The gene structure analysis showed that the number of exons ranged from 6 to 16. Amino acid sequence alignment, conserved domain, and conserved motif analyses showed that all seven PsRBOHs had typical features of plant RBOHs. The expression patterns of PsRBOH genes in different tissues provided suggested their roles in plant growth and organ development. In addition, the expression levels of PsRBOH genes under different abiotic stresses were analyzed via reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The results demonstrated that PsRBOH genes exhibited unique stress-response characteristics, which allowed for functional diversity in response to different abiotic stresses. Furthermore, four PsRBOHs had a high probability of localization in the plasma membrane, and PsRBOH6 was localized to the plasma membrane and endoplasmic reticulum. The results of this study provide valuable information for further functional analysis of pea RBOH genes and their role in plant adaptation to climate-driven environmental constraints.

cGMP functions as an important messenger involved in SlSAMS1-regulated salt stress tolerance in tomato

Salt stress adversely affects the growth, development, and yield of tomato (Solanum lycopersicum). SAM Synthetase (SAMS), which is responsible for the biosynthesis of S-adenosylmethionine (SAM, a precursor of polyamine biosynthesis), participates in plant response to abiotic stress. However, the regulatory mechanism of SAMS-mediated salt stress tolerance remains elusive. In this study, we characterized a SAMS homologue SlSAMS1 in tomato. We found that SlSAMS1 is highly expressed in tomato roots, and its expression can be induced by salt stress. Crucially, overexpression of SlSAMS1 in tomato enhances salt stress tolerance. Through metabolomic profiling, we identified some differentially accumulated metabolites, especially, a secondary messenger guanosine 3',5'-cyclic monophosphate (cGMP) which may play a key role in SlSAMS1-regulated salt tolerance. A series of physiological and biochemical data suggest that cGMP alleviates salt stress-induced growth inhibition, and potentially acts downstream of the polyamine-nitric oxide (PA-NO) signaling pathway to trigger H2O2 signaling in response to salt stress. Taken together, the study reveals that SlSAMS1 regulates tomato salt tolerance via the PA-NO-cGMP-H2O2 signal module. Our findings elucidate the regulatory pathway of SlSAMS1-induced plant response to salt stress and indicate a pivotal role of cGMP in salt tolerance.

Comparative metabolomic analysis between tomato and halophyte plants under intercropping conditions

Halophyte-based intercropping appears nowadays as a valuable approach in soil remediation and agriculture. In this work, intercropping between the halophyte Arthrocaulon macrostachyum and tomato (Solanum lycopersicum var. Sargento) was studied in both plant species using comparative mass spectrometry-based metabolomics coupled to metabolic pathway predictions. A significant number of changes in metabolites was observed in the halophyte. In terms of alteration of specific metabolic pathways, intercropping conditions stimulated sugar and starch metabolisms in tomato, whereas in the halophyte, intercropping mainly altered amino acid-related pathways. In addition, arginine and proline metabolism were commonly affected in both tomato and halophyte plants. Moreover, metabolomic changes were associated with physiological alterations in tomato. In this sense, mild oxidative stress was induced in intercropped tomato plants, which, in turn, could trigger signaling events leading to plant adjustment to intercropping conditions. This study represents the first approach toward understanding intercropping interactions at the metabolome level and its effect on plant physiology, opening up prospects for further characterization of this crop cultivation strategy.

H2 O2 , NO, and H2 S networks during root development and signalling under physiological and challenging environments: Beneficial or toxic?

Hydrogen peroxide (H₂ O₂ ) is a reactive oxygen species (ROS) and a key modulator of the development and architecture of the root system under physiological and adverse environmental conditions. Nitric oxide (NO) and hydrogen sulphide (H₂ S) also exert myriad functions on plant development and signalling. Accumulating pieces of evidence show that depending upon the dose and mode of applications, NO and H₂ S can have synergistic or antagonistic actions in mediating H₂ O₂ signalling during root development. Thus, H₂ O₂ -NO-H₂ S crosstalk might essentially impart tolerance to elude oxidative stress in roots. Growth and proliferation of root apex involve crucial orchestration of NO and H₂ S-mediated ROS signalling which also comprise other components including mitogen-activated protein kinase, cyclins, cyclin-dependent kinases, respiratory burst oxidase homolog (RBOH), and Ca²⁺ flux. This assessment provides a comprehensive update on the cooperative roles of NO and H₂ S in modulating H₂ O₂ homoeostasis during root development, abiotic stress tolerance, and root-microbe interaction. Furthermore, it also analyses the scopes of some fascinating future investigations associated with strigolactone and karrikins concerning H₂ O₂ -NO-H₂ S crosstalk in plant roots.

