Nitric oxide accumulation in Arabidopsis is independent of NOA1 in the presence of sucrose

Nitric Oxide Is Required for Primary Nitrate Response in Arabidopsis: Evidence for S-Nitrosation of NLP7

Aims: Nitrogen (N) is a necessary nutrient for plant development and seed production, with nitrate (NO3-) serving as the primary source of N in soils. Although several molecular players in plant responses to NO3- signaling were unraveled, it is still a complex process with gaps that require further investigation. The aim of our study is to analyze the role of nitric oxide (NO) in the primary nitrate response (PNR). Results: Using a combination of genetic and pharmacological approaches, we demonstrate that NO is required for the expression of the NO3--regulated genes nitrate reductase 1 (NIA1), nitrite reductase (NIR), and nitrate transporters (nitrate transporter 1.1 [NRT1.1] and nitrate transporter 2.1 [NRT2.1]) in Arabidopsis. The PNR is impaired in the Arabidopsis mutant noa1, defective in NO production. Our results also show that PHYTOGLOBIN 1 (PHYTOGLB1), involved in NO homeostasis, is rapidly induced during PNR in wild type (wt) but not in the mutants of the nitrate transceptor NTR1.1 and the transcription factor nodule inception-like protein 7 (NLP7), suggesting that the NRT1.1-NLP7 cascade modulates PHYTOGLB1 gene expression. Biotin switch experiments demonstrate that NLP7, the PNR-master regulator, is S-nitrosated in vitro. Depletion of NO during PNR intensifies the decrease in reactive oxygen species levels and the rise of catalase (CAT) and ascorbate peroxidase (APX) enzyme activity. Conclusion and Innovation: NO, a by-product of NO3- metabolism and a well-characterized signal molecule in plants, is an important player in the PNR.

Iron oxide nanoparticles impart cross tolerance to arsenate stress in rice roots through involvement of nitric oxide

The growth and development patterns of crop plants are being seriously threatened by arsenic (As) contamination in the soil, and it also acts as a major hurdle in crop productivity. This study focuses on arsenate As(V) mediated toxicity in rice plants. Further, among the different type of NPs, iron oxide nanoparticles (FeO NPs) display a dose-dependent effect but their potential role in mitigating As(V) stress is still elusive. FeO NPs (500 μM) play a role in imparting cross-tolerance against As(V) induced toxicity in rice. Growth attributes, photosynthetic performance, nutrient contents and biochemical parameters were significantly altered by As(V). But FeO NPs rescued the negative consequences of As(V) by restricting its entry with the possible involvement of NO in rice roots. Moreover, results related with gene expression of NO(OsNoA1 and OsNIA1) and proline metabolism were greatly inhibited by As(V) toxicity. But, FeO NPs reversed the toxic effect of As(V) by improving proline metabolism and stimulating NO mediated up-regulation of antioxidant enzymes particularly glutathione-S-transferase which may be possible reasons for the reduction of As(V) toxicity in rice roots. Overall, it can be stated that FeO NPs may act as an As(V) barrier to restrict the As(V) uptake by roots and have the ability to confer cross tolerance by modulating various morphological, biochemical and molecular characteristics with possible intrinsic involvement of NO.

COE2 Is Required for the Root Foraging Response to Nitrogen Limitation

There are numerous exchanges of signals and materials between leaves and roots, including nitrogen, which is one of the essential nutrients for plant growth and development. In this study we identified and characterized the Chlorophyll A/B-Binding Protein (CAB) (named coe2 for CAB overexpression 2) mutant, which is defective in the development of chloroplasts and roots under normal growth conditions. The phenotype of coe2 is caused by a mutation in the Nitric Oxide Associated (NOA1) gene that is implicated in a wide range of chloroplast functions including the regulation of metabolism and signaling of nitric oxide (NO). A transcriptome analysis reveals that expression of genes involved in metabolism and lateral root development are strongly altered in coe2 seedlings compared with WT. COE2 is expressed in hypocotyls, roots, root hairs, and root caps. Both the accumulation of NO and the growth of lateral roots are enhanced in WT but not in coe2 under nitrogen limitation. These new findings suggest that COE2-dependent signaling not only coordinates gene expression but also promotes chloroplast development and function by modulating root development and absorption of nitrogen compounds.

Evidence for a role of nitric oxide in iron homeostasis in plants

Nitric oxide (NO), once regarded as a poisonous air pollutant, is now understood as a regulatory molecule essential for several biological functions in plants. In this review, we summarize NO generation in different plant organs and cellular compartments, and also discuss the role of NO in iron (Fe) homeostasis, particularly in Fe-deficient plants. Fe is one of the most limiting essential nutrient elements for plants. Plants often exhibit Fe deficiency symptoms despite sufficient tissue Fe concentrations. NO appears to not only up-regulate Fe uptake mechanisms but also makes Fe more bioavailable for metabolic functions. NO forms complexes with Fe, which can then be delivered into target cells/tissues. NO generated in plants can alleviate oxidative stress by regulating antioxidant defense processes, probably by improving functional Fe status and by inducing post-translational modifications in the enzymes/proteins involved in antioxidant defense responses. It is hypothesized that NO acts in cooperation with transcription factors such as bHLHs, FIT, and IRO to regulate the expression of enzymes and proteins essential for Fe homeostasis. However, further investigations are needed to disentangle the interaction of NO with intracellular target molecules that leads to enhanced internal Fe availability in plants.

Nitric Oxide Overproduction by cue1 Mutants Differs on Developmental Stages and Growth Conditions

The cue1 nitric oxide (NO) overproducer mutants are impaired in a plastid phosphoenolpyruvate/phosphate translocator, mainly expressed in Arabidopsis thaliana roots. cue1 mutants present an increased content of arginine, a precursor of NO in oxidative synthesis processes. However, the pathways of plant NO biosynthesis and signaling have not yet been fully characterized, and the role of CUE1 in these processes is not clear. Here, in an attempt to advance our knowledge regarding NO homeostasis, we performed a deep characterization of the NO production of four different cue1 alleles (cue1-1, cue1-5, cue1-6 and nox1) during seed germination, primary root elongation, and salt stress resistance. Furthermore, we analyzed the production of NO in different carbon sources to improve our understanding of the interplay between carbon metabolism and NO homeostasis. After in vivo NO imaging and spectrofluorometric quantification of the endogenous NO levels of cue1 mutants, we demonstrate that CUE1 does not directly contribute to the rapid NO synthesis during seed imbibition. Although cue1 mutants do not overproduce NO during germination and early plant development, they are able to accumulate NO after the seedling is completely established. Thus, CUE1 regulates NO homeostasis during post-germinative growth to modulate root development in response to carbon metabolism, as different sugars modify root elongation and meristem organization in cue1 mutants. Therefore, cue1 mutants are a useful tool to study the physiological effects of NO in post-germinative growth.

