Nitric oxide signalling in plants

Involvement of nitric oxide signaling in mammalian Bax-induced terpenoid indole alkaloid production of Catharanthus roseus cells

Bax, a mammalian pro-apoptotic member of the Bcl-2 family, has been demonstrated to be a potential regulatory factor for plant secondary metabolite biosynthesis recently. To investigate the molecular mechanism of Bax-induced secondary metabolite biosynthesis, we determined the contents of nitric oxide (NO) of the transgenic Catharanthus roseus cells overexpressing a mouse Bax protein and checked the effects of NO specific scavenger 2,4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPITO) on Bax-induced terpenoid indole alkaloid (TIA) production of the cells. The data showed that overexpression of the mouse Bax in C. roseus cells triggered NO generation of the cells. Treatment of cPITO not only inhibited the Bax-triggered NO burst but also suppressed the Bax-induced TIA production. The results indicated that the mouse Bax might activate the NO signaling in C. roseus cells and induce TIA production through the NO-dependent signal pathway in the cells. Furthermore, the activities of nitric oxide synthase (NOS) were significantly increased in the transgenic Bax cells as compared to those in the control cells, showing that the mouse Bax may induce NOS of C. roseus cells. Treatment of the transgenic Bax cells with NOS inhibitor PBITU blocked both Bax-induced NO generation and TIA production, which suggested that the mouse Bax might trigger NO generation and TIA production through NOS. However, the NOS-like activities and NO generation in the transgenic Bax cells did not match kinetically and the Bax-induced NOS-like activity was much later and lower than NO production. Moreover, the Bax-induced NO generation and TIA production were only partially inhibited by PBITU. Thus, our results suggested that the Bax-induced NO production and secondary metabolite biosynthesis in C. roseus cells was not entirely dependent on NOS or NOS-like enzymes.

The role of water channel proteins and nitric oxide signaling in rice seed germination

Previous studies have demonstrated the possible role of several aquaporins in seed germination. But systematic investigation of the role of aquaporin family members in this process is lacking. Here, the developmental regulation of plasma membrane intrinsic protein (PIP) expression throughout germination and post-germination processes in rice embryos was analyzed. The expression patterns of the PIPs suggest these aquaporins play different roles in seed germination and seedling growth. Partial silencing of the water channel genes, OsPIP1;1 and OsPIP1;3, reduced seed germination while over-expression of OsPIP1;3 promoted seed germination under water-stress conditions. Moreover, spatial expression analysis indicates that OsPIP1;3 is expressed predominantly in embryo during seed germination. Our data also revealed that the nitric oxide (NO) donors, sodium nitroprusside (SNP) and S-nitrosoglutathione (GSNO), promoted seed germination; furthermore, the NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, inhibited germination and reduced the stimulative effects of SNP and GSNO on rice germination. Exogenous NO stimulated the transcription of OsPIP1;1, OsPIP1;2, OsPIP1;3 and OsPIP2;8 in germinating seeds. These results suggest that water channels play an important role in seed germination, acting, at least partly, in response to the NO signaling pathway.

Reactive oxygen species and nitric oxide are involved in ABA inhibition of stomatal opening

Although nitric oxide (NO) and reactive oxygen species (ROS) are essential signalling molecules required for mediation of abscisic acid (ABA)-induced stomatal closure, it is not known whether these molecules also mediate the ABA inhibition of stomatal opening. In this study, we investigated the role of NO and ROS in the ABA inhibition of stomatal opening in Vicia faba. ABA induced both NO and ROS synthesis, and the NO scavenger reduced the ABA inhibition of stomatal opening. Exogenous NO and hydrogen peroxide (H2O2) also inhibited stomatal opening, indicating that NO and ROS are involved in the inhibition signalling process. An inhibitor of nitric oxide synthase (NOS) reversed the ABA inhibition of stomatal opening. Either the NO scavenger or the NOS inhibitor also reversed the process in the H2O2 inhibition of stomatal opening. We found that in the ABA inhibition of stomatal opening, NO is downstream of ROS in the signalling process, and NO is synthesized by a NOS-like enzyme.

