Nitric oxide as a key component in hormone-regulated processes

Cloning and characterization of a novel nitric oxide synthase gene from Exiguobacterium profundum and its expression in Escherichia coli

The recognition and characterization of gene-encoded nitric oxide synthase (NOS) from Exiguobacterium profundum are reported in this study. A new gene was sequenced and cloned from E. profundum and heterologously expressed in E. coli for functional identification, followed by protein purification using the His-tag. The stability and activity characteristics of the recombinant NOS were evaluated using different concentrations of IPTG at various time points. A band of approximately 42 kDa was observed by SDS-PAGE. The Km value of NOS, calculated based on the Michaelis-Menten equation was 0.59 μmol/L. Additionally, homologous sequence alignment analysis indicated that the new NOS shared 80.48 % similarity with the same protein from Bacillus subtilis and Umezawaea. The construction of the NOS expression vector and the purification of the recombinant protein provide a foundation for further functional research and inhibitor development.

Adaptive responses of nitric oxide (NO) and its intricate dialogue with phytohormones during salinity stress

Nitric oxide (NO) is a gaseous free radical that acts as a messenger for various plant phenomena corresponding to photomorphogenesis, fertilisation, flowering, germination, growth, and productivity. Recent developments have suggested the critical role of NO in inducing adaptive responses in plants during salinity. NO minimises salinity-induced photosynthetic damage and improves plant-water relation, nutrient uptake, stomatal conductance, electron transport, and ROS and antioxidant metabolism. NO contributes active participation in ABA-mediated stomatal regulation. Similar crosstalk of NO with other phytohormones such as auxins (IAAs), gibberellins (GAs), cytokinins (CKs), ethylene (ET), salicylic acid (SA), strigolactones (SLs), and brassinosteroids (BRs) were also observed. Additionally, we discuss NO interaction with other gaseous signalling molecules such as reactive oxygen species (ROS) and reactive sulphur species (RSS). Conclusively, the present review traces critical events in NO-induced morpho-physiological adjustments under salt stress and discusses how such modulations upgrade plant resilience.

Brassinosteroid enhances salt tolerance via S-nitrosoglutathione reductase and nitric oxide signaling pathway in mangrove Kandelia obovata

Brassinosteroid (BR) has been shown to modulate plant tolerance to various stresses. S-nitrosoglutathione reductase (GSNOR) is involved in the plant response to environment stress by fine-turning the level of nitric oxide (NO). However, whether GSNOR is involved in BR-regulated Na+ /K+ homeostasis to improve the salt tolerance in halophyte is unknown. Here, we firstly reported that high salinity increases the expression of BR-biosynthesis genes and the endogenous levels of BR in mangrove Kandelia obovata. Then, salt-induced BR triggers the activities and gene expressions of GSNOR and antioxidant enzymes, thereafter decrease the levels of malondialdehyde, hydrogen peroxide. Subsequently, BR-mediated GSNOR negatively regulates NO contributions to the reduction of reactive oxygen species generation and induction of the gene expression related to Na+ and K+ transport, leading to the decrease of Na+ /K+ ratio in the roots of K. obovata. Finally, the applications of exogenous BR, NO scavenger, BR biosynthetic inhibitor and GSNOR inhibitor further confirm the function of BR. Taken together, our result provides insight into the mechanism of BR in the response of mangrove K. obovata to high salinity via GSNOR and NO signaling pathway by reducing oxidative damage and modulating Na+ /K+ homeostasis.

Strigolactone and nitric oxide collaborate synergistically to boost tomato seedling resilience to arsenic toxicity via modulating physiology and antioxidant system

Arsenic (As) poses a significant environmental threat as a metalloid toxin, adversely affecting the health of both plants and animals. Strigolactones (SL) and nitric oxide (NO) are known to play crucial roles in plant physiology. Therefore, the present experiment was designed to investigate the potential cumulative role of SL (GR24-0.20 μM) and NO (100 μM) in mitigating the adverse effect of AsV (53 μM) by modulating physiological mechanisms in two genotypes of tomato (Riogrand and Super Strain 8). A sample randomized design with four replicates was used to arrange the experimental pots in the growth chamber. 45-d old both tomato cultivars under AsV toxicity exhibited reduced morphological attributes (root and shoot length, root and shoot fresh weight, and root and shoot dry weight) and physiological and biochemical characteristics [chlorophyll (Chl) a and b content, activity of δ-aminolevulinic acid dehydratase activity (an enzyme responsible for Chl biosynthesis), and carbonic anhydrase activity (an enzyme responsible for photosynthesis), and enhanced Chl degradation, overproduction of reactive oxygen species (ROS) and lipid peroxidation due to enhanced malondialdehyde (MDA) content. However, the combined application of SL and NO was more effective in enhancing the tolerance of both varieties to AsV toxicity compared to individual application. The combined application of SL and NO improved growth parameters, biosynthesis of Chls, NO and proline. However, the combined application significantly suppressed cellular damage by inhibiting MDA and overproduction of ROS in leaves and roots, as confirmed by the fluorescent microscopy study and markedly upregulated the antioxidant enzymes (catalase, peroxidase, superoxide dismutase, ascorbate dismutase and glutathione reductase) activity. This study provides clear evidence that the combined application of SL and NO supplementation significantly improves the resilience of tomato seedlings against AsV toxicity. The synergistic effect of SL and NO was confirmed by the application of cPTIO (an NO scavenger) with SL and NO. However, further molecular studies could be imperative to conclusively validate the simultaneous role of SL and NO in enhancing plant tolerance to abiotic stress.

The ROP2 GTPase Participates in Nitric Oxide (NO)-Induced Root Shortening in Arabidopsis

Nitric oxide (NO) is a versatile signal molecule that mediates environmental and hormonal signals orchestrating plant development. NO may act via reversible S-nitrosation of proteins during which an NO moiety is added to a cysteine thiol to form an S-nitrosothiol. In plants, several proteins implicated in hormonal signaling have been reported to undergo S-nitrosation. Here, we report that the Arabidopsis ROP2 GTPase is a further potential target of NO-mediated regulation. The ROP2 GTPase was found to be required for the root shortening effect of NO. NO inhibits primary root growth by altering the abundance and distribution of the PIN1 auxin efflux carrier protein and lowering the accumulation of auxin in the root meristem. In rop2-1 insertion mutants, however, wild-type-like root size of the NO-treated roots were maintained in agreement with wild-type-like PIN1 abundance in the meristem. The ROP2 GTPase was shown to be S-nitrosated in vitro, suggesting that NO might directly regulate the GTPase. The potential mechanisms of NO-mediated ROP2 GTPase regulation and ROP2-mediated NO signaling in the primary root meristem are discussed.

SlMYC2 mediates stomatal movement in response to drought stress by repressing SlCHS1 expression

Drought stress limits plant development and reproduction. Multiple mechanisms in plants are activated to respond to stress. The MYC2 transcription factor is a core regulator of the jasmonate (JA) pathway and plays a vital role in the crosstalk between abscisic acid (ABA) and JA. In this study, we found that SlMYC2 responded to drought stress and regulated stomatal aperture in tomato (Solanum lycopersicum). Overexpression of SlMYC2 repressed SlCHS1 expression and decreased the flavonol content, increased the reactive oxygen species (ROS) content in guard cells and promoted the accumulation of JA and ABA in leaves. Additionally, silencing the SlCHS1 gene produced a phenotype that was similar to that of the MYC2-overexpressing (MYC2-OE) strain, especially in terms of stomatal dynamics and ROS levels. Finally, we confirmed that SlMYC2 directly repressed the expression of SlCHS1. Our study revealed that SlMYC2 drove stomatal closure by modulating the accumulation of flavonol and the JA and ABA contents, helping us decipher the mechanism of stomatal movement under drought stress.