Nitric Oxide Boosts Bemisia tabaci Performance Through the Suppression of Jasmonic Acid Signaling Pathway in Tobacco Plants

The intimate relationships between plants and insects start with herbivory, which can be traced back to approximately 420 million year ago. Like many other relationships, a plant–insect interaction can be mutualistic, commensalistic, or antagonistic. Within antagonistic relationships, plants deploy inducible defense to insect phytophagy. Insects, however, can evade/suppress effectual plant defenses by manipulating plant defense signaling. Previously, we showed that the sweet potato whitefly, Bemisia tabaci, a global invasive insect pest, can suppress jasmonic acid (JA)-dependent defenses, thereby enhancing their performance on host plants. Given that nitric oxide (NO), a multifunctional signaling molecule, interacts closely with JA signaling pathway, we hypothesized that NO is involved in the suppression of JA defensive responses. Equipped with an integrated approach, we comprehensively examined this overarching hypothesis. We showed that: (1) tobacco plants responded to B. tabaci infestation by accumulating high levels of NO, (2) the exogenous application of sodium nitroprusside, a NO donor, in tobacco plants attracted B. tabaci adults and accelerated nymphal development, whereas plants treated with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), a NO scavenger, repelled B. tabaci adults and prolonged nymphal development, and, more importantly, (3) silencing of NO-associated protein 1, a gene associated with NO accumulation, and cPTIO application disrupted the B. tabaci-mediated suppression of JA in plants. Collectively, these results suggest that: (1) NO signaling is activated by B. tabaci infestation, (2) NO is involved in the suppression of JA-dependent plant defense, and, consequently, (3) NO improves B. tabaci performance on host plants. Our study reflects the remarkable arm race that co-evolved for millions of years between plants and insects and offers a potential novel target (nitric oxide) for the long-term sustainable management of this global invasive pest.

Nitric oxide-dependent regulation of sweet pepper fruit ripening

Ripening is a complex physiological process that involves changes in reactive nitrogen and oxygen species that govern the shelf-life and quality of fruits. Nitric oxide (NO)-dependent changes in the sweet pepper fruit transcriptome were determined by treating fruits at the initial breaking point stage with NO gas. Fruits were also harvested at the immature (green) and ripe (red) stages. Fruit ripening in the absence of NO resulted in changes in the abundance of 8805 transcripts whose function could be identified. Among these, functional clusters associated with reactive oxygen/nitrogen species and lipid metabolism were significantly modified. NO treatment resulted in the differential expression of 498 genes framed within these functional categories. Biochemical analysis revealed that NO treatment resulted in changes in fatty acid profiling, glutathione and proline contents, and the extent of lipid peroxidation, as well as increases in the activity of ascorbate peroxidase and lipoxygenase. These data provide supporting evidence for the crucial role of NO in the ripening of pepper fruit.

Nitrogen and nitric oxide regulate Arabidopsis flowering differently

Both nitrogen (N) and nitric oxide (NO) postpone plant flowering. However, we still don't know whether N and NO trigger the same signaling pathways leading to flowering delay. Our previous study found that ferredoxin NADP⁺ oxidoreductase (FNR1) and the blue-light receptor cryptochrome 1 (CRY1) are involved in nitrogen-regulated flowering-time control. However, NO-induced late-flowering does not require FNR1 or CRY1. Sucrose supply counteracts the flowering delay induced by NO. However high-N-induced late-flowering could not be reversed by 5% sucrose supplementation. The high nitrogen condition decreased the amplitudes of all transcripts of the circadian clock. While NO increased the amplitudes of circadian transcripts of CRY1, LHY (LATE ELONGATED HYPOCOTYL), CCA1 (CIRCADIAN CLOCK ASSOCIATED 1) and TOC1 (TIMING OF CAB EXPRESSION 1), but decreased the amplitudes of circadian transcripts of CO (CONSTANS) and GI (GIGANTEA). 5% sucrose supplementation reversed the declines in amplitudes of circadian transcripts of CO and GI after the NO treatment. NO induced S-nitrosation modification on oscillators CO and GI, but not on the other oscillators of the circadian clock. Sucrose supply interestingly reduced S-nitrosation levels of GI and CO proteins. Thus N and NO rely on overlapping but distinct signaling pathways on plant flowering.

The nitric oxide suppressed Arabidopsis mutants- Atnoa1 and Atnia1nia2noa1-2 produce nitric oxide in MS growth medium and on uranium exposure

The mutants Atnoa1 and Atnia1nia2noa1-2 having a defective chloroplast developmental process, showed enhanced chlorophyll levels when they were grown on Murashige and Skoog (MS) medium and on exposure with uranium (U) on Hoagland medium. Thus we hypothesized that these mutants probably produced NO in MS medium and on exposure with U. Wild-type Col-0, Atnoa1, Atnia1nia2noa1-2 plants were cultured on modified Hoagland and 1/10 MS media and NO generation in the roots of these mutants was monitored using NO selective fluorescent dyes, DAF-2DA and Fl2E. Both Atnoa1 and Atnia1nia2noa1-2 triple mutants produced NO as observed by increases in DAF-2T and Fl2E fluorescence when these mutants were grown on MS medium but not on Hoagland medium. In presence of NO scavenger, methylene blue (MB, 200 μM), DAF-2T and Fl2E fluorescence was completely abolished. On the other hand treatment of the plants with 25 μM U triggered NO generation. U-treated Atnoa1 and Atnia1nia2noa1-2 plants upregulated genes (POR B, POR D, CHL D) involved in the chlorophyll biosynthesis. From these results it was concluded that Atnoa1 and Atnia1nia2noa1-2 are conditional NO producers and it appears that NO generation in plants substantially depends on growth medium and NIA1, NIA2 or NOA1 does not appear to be really involved in NO generation in MS medium or after U exposure.