Aluminum-induced ethylene production is associated with inhibition of root elongation in Lotus japonicus L

Inhibition of root elongation by toxic aluminum (Al(3+)) occurs rapidly and is one of the most distinct and earliest symptoms of Al toxicity. To elucidate mechanism underlying Al(3+)-induced inhibition of root elongation, we investigated the involvement of ethylene in Al(3+)-induced inhibition of root elongation using the legume model plants Lotus japonicus and Medicago truncatula. Root elongation of L. japonicus and M. truncatula was rapidly inhibited by exposure to AlCl(3). A similar rapid inhibition of root elongation by the ethylene-releasing substance, ethephon, and the ethylene precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), was also observed. The Al(3+)-induced inhibition of root elongation was substantially ameliorated in the presence of antagonists of ethylene biosynthesis [Co(2+) and aminoethoxyvinylglycine (AVG)]. Al(3+) increased the activity of ACC oxidase (ACO), and induced a rapid evolution of ethylene from root apices and expression of genes of ACC synthase (ACS) and ACO. These findings suggest that induction of ethylene evolution resulting from up-regulation of ACS and ACO plays a critical role in Al(3+)-induced inhibition of root elongation.

Perception and transduction of abscisic acid signals: keys to the function of the versatile plant hormone ABA

During the past decade, much progress has been made toward understanding the mechanisms underlying plant hormone activity, from perception to nuclear events. However, the signaling mechanisms for abscisic acid (ABA) have remained largely obscure. Recent breakthroughs identifying FCA, which is an RNA-binding protein, the Mg-chelatase H subunit, and a G protein-coupled receptor as receptors for ABA provide a major leap forward in understanding the initial steps of ABA signaling mechanisms. Recent studies have also revealed the molecular mechanisms of second messenger production, protein modifications such as phosphorylation, and regulatory mechanisms of gene expression in the ABA response. Therefore, the connections between these events are also beginning to be determined. Here, we review recent progress and discuss the overall scheme of the ABA response mechanisms.

Interference with the citrulline-based nitric oxide synthase assay by argininosuccinate lyase activity inArabidopsisextracts

There are many reports of an arginine-dependent nitric oxide synthase activity in plants; however, the gene(s) or protein(s) responsible for this activity have yet to be convincingly identified. To measure nitric oxide synthase activity, many studies have relied on a citrulline-based assay that measures the formation of L-citrulline from L-arginine using ion exchange chromatography. In this article, we report that when such assays are used with protein extracts from Arabidopsis, an arginine-dependent activity was observed, but it produced a product other than citrulline. TLC analysis identified the product as argininosuccinate. The reaction was stimulated by fumarate (> 500 microM), implicating the urea cycle enzyme argininosuccinate lyase (EC 4.3.2.1), which reversibly converts arginine and fumarate to argininosuccinate. These results indicate that caution is needed when using standard citrulline-based assays to measure nitric oxide synthase activity in plant extracts, and highlight the importance of verifying the identity of the product as citrulline.

Involvement of hydrogen peroxide and nitric oxide in salt resistance in the calluses from Populus euphratica

Nitric oxide (NO) and hydrogen peroxide (H2O2) function as signalling molecules in plants under abiotic and biotic stresses. Calluses from Populus euphratica, which show salt tolerance, were used to study the interaction of NO and H2O2 in plant adaptation to salt resistance. The nitric oxide synthase (NOS) activity was identified in the calluses, and this activity was induced under 150 mM NaCl treatment. Under 150 mM NaCl treatment, the sodium (Na) percentage decreased, but the potassium (K) percentage and the K/Na ratio increased in P. euphratica calluses. Application of glucose/glucose oxidase (G/GO, a H2O2 donor) and sodium nitroprusside (SNP, a NO donor) revealed that both H2O2 and NO resulted in increased K/Na ratio in a concentration-dependent manner. Diphenylene iodonium (DPI, an NADPH oxidase inhibitor) counteracted H2O2 and NO effect by increasing the Na percentage, decreasing the K percentage and K/Na ratio. NG-monomethyl-L-Arg monoacetate (NMMA, an NO synthase inhibitor) and 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxyde (PTIO, a specific NO scavenger) only reversed NO effect, but did not block H2O2 effect. The increased activity of plasma membrane (PM) H+ -ATPase caused by salt stress was reversed by treatment with DPI and NMMA. Exogenous H2O2 increased the activity of PM H+ -ATPase, but the effect could not be diminished by NMMA and PTIO. The NO-induced increase of PM H+ -ATPase can be reversed by NMMA and PTIO, but not by DPI. Western blot analysis demonstrated that NO and H2O2 stimulated the expression of PM H+ -ATPase in P. euphratica calluses. These results indicate that NO and H2O2 served as intermediate molecules in inducing salt resistance in the calluses from P. euphratica under slat stress by increasing the K/Na ratio, which was dependent on the increased PM H+ -ATPase activity.