Alteration of proteome in germinating seedlings of piegonpea (Cajanus cajan) after salt stress

Pigeonpea (Cajanus cajan) is an important crop in semi-arid regions and a significant source of dietary proteins in India. The plant is sensitive to salinity stress, which adversely affects its productivity. Based on the dosage-dependent influence of salinity stress on the growth and ion contents in the young seedlings of pigeonpea, a comparative proteome analysis of control and salt stressed (150 mM NaCl) plants was conducted using 7 days-old seedlings. Among various amino acids, serine, aspartate and asparagine were the amino acids that showed increment in the root, whereas serine, aspartate and phenylalanine showed an upward trend in shoots under salt stress. Furthermore, a label-free and gel-free comparative Q-Tof, Liquid Chromatography-Mass spectrometry (LC-MS) revealed total of 118 differentially abundant proteins in roots and shoots with and without salt stress conditions. Proteins related to DNA-binding with one finger (Dof) transcription factor family and glycine betaine (GB) biosynthesis were differentially expressed in the shoot and root of the salinity-stressed seedlings. Exogenous application of choline on GB accumulation under salt stress showed the increase of GB pathway in C. cajan. Gene expression analysis for differentially abundant proteins revealed the higher induction of ethanolamine kinase (CcEthKin), choline-phosphate cytidylyltransferase 1-like (CcChoPh), serine hydroxymethyltransferase (CcSHMT) and Dof protein (CcDof29). The results indicate the importance of, choline precursor, serine biosynthetic pathways and glycine betaine synthesis in salinity stress tolerance. The glycine betaine protects plant from cellular damages and acts as osmoticum under stress condition. Protein interaction network (PIN) analysis demonstrated that 61% of the differentially expressed proteins exhibited positive interactions and 10% of them formed the center of the PIN. Further, The PIN analysis also highlighted the potential roles of the cytochrome c oxidases in sensing and signaling cascades governing salinity stress responses in pigeonpea.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01116-w.

Plant Transcription Factors Involved in Drought and Associated Stresses

Transcription factors (TFs) play a significant role in signal transduction networks spanning the perception of a stress signal and the expression of corresponding stress-responsive genes. TFs are multi-functional proteins that may simultaneously control numerous pathways during stresses in plants-this makes them powerful tools for the manipulation of regulatory and stress-responsive pathways. In recent years, the structure-function relationships of numerous plant TFs involved in drought and associated stresses have been defined, which prompted devising practical strategies for engineering plants with enhanced stress tolerance. Vast data have emerged on purposely basic leucine zipper (bZIP), WRKY, homeodomain-leucine zipper (HD-Zip), myeloblastoma (MYB), drought-response elements binding proteins/C-repeat binding factor (DREB/CBF), shine (SHN), and wax production-like (WXPL) TFs that reflect the understanding of their 3D structure and how the structure relates to function. Consequently, this information is useful in the tailored design of variant TFs that enhances our understanding of their functional states, such as oligomerization, post-translational modification patterns, protein-protein interactions, and their abilities to recognize downstream target DNA sequences. Here, we report on the progress of TFs based on their interaction pathway participation in stress-responsive networks, and pinpoint strategies and applications for crops and the impact of these strategies for improving plant stress tolerance.

Up-regulated 2-alkenal reductase expression improves low-nitrogen tolerance in maize by alleviating oxidative stress

In plants, cellular lipid peroxidation is enhanced under low nitrogen (LN) stress; this increases the lipid-derived reactive carbonyl species (RCS) levels. The cellular toxicity of RCS can be reduced by various RCS-scavenging enzymes. However, the roles of these enzymes in alleviating oxidative stress and improving nutrient use efficiency (NUE) under nutrient stress remain unknown. Here, we overexpressed maize endogenous NADPH-dependent 2-alkenal reductase (ZmAER) in maize; it significantly increased the tolerance of transgenic plants (OX-AER) to LN stress. Under LN condition, the biomass, nitrogen accumulation, NUE, and leaf photosynthesis of the OX-AER plants were significantly higher than those of the wild-type (WT) plants. The leaf and root malondialdehyde and H₂ O₂ levels in the transgenic plants were significantly lower than those in WT. The expression of antioxidant enzyme-related genes ZmCAT3, ZmPOD5 and ZmPOD13 was significantly higher in the transgenic lines than in WT. Under LN stress, the nitrate reductase activity in the OX-AER leaves was significantly increased compared with that in the WT leaves. Furthermore, under LN stress, ZmNRT1.1 and ZmNRT2.5 expression was upregulated in the OX-AER plants compared with that in WT. Overall, up-regulated ZmAER expression could enhance maize's tolerance to LN stress by alleviating oxidative stress and improve NUE.