OsNOA1 functions in a threshold-dependent manner to regulate chloroplast proteins in rice at lower temperatures

BACKGROUND

Although decreased protein expressions have been observed in NOA1 (Nitric Oxide Associated protein 1) deficient plants, the molecular mechanisms of how NOA1 regulates protein metabolism remain poorly understood. In this study, we have used a global comparative proteomic approach for both OsNOA1 suppression and overexpression transgenic lines under two different temperatures, in combination with physiological and biochemical analyses to explore the regulatory mechanisms of OsNOA1 in rice.

RESULTS

In OsNOA1-silenced or highly overexpressed rice, considerably different expression patterns of both chlorophyll and Rubisco as well as distinct phenotypes were observed between the growth temperatures at 22 °C and 30 °C. These observations led us to hypothesize there appears a narrow abundance threshold for OsNOA1 to function properly at lower temperatures, while higher temperatures seem to partially compensate for the changes of OsNOA1 abundance. Quantitative proteomic analyses revealed higher temperatures could restore 90% of the suppressed proteins to normal levels, whereas almost all of the remaining suppressed proteins were chloroplast ribosomal proteins. Additionally, our data showed 90% of the suppressed proteins in both types of transgenic plants at lower temperatures were located in the chloroplast, suggesting a primary effect of OsNOA1 on chloroplast proteins. Transcript analyses, along with in vitro pull-down experiments further demonstrated OsNOA1 is associated with the function of chloroplast ribosomes.

CONCLUSIONS

Our results suggest OsNOA1 functions in a threshold-dependent manner for regulation of chloroplast proteins at lower temperatures, which may be mediated by interactions between OsNOA1 and chloroplast ribosomes.

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.

Overexpressing sweetpotato peroxidase gene swpa4 affects nitric oxide production by activating the expression of reactive oxygen species- and nitric oxide-related genes in tobacco

Reactive oxygen species (ROS) and nitric oxide (NO) are key signaling molecules involved in various developmental and stress responses in plants. NO and ROS production, which is triggered by various stimuli, activates downstream signaling pathways to help plants cope with abiotic and biotic stresses. Recent evidence suggests that the interplay between NO and ROS signaling plays a critical role in regulating stress responses. However, the underlying molecular mechanism remains poorly understood. We previously reported that transgenic tobacco overexpressing the swpa4 peroxidase (POD) gene from sweetpotato exhibits increased tolerance to stress. Overexpression of swpa4 also induces the generation of H2O2 and activates the expression of various extracellular acidic pathogenesis-related (PR) genes. Here, we show that swpa4 positively regulates the expression of ROS- and NO-related genes in transgenic tobacco plants. Plants expressing swpa4 exhibited increased expression of ROS-related genes and increased ROS-related enzyme activity under normal conditions and H2O2 treatment, whereas the expression of NO associated 1 (NOA1) only increased under normal conditions. Moreover, plants overexpressing swpa4 showed increased NO levels under normal conditions and after treatment with the NO donor sodium nitroprusside (SNP). Interestingly, treatment with a POD inhibitor dramatically reduced NO levels in swpa4 transgenic plants. These findings suggest that swpa4 regulates H2O2 and NO homeostasis in plants under stress conditions, thereby establishing a possible molecular link between the NO and ROS signaling pathways.

Nitric oxide induces monosaccharide accumulation through enzyme S-nitrosylation

Nitric oxide (NO) is extensively involved in various growth processes and stress responses in plants; however, the regulatory mechanism of NO-modulated cellular sugar metabolism is still largely unknown. Here, we report that NO significantly inhibited monosaccharide catabolism by modulating sugar metabolic enzymes through S-nitrosylation (mainly by oxidizing dihydrolipoamide, a cofactor of pyruvate dehydrogenase). These S-nitrosylation modifications led to a decrease in cellular glycolysis enzymes and ATP synthase activities as well as declines in the content of acetyl coenzyme A, ATP, ADP-glucose and UDP-glucose, which eventually caused polysaccharide-biosynthesis inhibition and monosaccharide accumulation. Plant developmental defects that were caused by high levels of NO included delayed flowering time, retarded root growth and reduced starch granule formation. These phenotypic defects could be mediated by sucrose supplementation, suggesting an essential role of NO-sugar cross-talks in plant growth and development. Our findings suggest that molecular manipulations could be used to improve fruit and vegetable sweetness.

Nitric oxide associated protein 1 is associated with chloroplast perturbation and disease symptoms in Nicotiana benthamiana infected with South African cassava mosaic virus

Nitric oxide associated 1 (NOA1) in plants is a cyclic GTPase involved in protein translation in the chloroplast and has been indirectly linked to nitric oxide (NO) accumulation and response to biotic stress. The association between NOA1 and NO accumulation in Arabidopsis noa1 mutants has been linked to the inability of noa1 mutants to accumulate carbon reserves such as fumarate, leading to chloroplast dysfunction and a pale green leaf phenotype. To understand the role played by NOA1 in response to South African cassava mosaic virus infection in Nicotiana benthamiana, the expression of NbNOA1 and the accumulation of NO in leaf samples was compared between south african cassava mosaic (SACMV)-infected and mock-infected plants at 14 and 28 dpi. Real-time qPCR was used to measure SACMV viral load which increased significantly by 20% from 14 to 28 dpi as chlorosis and symptom severity progressed. At 14 and 28 dpi, NbNOA1 expression was significantly lower than mock inoculated plants (2-fold lower at 14 dpi, p-value=0.01 and 5-fold lower at 28, p-value=0.00). At 14 dpi, NO accumulation remained unchanged in infected leaf tissue compared to mock inoculated, while at 28 dpi, NO accumulation was 40% lower (p-value=0.01). At 28 dpi, the decrease in NbNOA1 expression and NO accumulation was accompanied by chloroplast dysfunction, evident from the significant reduction in chlorophylls a and b and carotenoids in SACMV-infected leaves. Furthermore, the expression of chloroplast translation factors (chloroplast RNA binding, chloroplast elongation factor G, translation elongation factor Tu, translation initiation factor 3-2, plastid-specific ribosomal protein 6 and plastid ribosome recycling factor) were found to be repressed in infected N. benthamiana. GC-MS analysis showed a decrease in fumarate and an increase in glucose in SACMV-infected N. benthamiana in comparison to mock samples suggesting a decrease in carbon stores. Collectively, these results provide evidence that in response to SACMV infection, a decrease in photopigments and carbon stores, accompanied by an increase in glucose and decrease in fumarate, leads to a decline in NbNOA1expression and NO levels. This is manifested by suppressed translation factors and disruption of chloroplast function, thereby contributing to chlorotic disease symptoms.