Enhanced sensitivity to oxidative stress in an Arabidopsis nitric oxide synthase mutant

The possible involvement of nitric oxide (NO) in oxidative stress tolerance was studied using Arabidopsis thaliana wild type (WT) and Atnos1 mutant plants, in which endogenous NO production is greatly diminished because 80% of nitric oxide synthase (NOS) activity is eliminated due to T-DNA insertion in the first exon of the NOS1 gene. Compared with WT, Atnos1 mutant plants showed increased hypersensitivity to salt stress and methyl viologen (MV) treatment. The maximal photochemical efficiency of photosystem II (F(v)/F(m)) and membrane integrity decreased in WT and Atnos1 mutant plants under stresses, but the extent was higher in the mutant. Treatment with sodium nitroprusside (SNP) (a NO donor) to Atnos1 mutant plants alleviated the damage. Instead, inhibition of nitric oxide accumulation in the WT plants produced opposite effects. Hydrogen peroxide and lipid peroxidation increased and the extent was higher in Atnos1 mutant plants than that in WT plants under MV stress. These results indicated that nitric oxide could protect the damage against NaCl and MV treatments.

Nitric oxide synthase-dependent nitric oxide production is associated with salt tolerance in Arabidopsis

Nitric oxide (NO) has emerged as a key molecule involved in many physiological processes in plants. To characterize roles of NO in tolerance of Arabidopsis (Arabidopsis thaliana) to salt stress, effect of NaCl on Arabidopsis wild-type and mutant (Atnoa1) plants with an impaired in vivo NO synthase (NOS) activity and a reduced endogenous NO level was investigated. Atnoa1 mutant plants displayed a greater Na+ to K+ ratio in shoots than wild-type plants due to enhanced accumulation of Na+ and reduced accumulation of K+ when exposed to NaCl. Germination of Atnoa1 seeds was more sensitive to NaCl than that of wild-type seeds, and wild-type plants exhibited higher survival rates than Atnoa1 plants when grown under salt stress. Atnoa1 plants had higher levels of hydrogen peroxide than wild-type plants under both control and salt stress, suggesting that Atnoa1 is more vulnerable to salt and oxidative stress than wild-type plants. Treatments of wild-type plants with NOS inhibitor and NO scavenger reduced endogenous NO levels and enhanced NaCl-induced increase in Na+ to K+ ratio. Exposure of wild-type plants to NaCl inhibited NOS activity and reduced quantity of NOA1 protein, leading to a decrease in endogenous NO levels measured by NO-specific fluorescent probe. Treatment of Atnoa1 plants with NO donor sodium nitroprusside attenuated the NaCl-induced increase in Na+ to K+ ratio. Therefore, these findings provide direct evidence to support that disruption of NOS-dependent NO production is associated with salt tolerance in Arabidopsis.