Differential expression of AtWAKL10 in response to nitric oxide suggests a putative role in biotic and abiotic stress responses

Plant defense against pathogens and abiotic stresses is regulated differentially by communicating signal transduction pathways in which nitric oxide (NO) plays a key role. Here, we show the biological role of Arabidopsis thaliana wall-associated kinase (AtWAK) Like10 (AtWAKL10) that exhibits greater than a 100-fold change in transcript accumulation in response to the NO donor S-nitroso-L-cysteine (CysNO), identified from high throughput RNA-seq based transcriptome analysis. Loss of AtWAKL10 function showed a similar phenotype to wild type (WT) with, however, less branching. The growth of atwakl10 on media supplemented with oxidative or nitrosative stress resulted in differential results with improved growth following treatment with CysNO but reduced growth in response to S-nitrosoglutatione (GSNO) and methyl-viologen. Further, atwakl10 plants exhibited increased susceptibility to virulent Pseudomonas syringae pv tomato (Pst) DC3000 with a significant increase in pathogen growth and decrease in PR1 transcript accumulation compared to WT overtime. Similar results were found in response to Pst DC3000 avrB, resulting in increased cell death as shown by increased electrolyte leakage in atwakl10. Furthermore, atwakl10 also showed increased reactive oxygen species accumulation following Pst DC3000 avrB inoculation. Promoter analysis of AtWAKL10 showed transcription factor (TF) binding sites for biotic and abiotic stress-related TFs. Further investigation into the role of AtWAKL10 in abiotic stresses showed that following two weeks water-withholding drought condition most of the atwakl10 plants got wilted; however, the majority (60%) of these plants recovered following re-watering. In contrast, in response to salinity stress, atwakl10 showed reduced germination under 150 mM salt stress compared to WT, suggesting that NO-induced AtWAKL10 differentially regulates different abiotic stresses. Taken together, this study further elucidates the importance of NO-induced changes in gene expression and their role in plant biotic and abiotic stress tolerance.

Genome-Wide Identification and Functional Analysis of NADPH Oxidase Family Genes in Wheat During Development and Environmental Stress Responses

As the key producers of reactive oxygen species (ROS), NADPH oxidases (NOXs), also known as respiratory burst oxidase homologs (RBOHs), play crucial roles in various biological processes in plants with considerable evolutionary selection and functional diversity in the entire terrestrial plant kingdom. However, only limited resources are available on the phylogenesis and functions of this gene family in wheat. Here, a total of 46 NOX family genes were identified in the wheat genome, and these NOXs could be classified into three subgroups: typical TaNOXs, TaNOX-likes, and ferric reduction oxidases (TaFROs). Phylogenetic analysis indicated that the typical TaNOXs might originate from TaFROs during evolution, and the TaFROs located on Chr 2 might be the most ancient forms of TaNOXs. TaNOXs are highly expressed in wheat with distinct tissue or organ-specificity and stress-inducible diversity. A large-scale expression and/or coexpression analysis demonstrated that TaNOXs can be divided into four functional groups with different expression patterns under a broad range of environmental stresses. Different TaNOXs are coexpressed with different sets of other genes, which widely participate in several important intracellular processes such as cell wall biosynthesis, defence response, and signal transduction, suggesting their vital but diversity of roles in plant growth regulation and stress responses of wheat.

Proteomic Analysis of Calcium Effects on Soybean Root Tip under Flooding and Drought Stresses

Flooding and drought are disadvantageous environmental conditions that induce cytosolic calcium in soybean. To explore the effects of flooding- and drought-induced increases in calcium, a gel-free/label-free proteomic analysis was performed. Cytosolic calcium was decreased by blocking calcium channels in the endoplasmic reticulum (ER) and plasma membrane under both stresses. Calnexin, protein disulfide isomerase, heat shock proteins and thioredoxin were predominantly affected as the ER proteins in response to calcium, and ER-associated degradation-related proteins of HCP-like superfamily protein were up-regulated under stress exposure and then down-regulated. Glycolysis, fermentation, the tricarboxylic acid cycle and amino acid metabolism were mainly induced as the types of cellular metabolism in response to calcium under both stresses. Pyruvate decarboxylase was increased and decreased under flooding and drought, respectively, and was further decreased by the reduction of cytosolic calcium; however, it was recovered by exogenous calcium under both stresses. Furthermore, pyruvate decarboxylase activity was increased under flooding, but decreased under drought. These results suggest that calcium is involved in protein folding in the ER, and ER-associated degradation might alleviate ER stress during the early stage of both stresses. Furthermore, calcium appears to modify energy metabolism, and pyruvate decarboxylase may be a key enzyme in this process under flooding and drought.

Hydrogen Peroxide Is Involved in β-Cyclodextrin-hemin Complex-Induced Lateral Root Formation in Tomato Seedlings

Although previous results showed that β-cyclodextrin-hemin complex (β-CDH) could induce tomato lateral root (LR) formation, the corresponding downstream messengers are still not fully understood. In this report, similar to the inducing effects of exogenously applied hydrogen peroxide (H₂O₂), we discovered that β-CDH elicited RBOH1 transcript upregulation, endogenous H₂O₂ accumulation, and thereafter tomato LR development. Above responses were sensitive to dimethylthiourea (DMTU) and ascorbic acid (AsA), two membrane-permeable scavengers of H₂O₂, showing that accumulation of H₂O₂ and LR formation were significantly blocked. The test with diphenyleneiodonium (DPI; the inhibitor of NADPH oxidase) revealed that H₂O₂ mainly produced by NADPH oxidase, might be involved in LR formation triggered by β-CDH. qPCR combined with pharmacological and anatomical analyses showed that β-CDH-modulated several marker genes responsible for LR formation, such as CYCA3;1, CYCA2;1, CYCD3;1, and CDKA1 (four cell cycle regulatory genes), ARF7 and RSI-1 (two auxin signaling genes), LAX3 (an auxin influx carrier), IAA14 (encoding a member of the Aux/IAA protein family), PIN3 and PIN7 (two auxin efflux carriers), isocitrate dehydrogenase [NADP], NADH-cytochrome b₅ reductase 1, and L-ascorbate oxidase homolog genes (two reactive oxygen species-associated genes and one LR formation-related gene), were causally related to above H₂O₂ signaling. Particularly, representative proteins related to H₂O₂ metabolism and lateral rooting, were specifically induced in β-CDH-treated tomato seedlings. Overall, the results clearly suggested a vital role of H₂O₂ in the β-CDH-induced tomato LR formation, and β-CDH-elicited H₂O₂-related target proteins responsible for LR formation might be, at least partially, regulated at transcriptional and translational levels.