Nitric oxide participates in plant flowering repression by ascorbate

In Oncidium, redox homeostasis involved in flowering is mainly due to ascorbic acid (AsA). Here, we discovered that Oncidium floral repression is caused by an increase in AsA-mediated NO levels, which is directed by the enzymatic activities of nitrate reductase (NaR) and nitrite reducatase (NiR). Through Solexa transcriptomic analysis of two libraries, ‘pseudobulb with inflorescent bud’ (PIB) and ‘pseudobulb with axillary bud’ (PAB), we identified differentially expressed genes related to NO metabolism. Subsequently, we showed a significant reduction of NaR enzymatic activities and NO levels during bolting and blooming stage, suggesting that NO controlled the phase transition and flowering process. Applying AsA to Oncidium PLB (protocorm-like bodies) significantly elevated the NO content and enzyme activities. Application of sodium nitroprusside (-NO donor) on Arabidopsis vtc1 mutant caused late flowering and expression level of flowering-associated genes (CO, FT and LFY) were reduced, suggesting NO signaling is vital for flowering repression. Conversely, the flowering time of noa1, an Arabidopsis NO-deficient mutant, was not altered after treatment with L-galacturonate, a precursor of AsA, suggesting AsA is required for NO-biosynthesis involved in the NO-mediated flowering-repression pathway. Altogether, Oncidium bolting is tightly regulated by AsA-mediated NO level and downregulation of transcriptional levels of NO metabolism genes.

Low concentrations of salicylic acid delay methyl jasmonate-induced leaf senescence by up-regulating nitric oxide synthase activity

In plants, extensive efforts have been devoted to understanding the crosstalk between salicylic acid (SA) and jasmonic acid (JA) signaling in pathogen defenses, but this crosstalk has scarcely been addressed during senescence. In this study, the effect of SA application on methyl jasmonate (MeJA)-induced leaf senescence was assessed. We found that low concentrations of SA (1-50 μM) played a delayed role against the senescence promoted by MeJA. Furthermore, low concentrations of SA enhanced plant antioxidant defenses and restricted reactive oxygen species (ROS) accumulation in MeJA-treated leaves. When applied simultaneously with MeJA, low concentrations of SA triggered a nitric oxide (NO) burst, and the elevated NO levels were linked to the nitric oxide associated 1 (NOA1)-dependent pathway via nitric oxide synthase (NOS) activity. The ability of SA to up-regulate plant antioxidant defenses, reduce ROS accumulation, and suppress leaf senescence was lost in NO-deficient Atnoa1 plants. In a converse manner, exogenous addition of NO donors increased the plant antioxidant capacity and lowered the ROS levels in MeJA-treated leaves. Taken together, the results indicate that SA at low concentrations counteracts MeJA-induced leaf senescence through NOA1-dependent NO signaling and strengthening of the antioxidant defense.

Plant Survival in a Changing Environment: The Role of Nitric Oxide in Plant Responses to Abiotic Stress

Nitric oxide in plants may originate endogenously or come from surrounding atmosphere and soil. Interestingly, this gaseous free radical is far from having a constant level and varies greatly among tissues depending on a given plant's ontogeny and environmental fluctuations. Proper plant growth, vegetative development, and reproduction require the integration of plant hormonal activity with the antioxidant network, as well as the maintenance of concentration of reactive oxygen and nitrogen species within a narrow range. Plants are frequently faced with abiotic stress conditions such as low nutrient availability, salinity, drought, high ultraviolet (UV) radiation and extreme temperatures, which can influence developmental processes and lead to growth restriction making adaptive responses the plant's priority. The ability of plants to respond and survive under environmental-stress conditions involves sensing and signaling events where nitric oxide becomes a critical component mediating hormonal actions, interacting with reactive oxygen species, and modulating gene expression and protein activity. This review focuses on the current knowledge of the role of nitric oxide in adaptive plant responses to some specific abiotic stress conditions, particularly low mineral nutrient supply, drought, salinity and high UV-B radiation.

Switch from heterotrophy to autotrophy of apple cotyledons depends on NO signal

NO accelerates transition of germinated embryos from heterotrophy to autotrophy by stimulation of chloroplasts maturation. NO-mediated autotrophy of apple seedlings correlates to increased content of RuBisCO small subunit and improvement of parameters of chlorophyll a fluorescence. Nitric oxide (NO) acts as signaling molecule involved in regulation of various physiological processes in plants, although its involvement in cotyledons greening is poorly recognized. To identify the importance of NO signal for plant growth and development we investigated the effects of short-term application of NO at various developmental stages of seedlings of apple (Malus domestica Borkh.) on cotyledons' chlorophyll a to b ratio, chlorophyll a fluorescence, photosynthetic activity, carbohydrates and RuBisCO both subunits content. NO-dependent biochemical alterations were linked to cytological observation of developing plastids in cotyledons of apple plants. Abnormal plantlets developing from dormant apple embryos are characterized by anatomical malformations of cotyledons. Short-term pre-treatment with NO of isolated embryos or seedlings with developmental anomalies resulted in formation of plants with cotyledons of equal size and chlorophyll content; these responses were blocked by NO scavenger. NO independently of time point of application accelerated embryos transition from heterotrophy to autotrophy by stimulation of photosynthetic activity, improvement of parameters of chlorophyll a fluorescence (F v/F m, F v/F 0) and increased content of RuBisCO small subunit. Further analysis showed that NO application modified glucose and hydrogen peroxide concentration in cotyledons. Beneficial effect of NO on development of seedlings without any abnormalities was manifested at ultrastructural level by decline in amount of proplastids and induction of formation and maturation of chloroplasts. Our data suggest that progress of autotrophy of young seedlings is governed by NO acting as stimulator of chloroplast-to-nucleus signaling.