Physiology and biochemistry of source-regulated protein accumulation in the wheat grain

Wheat is unique among cereals for the baking qualities of its flour, which are dependent upon the type and concentration of its proteins. As a consequence, the grain protein concentration (GPC) is one of the main determinants of wheat international market price. More than 50-70% of the final grain N is accumulated before flowering and later remobilized to the grain, N fertilization being the common practice used to produce high GPC. However, after incremental additions of N fertilizer, GPC reaches a maximum and then remains constant, without any increase in N uptake or remobilization by the crop, thus decreasing the efficiency of N fertilizer. Although, the genetic and molecular mechanisms that regulate N uptake by the roots are being clarified quickly, the regulation and physiology of N transport from the leaves to the grain remains less clear. In this review, the possible regulatory points involved in N transport to the grain and the difficulties for increasing GPC are discussed. It has been demonstrated that protein synthesis in the grain is source-limited, and that the grain can accumulate protein limited only by the amino acids provided by the phloem. It has also been shown that there is no limitation in the amino acid/sugar ratios that can be exported to the phloem. On the other hand, NO(3)(-) uptake transporters are depressed when the plant concentration of some amino acids, such as glutamine, is high. It has also been shown that a high N supply increases cytokinins concentration, preventing leaf senescence and proteolysis. Based on this information, it is postulated that there are two main regulatory points during grain filling when plant N status is ample. On the one hand, the N uptake transporters in the roots are depressed due to the high amino acids concentration in the tissues, and N uptake is low. On the other, a high amino acids concentration keeps the cytokinins level high, repressing leaf protein degradation and decreasing amino acid export to the phloem. As a consequence, GPC cannot be increased despite the ample N supply.

Involvement of nitric oxide in cerebroside-induced defense responses and taxol production in Taxus yunnanensis suspension cells

This work was to characterize the generation of nitric oxide (NO) in Taxus yunnanensis cells induced by a fungal-derived cerebroside and the signal role of NO in the elicitation of plant defense responses and taxol production. (2S,2'R,3R,3'E,4E,8E)-1-O-beta-D-glucopyranosyl-2-N-(2'-hydroxy-3'-octadecenoyl)-3-hydroxy-9-methyl-4,8-sphingadienine at 10 microg/ml induced a rapid and dose-dependent NO production in the Taxus cell culture, reaching a maximum within 5 h of the treatment. The NO donor sodium nitroprusside (SNP) potentiated cerebroside-induced H(2)O(2) production and cell death. Inhibition of nitric oxide synthase activity by phenylene-1,3-bis(ethane-2-isothiourea) dihydrobromide or scavenging NO by 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide partially blocked the cerebroside-induced H(2)O(2) production and cell death. Moreover, NO enhanced cerebroside-induced activation of phenylalanine ammonium-lyase and accumulation of taxol in cell cultures. These results are suggestive of a role for NO as a new signal component for activating the cerebroside-induced defense responses and secondary metabolism activities of plant cells.

Glucose-6-phosphate dehydrogenase plays a pivotal role in nitric oxide-involved defense against oxidative stress under salt stress in red kidney bean roots

The pivotal role of glucose-6-phosphate dehydrogenase (G-6-PDH)-mediated nitric oxide (NO) production in the tolerance to oxidative stress induced by 100 mM NaCl in red kidney bean (Phaseolus vulgaris) roots was investigated. The results show that the G-6-PDH activity was enhanced rapidly in the presence of NaCl and reached a maximum at 100 mM. Western blot analysis indicated that the increase of G-6-PDH activity in the red kidney bean roots under 100 mM NaCl was mainly due to the increased content of the G-6-PDH protein. NO production and nitrate reductase (NR) activity were also induced by 100 mM NaCl. The NO production was reduced by NaN(3) (an NR inhibitor), but not affected by N(omega)-nitro-L-arginine (L-NNA) (an NOS inhibitor). Application of 2.5 mM Na(3)PO(4), an inhibitor of G-6-PDH, blocked the increase of G-6-PDH and NR activity, as well as NO production in red kidney bean roots under 100 mM NaCl. The activities of antioxidant enzymes in red kidney bean roots increased in the presence of 100 mM NaCl or sodium nitroprusside (SNP), an NO donor. The increased activities of all antioxidant enzymes tested at 100 mM NaCl were completely inhibited by 2.5 mM Na(3)PO(4). Based on these results, we conclude that G-6-PDH plays a pivotal role in NR-dependent NO production, and in establishing tolerance of red kidney bean roots to salt stress.