The arabidopsis cyclic nucleotide interactome

BACKGROUND

Cyclic nucleotides have been shown to play important signaling roles in many physiological processes in plants including photosynthesis and defence. Despite this, little is known about cyclic nucleotide-dependent signaling mechanisms in plants since the downstream target proteins remain unknown. This is largely due to the fact that bioinformatics searches fail to identify plant homologs of protein kinases and phosphodiesterases that are the main targets of cyclic nucleotides in animals.

METHODS

An affinity purification technique was used to identify cyclic nucleotide binding proteins in Arabidopsis thaliana. The identified proteins were subjected to a computational analysis that included a sequence, transcriptional co-expression and functional annotation analysis in order to assess their potential role in plant cyclic nucleotide signaling.

RESULTS

A total of twelve cyclic nucleotide binding proteins were identified experimentally including key enzymes in the Calvin cycle and photorespiration pathway. Importantly, eight of the twelve proteins were shown to contain putative cyclic nucleotide binding domains. Moreover, the identified proteins are post-translationally modified by nitric oxide, transcriptionally co-expressed and annotated to function in hydrogen peroxide signaling and the defence response. The activity of one of these proteins, GLYGOLATE OXIDASE 1, a photorespiratory enzyme that produces hydrogen peroxide in response to Pseudomonas, was shown to be repressed by a combination of cGMP and nitric oxide treatment.

CONCLUSIONS

We propose that the identified proteins function together as points of cross-talk between cyclic nucleotide, nitric oxide and reactive oxygen species signaling during the defence response.

The plasma membrane NADPH oxidase OsRbohA plays a crucial role in developmental regulation and drought-stress response in rice

Plasma membrane NADPH oxidases are major producers of reactive oxygen species (ROS) in plant cells under normal growth and stress conditions. In the present study the total activity of rice NADPH oxidases and the transcription of OsRbohA, which encodes an Oryza sativa plasma membrane NADPH oxidase, were stimulated by drought. OsRbohA was expressed in all tissues examined throughout development. Its mRNA was upregulated by a number of factors, including heat, drought, salt, oxidative stress and methyl jasmonate treatment. Compared with wild-type (WT), the OsRbohA-knockout mutant osrbohA exhibited upregulated expression of other respiratory burst oxidase homolog genes and multiple abnormal agronomic traits, including reduced biomass, low germination rate and decreased pollen viability and seed fertility. However, OsRbohA-overexpressing transgenic plants showed no differences in these traits compared with WT. Although osrbohA leaves and roots produced more ROS than WT, the mutant had lesser intracellular ROS. In contrast, OsRbohA-overexpressing transgenic plants exhibited higher ROS production at the intracellular level and in tissues. Ablation of OsRbohA impaired the tolerance of plants to various water stresses, whereas its overexpression enhanced the tolerance. In addition, a number of genes related to energy supply, substrate transport, stress response and transcriptional regulation were differentially expressed in osrbohA plants even under normal growth conditions, suggesting that OsRbohA has fundamental and broad functions in rice. These results indicate that OsRbohA-mediated processes are governed by complex signaling pathways that function during the developmental regulation and drought-stress response in rice.

Hydrogen Peroxide Signaling in Plant Development and Abiotic Responses: Crosstalk with Nitric Oxide and Calcium

Hydrogen peroxide (H2O2), as a reactive oxygen species, is widely generated in many biological systems. It has been considered as an important signaling molecule that mediates various physiological and biochemical processes in plants. Normal metabolism in plant cells results in H2O2 generation, from a variety of sources. Also, it is now clear that nitric oxide (NO) and calcium (Ca(2+)) function as signaling molecules in plants. Both H2O2 and NO are involved in plant development and abiotic responses. A wide range of evidences suggest that NO could be generated under similar stress conditions and with similar kinetics as H2O2. The interplay between H2O2 and NO has important functional implications to modulate transduction processes in plants. Moreover, close interaction also exists between H2O2 and Ca(2+) in response to development and abiotic stresses in plants. Cellular responses to H2O2 and Ca(2+) signaling systems are complex. There is quite a bit of interaction between H2O2 and Ca(2+) signaling in responses to several stimuli. This review aims to introduce these evidences in our understanding of the crosstalk among H2O2, NO, and Ca(2+) signaling which regulates plant growth and development, and other cellular and physiological responses to abiotic stresses.