Cloning of nitric oxide associated 1 (NOA1) transcript from oil palm (Elaeis guineensis) and its expression during Ganoderma infection

Nitric oxide associated 1 (NOA1) protein is implicated in plant disease resistance and nitric oxide (NO) biosynthesis. A full-length cDNA encoding of NOA1 protein from oil palm (Elaeis guineensis) was isolated and designated as EgNOA1. Sequence analysis suggested that EgNOA1 was a circular permutated GTPase with high similarity to the bacterial YqeH protein of the YawG/YlqF family. The gene expression of EgNOA1 and NO production in oil palm root tissues treated with Ganoderma boninense, the causal agent of basal stem rot (BSR) disease were profiled to investigate the involvement of EgNOA1 during fungal infection and association with NO biosynthesis. Real-time PCR (qPCR) analysis revealed that the transcript abundance of EgNOA1 in root tissues was increased by G. boninense treatment. NO burst in Ganoderma-treated root tissue was detected using Griess reagent, in advance of the up-regulation of the EgNOA1 transcript. This indicates that NO production was independent of EgNOA1. However, the induced expression of EgNOA1 in Ganoderma-treated root tissues implies that it might be involved in plant defense responses against pathogen infection.

Reactive Oxygen Species-Dependent Nitric Oxide Production Contributes to Hydrogen-Promoted Stomatal Closure in Arabidopsis

The signaling role of hydrogen gas (H₂) has attracted increasing attention from animals to plants. However, the physiological significance and molecular mechanism of H₂ in drought tolerance are still largely unexplored. In this article, we report that abscisic acid (ABA) induced stomatal closure in Arabidopsis (Arabidopsis thaliana) by triggering intracellular signaling events involving H₂, reactive oxygen species (ROS), nitric oxide (NO), and the guard cell outward-rectifying K⁺ channel (GORK). ABA elicited a rapid and sustained H₂ release and production in Arabidopsis. Exogenous hydrogen-rich water (HRW) effectively led to an increase of intracellular H₂ production, a reduction in the stomatal aperture, and enhanced drought tolerance. Subsequent results revealed that HRW stimulated significant inductions of NO and ROS synthesis associated with stomatal closure in the wild type, which were individually abolished in the nitric reductase mutant nitrate reductase1/2 (nia1/2) or the NADPH oxidase-deficient mutant rbohF (for respiratory burst oxidase homolog). Furthermore, we demonstrate that the HRW-promoted NO generation is dependent on ROS production. The rbohF mutant had impaired NO synthesis and stomatal closure in response to HRW, while these changes were rescued by exogenous application of NO. In addition, both HRW and hydrogen peroxide failed to induce NO production or stomatal closure in the nia1/2 mutant, while HRW-promoted ROS accumulation was not impaired. In the GORK-null mutant, stomatal closure induced by ABA, HRW, NO, or hydrogen peroxide was partially suppressed. Together, these results define a main branch of H₂-regulated stomatal movement involved in the ABA signaling cascade in which RbohF-dependent ROS and nitric reductase-associated NO production, and subsequent GORK activation, were causally involved.

Arabidopsis HY1 confers cadmium tolerance by decreasing nitric oxide production and improving iron homeostasis

Up-regulation of the gene that encodes intracellular heme oxygenase 1 (HO1) benefits plants under cadmium (Cd(2+)) stress; however, the molecular mechanisms remain unclear. Here, we elucidate the role of Arabidopsis HY1 (AtHO1) in Cd(2+) tolerance by using genetic and molecular approaches. Analysis of two HY1 null mutants, three HY1 overexpression lines, HO double or triple mutants, as well as phyA and phyB mutants revealed the specific hypersensitivity of hy1 to Cd(2+) stress. Supplementation with two enzymatic by-products of HY1, carbon monoxide (CO) and iron (Fe, especially), rescued the Cd(2+)-induced inhibition of primary root (PR) elongation in hy1-100. The mutation of HY1, which exhibited lower glutathione content than Col-0 in root tissues, was able to induce nitric oxide (NO) overproduction, Cd(2+) accumulation, and severe Fe deficiency in root tissues. However, the contrasting responses appeared in 35S:HY1-4. Additionally, reduced levels of Ferric Reduction Oxidase 2 (FRO2) and Iron-Regulated Transporter 1 (IRT1) transcripts, and increased levels of Heavy Metal ATPase 2/4 (HMA2/4) transcripts bolster the notion that HY1 up-regulation ameliorates Fe deficiency, and might increase Cd(2+) exclusion. Taken together, these results showed that HY1 plays a common link in Cd(2+) tolerance by decreasing NO production and improving Fe homeostasis in Arabidopsis root tissues.

Arabidopsis chlorophyll biosynthesis: an essential balance between the methylerythritol phosphate and tetrapyrrole pathways

Chlorophyll, essential for photosynthesis, is composed of a chlorin ring and a geranylgeranyl diphosphate (GGPP)-derived isoprenoid, which are generated by the tetrapyrrole and methylerythritol phosphate (MEP) biosynthesis pathways, respectively. Although a functional MEP pathway is essential for plant viability, the underlying basis of the requirement has been unclear. We hypothesized that MEP pathway inhibition is lethal because a reduction in GGPP availability results in a stoichiometric imbalance in tetrapyrrolic chlorophyll precursors, which can cause deadly photooxidative stress. Consistent with this hypothesis, lethality of MEP pathway inhibition in Arabidopsis thaliana by fosmidomycin (FSM) is light dependent, and toxicity of MEP pathway inhibition is reduced by genetic and chemical impairment of the tetrapyrrole pathway. In addition, FSM treatment causes a transient accumulation of chlorophyllide and transcripts associated with singlet oxygen-induced stress. Furthermore, exogenous provision of the phytol molecule reduces FSM toxicity when the phytol can be modified for chlorophyll incorporation. These data provide an explanation for FSM toxicity and thereby provide enhanced understanding of the mechanisms of FSM resistance. This insight into MEP pathway inhibition consequences underlines the risk plants undertake to synthesize chlorophyll and suggests the existence of regulation, possibly involving chloroplast-to-nucleus retrograde signaling, that may monitor and maintain balance of chlorophyll precursor synthesis.