Unveiling Molecular Mechanisms of Nitric Oxide-Induced Low-Temperature Tolerance in Cucumber by Transcriptome Profiling

Cucumber (Cucumis sativus L.) is one of the most popular cultivated vegetable crops but it is intrinsically sensitive to cold stress due to its thermophilic nature. To explore the molecular mechanism of plant response to low temperature (LT) and the mitigation effect of exogenous nitric oxide (NO) on LT stress in cucumber, transcriptome changes in cucumber leaves were compared. The results showed that LT stress regulated the transcript level of genes related to the cell cycle, photosynthesis, flavonoid accumulation, lignin synthesis, active gibberellin (GA), phenylalanine metabolism, phytohormone ethylene and salicylic acid (SA) signaling in cucumber seedlings. Exogenous NO improved the LT tolerance of cucumber as reflected by increased maximum photochemical efficiency (Fv/Fm) and decreased chilling damage index (CI), electrolyte leakage and malondialdehyde (MDA) content, and altered transcript levels of genes related to phenylalanine metabolism, lignin synthesis, plant hormone (SA and ethylene) signal transduction, and cell cycle. In addition, we found four differentially expressed transcription factors (MYB63, WRKY21, HD-ZIP, and b-ZIP) and their target genes such as the light-harvesting complex I chlorophyll a/b binding protein 1 gene (LHCA1), light-harvesting complex II chlorophyll a/b binding protein 1, 3, and 5 genes (LHCB1, LHCB3, and LHCB5), chalcone synthase gene (CSH), ethylene-insensitive protein 3 gene (EIN3), peroxidase, phenylalanine ammonia-lyase gene (PAL), DNA replication licensing factor gene (MCM5 and MCM6), gibberellin 3 beta-dioxygenase gene (GA3ox), and regulatory protein gene (NPRI), which are potentially associated with plant responses to NO and LT stress. Notably, HD-ZIP and b-ZIP specifically responded to exogenous NO under LT stress. Taken together, these results demonstrate that cucumber seedlings respond to LT stress and exogenous NO by modulating the transcription of some key transcription factors and their downstream genes, thereby regulating photosynthesis, lignin synthesis, plant hormone signal transduction, phenylalanine metabolism, cell cycle, and GA synthesis. Our study unveiled potential molecular mechanisms of plant response to LT stress and indicated the possibility of NO application in cucumber production under LT stress, particularly in winter and early spring.

Scavenging of nitric oxide up-regulates photosynthesis under drought in Festuca arundinacea and F. glaucescens but reduces their drought tolerance

Nitric oxide (NO) has been proven to be involved in the regulation of many physiological processes in plants. Though the contribution of NO in plant response to drought has been demonstrated in numerous studies, this phenomenon remains still not fully recognized. The research presented here was performed to decipher the role of NO metabolism in drought tolerance and the ability to recover after stress cessation in two closely related species of forage grasses, important for agriculture in European temperate regions: Festuca arundinacea and F. glaucescens. In both species, two genotypes with distinct levels of drought tolerance were selected to compare their physiological reactions to simulated water deficit and further re-watering, combined with a simultaneous application of NO scavenger, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO). The results clearly indicated a strong relationship between scavenging of NO in leaves and physiological response of both analyzed grass species to water deficit and re-watering. It was revealed that NO generated under drought was mainly located in mesophyll cells. In plants with reduced NO level a higher photosynthetic capacity and delay in stomatal closure under drought, were observed. Moreover, NO scavenging resulted also in the increased membrane permeability and higher accumulation of ROS in cells of analyzed plants both under drought and re-watering. This phenomena indicate that lower NO level might reduce drought tolerance and the ability of F. arundinacea and F. glaucescens to recover after stress cessation.

GSNOR Contributes to Demethylation and Expression of Transposable Elements and Stress-Responsive Genes

In the past, reactive nitrogen species (RNS) were supposed to be stress-induced by-products of disturbed metabolism that cause oxidative damage to biomolecules. However, emerging evidence demonstrates a substantial role of RNS as endogenous signals in eukaryotes. In plants, S-nitrosoglutathione (GSNO) is the dominant RNS and serves as the ^•NO donor for S-nitrosation of diverse effector proteins. Remarkably, the endogenous GSNO level is tightly controlled by S-nitrosoglutathione reductase (GSNOR) that irreversibly inactivates the glutathione-bound NO to ammonium. Exogenous feeding of diverse RNS, including GSNO, affected chromatin accessibility and transcription of stress-related genes, but the triggering function of RNS on these regulatory processes remained elusive. Here, we show that GSNO reductase-deficient plants (gsnor1-3) accumulate S-adenosylmethionine (SAM), the principal methyl donor for methylation of DNA and histones. This SAM accumulation triggered a substantial increase in the methylation index (MI = [SAM]/[S-adenosylhomocysteine]), indicating the transmethylation activity and histone methylation status in higher eukaryotes. Indeed, a mass spectrometry-based global histone profiling approach demonstrated a significant global increase in H3K9me2, which was independently verified by immunological detection using a selective antibody. Since H3K9me2-modified regions tightly correlate with methylated DNA regions, we also determined the DNA methylation status of gsnor1-3 plants by whole-genome bisulfite sequencing. DNA methylation in the CG, CHG, and CHH contexts in gsnor1-3 was significantly enhanced compared to the wild type. We propose that GSNOR1 activity affects chromatin accessibility by controlling the transmethylation activity (MI) required for maintaining DNA methylation and the level of the repressive chromatin mark H3K9me2.

Nitric oxide production and signalling in algae

Nitric oxide (NO) was the first identified gaseous messenger and is now well established as a major ubiquitous signalling molecule. The rapid development of our understanding of NO biology in embryophytes came with the partial characterization of the pathways underlying its production and with the decrypting of signalling networks mediating its effects. Notably, the identification of proteins regulated by NO through nitrosation greatly enhanced our perception of NO functions. In comparison, the role of NO in algae has been less investigated. Yet, studies in Chlamydomonas reinhardtii have produced key insights into NO production through the identification of NO-forming nitrite reductase and of S-nitrosated proteins. More intriguingly, in contrast to embryophytes, a few algal species possess a conserved nitric oxide synthase, the main enzyme catalysing NO synthesis in metazoans. This latter finding paves the way for a deeper characterization of novel members of the NO synthase family. Nevertheless, the typical NO-cyclic GMP signalling module transducing NO effects in metazoans is not conserved in algae, nor in embryophytes, highlighting a divergent acquisition of NO signalling between the green and the animal lineages.

Crosstalk between melatonin and nitric oxide in plant development and stress responses

Melatonin is widely involved in plant growth and stress responses as a master regulator. Melatonin treatment alters the levels of endogenous nitric oxide (NO) and NO affects endogenous melatonin content. Melatonin and NO may induce various plant physiological behavior through interaction mechanism. However, the interactions between melatonin and NO in plants are largely unknown. The review presented the metabolism of endogenous melatonin and NO and their relationship in plants. The interactions between melatonin and NO in plant growth and development and responses to environmental stress were summarized. The molecular mechanisms of interaction between melatonin and NO in plants were also proposed.

The role of NO in plant response to salt stress: interactions with polyamines

Soil salinity is a major abiotic stress that limits plant growth and productivity. High concentrations of sodium chloride can cause osmotic and ionic effects. This stress minimises a plant's ability to uptake water and minerals, and increases Na+ accumulation in the cytosol, thereby disturbing metabolic processes. Prolonged plant exposure to salt stress can lead to oxidative stress and increased production of reactive oxygen species (ROS). Higher plants developed some strategies to cope with salt stress. Among these, mechanisms involving nitric oxide (NO) and polyamines (PAs) are particularly important. NO is a key signalling molecule that mediates a variety of physiological functions and defence responses against abiotic stresses in plants. Under salinity conditions, NO donors increase growth parameters, reduce Na+ toxicity, maintain ionic homeostasis, stimulate osmolyte accumulation and prevent damages caused by ROS. NO enhances salt tolerance of plants via post-translational protein modifications through S-nitrosylation of thiol groups, nitration of tyrosine residues and modulation of multiple gene expression. Several reviews have reported on the role of polyamines in modulating salt stress plant response and the capacity to enhance PA synthesis upon salt stress exposure, and it is known that NO and PAs interact under salinity. In this review, we focus on the role of NO in plant response to salt stress, paying particular attention to the interaction between NO and PAs.