Comprehensive Genomic Analysis and Expression Profiling of the NOX Gene Families under Abiotic Stresses and Hormones in Plants

Plasma membrane NADPH oxidases (NOXs) are key producers of reactive oxygen species under both normal and stress conditions in plants and they form functional subfamilies. Studies of these subfamilies indicated that they show considerable evolutionary selection. We performed a comparative genomic analysis that identified 50 ferric reduction oxidases (FRO) and 77 NOX gene homologs from 20 species representing the eight major plant lineages within the supergroup Plantae: glaucophytes, rhodophytes, chlorophytes, bryophytes, lycophytes, gymnosperms, monocots, and eudicots. Phylogenetic and structural analysis classified these FRO and NOX genes into four well-conserved groups represented as NOX, FRO I, FRO II, and FRO III. Further analysis of NOXs of phylogenetic and exon/intron structures showed that single intron loss and gain had occurred, yielding the diversified gene structures during the evolution of NOXs family genes and which were classified into four conserved subfamilies which are represented as Sub.I, Sub.II, Sub.III, and Sub.IV. Additionally, both available global microarray data analysis and quantitative real-time PCR experiments revealed that the NOX genes in Arabidopsis and rice (Oryza sativa) have different expression patterns in different developmental stages, various abiotic stresses and hormone treatments. Finally, coexpression network analysis of NOX genes in Arabidopsis and rice revealed that NOXs have significantly correlated expression profiles with genes which are involved in plants metabolic and resistance progresses. All these results suggest that NOX family underscores the functional diversity and divergence in plants. This finding will facilitate further studies of the NOX family and provide valuable information for functional validation of this family in plants.

The hydroxyl radical in plants: from seed to seed

The hydroxyl radical (OH(•)) is the most potent yet short-lived of the reactive oxygen species (ROS) radicals. Just as hydrogen peroxide was once considered to be simply a deleterious by-product of oxidative metabolism but is now acknowledged to have signalling roles in plant cells, so evidence is mounting for the hydroxyl radical as being more than merely an agent of destruction. Its oxidative power is harnessed to facilitate germination, growth, stomatal closure, reproduction, the immune response, and adaptation to stress. It features in plant cell death and is a key tool in microbial degradation of plant matter for recycling. Production of the hydroxyl radical in the wall, at the plasma membrane, and intracellularly is facilitated by a range of peroxidases, superoxide dismutases, NADPH oxidases, and transition metal catalysts. The spatio-temporal activity of these must be tightly regulated to target substrates precisely to the site of radical production, both to prevent damage and to accommodate the short half life and diffusive capacity of the hydroxyl radical. Whilst research has focussed mainly on the hydroxyl radical's mode of action in wall loosening, studies now extend to elucidating which proteins are targets in signalling systems. Despite the difficulties in detecting and manipulating this ROS, there is sufficient evidence now to acknowledge the hydroxyl radical as a potent regulator in plant cell biology.

Hydrogen sulfide is involved in maintaining ion homeostasis via regulating plasma membrane Na+/H+ antiporter system in the hydrogen peroxide-dependent manner in salt-stress Arabidopsis thaliana root

Hydrogen sulfide (H2S) and hydrogen peroxide (H2O2) function as the signaling molecules in plants responding to salt stresses. The present study presents a signaling network involving H2S and H2O2 in salt resistance pathway of the Arabidopsis root. Arabidopsis roots were sensitive to 100 mM NaCl treatment, which displayed a great increase in electrolyte leakage (EL) and Na(+)/K(+) ratio under salt stress. The treatment of H2S donors sodium hydrosulfide (NaHS) enhanced the salt tolerance by maintaining a lower Na(+)/K(+) ratio. In addition, the inhibition of root growth under salt stress was removed by H2S. Further studies indicated that H2O2 was involved in H2S-induced salt tolerance pathway. H2S induced the production of the endogenous H2O2 via regulating the activities of glucose-6-phosphate dehydrogenase (G6PDH) and plasma membrane (PM) NADPH oxidase, with the treatment with dimethylthiourea (DMTU, an ROS scavenger), diphenylene iodonium (DPI, a PM NADPH oxidase inhibitor), or glycerol (G6PDH inhibitor) removing the effect of H2S. Treatment with amiloride (an inhibitor of PM Na(+)/H(+) antiporter) and vanadate (an inhibitor of PM H(+)-ATPase) also inhibited the activity of H2S on Na(+)/K(+) ratio. Through an analysis of quantitative real-time polymerase chain reaction and Western blot, we found that H2S promoted the genes expression and the phosphorylation level of PM H(+)-ATPase and Na(+)/H(+) antiporter protein level. However, when the endogenous H2O2 level was inhibited by DPI or DMTU, the effect of H2S on the PM Na(+)/H(+) antiporter system was removed. Taken together, H2S maintains ion homeostasis in the H2O2-dependent manner in salt-stress Arabidopsis root.

cGMP and ethylene are involved in maintaining ion homeostasis under salt stress in Arabidopsis roots

KEY MESSAGE

cGMP promotes ethylene production and enhances the perception of ethylene. Endogenous ethylene or cGMP accumulation maintains ion homeostasis to enhancing salt resistance. etr1 - 3 is insensitive to cGMP under salt stress.