Cross-talk interactions of exogenous nitric oxide and sucrose modulates phenylpropanoid metabolism in yellow lupine embryo axes infected with Fusarium oxysporum

The aim of the study was to examine cross-talk of exogenous nitric oxide (NO) and sucrose in the mechanisms of synthesis and accumulation of isoflavonoids in embryo axes of Lupinus luteus L. cv. Juno. It was verified whether the interaction of these molecules can modulate the defense response of axes to infection and development of the pathogenic fungus Fusarium oxysporum f. sp. lupini. Sucrose alone strongly stimulated a high level of genistein glucoside in axes pretreated with exogenous nitric oxide (SNP or GSNO) and non-pretreated axes. As a result of amplification of the signal coming from sucrose and GSNO, high isoflavonoids accumulation was observed (+Sn+GSNO). It needs to be stressed that infection in tissues pretreated with SNP/GSNO and cultured on the medium with sucrose (+Si+SNP/+Si+GSNO) very strongly enhances the accumulation of free isoflavone aglycones. In +Si+SNP axes phenylalanine ammonia-lyase activity was high up to 72h. As early as at 12h in +Si+SNP axes an increase was recorded in gene expression level of the specific isoflavonoid synthesis pathway. At 24h in +Si+SNP axes a very high total antioxidant capacity dependent on the pool of fast antioxidants was noted. Post-infection generation of semiquinone radicals was lower in axes with a high level of sucrose than with a deficit.

Roles of NIA/NR/NOA1-dependent nitric oxide production and HY1 expression in the modulation of Arabidopsis salt tolerance

Despite substantial evidence on the separate roles of Arabidopsis nitric oxide-associated 1 (NOA1)-associated nitric oxide (NO) production and haem oxygenase 1 (HY1) expression in salt tolerance, their integrative signalling pathway remains largely unknown. To fill this knowledge gap, the interaction network among nitrate reductase (NIA/NR)- and NOA1-dependent NO production and HY1 expression was studied at the genetic and molecular levels. Upon salinity stress, the majority of NO production was attributed to NIA/NR/NOA1. Further evidence confirmed that HY1 mutant hy1-100, nia1/2/noa1, and nia1/2/noa1/hy1-100 mutants exhibited progressive salt hypersensitivity, all of which were significantly rescued by three NO-releasing compounds. The salinity-tolerant phenotype and the stronger NO production in gain-of-function mutant of HY1 were also blocked by the NO synthetic inhibitor and scavenger. Although NO- or HY1-deficient mutants showed a compensatory mode of upregulation of HY1 or slightly increased NO production, respectively, during 2 d of salt treatment, downregulation of ZAT10/12-mediated antioxidant gene expression (cAPX1/2 and FSD1) was observed after 7 d of treatment. The hypersensitive phenotypes and stress-related genes expression profiles were differentially rescued or blocked by the application of NO- (in particular) or carbon monoxide (CO)-releasing compounds, showing a synergistic mode. Similar reciprocal responses were observed in the nia1/2/noa1/hy1-100 quadruple mutant, with the NO-releasing compounds exhibit the maximal rescuing responses. Overall, the findings present the combination of compensatory and synergistic modes, linking NIA/NR/NOA1-dependent NO production and HY1 expression in the modulation of plant salt tolerance.

Functional Redundancy and Divergence within the Arabidopsis RETICULATA-RELATED Gene Family

A number of Arabidopsis (Arabidopsis thaliana) mutants exhibit leaf reticulation, having green veins that stand out against paler interveinal tissues, fewer cells in the interveinal mesophyll, and normal perivascular bundle sheath cells. Here, to examine the basis of leaf reticulation, we analyzed the Arabidopsis RETICULATA-RELATED (RER) gene family, several members of which cause leaf reticulation when mutated. Although transcripts of RE, RER1, and RER3 were mainly detected in the bundle sheath cells of expanded leaves, functional RER3:GREEN FLUORESCENT PROTEIN was visualized in the chloroplast membranes of all photosynthetic cells. Leaf reticulation in the re and rer3 loss-of-function mutants occurred, along with accumulation of reactive oxygen species, in a photoperiod-dependent manner. A comparison of re and rer3 leaf messenger RNA expression profiles showed more than 200 genes were similarly misexpressed in both mutants. In addition, metabolic profiles of mature leaves revealed that several biosynthetic pathways downstream of pyruvate are altered in re and rer3. Double mutant analysis showed that only re rer1 and rer5 rer6 exhibited synergistic phenotypes, indicating functional redundancy. The redundancy between RE and its closest paralog, RER1, was confirmed by overexpressing RER1 in re mutants, which partially suppressed leaf reticulation. Our results show that RER family members can be divided into four functional modules with divergent functions. Moreover, these results provide insights into the origin of the reticulated phenotype, suggesting that the RER proteins functionally interconnect photoperiodic growth, amino acid homeostasis, and reactive oxygen species metabolism during Arabidopsis leaf growth.

Chloroplast functionality has a positive effect on nitric oxide level in soybean cotyledons

The subcellular localization of NO generation in soybean cotyledons, and the relationship between NO synthesis and in vivo chloroplast performance were studied. Employing the NO probe 4-aminomethyl-2',7'-difluorofluorescein diacetate (DAF-FM DA) and fluorescence microscopy, a strongly punctuated fluorescence was detected in mesophyll cells. The co-localization of DAF-FM and chlorophyll fluorescence, in confocal laser microscopy images, indicated the presence of NO in the chloroplasts. NO visualization was dependent on light, seedling age, and chloroplast function throughout cotyledons lifespan. The addition of herbicides with action in chloroplasts (DCMU and paraquat) dramatically reduced the quantum yield of photosystem II (φ(PSII)), and lead to images with absence of punctuated green fluorescence. Moreover, electron paramagnetic resonance signals corresponding to NO-spin trap adduct observed in cotyledon homogenates decreased significantly by the treatment with herbicides, as compared to controls. Neither chloroplast function nor NO content were significantly different in cotyledons from plants growing in the presence of ammonium or nitrate as the nitrogen source. These findings suggest that chloroplasts are organelles that contribute to NO synthesis in vivo, and that their proper functionality is essential for maintaining NO levels in soybean cotyledons.