Nitrosative stress in plants

Nitrosative stress has become a usual term in the physiology of nitric oxide in mammalian systems. However, in plants there is much less information on this type of stress. Using olive leaves as experimental model, the effect of salinity on the potential induction of nitrosative stress was studied. The enzymatic l-arginine-dependent production of nitric oxide (NOS activity) was measured by ozone chemiluminiscence. The specific activity of NOS in olive leaves was 0.280nmol NOmg(-1) proteinmin(-1), and was dependent on l-arginine, NADPH and calcium. Salt stress (200mM NaCl) caused an increase of the l-arginine-dependent production of nitric oxide (NO), total S-nitrosothiols (RSNO) and number of proteins that underwent tyrosine nitration. Confocal laser scanning microscopy analysis using either specific fluorescent probes for NO and RSNO or antibodies to S-nitrosoglutathione and 3-nitrotyrosine, showed also a general increase of these reactive nitrogen species (RNS) mainly in the vascular tissue. Taken together, these findings show that in olive leaves salinity induces nitrosative stress, and vascular tissues could play an important role in the redistribution of NO-derived molecules during nitrosative stress.

Signaling on the Stigma: Potential New Roles for ROS and NO in Plant Cell Signaling

Reactive oxygen species (ROS) and reactive nitrogen species, particularly NO, are key components of diverse signaling networks in animals and plants. We have recently shown that epidermal cells of stigmas from a range of different angiosperms accumulate relatively large amounts of ROS, principally H(2)O(2), whereas pollen produces NO. Importantly, ROS/H(2)O(2) levels appeared reduced in stigma cells supporting developing pollen grains compared to cells without pollen grains attached. To explore a possible link between pollen NO production and reduced levels of stigmatic ROS/H(2)O(2), we supplied stigmas with NO and observed an overall reduction in levels of stigmatic ROS/H(2)O(2). These new and unexpected data suggest a potential new signaling role for ROS/H(2)O(2) and NO in pollen-stigma recognition processes.

Expression of a nitric oxide degrading enzyme induces a senescence programme in Arabidopsis

Nitric oxide (NO) has been proposed to act as a factor delaying leaf senescence and fruit maturation in plants. Here we show that expression of a NO degrading dioxygenase (NOD) in Arabidopsis thaliana initiates a senescence-like phenotype, an effect that proved to be more pronounced in older than in younger leaves. This senescence phenotype was preceded by a massive switch in gene expression in which photosynthetic genes were down-regulated, whereas many senescence-associated genes (SAGs) and the 1-aminocyclopropane-1-carboxylic acid (ACC) synthase gene ACS6 involved in ethylene synthesis were up-regulated. External fumigation of NOD plants with NO as well as environmental conditions known to stimulate endogenous NO production attenuated the induced senescence programme. For instance, both high light conditions and nitrate feeding reduced the senescence phenotype and attenuated the down-regulation of photosynthetic genes as well as the up-regulation of SAGs. Treatment of plants with the cytokinin 6-benzylaminopurin (BAP) reduced the down-regulation of photosynthesis, although it had no consistent effect on SAG expression. Metabolic changes during NOD-induced senescence comprehended increases in salicylic acid (SA) levels, accumulation of the phytoalexin camalexin and elevation of leaf gamma-tocopherol contents, all of which occurred during natural senescence in Arabidopsis leaves as well. Moreover, NO fumigation delayed the senescence process induced by darkening individual Arabidopsis Columbia-0 (Col-0) leaves. Our data thus support the notion that NO acts as a negative regulator of leaf senescence.