ABSTRACT

In the present study, we presented a signaling network involving ethylene and cGMP in salt resistance pathway of Arabidopsis roots. Results showed that the ethylene-insensitive mutant etr1-3 was more sensitive to salt stress than the wild type (WT). etr1-3 displayed a greater electrolyte leakage, thiobarbituric acid reactive substances and Na(+)/K(+) ratio, but a lower plasma membrane (PM) H(+)-ATPase activity compared to WT under the different NaCl contents. Application of 1-aminocyclopropane-1-carboxylic acid (ACC, an ethylene precursor) or 8-Br-cGMP (the cGMP analog) alleviated NaCl-induced injury by maintaining a lower Na(+)/K(+) ratio and increasing PM H(+)-ATPase activity in WT, but not in etr1-3. Roots treated with 8-Br-cGMP could promote ethylene production and enhance the expression of ACC synthase gene in WT. In addition, the 8-Br-cGMP action in NaCl stress was inhibited by aminooxyacetic acid (an inhibitor of ethylene biosynthesis), but 6-Anilino-5,8-quinolinedione (Ly83583, a guanylate cyclase inhibitor) could not affect ACC action in WT. These results suggest that ethylene functions as a downstream signal of cGMP that stimulates the PM H(+)-ATPase activity, which finally results in regulating ion homeostasis in Arabidopsis tolerance to salt. Moreover, cGMP enhanced the perception of ethylene in Arabidopsis under salt stress, which reversed the salt-induced increase of ETR1 and increased ERF1 at the transcript levels in WT. In a word, cGMP modulates salt resistance pathway of ethylene through regulating biosynthesis and perception of ethylene in Arabidopsis roots.

Annexin 1 regulates the H2O2-induced calcium signature in Arabidopsis thaliana roots

Hydrogen peroxide is the most stable of the reactive oxygen species (ROS) and is a regulator of development, immunity and adaptation to stress. It frequently acts by elevating cytosolic free Ca(2+) ([Ca(2+) ]cyt ) as a second messenger, with activation of plasma membrane Ca(2+) -permeable influx channels as a fundamental part of this process. At the genetic level, to date only the Ca(2) (+) -permeable Stelar K(+) Outward Rectifier (SKOR) channel has been identified as being responsive to hydrogen peroxide. We show here that the ROS-regulated Ca(2+) transport protein Annexin 1 in Arabidopsis thaliana (AtANN1) is involved in regulating the root epidermal [Ca(2+) ]cyt response to stress levels of extracellular hydrogen peroxide. Peroxide-stimulated [Ca(2+) ]cyt elevation (determined using aequorin luminometry) was aberrant in roots and root epidermal protoplasts of the Atann1 knockout mutant. Similarly, peroxide-stimulated net Ca(2+) influx and K(+) efflux were aberrant in Atann1 root mature epidermis, determined using extracellular vibrating ion-selective microelectrodes. Peroxide induction of GSTU1 (Glutathione-S-Transferase1 Tau 1), which is known to be [Ca(2+) ]cyt -dependent was impaired in mutant roots, consistent with a lesion in signalling. Expression of AtANN1 in roots was suppressed by peroxide, consistent with the need to restrict further Ca(2+) influx. Differential regulation of annexin expression was evident, with AtANN2 down-regulation but up-regulation of AtANN3 and AtANN4. Overall the results point to involvement of AtANN1 in shaping the root peroxide-induced [Ca(2+) ]cyt signature and downstream signalling.

A cis-element responsible for cGMP in the promoter of the soybean chalcone synthase gene

The cyclic nucleotides cGMP and cAMP have been reported to play key roles in the regulation of plant processes and responses. We have previously reported that several genes encoding flavonoid biosynthetic enzymes, including chalcone synthase (CHS) in soybean (Glycine max L.), were induced by cGMP but not cAMP. The soybean genome contains nine CHS gene copies (GmCHS1-9). We investigated the responsiveness of several GmCHS genes to cGMP, cAMP, NO, and white light. Quantitative RT-PCR analysis showed that the transcript levels of GmCHS7 and GmCHS8 were increased by 3.6- and 3.8-fold, respectively, with cGMP whereas the transcript levels of GmCHS2 remained constant. Although cAMP had no effect on the transcript levels of the three genes, NO had an activation effect on all three. White light activated the three genes in a transient manner, with GmCHS2, GmCHS7, and GmCHS8 transcript levels increasing 3-fold after 3 h and decreasing to basal levels after 9 h. The GmCHS8 promoter contains several important cis-elements, including the G-box and H-box forming the Unit-I-like sequence and the MYB binding sequence, a target of the GmMYB176 transcription factor regulating the expression of GmCHS8. A transient gene expression assay revealed the activation of the Unit-I-like sequence, but not of the MYB binding sequence, by cGMP. The combination of G-box and H-box was necessary for cGMP responsiveness. Taken together, these results suggest that the Unit-I-like sequence in the promoters of GmCHS7 and GmCHS8 is a cGMP responsive cis-element in these genes and that NO exerts its effect via cis-elements other than the Unit-I-like sequence.