Bis-aryl methanone compound is a candidate of nitric oxide producing elicitor and induces resistance in Nicotiana benthamiana against Phytophthora infestans

Nitric oxide (NO) is important in some physiological responses of plants and plays a crucial role in the regulation of both defense responses and inducing resistance to fungal pathogens. NUBS-4190, a new bis-aryl-methanone compound elicited NO production and defense responses in Nicotiana benthamiana against Phytophthora infestans. NUBS-4190 induced resistance in N. benthamiana to P. infestans, without association of reactive oxygen generation and hypersensitive cell death. Callose induction was reduced in NUBS-4190-treated N. benthamiana leaves after challenge inoculation of P. infestans indicating the penetration resistance. Involvement of pathogenesis-related 1a (NbPR1a) and nitric oxide associated 1 (NbNOA1) genes in the induced resistance to N. benthamiana against P. infestans was found to be associated with resistance. Increased susceptibility in NbPR1a- and NbNOA1-silenced plants correlated with the constitutive accumulation of PR1a transcripts and NO associated salicylic acid. Moreover, reduced NO generation in NOA1 silenced N. benthamiana plants treated with NUBS-4190 indicated that NbNOA1 is involved in NUBS-4190-mediated NO production and is required for defense responses.

Nitric oxide deficiency accelerates chlorophyll breakdown and stability loss of thylakoid membranes during dark-induced leaf senescence in Arabidopsis

Nitric oxide (NO) has been known to preserve the level of chlorophyll (Chl) during leaf senescence. However, the mechanism by which NO regulates Chl breakdown remains unknown. Here we report that NO negatively regulates the activities of Chl catabolic enzymes during dark-induced leaf senescence. The transcriptional levels of the major enzyme genes involving Chl breakdown pathway except for RED CHL CATABOLITE REDUCTASE (RCCR) were dramatically up-regulated during dark-induced Chl degradation in the leaves of Arabidopsis NO-deficient mutant nos1/noa1 that exhibited an early-senescence phenotype. The activity of pheide a oxygenase (PAO) was higher in the dark-induced senescent leaves of nos1/noa1 compared with wild type. Furthermore, the knockout of PAO in nos1/noa1 background led to pheide a accumulation in the double mutant pao1 nos1/noa1, which retained the level of Chl during dark-induced leaf senescence. The accumulated pheide a in darkened leaves of pao1 nos1/noa1 was likely to inhibit the senescence-activated transcriptional levels of Chl catabolic genes as a feed-back inhibitory effect. We also found that NO deficiency led to decrease in the stability of photosynthetic complexes in thylakoid membranes. Importantly, the accumulation of pheide a caused by PAO mutations in combination with NO deficiency had a synergistic effect on the stability loss of thylakoid membrane complexes in the double mutant pao1 nos1/noa1 during dark-induced leaf senescence. Taken together, our findings have demonstrated that NO is a novel negative regulator of Chl catabolic pathway and positively functions in maintaining the stability of thylakoid membranes during leaf senescence.

The 14-3-3 protein GENERAL REGULATORY FACTOR11 (GRF11) acts downstream of nitric oxide to regulate iron acquisition in Arabidopsis thaliana

Here we report the function of a general regulatory factor, GENERAL REGULATORY FACTOR11 (GRF11), in terms of the iron (Fe) deficiency response. Physiological and molecular responses of the loss-of-function Arabidopsis thaliana grf11 mutant to Fe supply were investigated. Genes involved in posttranscriptional regulation of FER-LIKE FE DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT) were also analyzed. In addition, the molecular link between the signaling molecule nitric oxide (NO) and Fe deficiency responses was further dissected. Our results suggest that GRF11 is necessary for induction of Fe-deficiency-tolerance mechanisms. The FIT protein can bind to the promoter of GRF11, which contains an E-box motif. GRF11 also positively affects FIT transcription but has no influence on the genes involved in posttranscriptional regulation of FIT. Furthermore, NO positively regulates GRF11 induction upon the onset of Fe deficiency. We propose that, upon the onset of Fe deficiency, induction of FIT expression is dependent on GRF11, which acts downstream of NO to mediate Fe deficiency responses.

Endogenous nitric oxide generation in protoplast chloroplasts

KEY MESSAGE : NO generation is studied in the protoplast chloroplasts. NO, ONOO ( - ) and ROS (O ( 2 ) ( - ) and H ( 2 ) O ( 2 ) ) are generated in chloroplasts. Nitric oxide synthase-like protein appears to be involved in NO generation. Nitric oxide stimulates chlorophyll biosynthesis and chloroplast differentiation. The present study was conducted to better understand the process of NO generation in the leaf chloroplasts and protoplasts. NO, peroxynitrite and superoxide anion were investigated in the protoplasts and isolated chloroplasts using specific dyes, confocal laser scanning and light microscopy. The level of NO was highest after protoplast isolation and subsequently decreased during culture. Suppression of NO signal in the presence of PTIO, suggests that diaminofluorescein-2 diacetate (DAF-2DA) detected NO. Detection of peroxynitrite, a reaction product of NO and superoxide anion, further suggests NO generation. Moreover, generation of NO and peroxynitrite in the chloroplasts of wild-type Arabidopsis and their absence or weak signals in the leaf-derived protoplasts of Atnoa1 mutants confirmed the reactivity of DAF-2DA and aminophenyl fluorescein to NO and peroxynitrite, respectively. Isolated chloroplasts also showed signal of NO. Suppression of NO signal in the presence of 100 μM nitric oxide synthase inhibitors [L-NNA, Nω-nitro-L-arginine and PBIT, S,S'-1,3-phenylene-bis(1,2-ethanediyl)-bis-isothiourea] revealed that nitric oxide synthase-like system is involved in NO synthesis. Suppression of NO signal in the protoplasts isolated in the presence of cycloheximide suggests de novo synthesis of NO generating protein during the process of protoplast isolation. Furthermore, the lack of inhibition of NO production by sodium tungstate (250 μM) and inhibition by L-NNA, and PBIT suggest involvement NOS-like protein, but not nitrate reductase, in NO generation in the leaf chloroplasts and protoplasts.

Arbuscular mycorrhizal fungal inoculation increases phenolic synthesis in clover roots via hydrogen peroxide, salicylic acid and nitric oxide signaling pathways

Arbuscular mycorrhizal fungi can increase the host resistance to pathogens via promoted phenolic synthesis, however, the signaling pathway responsible for it still remains unclear. In this study, in order to reveal the signaling molecules involved in this process, we inoculated Trifolium repense L. with an arbuscular mycorrhizal fungus (AMF), Glomus mosseae, and monitored the contents of phenolics and signaling molecules (hydrogen peroxide (H(2)O(2)), salicylic acid (SA), and nitric oxide (NO)) in roots, measured the activities of l-phenylalanine ammonia-lyase (PAL) and nitric oxide synthase (NOS), and the expression of pal and chs genes. Results demonstrated that AMF colonization promoted the phenolic synthesis, in parallel with the increase in related enzyme activity and gene expression. Meanwhile, the accumulation of all three signaling molecules was also up-regulated by AMF. This study suggested that AMF increased the phenolic synthesis in roots probably via signaling pathways of H(2)O(2), SA and NO in a signaling cascade.