Nitric oxide induced by hydrogen peroxide mediates abscisic acid-induced activation of the mitogen-activated protein kinase cascade involved in antioxidant defense in maize leaves

* The role of nitric oxide (NO) and the relationship between NO, hydrogen peroxide (H(2)O(2)) and mitogen-activated protein kinase (MAPK) in abscisic acid (ABA)-induced antioxidant defense in leaves of maize (Zea mays) plants were investigated. * Both ABA and H(2)O(2) induced increases in the generation of NO in mesophyll cells of maize leaves, and H(2)O(2) was required for the ABA-induced generation of NO. Pretreatment with NO scavenger and nitric oxide synthase (NOS) inhibitor substantially reduced the ABA-induced production of NO, and partly blocked the activation of a 46 kDa MAPK and the expression and the activities of several antioxidant enzymes induced by ABA. Treatment with the NO donor sodium nitroprusside (SNP) also induced the activation of the MAPK, and enhanced the antioxidant defense systems. * Conversely, SNP treatment did not induce the production of H(2)O(2), and pretreatments with NO scavenger and NOS inhibitor did not affect ABA-induced H(2)O(2) production. * Our results suggest that ABA-induced H(2)O(2) production mediates NO generation, which, in turn, activates MAPK and results in the upregulation in the expression and the activities of antioxidant enzymes in ABA signaling.

Nitric oxide promotes the wound-healing response of potato leaflets

Nitric oxide (NO) is an essential regulatory molecule in several developmental and (patho) physiological processes. In this work, it is demonstrated that NO participates in the wound-healing response of potato leaves. The experimental approaches showed that the deposition of the cell-wall glucan callose was induced by the application of the NO donor sodium nitroprusside (SNP), and such induction was additive to the wound-induced callose production. Additionally, the expression of wound-related genes as phenylalanine ammonia-lyase (PAL) and extensin showed an accumulation of their transcript levels by SNP treatment. Moreover, the SNP-mediated increase of the PAL transcript level was additive to the induction mediated by wounding. These results indicate that increased levels of NO might potentiate the healing responses in plants leading to a rapid restoration of the damaged tissue.

Inhibition of nitric oxide synthase (NOS) underlies aluminum-induced inhibition of root elongation in Hibiscus moscheutos

Aluminum (Al) is toxic to plants when solubilized into Al(3+) in acidic soils, and becomes a major factor limiting plant growth. However, the primary cause for Al toxicity remains unknown. Nitric oxide (NO) is an important signaling molecule modulating numerous physiological processes in plants. Here, we investigated the role of NO in Al toxicity to Hibiscus moscheutos. Exposure of H. moscheutos to Al(3+) led to a rapid inhibition of root elongation, and the inhibitory effect was alleviated by NO donor sodium nitroprusside (SNP). NO scavenger and inhibitors of NO synthase (NOS) and nitrate reductase had a similar inhibitory effect on root elongation. The inhibition of root elongation by these treatments was ameliorated by SNP. Aluminum inhibited activity of NOS and reduced endogenous NO concentrations. The alleviation of inhibition of root elongation induced by Al, NO scavenger and NOS inhibitor was correlated with endogenous NO concentrations in root apical cells, suggesting that reduction of endogenous NO concentrations resulting from inhibition of NOS activity could underpin Al-induced arrest of root elongation in H. moscheutos.

Chloroplasts as a nitric oxide cellular source. Effect of reactive nitrogen species on chloroplastic lipids and proteins

Nitric oxide (NO) generation by soybean (Glycine max var. ADM 4800) chloroplasts was studied as an endogenous product assessed by the electron paramagnetic resonance spin-trapping technique. Nitrite and l-arginine (Arg) are substrates for enzymatic activities considered to be the possible sources of NO in plants. Soybean chloroplasts showed a NO production of 3.2 +/- 0.2 nmol min(-1) mg(-1) protein in the presence of 1 mm NaNO(2). Inhibition of photosynthetic electron flow by 3-(3,4-dichlorophenyl)-1,1-dimethyl urea resulted in a lower rate (1.21 +/- 0.04 nmol min(-1) mg(-1) protein) of NO generation. Chloroplasts incubated with 1 mm Arg showed NO production of 0.76 +/- 0.04 nmol min(-1) mg(-1) protein that was not affected either by omission of Ca(2+) or by supplementation with Ca(2+) and calmodulin to the incubation medium. This production was inhibited when chloroplasts were incubated in the presence of NO synthase inhibitors N(omega)-nitro-l-Arg methyl ester hydrochloride and N(omega)-nitro-l-Arg. In vitro exposure of chloroplasts to an NO donor (250 mum S-nitrosoglutathione) decreased lipid radical content in membranes by 29%; however, incubation in the presence of 25 mum peroxynitrite (ONOO(-)) led to an increase in lipid-derived radicals (34%). The effect of ONOO(-) on protein oxidation was determined by western blotting, showing an increase in carbonyl content either in stroma or thylakoid proteins as compared to controls. Moreover, ONOO(-) treatment significantly affected both O(2) evolution and chlorophyll fluorescence in thylakoids. Data reported here suggest that NO is an endogenous metabolite in soybean chloroplasts and that reactive nitrogen species could exert either antioxidant or prooxidant effects on chloroplast macromolecules.