Comparative transcriptome combined with metabolome analyses revealed key factors involved in nitric oxide (NO)-regulated cadmium stress adaptation in tall fescue

BACKGROUND

It has been reported that nitric oxide (NO) could ameliorate cadmium (Cd) toxicity in tall fescue; however, the underlying mechanisms of NO mediated Cd detoxification are largely unknown. In this study, we investigated the possible molecular mechanisms of Cd detoxification process by comparative transcriptomic and metabolomic approaches.

RESULTS

The application of Sodium nitroprusside (SNP) as NO donor decreased the Cd content of tall fescue by 11% under Cd stress (T1 treatment), but the Cd content was increased by 24% when treated with Carboxy-PTIO (c-PTIO) together with Nitro-L-arginine methyl ester (L-NAME) (T2 treatment). RNA-seq analysis revealed that 904 (414 up- and 490 down-regulated) and 118 (74 up- and 44 down-regulated) DEGs were identified in the T1 vs Cd (only Cd treatment) and T2 vs Cd comparisons, respectively. Moreover, metabolite profile analysis showed that 99 (65 up- and 34-down- regulated) and 131 (45 up- and 86 down-regulated) metabolites were altered in the T1 vs Cd and T2 vs Cd comparisons, respectively. The integrated analyses of transcriptomic and metabolic data showed that 81 DEGs and 15 differentially expressed metabolites were involved in 20 NO-induced pathways. The dominant pathways were antioxidant activities such as glutathione metabolism, arginine and proline metabolism, secondary metabolites such as flavone and flavonol biosynthesis and phenylpropanoid biosynthesis, ABC transporters, and nitrogen metabolism.

CONCLUSIONS

In general, the results revealed that there are three major mechanisms involved in NO-mediated Cd detoxification in tall fescue, including (a) antioxidant capacity enhancement; (b) accumulation of secondary metabolites related to cadmium chelation and sequestration; and (c) regulation of cadmium ion transportation, such as ABC transporter activation. In conclusion, this study provides new insights into the NO-mediated cadmium stress response.

Regulation of the biosynthesis of endogenous nitric oxide and abscisic acid in stored peaches by exogenous nitric oxide and abscisic acid

BACKGROUND

Nitric oxide (NO) and abscisic acid (ABA) are important regulators of plant response to cold stress, and they interact in response to cold signals. The primary goal of this study was to determine the roles of exogenous NO and ABA on the synthesis of endogenous NO and ABA in cold-stored peach fruit.

RESULTS

Exogenous NO and ABA maintained a relatively high content of NO, increased nitrate reductase (NR) activity, and inhibited the activity of NO synthase (NOS)-like and the levels of polyamine biosynthesis in peaches during cold storage. Treatments of potassium 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO), NO, N-nitro-l-Arg-methyl ester (_L -NAME), and sodium tungstate did not influence ABA content. Exogenous ABA increased the content of carotenoids and the activities of aldehyde oxidase (AO), 9-cis-epoxycarotenoid dioxygenase (NCED), and zeaxanthin epoxidase (ZEP) of ABA synthesis in peaches during cold storage, and upregulated the gene expression of PpAO1, PpNCED1, PpNCED2, and PpZEP. The production of endogenous NO was differentially inhibited by NO scavengers, ABA inhibitors, and NR inhibitors, but not affected by NOS-like inhibitors during cold storage.

CONCLUSION

Exogenous NO and ABA can induce endogenous NO synthesis in cold-stored peaches by the nitrate reductase pathway, and ABA can mediate endogenous ABA synthesis by the autocatalytic reaction. NO does not regulate ABA synthesis. © 2019 Society of Chemical Industry.

Nitric oxide and hydrogen sulfide crosstalk during heavy metal stress in plants

Gases such as ethylene, hydrogen peroxide (H₂ O₂ ), nitric oxide (NO), carbon monoxide (CO) and hydrogen sulfide (H₂ S) have been recognized as vital signaling molecules in plants and animals. Of these gasotransmitters, NO and H₂ S have recently gained momentum mainly because of their involvement in numerous cellular processes. It is therefore important to study their various attributes including their biosynthetic and signaling pathways. The present review provides an insight into various routes for the biosynthesis of NO and H₂ S as well as their signaling role in plant cells under different conditions, more particularly under heavy metal stress. Their beneficial roles in the plant's protection against abiotic and biotic stresses as well as their adverse effects have been addressed. This review describes how H₂ S and NO, being very small-sized molecules, can quickly pass through the cell membranes and trigger a multitude of responses to various factors, notably to various stress conditions such as drought, heat, osmotic, heavy metal and multiple biotic stresses. The versatile interactions between H₂ S and NO involved in the different molecular pathways have been discussed. In addition to the signaling role of H₂ S and NO, their direct role in posttranslational modifications is also considered. The information provided here will be helpful to better understand the multifaceted roles of H₂ S and NO in plants, particularly under stress conditions.

Present knowledge and controversies, deficiencies, and misconceptions on nitric oxide synthesis, sensing, and signaling in plants

After 30 years of intensive work, nitric oxide (NO) has just started to be characterized as a relevant regulatory molecule on plant development and responses to stress. Its reactivity as a free radical determines its mode of action as an inducer of posttranslational modifications of key target proteins through cysteine S-nitrosylation and tyrosine nitration. Many of the NO-triggered regulatory actions are exerted in tight coordination with phytohormone signaling. This review not only summarizes and updates the information accumulated on how NO is synthesized, sensed, and transduced in plants but also makes emphasis on controversies, deficiencies, and misconceptions that are hampering our present knowledge on the biology of NO in plants. The development of noninvasive accurate tools for the endogenous NO quantitation as well as the implementation of genetic approaches that overcome misleading pharmacological experiments will be critical for getting significant advances in better knowledge of NO homeostasis and regulatory actions in plants.

Multilevel Regulation of Peroxisomal Proteome by Post-Translational Modifications

Peroxisomes, which are ubiquitous organelles in all eukaryotes, are highly dynamic organelles that are essential for development and stress responses. Plant peroxisomes are involved in major metabolic pathways, such as fatty acid β-oxidation, photorespiration, ureide and polyamine metabolism, in the biosynthesis of jasmonic, indolacetic, and salicylic acid hormones, as well as in signaling molecules such as reactive oxygen and nitrogen species (ROS/RNS). Peroxisomes are involved in the perception of environmental changes, which is a complex process involving the regulation of gene expression and protein functionality by protein post-translational modifications (PTMs). Although there has been a growing interest in individual PTMs in peroxisomes over the last ten years, their role and cross-talk in the whole peroxisomal proteome remain unclear. This review provides up-to-date information on the function and crosstalk of the main peroxisomal PTMs. Analysis of whole peroxisomal proteomes shows that a very large number of peroxisomal proteins are targeted by multiple PTMs, which affect redox balance, photorespiration, the glyoxylate cycle, and lipid metabolism. This multilevel PTM regulation could boost the plasticity of peroxisomes and their capacity to regulate metabolism in response to environmental changes.

Reactive oxygen species, auxin and nitric oxide in metal-stressed roots: toxicity or defence

The presented review is a summary on the current knowledge about metal induced stress response in plants, focusing on the roles of reactive oxygen species, auxin and nitric oxide in roots. The article focuses mainly on the difference between defence and toxicity symptoms of roots during metal-induced stress. Nowadays, pollution of soils by heavy metals is a rapidly growing issue, which affects agriculture and human health. In order to deal with these problems, we must first understand the basic mechanisms and responses to environmental conditions in plants growing under such conditions. Studies so far show somewhat conflicting data, interpreting the same stress responses as both symptoms of defence and toxicity. Therefore, the aim of this review is to give a report about current knowledge of heavy metal-induced stress research, and also to differentiate between toxicity and defence, and outline the challenges of research, focusing on reactive oxygen and nitrogen species, auxin, and the interplay among them. There are still remaining questions on how reactive oxygen and nitrogen species, as well as auxin, can activate either symptoms of toxicity or defence, and adaptation responses.