Indole-3-butyric acid induces lateral root formation via peroxisome-derived indole-3-acetic acid and nitric oxide

Controlled plant growth requires regulation through a variety of signaling molecules, including steroids, peptides, radicals of oxygen and nitrogen, as well as the 'classical' phytohormone groups. Auxin is critical for the control of plant growth and also orchestrates many developmental processes, such as the formation of new roots. It modulates root architecture both slowly, through actions at the transcriptional level and, more rapidly, by mechanisms targeting primarily plasma membrane sensory systems and intracellular signaling pathways. The latter reactions use several second messengers, including Ca(2+) , nitric oxide (NO) and reactive oxygen species (ROS). Here, we investigated the different roles of two auxins, the major auxin indole-3-acetic acid (IAA) and another endogenous auxin indole-3-butyric acid (IBA), in the lateral root formation process of Arabidopsis and maize. This was mainly analyzed by different types of fluorescence microscopy and inhibitors of NO production. This study revealed that peroxisomal IBA to IAA conversion is followed by peroxisomal NO, which is important for IBA-induced lateral root formation. We conclude that peroxisomal NO emerges as a new player in auxin-induced root organogenesis. In particular, the spatially and temporally coordinated release of NO and IAA from peroxisomes is behind the strong promotion of lateral root formation via IBA.

Hydrogen peroxide is involved in cGMP modulating the lateral root development of Arabidopsis thaliana

3',5'-cyclic guanosine monophosphate (cGMP) and hydrogen peroxide (H₂O₂) function as the important signaling molecule which promote the lateral root development of Arabidopsis thaliana. In this study, interestingly, application of 8-Br-cGMP (the membrane permeable cGMP analog) promoted the endogenous H₂O₂ production. In addition, the decrease of endogenous H₂O₂ also inhibited the effect of cGMP on the lateral root development. Thus, H₂O₂ maybe act as a downstream signaling of cGMP molecule which is involved in the lateral root development of Arabidopsis. We further found that H₂O₂ affected cGMP modulating polar auxin transport. When the endogenous H₂O₂ level was inhibited, the effect of cGMP on the acropetal auxin transport and the basipetal auxin transport was removed. Moreover, pin2 was insensitive for cGMP and H₂O₂ suggesting that PIN2 protein plays an important role in cGMP and H₂O₂ modulating the lateral root development of Arabidopsis.

cGMP modulates Arabidopsis lateral root formation through regulation of polar auxin transport

The phytohormone auxin participates in lateral root formation and primary root growth in plants. The auxin gradient formation is mainly regulated by the direction of polar auxin transport (PAT). PAT requires PIN family proteins, which are auxin transport facilitators and contribute to the establishment and maintenance of auxin gradients and mediate multiple developmental processes. Here, we report the effect of the 3', 5'-cyclic guanosine monophosphate (cGMP), an important second messenger, on postembryonic developmental of Arabidopsis lateral root. We find that enhanced cGMP level through the application of the membrane permeable cGMP analog 8-Br-cGMP, promotes the initiation of lateral root primordia and formation of lateral root. 6-Anilino-5,8-quinolinedione (Ly83583, the guanylate cyclase inhibitor) negatively regulates the process. cGMP also mediates acropetal auxin transport and basipetal auxin transport in the root. We further find that 8-Br-cGMP and Ly83583 change the expression of auxin transport genes and alter the polar localization and expression of PIN1 and PIN2 proteins. Moreover, Ly83583 affects actin organization and localization. Taken together, we propose that cGMP affects auxin transport and auxin gradient through modulation PINs proteins localization and expression. cGMP regulates postembryonic formation of Arabidopsis lateral root through the crosstalk with PAT.

Characterization of Rice NADPH oxidase genes and their expression under various environmental conditions

Plasma membrane NADPH oxidases (Noxs) are key producers of reactive oxygen species under both normal and stress conditions in plants. We demonstrate that at least eleven genes in the genome of rice (Oryza sativa L.) were predicted to encode Nox proteins, including nine genes (OsNox1–9) that encode typical Noxs and two that encode ancient Nox forms (ferric reduction oxidase 1 and 7, OsFRO1 and OsFRO7). Phylogenetic analysis divided the Noxs from nine plant species into six subfamilies, with rice Nox genes distributed among subfamilies I to V. Gene expression analysis using semi-quantitative RT-PCR and real-time qRT-PCR indicated that the expression of rice Nox genes depends on organs and environmental conditions. Exogenous calcium strongly stimulated the expression of OsNox3, OsNox5, OsNox7, and OsNox8, but depressed the expression of OsFRO1. Drought stress substantially upregulated the expression of OsNox1–3, OsNox5, OsNox9, and OsFRO1, but downregulated OsNox6. High temperature upregulated OsNox5–9, but significantly downregulated OsNox1–3 and OsFRO1. NaCl treatment increased the expression of OsNox2, OsNox8, OsFRO1, and OsFRO7, but decreased that of OsNox1, OsNox3, OsNox5, and OsNox6. These results suggest that the expression profiles of rice Nox genes have unique stress-response characteristics, reflecting their related but distinct functions in response to different environmental stresses.