Do nitric oxide donors mimic endogenous NO-related response in plants?

Huge advances achieved recently in elucidating the role of NO in plants have been made possible by the application of NO donors. However, the application of NO to plants in various forms and doses should be subjected to detailed verification criteria. Not all metabolic responses induced by NO donors are reliable and reproducible in other experimental designs. The aim of the presented studies was to investigate the half-life of the most frequently applied donors (SNP, SNAP and GSNO), the rate of NO release under the influence of light and reducing agents. At a comparable donor concentration (500 microM) and under light conditions the highest rate of NO generation was found for SNAP, followed by GSNO and SNP. The measured half-life of the donor in the solution was 3 h for SNAP, 7 h for GSNO and 12 h for SNP. A temporary lack of light inhibited NO release from SNP, both in the solution and SNP-treated leaf tissue, which was measured by the electrochemical method. Also a NO, selective fluorescence indicator DAF-2DA in leaves supplied with different donors showed green fluorescence spots in the epidermal cells mainly in the light. SNP as a NO donor was the most photosensitive. The activity of PAL, which plays an important role in plant defence, was also activated by SNP in the light, not in the dark. S-nitrosothiols (SNAP and GSNO) also underwent photodegradation, although to a lesser degree than SNP. Additionally, NO generation capacity from S-nitrosothiols was shown in the presence of reducing agents, i.e. ascorbic acid and GSH, and the absence of light. The authors of this paper would like to polemicize with the commonly cited statement that "donors are compounds that spontaneously break down to release NO" and wish to point out the fact that the process of donor decomposition depends on the numerous external factors. It may be additionally stimulated or inhibited by live plant tissue, thus it is necessary to take into consideration these aspects and monitor the amount of NO released by the donor.

Nitric oxide protects against oxidative stress under heat stress in the calluses from two ecotypes of reed

Calluses from two ecotypes of reed (Phragmites communis Trin.) plant (dune reed [DR] and swamp reed [SR]), which show different sensitivity to heat stress, were used to study plant acclimations to heat stress. SR callus suffered more oxidative damage than DR callus, and DR callus maintained higher relative growth rate and cell viability and lower ion leakage than SR callus under heat stress. Application of two nitric oxide donors, sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine (SNAP), dramatically alleviated heat stress induced ion leakage increase, growth suppression and cell viability decrease in both calluses under heat stress. H2O2 and MDA contents were decreased and the activities of superoxide dismutase, catalase, ascorbate peroxidase and peroxidase increased in both calluses in the presence of NO donors under heat stress. The potassium salt of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), a specific NO scavenger, arrested NO donors mediated protective effects. Moreover, measurement of the rate of NO release showed that NO production increased significantly in DR callus while kept stable in SR callus under heat stress. Pretreatment with cPTIO had no effect on SR callus under heat stress while it significantly enhanced oxidative stress in DR callus compared with that of heat stress alone. These results suggest that NO can effectively protect both calluses from oxidative stress induced by heat stress and that NO might act as a signal in activating active oxygen scavenging enzymes under heat stress and thus confer DR callus thermotolerance.