Role of nitric oxide in plant responses to heavy metal stress: exogenous application versus endogenous production

Anthropogenic activities, such as industrial processes, mining, and agriculture, lead to an increase in heavy metal concentrations in soil, water, and air. Given their stability in the environment, heavy metals are difficult to eliminate and can constitute a human health risk by entering the food chain through uptake by crop plants. An excess of heavy metals is toxic for plants, which have various mechanisms to prevent their accumulation. However, once metals enter the plant, oxidative damage sometimes occurs, which can lead to plant death. Initial production of nitric oxide (NO), which may play a role in plant perception, signalling, and stress acclimation, has been shown to protect against heavy metals. Very little is known about NO-dependent mechanisms downstream from signalling pathways in plant responses to heavy metal stress. In this review, using bioinformatic techniques, we analyse studies of the involvement of NO in plant responses to heavy metal stress, its possible role as a cytoprotective molecule, and its relationship with reactive oxygen species. Some conclusions are drawn and future research perspectives are outlined to further elucidate the signalling mechanisms underlying the role of NO in plant responses to heavy metal stress.

Nitric oxide in plants: pro- or anti-senescence

Senescence is a regulated process of tissue degeneration that can affect any plant organ and consists of the degradation and remobilization of molecules to other growing tissues. Senescent organs display changes at the microscopic level as well as modifications to internal cellular structure and differential gene expression. A large number of factors influencing senescence have been described including age, nutrient supply, and environmental interactions. Internal factors such as phytohormones also affect the timing of leaf senescence. A link between the senescence process and the production of nitric oxide (NO) in senescing tissues has been known for many years. Remarkably, this link can be either a positive or a negative correlation depending upon the organ. NO can be both a signaling or a toxic molecule and is known to have multiple roles in plants; this review considers the duality of NO roles in the senescence process of two different plant organs, namely the leaves and root nodules.

Effect of Long-Term Farming Practices on Agricultural Soil Microbiome Members Represented by Metagenomically Assembled Genomes (MAGs) and Their Predicted Plant-Beneficial Genes

To follow the hypothesis that agricultural management practices affect structure and function of the soil microbiome regarding soil health and plant-beneficial traits, high-throughput (HT) metagenome analyses were performed on Chernozem soil samples from a long-term field experiment designated LTE-1 carried out at Bernburg-Strenzfeld (Saxony-Anhalt, Germany). Metagenomic DNA was extracted from soil samples representing the following treatments: (i) plough tillage with standard nitrogen fertilization and use of fungicides and growth regulators, (ii) plough tillage with reduced nitrogen fertilization (50%), (iii) cultivator tillage with standard nitrogen fertilization and use of fungicides and growth regulators, and (iv) cultivator tillage with reduced nitrogen fertilization (50%). Bulk soil (BS), as well as root-affected soil (RS), were considered for all treatments in replicates. HT-sequencing of metagenomic DNA yielded approx. 100 Giga bases (Gb) of sequence information. Taxonomic profiling of soil communities revealed the presence of 70 phyla, whereby Proteobacteria, Actinobacteria, Bacteroidetes, Planctomycetes, Acidobacteria, Thaumarchaeota, Firmicutes, Verrucomicrobia and Chloroflexi feature abundances of more than 1%. Functional microbiome profiling uncovered, i.a., numerous potential plant-beneficial, plant-growth-promoting and biocontrol traits predicted to be involved in nutrient provision, phytohormone synthesis, antagonism against pathogens and signal molecule synthesis relevant in microbe-plant interaction. Neither taxonomic nor functional microbiome profiling based on single-read analyses revealed pronounced differences regarding the farming practices applied. Soil metagenome sequences were assembled and taxonomically binned. The ten most reliable and abundant Metagenomically Assembled Genomes (MAGs) were taxonomically classified and metabolically reconstructed. Importance of the phylum Thaumarchaeota for the analyzed microbiome is corroborated by the fact that the four corresponding MAGs were predicted to oxidize ammonia (nitrification), thus contributing to the cycling of nitrogen, and in addition are most probably able to fix carbon dioxide. Moreover, Thaumarchaeota and several bacterial MAGs also possess genes with predicted functions in plant-growth-promotion. Abundances of certain MAGs (species resolution level) responded to the tillage practice, whereas the factors compartment (BS vs. RS) and nitrogen fertilization only marginally shaped MAG abundance profiles. Hence, soil management regimes promoting plant-beneficial microbiome members are very likely advantageous for the respective agrosystem, its health and carbon sequestration and accordingly may enhance plant productivity. Since Chernozem soils are highly fertile, corresponding microbiome data represent a valuable reference resource for agronomy in general.

Transcriptomic Analysis Reveals a Comprehensive Calcium- and Phytohormone-Dominated Signaling Response in Leymus chinensis Self-Incompatibility

Sheepgrass (Leymus chinensis (Trin.) Tzvel.) is an economically and ecologically important forage in the grass family. Self-incompatibility (SI) limits its seed production due to the low seed-setting rate after self-pollination. However, investigations into the molecular mechanisms of sheepgrass SI are lacking. Therefore, microscopic observation of pollen germination and pollen tube growth, as well as transcriptomic analyses of pistils after self- and cross-pollination, were performed. The results indicated that pollen tube growth was rapidly inhibited from 10 to 30 min after self-pollination and subsequently stopped but preceded normally after cross-pollination. Time course comparative transcriptomics revealed different transcriptome dynamics between self- and cross-pollination. A pool of SI-related signaling genes and pathways was generated, including genes related to calcium (Ca2+) signaling, protein phosphorylation, plant hormone, reactive oxygen species (ROS), nitric oxide (NO), cytoskeleton, and programmed cell death (PCD). A putative SI response molecular model in sheepgrass was presented. The model shows that SI may trigger a comprehensive calcium- and phytohormone-dominated signaling cascade and activate PCD, which may explain the rapid inhibition of self-pollen tube growth as observed by cytological analyses. These results provided new insight into the molecular mechanisms of sheepgrass (grass family) SI.

Interactive effects of abscisic acid and nitric oxide on chilling resistance and active oxygen metabolism in peach fruit during cold storage

BACKGROUND

Cold conditions can accelerate the production of reactive oxygen species (ROS), and excessive ROS may attack biological macromolecules, disrupt related signal pathways, induce oxidative stress and influence plant metabolism. The cross-talk between nitric oxide (NO) and abscisic acid (ABA) and the inhibitions by NO or ABA on oxidative damage have been reported in fruits. However, there are few reports about the roles of NO-ABA interactions in chilling stress and antioxidant defense in fruits during cold storage. This study was conducted to investigate the roles of NO, ABA and interactions between NO and ABA in response to chilling stress on peach fruit (Prunus persica (L.) Batsch, cv. 'Xintaihong').

RESULTS

Treatments with 15 µmol L^-1 NO, 100 µmol L^-1 ABA and 15 µmol L^-1 NO + 5 mmol L^-1 sodium tungstate solution could reduce ROS content, alleviate lipid peroxidation and enhance antioxidant enzyme activities and antioxidant capacities. However, treatments with 5 µmol L^-1 potassium 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO), 5 mmol L^-1 sodium tungstate and 100 µmol L^-1 ABA + 5 µmol L^-1 c-PTIO differentially blocked these protective effects and significantly increased ROS content and lipid peroxidation of peaches under low-temperature conditions.

CONCLUSIONS

Application of exogenous ABA could increase the resistance to cold-induced oxidative stress by enhancing the efficiency of enzymatic and non-enzymatic systems, which were partially mediated by NO. © 2018 Society of Chemical Industry.