A cyclic nucleotide-gated channel (CNGC16) in pollen is critical for stress tolerance in pollen reproductive development

Cyclic nucleotide-gated channels (CNGCs) have been implicated in diverse aspects of plant growth and development, including responses to biotic and abiotic stress, as well as pollen tube growth and fertility. Here, genetic evidence identifies CNGC16 in Arabidopsis (Arabidopsis thaliana) as critical for pollen fertility under conditions of heat stress and drought. Two independent transfer DNA disruptions of cngc16 resulted in a greater than 10-fold stress-dependent reduction in pollen fitness and seed set. This phenotype was fully rescued through pollen expression of a CNGC16 transgene, indicating that cngc16-1 and 16-2 were both loss-of-function null alleles. The most stress-sensitive period for cngc16 pollen was during germination and the initiation of pollen tube tip growth. Pollen viability assays indicate that mutant pollen are also hypersensitive to external calcium chloride, a phenomenon analogous to calcium chloride hypersensitivities observed in other cngc mutants. A heat stress was found to increase concentrations of 3',5'-cyclic guanyl monophosphate in both pollen and leaves, as detected using an antibody-binding assay. A quantitative PCR analysis indicates that cngc16 mutant pollen have attenuated expression of several heat-stress response genes, including two heat shock transcription factor genes, HsfA2 and HsfB1. Together, these results provide evidence for a heat stress response pathway in pollen that connects a cyclic nucleotide signal, a Ca(2+)-permeable ion channel, and a signaling network that activates a downstream transcriptional heat shock response.

The cyclic nucleotide cGMP is involved in plant hormone signalling and alters phosphorylation of Arabidopsis thaliana root proteins

The cyclic nucleotide cGMP has been shown to play important roles in plant development and responses to abiotic and biotic stress. Yet much controversy remains regarding the exact role of this second messenger. Progress in unravelling cGMP function in plants was hampered by laborious and time-consuming methodology to measure changes in cellular [cGMP] but the development of fluorescence-based reporters has removed this disadvantage. This study used the FlincG cGMP reporter to investigate potential interactions between phytohormone and cGMP signalling and found a rapid and significant effect of the hormones abscisic acid (ABA), auxin (IAA), and jasmonic acid (JA) on cytoplasmic cGMP levels. In contrast, brassinosteroids and cytokinin did not evoke a cGMP signal. The effects of ABA, IAA, and JA were apparent at external concentrations in the nanomolar range with EC50 values of around 1000, 300, and 0.03 nmoles for ABA, IAA, and JA respectively. To examine potential mechanisms for how hormone-induced cGMP signals are propagated, the role of protein phosphorylation was tested. A phosphoproteomics analysis on Arabidopsis thaliana root microsomal proteins in the absence and presence of membrane-permeable cGMP showed 15 proteins that rapidly (within minutes) changed in phosphorylation status. Out of these, nine were previously shown to also alter phosphorylation status in response to plant hormones, pointing to protein phosphorylation as a target for hormone-induced cGMP signalling.

Phosphatidylinositol 3-kinase plays a vital role in regulation of rice seed vigor via altering NADPH oxidase activity

Phosphatidylinositol 3-kinase (PI3K) has been reported to be important in normal plant growth and stress responses. In this study, it was verified that PI3K played a vital role in rice seed germination through regulating NADPH oxidase activity. Suppression of PI3K activity by inhibitors wortmannin or LY294002 could abate the reactive oxygen species (ROS) formation, which resulted in disturbance to the seed germination. And then, the signal cascades that PI3K promoted the ROS liberation was also evaluated. Diphenylene iodonium (DPI), an NADPH oxidase inhibitor, suppressed most of ROS generation in rice seed germination, which suggested that NADPH oxidase was the main source of ROS in this process. Pharmacological experiment and RT-PCR demonstrated that PI3K promoted the expression of Os rboh9. Moreover, functional analysis by native PAGE and the measurement of the 2, 3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazo-lium-5- carboxanilide (XTT) formazan concentration both showed that PI3K promoted the activity of NADPH oxidase. Furthermore, the western blot analysis of OsRac-1 demonstrated that the translocation of Rac-1 from cytoplasm to plasma membrane, which was known as a key factor in the assembly of NADPH oxidase, was suppressed by treatment with PI3K inhibitors, resulting in the decreased activity of NADPH oxidase. Taken together, these data favored the novel conclusion that PI3K regulated NADPH oxidase activity through modulating the recruitment of Rac-1 to plasma membrane and accelerated the process of rice seed germination.