Constitutive expression of mammalian nitric oxide synthase in tobacco plants triggers disease resistance to pathogens

Nitric oxide (NO) is known for its role in the activation of plant defense responses. To examine the involvement and mode of action of NO in plant defense responses, we introduced calmodulin-dependent mammalian neuronal nitric oxide synthase (nNOS), which controls the CaMV35S promoter, into wild-type and NahG tobacco plants. Constitutive expression of nNOS led to NO production and triggered spontaneous induction of leaf lesions. Transgenic plants accumulated high amounts of H(2)O(2), with catalase activity lower than that in the wild type. nNOS transgenic plants contained high levels of salicylic acid (SA), and they induced an array of SA-, jasmonic acid (JA)-, and/or ethylene (ET)-related genes. Consequently, NahG co-expression blocked the induction of systemic acquired resistance (SAR)-associated genes in transgenic plants, implying SA is involved in NO-mediated induction of SAR genes. The transgenic plants exhibited enhanced resistance to a spectrum of pathogens, including bacteria, fungi, and viruses. Our results suggest a highly ranked regulatory role for NO in SA-, JA-, and/or ET-dependent pathways that lead to disease resistance.

Nitric oxide-mediated maintenance of redox homeostasis contributes to NPR1-dependent plant innate immunity triggered by lipopolysaccharides

The perception of lipopolysaccharides (LPS) by plant cells can lead to nitric oxide (NO) production and defense gene induction. However, the signaling cascades underlying these cellular responses have not yet been resolved. This work investigated the biosynthetic origin of NO and the role of NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1) to gain insight into the mechanism involved in LPS-induced resistance of Arabidopsis (Arabidopsis thaliana). Analysis of inhibitors and mutants showed that LPS-induced NO synthesis was mainly mediated by an arginine-utilizing source of NO generation. Furthermore, LPS-induced NO caused transcript accumulation of alternative oxidase genes and increased antioxidant enzyme activity, which enhanced antioxidant capacity and modulated redox state. We also analyzed the subcellular localization of NPR1 to identify the mechanism for protein-modulated plant innate immunity triggered by LPS. LPS-activated defense responses, including callose deposition and defense-related gene expression, were found to be regulated through an NPR1-dependent pathway. In summary, a significant NO synthesis induced by LPS contributes to the LPS-induced defense responses by up-regulation of defense genes and modulation of cellular redox state. Moreover, NPR1 plays an important role in LPS-triggered plant innate immunity.

Nitric oxide and hydrogen peroxide alleviate drought stress in marigold explants and promote its adventitious root development

Drought stress is one of the most important environmental factors that regulates plant growth and development. In this study, we examined the effects of nitric oxide (NO) and hydrogen peroxide (H(2)O(2)) on adventitious rooting in marigold (Tagetes erecta L.) under drought stress. The results showed that the promoting effect of NO or H(2)O(2) on rooting under drought stress was dose-dependent, with a maximal biological response at 10 μM NO donor sodium nitroprusside (SNP) or 600 μM H(2)O(2). Results also indicated that endogenous NO and H(2)O(2) may play crucial roles in rooting under drought conditions, and H(2)O(2) may be involved in rooting promoted by NO under drought stress. NO or H(2)O(2) treatment attenuated the destruction of mesophyll cells ultrastructure by drought stress. Similarly, NO or H(2)O(2) increased leaf chlorophyll content, chlorophyll fluorescence parameters (Fv/Fm, ΦPS II and qP), and hypocotyls soluble carbohydrate and protein content, while decreasing starch content. Results suggest that the protection of mesophyll cells ultrastructure by NO or H(2)O(2) under drought conditions improves the photosynthetic performance of leaves and alleviates the negative effects of drought on carbohydrate and nitrogen accumulation in explants, thereby adventitious rooting being promoted.

Nitric oxide is involved in hemin-induced cucumber adventitious rooting process

Hemin, a heme oxygenase-1 (HO-1) inducer, was shown to exert numerous beneficial physiological functions in animals. Our previous study suggests that HO-1/carbon monoxide (CO) acts as a novel downstream signal system in the auxin-induced adventitious rooting. The objective of this study was to test whether nitric oxide (NO) is involved in hemin-induced cucumber adventitious rooting. Applications of hemin or CO aqueous solution to auxin-depleted cucumber explant induced up-regulation of cucumber HO-1 transcripts (CsHO1), NO production, and thereafter adventitious root formation, and some above responses were blocked by the combination treatment with two nitric oxide synthase (NOS)-like enzyme inhibitors N(G)-nitro-L-arginine methylester hydrochloride and N(G)-nitro-L-arginine, a HO-1 specific inhibitor zinc protoporphyrin IX, and a specific NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt. However, these blocking responses were not observed using tungstate, an inhibitor of nitrate reductase, another NO producing enzyme in plants. Furthermore, the guanylate cyclase inhibitors 1H-(1,2,4)-oxadiazole[4,3-a]quinoxalin-1-one and 6-anilino-5,8-quinolinedione reduced root development induced by hemin, whereas the cell-permeable cyclic guanosine monophosphate (cGMP) derivative 8-Br-cGMP reversed this effect. Together, our results indicated that at least in our experimental conditions, NO might operate downstream of hemin promoting adventitious root formation probably in a cGMP-dependent manner.

GSH threshold requirement for NO-mediated expression of the Arabidopsis AtFer1 ferritin gene in response to iron

Iron treatment of Arabidopsis cultured cells promotes a rapid NO burst within chloroplasts, necessary for up-regulation of the AtFer1 ferritin gene expression. The same occurs in Arabidopsis leaf chloroplasts, and is dependent upon the GSH content of plants. A leaf GSH concentration threshold between 10 and 50 nmol GSHg(-1) FW is required for full induction of AtFer1 gene expression in response to iron.