Nitric oxide responses in Arabidopsis hypocotyls are mediated by diverse phytohormone pathways

Plants are often exposed to high levels of nitric oxide (NO) that affects development and stress-triggered responses. However, the way in which plants sense NO is still largely unknown. Here we combine the analysis of early changes in the transcriptome of plants exposed to a short acute pulse of exogenous NO with the identification of transcription factors (TFs) involved in NO sensing. The NO-responsive transcriptome was enriched in hormone homeostasis- and signaling-related genes. To assess events involved in NO sensing in hypocotyls, we used a functional sensing assay based on the NO-induced inhibition of hypocotyl elongation in etiolated seedlings. Hormone-related mutants and the TRANSPLANTA collection of transgenic lines conditionally expressing Arabidopsis TFs were screened for NO-triggered hypocotyl shortening. These approaches allowed the identification of hormone-related TFs, ethylene perception and signaling, strigolactone biosynthesis and signaling, and salicylate production and accumulation that are essential for or modulate hypocotyl NO sensing. Moreover, NO inhibits hypocotyl elongation through the positive and negative regulation of some abscisic acid (ABA) receptors and transcripts encoding brassinosteroid signaling components thereby also implicating these hormones in NO sensing.

Regulation of SCFTIR1/AFBs E3 ligase assembly by S-nitrosylation of Arabidopsis SKP1-like1 impacts on auxin signaling

The F-box proteins (FBPs) TIR1/AFBs are the substrate recognition subunits of SKP1–cullin–F-box (SCF) ubiquitin ligase complexes and together with Aux/IAAs form the auxin co-receptor. Although tremendous knowledge on auxin perception and signaling has been gained in the last years, SCF^TIR1/AFBs complex assembly and stabilization are emerging as new layers of regulation. Here, we investigated how nitric oxide (NO), through S-nitrosylation of ASK1 is involved in SCF^TIR1/AFBs assembly. We demonstrate that ASK1 is S-nitrosylated and S-glutathionylated in cysteine (Cys) 37 and Cys118 residues in vitro. Both, in vitro and in vivo protein-protein interaction assays show that NO enhances ASK1 binding to CUL1 and TIR1/AFB2, required for SCF^TIR1/AFB2 assembly. In addition, we demonstrate that Cys37 and Cys118 are essential residues for proper activation of auxin signaling pathway in planta. Phylogenetic analysis revealed that Cys37 residue is only conserved in SKP proteins in Angiosperms, suggesting that S-nitrosylation on Cys37 could represent an evolutionary adaption for SKP1 function in flowering plants. Collectively, these findings indicate that multiple events of redox modifications might be part of a fine-tuning regulation of SCF^TIR1/AFBs for proper auxin signal transduction.

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ASK1 adaptor protein of the SCF^TIR1/AFB E3 ligase complex is redox regulated.

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NO regulates ASK1 function by S-nitrosylation in Cys37 and Cys118 residues.

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NO enhances ASK1-CUL1 and ASK1-TIR1/AFB2 protein-protein interactions required for SCF^TIR1/AFB2 assembly in vitro and in vivo.

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S-nitrosylated residues in ASK1 are essential for activation of auxin signaling pathway in plants.

Transcriptome analysis of nitric oxide-responsive genes in upland cotton (Gossypium hirsutum)

Nitric oxide (NO) is an important signaling molecule with diverse physiological functions in plants. It is therefore important to characterize the downstream genes and signal transduction networks modulated by NO. Here, we identified 1,932 differentially expressed genes (DEGs) responding to NO in upland cotton using high throughput tag sequencing. The results of quantitative real-time polymerase chain reaction (qRT-PCR) analysis of 25 DEGs showed good consistency. Gene Ontology (GO) and KEGG pathway were analyzed to gain a better understanding of these DEGs. We identified 157 DEGs belonging to 36 transcription factor (TF) families and 72 DEGs related to eight plant hormones, among which several TF families and hormones were involved in stress responses. Hydrogen peroxide and malondialdehyde (MDA) contents were increased, as well related genes after treatment with sodium nitroprusside (SNP) (an NO donor), suggesting a role for NO in the plant stress response. Finally, we compared of the current and previous data indicating a massive number of NO-responsive genes at the large-scale transcriptome level. This study evaluated the landscape of NO-responsive genes in cotton and identified the involvement of NO in the stress response. Some of the identified DEGs represent good candidates for further functional analysis in cotton.

Involvement of NR and PM-NR in NO biosynthesis in cucumber plants subjected to salt stress

Nitrate reductase (NR) mainly reduces nitrate to nitrite. However, in certain conditions it can reduce nitrite to NO. In plants, a plasma membrane-associated form of NR (PM-NR) is present. It produces NO₂^- for nitrite NO/reductase (Ni-NOR), which can release NO into the apoplastic space. The effect of 50 mM NaCl on NO formation and the involvement of NR in NO biosynthesis were studied in cucumber seedling roots under salt stress. In salt-stressed roots, the amount of NO was higher than in control. The application of tungstate abolished the increase of NO level in stressed roots, indicating that NR was responsible for NO biosynthesis under the test conditions. The involvement of other molybdoenzymes was excluded using specific inhibitors. Furthermore, higher cNR and PM-NR activities were observed in NaCl-treated roots. The increase in NR activity was due to the stimulation of CsNR genes expression and posttranslational modifications, such as enzyme dephosphorylation. This was confirmed by Western blot analysis. Moreover, the increase of nitrite tissue level in short-term stressed roots and the nitrite/nitrate ratio, with a simultaneous decrease of nitrite reductase (NiR) activity, in both short- and long-term stressed roots, could promote the production of NO by NR in roots under salt stress.

Nitric oxide-mediated integrative alterations in plant metabolism to confer abiotic stress tolerance, NO crosstalk with phytohormones and NO-mediated post translational modifications in modulating diverse plant stress

Nitric oxide (NO) is a major signaling biomolecule associated with signal transduction in plants. The beneficial role of NO in plants, exposed to several abiotic stresses shifted our understanding as it being not only free radical, released from the toxic byproducts of oxidative metabolism but also helps in plant sustenance. An explosion of research in plant NO biology during the last two decades has revealed that NO is a key signal associated with plant growth, germination, photosynthesis, leaf senescence, pollen growth and reorientation. NO is beneficial as well as harmful to plants in a dose-dependent manner. Exogenous application of NO at lower concentrations promotes seed germination, hypocotyl elongation, pollen development, flowering and delays senescence but at higher concentrations it causes nitrosative damage to plants. However, this review concentrates on the beneficial impact of NO in lower concentrations in the plants and also highlights the NO crosstalk of NO with other plant hormones, such as auxins, gibberellins, abscisic acid, cytokinins, ethylene, salicylic acid and jasmonic acid, under diverse stresses. While concentrating on the multidimensional role of NO, an attempt has been made to cover the role of NO-mediated genes associated with plant developmental processes, metal uptake, and plant defense responses as well as stress-related genes. More recently, several NO-mediated post translational modifications, such as S-nitrosylation, N-end rule pathway operates under hypoxia and tyrosine nitration also occurs to modulate plant physiology.

Nitric oxide mediates brassinosteroid-induced flavonoid biosynthesis in Camellia sinensis L

Flavonoids are one of the key secondary metabolites determining the quality of tea. Although exogenous brassinosteroid (BR), a steroidal plant hormone, can stimulate polyphenol biosynthesis in tea plants (Camellia sinensis L.), the relevance of endogenous BR in flavonoid accumulation and the underlying mechanisms remain largely unknown. Here we show that BR enhances flavonoid concentration in tea leaves by inducing an increase in the endogenous concentration of nitric oxide (NO). Notably, exogenous BR increased levels of flavonoids as well as NO in a concentration dependent manner, while suppression of BR levels by an inhibitor of BR biosynthesis, brassinazole (BRz), decreased the concentrations of both flavonoids and NO in tea leaves. Interestingly, combined treatment of BR and BRz reversed the inhibitory effect of BRz alone on the concentrations of flavonoids and NO. Likewise, exogenous NO also increased flavonoids and NO levels dose-dependently. When the NO level in tea leaves was suppressed by using a NO scavenger, 2,4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), flavonoid concentration dramatically decreased. Although individual application of 0.1μM BR increased the concentrations of flavonoids and NO, combined treatment with exogenous NO scavenger, cPTIO, reversed the effect of BR on flavonoid concentration. Furthermore, BR or sodium nitroprusside (SNP) promoted but cPTIO inhibited the transcription and activity of phenylalanine ammonia-lyase (PAL) in leaves, while combined treatment of BR with SNP or cPTIO had no additive effect. The results of this study suggest that an optimal level of endogenous NO is essential for BR-induced promotion of flavonoid biosynthesis in tea leaves. In conclusion, this study unveiled a crucial mechanism of BR-induced flavonoid biosynthesis, which might have potential implication in improving the quality of tea.

Nitric oxide signaling and its crosstalk with other plant growth regulators in plant responses to abiotic stress

Nitric oxide (NO) is a free radical molecule involved in an array of functions under physiological and adverse environmental conditions. As other free radical molecules, NO biological action depends on its cellular concentration, acting as a signal molecule when produced at low concentration or resulting in cellular damage when produced at sufficiently high levels to trigger nitro-oxidative stress. Over the last decade, significant progress has been made in characterizing NO metabolism and action mechanism, revealing that diverse biosynthetic routes can generate this free radical in plants and its action mainly occurs through posttranslational modification (nitration and S-nitrosylation) of target proteins. Intricate crosstalk networks between NO and other signaling molecules have been described involving phytohormones, other second messengers, and key transcription factors. This review will focus on our current understanding of NO interplay with phytohormones and other plant growth regulators under abiotic stress conditions.

Brassinosteroids modulate ABA-induced stomatal closure in Arabidopsis

Stomatal movement in response to water availability is an important physiological process in the survival of land plants. The plant hormone abscisic acid (ABA) and brassinosteroids (BRs) regulate stomatal closure. The physiological functions of ABA and BRs, including germination, cell elongation and stomatal movement, are generally known to be antagonistic. Here, we investigated how BRs affect stomatal movement alone and in combination with ABA. We demonstrate that brassinoslide (BL), the most active BR, promotes stomatal closure in an ABA-independent manner. Interestingly, BL also inhibited ABA-induced stomatal closure when a high concentration of BL was added to ABA. Furthermore, we found that the induction of some genes for reactive oxygen species (ROS) generation by ABA (AtrbohD, NIA1 and NIA2) and subsequent ROS production were repressed by BL treatment. The BR signaling mutant bri1-301 failed to inhibit ABA-induced stomatal closure upon BL treatment. However, BRI1-overexpressing transgenic plants were hypersensitive to ABA during stomatal closure, and BL reversed ABA-induced stomatal closure more completely than in wild type plants. Taken together, these results suggest that BRs can positively and negatively modulate ABA-induced stomatal closure. Therefore, interactions between ABA and BR signaling are important for the regulation of stomatal closure.

A dual system formed by the ARC and NR molybdoenzymes mediates nitrite-dependent NO production in Chlamydomonas

Nitric oxide (NO) is a relevant signal molecule involved in many plant processes. However, the mechanisms and proteins responsible for its synthesis are scarcely known. In most photosynthetic organisms NO synthases have not been identified, and Nitrate Reductase (NR) has been proposed as the main enzymatic NO source, a process that in vitro is also catalysed by other molybdoenzymes. By studying transcriptional regulation, enzyme approaches, activity assays with in vitro purified proteins and in vivo and in vitro NO determinations, we have addressed the role of NR and Amidoxime Reducing Component (ARC) in the NO synthesis process. N\R and ARC were intimately related both at transcriptional and activity level. Thus, arc mutants showed high NIA1 (NR gene) expression and NR activity. Conversely, mutants without active NR displayed an increased ARC expression in nitrite medium. Our results with nia1 and arc mutants and with purified enzymes support that ARC catalyses the NO production from nitrite taking electrons from NR and not from Cytb5-1/Cytb5-Reductase, the component partners previously described for ARC (proposed as NOFNiR, Nitric Oxide-Forming Nitrite Reductase). This NR-ARC dual system would be able to produce NO in the presence of nitrate, condition under which NR is unable to do it.

Jasmonic acid is a downstream component in the modulation of somatic embryogenesis by Arabidopsis Class 2 phytoglobin

Previous studies have shown that the beneficial effect of suppression of the Arabidopsis phytoglobin 2 gene, PGB2, on somatic embryogenesis occurs through the accumulation of nitric oxide (NO) within the embryogenic cells originating from the cultured explant. NO activates the expression of Allene oxide synthase (AOS) and Lipoxygenase 2 (LOX2), genes encoding two key enzymes of the jasmonic acid (JA) biosynthetic pathway, elevating JA content within the embryogenic tissue. The number of embryos in the single aos1-1 mutant and pgb2-aos1-1 double mutant declined, and was not rescued by increasing levels of NO stimulating embryogenesis in wild-type tissue. NO also influenced JA responses by up-regulating PLANT DEFENSIN 1 (PDF1) and JASMONATE-ZIM-PROTEIN (JAZ1), as well as down-regulating MYC2. The NO and JA modulation of MYC2 and JAZ1 controlled embryogenesis. Ectopic expression of JAZ1 or suppression of MYC2 promoted the formation of somatic embryos, while repression of JAZ1 and up-regulation of MYC2 reduced the embryogenic performance. Sustained expression of JAZ1 induced the transcription of several indole acetic acid (IAA) biosynthetic genes, resulting in higher IAA levels in the embryogenic cells. Collectively these data fit a model integrating JA in the PGB2 regulation of Arabidopsis embryogenesis. Suppression of PGB2 increases JA through NO. Elevated levels of JA repress MYC2 and induce JAZ1, favoring the accumulation of IAA in the explants and the subsequent production of somatic embryos.

The Role of the Atypical Kinases ABC1K7 and ABC1K8 in Abscisic Acid Responses

The ABC1K family of atypical kinases (activity of bc1 complex kinase) is represented in bacteria, archaea, and eukaryotes. In plants they regulate diverse physiological processes in the chloroplasts and mitochondria, but their precise functions are poorly defined. ABC1K7 and ABC1K8 are probably involved in oxidative stress responses, isoprenyl lipid synthesis and distribution of iron within chloroplasts. Because reactive oxygen species take part in abscisic acid (ABA)-mediated processes, we investigated the functions of ABC1K7 and ABC1K8 during germination, stomatal movement, and leaf senescence. Both genes were upregulated by ABA treatment and some ABA-responsive physiological processes were affected in abc1k7 and abc1k8 mutants. Germination was more severely affected by ABA, osmotic stress and salt stress in the single and double mutants; the stomatal aperture was smaller in the mutants under standard growth conditions and was not further reduced by exogenous ABA application; ABA-induced senescence symptoms were more severe in the leaves of the single and double mutants compared to wild type leaves. Taken together, our results suggest that ABC1K7 and ABC1K8 might be involved in the cross-talk between ABA and ROS signaling.

Elevated CO2-induced production of nitric oxide (NO) by NO synthase differentially affects nitrate reductase activity in Arabidopsis plants under different nitrate supplies

CO2 elevation often alters the plant's nitrate reductase (NR) activity, the first enzyme acting in the nitrate assimilation pathway. However, the mechanism underlying this process remains unknown. The association between elevated CO2-induced alterations of NR activity and nitric oxide (NO) was examined in Col-0 Arabidopsis fed with 0.2-10 mM nitrate, using NO donors, NO scavenger, and NO synthase (NOS) inhibitor. The noa1 mutant, in which most NOS activity was lost, and the NR activity-null mutant nia1 nia2 were also used to examine the above association. In response to CO2 elevation, NR activity increased in low-nitrate Col-0 plants but was inhibited in high-nitrate Col-0 plants. NO scavenger and NOS inhibitor could eliminate these two responses, whereas the application of NO donors mimicked these distinct responses in ambient CO2-grown Col-0 plants. Furthermore, in both low- and high-nitrate conditions, elevated CO2 increased NOS activity and NO levels in Col-0 and nia1 nia2 plants but had little effect on NO level and NR activity in noa1 plants. Considering all of these findings, this study concluded that, in response to CO2 elevation, either the NR activity induction in low-nitrate plants or the NR activity inhibition in high-nitrate plants is regulated by NOS-generated NO.

Plant Hormone Homeostasis, Signaling, and Function during Adventitious Root Formation in Cuttings

Adventitious root (AR) formation in cuttings is a multiphase developmental process, resulting from wounding at the cutting site and isolation from the resource and signal network of the whole plant. Though, promotive effects of auxins are widely used for clonal plant propagation, the regulation and function of plant hormones and their intricate signaling networks during AR formation in cuttings are poorly understood. In this focused review, we discuss our recent publications on the involvement of polar auxin transport (PAT) and transcriptional regulation of auxin and ethylene action during AR formation in petunia cuttings in a broad context. Integrating new findings on cuttings of other plant species and general models on plant hormone networks, a model on the regulation and function of auxin, ethylene, and jasmonate in AR formation of cuttings is presented. PAT and cutting off from the basipetal auxin drain are considered as initial principles generating early accumulation of IAA in the rooting zone. This is expected to trigger a self-regulatory process of auxin canalization and maximization to responding target cells, there inducing the program of AR formation. Regulation of auxin homeostasis via auxin influx and efflux carriers, GH3 proteins and peroxidases, of flavonoid metabolism, and of auxin signaling via AUX/IAA proteins, TOPLESS, ARFs, and SAUR-like proteins are postulated as key processes determining the different phases of AR formation. NO and H2O2 mediate auxin signaling via the cGMP and MAPK cascades. Transcription factors of the GRAS-, AP2/ERF-, and WOX-families link auxin signaling to cell fate specification. Cyclin-mediated governing of the cell cycle, modifications of sugar metabolism and microtubule and cell wall remodeling are considered as important implementation processes of auxin function. Induced by the initial wounding and other abiotic stress factors, up-regulation of ethylene biosynthesis, and signaling via ERFs and early accumulation of jasmonic acid stimulate AR formation, while both pathways are linked to auxin. Future research on the function of candidate genes should consider their tissue-specific role and regulation by environmental factors. Furthermore, the whole cutting should be regarded as a system of physiological units with diverse functions specifically responding to the environment and determining the rooting response.

POLYAMINE OXIDASE2 of Arabidopsis contributes to ABA mediated plant developmental processes

Polyamines (PA) are catabolised by two groups of amine oxidases, the copper-binding amine oxidases (CuAOs) and the FAD-binding polyamine oxidases (PAOs). Previously, we have shown that CuAO1 is involved in ABA associated growth responses and ABA- and PA-mediated rapid nitric oxide (NO) production. Here we report the differential regulation of expression of POLYAMINE OXIDASE2 of Arabidopsis (AtPAO2) in interaction with ABA, nitrate and ammonium. Without ABA treatment germination, cotyledon growth and fresh weight of pao2 knockdown mutants as well as PAO2OX over-expressor plants were comparable to those of the wild type (WT) plants irrespective of the N source. In the presence of ABA, in pao2 mutants cotyledon growth and fresh weights were more sensitive to inhibition by ABA while PAO2OX over-expressor plants showed a rather similar response to WT. When NO3(-) was the only N source primary root lengths and lateral root numbers were lower in pao2 mutants both without and with exogenous ABA. PAO2OX showed enhanced primary and lateral root growth in media with NO3(-) or NH4(+). Vigorous root growth of PAO2OX and the hypersensitivity of pao2 mutants to ABA suggest a positive function of AtPAO2 in root growth. ABA-induced NO production in pao2 mutants was lower indicating a potential contributory function of AtPAO2 in NO-mediated effects on root growth.

A bioassay for brassinosteroid activity based on the in vitro fluorimetric detection of nitric oxide production

Recent studies have shown that low concentrations of brassinolide induce a rapid generation of nitric oxide in mesophyll cells of maize leaves, which can be easily detected by fluorimetric methods. In this work we describe a series of natural and synthetic brassinosteroids that are able to trigger in vitro NO production in tomato cells that exhibits dose-response behavior. We propose that this effect can be used to develop a new rapid and very sensitive bioassay for brassinosteroid activity that offers several advantages when compared to the current methodologies.

Light-modulated seminal wavy roots in rice mediated by nitric oxide-dependent signaling

Rice (Oryza sativa L.) seminal roots from germinated seeds help establish seedlings, but the seminal root growth and morphology are sensitive to environmental factors. Our previous research showed that several indica-type rice varieties such as Taichung native 1 (TCN1) showed light-induced wavy roots. Also, auxin and oxylipins are two signaling factors regulating the wavy root photomorphology. To investigate the signaling pathway, here, we found that nitric oxide (NO) was a second messenger triggering the signal transduction of light stimuli to induce the wavy morphology of seminal roots in rice. Moreover, interactions between oxylipins and phytohormones such as ethylene and auxin participating in the NO-dependent regulatory pathway of light-induced wavy roots were examined. The order of action of signaling components in the pathway was NO, oxylipins, ethylene, and auxin.

The role of reactive oxygen species and nitric oxide in programmed cell death associated with self-incompatibility

Successful sexual reproduction often relies on the ability of plants to recognize self- or genetically-related pollen and prevent pollen tube growth soon after germination in order to avoid self-fertilization. Angiosperms have developed different reproductive barriers, one of the most extended being self-incompatibility (SI). With SI, pistils are able to reject self or genetically-related pollen thus promoting genetic variability. There are basically two distinct systems of SI: gametophytic (GSI) and sporophytic (SSI) based on their different molecular and genetic control mechanisms. In both types of SI, programmed cell death (PCD) has been found to play an important role in the rejection of self-incompatible pollen. Although reactive oxygen species (ROS) were initially recognized as toxic metabolic products, in recent years, a new role for ROS has become apparent: the control and regulation of biological processes such as growth, development, response to biotic and abiotic environmental stimuli, and PCD. Together with ROS, nitric oxide (NO) has become recognized as a key regulator of PCD. PCD is an important mechanism for the controlled elimination of targeted cells in both animals and plants. The major focus of this review is to discuss how ROS and NO control male-female cross-talk during fertilization in order to trigger PCD in self-incompatible pollen, providing a highly effective way to prevent self-fertilization.

NO signaling in plant immunity: a tale of messengers

Nitric oxide (NO) is a free radical gas involved in a myriad of plant physiological processes including immune responses. How NO mediates its biological effects in plant facing microbial pathogen attack is an unresolved question. Insights into the molecular mechanisms by which it propagates signals reveal the contribution of this simple gas in complex signaling pathways shared with reactive oxygen species (ROS) and the second messenger Ca(2+). Understanding of the subtle cross-talks operating between these signals was greatly improved by the recent identification and the functional analysis of proteins regulated through S-nitrosylation, a major NO-dependent post-translational protein modification. Overall, these findings suggest that NO is probably an important component of the mechanism coordinating and regulating Ca(2+) and ROS signaling in plant immunity.

The nitric oxide production in the moss Physcomitrella patens is mediated by nitrate reductase

During the last 20 years multiple roles of the nitric oxide gas (•NO) have been uncovered in plant growth, development and many physiological processes. In seed plants the enzymatic synthesis of •NO is mediated by a nitric oxide synthase (NOS)-like activity performed by a still unknown enzyme(s) and nitrate reductase (NR). In green algae the •NO production has been linked only to NR activity, although a NOS gene was reported for Ostreococcus tauri and O. lucimarinus, no other Viridiplantae species has such gene. As there is no information about •NO synthesis neither for non-vascular plants nor for non-seed vascular plants, the interesting question regarding the evolution of the enzymatic •NO production systems during land plant natural history remains open. To address this issue the endogenous •NO production by protonema was demonstrated using Electron Paramagnetic Resonance (EPR). The •NO signal was almost eliminated in plants treated with sodium tungstate, which also reduced the NR activity, demonstrating that in P. patens NR activity is the main source for •NO production. The analysis with confocal laser scanning microscopy (CLSM) confirmed endogenous NO production and showed that •NO signal is accumulated in the cytoplasm of protonema cells. The results presented here show for the first time the •NO production in a non-vascular plant and demonstrate that the NR-dependent enzymatic synthesis of •NO is common for embryophytes and green algae.

Methane-rich water induces cucumber adventitious rooting through heme oxygenase1/carbon monoxide and Ca(2+) pathways

Methane-rich water triggered adventitious rooting by regulating heme oxygenase1/carbon monoxide and calcium pathways in cucumber explants. Heme oxygenase1/carbon monoxide (HO1/CO) and calcium (Ca(2+)) were reported as the downstream signals in auxin-induced cucumber adventitious root (AR) formation. Here, we observed that application of methane-rich water (MRW; 80% saturation) obviously induced AR formation in IAA-depleted cucumber explants. To address the universality, we checked adventitious rooting in soybean and mung bean explants, and found that MRW (50 and 10% saturation, respectively) exhibited the similar inducing results. To further determine if the HO1/CO system participated in MRW-induced adventitious rooting, MRW, HO1 inducer hemin, its activity inhibitor zinc protoporphyrin IX (ZnPP), and its catalytic by-products CO, bilirubin, and Fe(2+) were used to detect their effects on cucumber adventitious rooting in IAA-depleted explants. Subsequent results showed that MRW-induced adventitious rooting was blocked by ZnPP and further reversed by 20% saturation CO aqueous solution. However, the other two by-products of HO1, bilirubin and Fe(2+), failed to induce AR formation. Above responses were consistent with the MRW-induced increases of HO1 transcript and corresponding protein level. Further molecular evidence indicted that expression of marker genes, including auxin signaling-related genes and cell cycle regulatory genes, were modulated by MRW alone but blocked by the cotreatment with ZnPP, the latter of which could be significantly rescued by the addition of CO. By using the Ca(2+)-channel blocker and Ca(2+) chelator, the involvement of Ca(2+) pathway in MRW-induced adventitious rooting was also suggested. Together, our results indicate that MRW might serve as a stimulator of adventitious rooting, which was partially mediated by HO1/CO and Ca(2+) pathways.

β-Cyclodextrin-hemin complex-induced lateral root formation in tomato: involvement of nitric oxide and heme oxygenase 1

β-Cyclodextrin-hemin complex-induced tomato lateral root formation was associated with nitric oxide and heme oxygenase 1 by modulating cell cycle regulatory genes. β-Cyclodextrin-hemin complex (β-CDH), a complex by combining β-cyclodextrin (β-CD) with hemin, a heme oxygenase 1 (HO1) inducer, was a trigger of cucumber adventitious root formation by enhancing HO1 gene expression. In this report, our results identified the previously unknown function of β-CDH in plants: the inducer of tomato lateral root (LR) formation. β-CDH-triggered LR formation is hemin-specific, since β-CD failed to induce LR development. Because nitric oxide (NO) is involved in LR formation, the correlation of β-CDH with NO and HO1 was investigated. Our analysis suggested that β-CDH induced an increase in endogenous NO production, followed by up-regulation of tomato HO1 gene and LR formation, all of which were mimicked by hemin and two NO-releasing compounds (SNP and GSNO). The induction of HO1 gene expression and LR formation triggered by β-CDH or hemin were significantly blocked by an inhibitor of HO1. Further results revealed that both β-CDH- and SNP-stimulated HO1 gene expression and thereafter LR formation were sensitive to the removal of NO with a potent NO scavenger, and the responses of SNP were significantly blocked by an inhibitor of HO1. Molecular evidence illustrated that representative cell cycle regulatory genes, including SlCDKA1, SlCYCA3;1, SlCYCA2;1, and SlCYCD3;1, were significantly up-regulated by β-CDH and SNP, but obviously blocked when seedlings were co-treated with the scavenger of NO or the inhibitor of HO1. In summary, our physiological and molecular evidence demonstrated that both NO and HO1 were involved in the β-CDH-induced LR formation with, at least partially, HO1 acting downstream of NO signaling.

The interaction of Arabidopsis with Piriformospora indica shifts from initial transient stress induced by fungus-released chemical mediators to a mutualistic interaction after physical contact of the two symbionts

BACKGROUND

Piriformospora indica, an endophytic fungus of Sebacinales, colonizes the roots of many plant species including Arabidopsis thaliana. The symbiotic interaction promotes plant performance, growth and resistance/tolerance against abiotic and biotic stress.

RESULTS

We demonstrate that exudated compounds from the fungus activate stress and defense responses in the Arabidopsis roots and shoots before the two partners are in physical contact. They induce stomata closure, stimulate reactive oxygen species (ROS) production, stress-related phytohormone accumulation and activate defense and stress genes in the roots and/or shoots. Once a physical contact is established, the stomata re-open, ROS and phytohormone levels decline, and the number and expression level of defense/stress-related genes decreases.

CONCLUSIONS

We propose that exudated compounds from P. indica induce stress and defense responses in the host. Root colonization results in the down-regulation of defense responses and the activation of genes involved in promoting plant growth, metabolism and performance.

An exogenous source of nitric oxide modulates zinc nutritional status in wheat plants

The effect of addition of the nitric oxide donor S-nitrosoglutathione (GSNO) on the Zn nutritional status was evaluated in hydroponically-cultured wheat plants (Triticum aestivum cv. Chinese Spring). Addition of GSNO in Zn-deprived plants did not modify biomass accumulation but accelerated leaf senescence in a mode concomitant with accelerated decrease of Zn allocation to shoots. In well-supplied plants, Zn concentration in both roots and shoots declined due to long term exposure to GSNO. A further evaluation of net Zn uptake rate (ZnNUR) during the recovery of long-term Zn-deprivation unveiled that enhanced Zn-accumulation was partially blocked when GSNO was present in the uptake medium. This effect on uptake was mainly associated with a change of Zn translocation to shoots. Our results suggest a role for GSNO in the modulation of Zn uptake and in root-to-shoot translocation during the transition from deficient to sufficient levels of Zn-supply.

Hypocotyl adventitious root organogenesis differs from lateral root development

Wound-induced adventitious root (AR) formation is a requirement for plant survival upon root damage inflicted by pathogen attack, but also during the regeneration of plant stem cuttings for clonal propagation of elite plant varieties. Yet, adventitious rooting also takes place without wounding. This happens for example in etiolated Arabidopsis thaliana hypocotyls, in which AR initiate upon de-etiolation or in tomato seedlings, in which AR initiate upon flooding or high water availability. In the hypocotyl AR originate from a cell layer reminiscent to the pericycle in the primary root (PR) and the initiated AR share histological and developmental characteristics with lateral roots (LRs). In contrast to the PR however, the hypocotyl is a determinate structure with an established final number of cells. This points to differences between the induction of hypocotyl AR and LR on the PR, as the latter grows indeterminately. The induction of AR on the hypocotyl takes place in environmental conditions that differ from those that control LR formation. Hence, AR formation depends on differentially regulated gene products. Similarly to AR induction in stem cuttings, the capacity to induce hypocotyl AR is genotype-dependent and the plant growth regulator auxin is a key regulator controlling the rooting response. The hormones cytokinins, ethylene, jasmonic acid, and strigolactones in general reduce the root-inducing capacity. The involvement of this many regulators indicates that a tight control and fine-tuning of the initiation and emergence of AR exists. Recently, several genetic factors, specific to hypocotyl adventitious rooting in A. thaliana, have been uncovered. These factors reveal a dedicated signaling network that drives AR formation in the Arabidopsis hypocotyl. Here we provide an overview of the environmental and genetic factors controlling hypocotyl-born AR and we summarize how AR formation and the regulating factors of this organogenesis are distinct from LR induction.