Nitric oxide is formed in Medicago truncatula-Sinorhizobium meliloti functional nodules

Nitric oxide (NO) has recently gained interest as a major signaling molecule during plant development and response to environmental cues. Its role is particularly crucial for plant-pathogen interactions, during which it participates in the control of plant defense response and resistance. Indication for the presence of NO during symbiotic interactions has also been reported. Here, we defined when and where NO is produced during Medicago truncatula-Sinorhizobium meliloti symbiosis. Using the NO-specific fluorescent probe 4,5-diaminofluorescein diacetate, NO production was detected by confocal microscopy in functional nodules. NO production was localized in the bacteroid-containing cells of the nodule fixation zone. The infection of Medicago roots with bacterial strains impaired in nitrogenase or nitrite reductase activities lead to the formation of nodules with an unaffected NO level, indicating that neither nitrogen fixation nor denitrification pathways are required for NO production. On the other hand, the NO synthase inhibitor N-methyl-L-arginine impaired NO detection, suggesting that a NO synthase may participate to NO production in nodules. These data indicate that a NO production occurs in functional nodules. The location of such a production in fully metabolically active cells raises the hypothesis of a new function for NO during this interaction unrelated to defense and cell-death activation.

Plant peroxisomes as a source of signalling molecules

Peroxisomes are pleiomorphic, metabolically plastic organelles. Their essentially oxidative function led to the adoption of the name 'peroxisome'. The dynamic and diverse nature of peroxisome metabolism has led to the realisation that peroxisomes are an important source of signalling molecules that can function to integrate cellular activity and multicellular development. In plants defence against predators and a hostile environment is of necessity a metabolic and developmental response--a plant has no place to hide. Mutant screens are implicating peroxisomes in disease resistance and signalling in response to light. Characterisation of mutants disrupted in peroxisomal beta-oxidation has led to a growing appreciation of the importance of this pathway in the production of jasmonic acid, conversion of indole butyric acid to indole acetic acid and possibly in the production of other signalling molecules. Likewise the role of peroxisomes in the production and detoxification of reactive oxygen, and possibly reactive nitrogen species and changes in redox status, suggests considerable scope for peroxisomes to contribute to perception and response to a wide range of biotic and abiotic stresses. Whereas the peroxisome is the sole site of beta-oxidation in plants, the production and detoxification of ROS in many cell compartments makes the specific contribution of the peroxisome much more difficult to establish. However progress in identifying peroxisome specific isoforms of enzymes associated with ROS metabolism should allow a more definitive assessment of these contributions in the future.

Cadmium effect on oxidative metabolism of pea (Pisum sativum L.) roots. Imaging of reactive oxygen species and nitric oxide accumulation in vivo

Growth of pea (Pisum sativum L.) plants with 50 microM CdCl2 for 15 d produced a reduction in the number and length of lateral roots, and changes in structure of the principal roots affecting the xylem vessels. Cadmium induced a reduction in glutathione (GSH) and ascorbate (ASC) contents, and catalase (CAT), GSH reductase (GR) and guaiacol peroxidase (GPX) activities. CuZn-superoxide dismutase (SOD) activity was also diminished by the Cd treatment, although Mn-SOD was slightly increased. CAT and CuZn-SOD were down-regulated at transcriptional level, while Mn-SOD, Fe-SOD and GR were up-regulated. Analysis of reactive oxygen species (ROS) and nitric oxide (NO) levels by fluorescence and confocal laser microscopy (CLM) showed an over-accumulation of O2*- and H2O2, and a reduction in the NO content in lateral and principal roots. ROS overproduction was dependent on changes in intracellular Ca+2 content, and peroxidases and NADPH oxidases were involved. Cadmium also produced an increase in salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) contents. The rise of ET and ROS, and the NO decrease are in accordance with senescence processes induced by Cd, and the increase of JA and SA could regulate the cellular response to cope with damages imposed by cadmium.

Nitric oxide mediates the fungal elicitor-induced puerarin biosynthesis in Pueraria thomsonii Benth. suspension cells through a salicylic acid (SA)-dependent and a jasmonic acid (JA)-dependent signal pathway

Nitric oxide (NO) has emerged as a key signaling molecule in plant secondary metabolite biosynthesis recently. In order to investigate the molecular basis of NO signaling in elicitor-induced secondary metabolite biosynthesis of plant cells, we determined the contents of NO, salicylic acid (SA), jasmonic acid (JA), and puerarin in Pueraria thomsonii Benth. suspension cells treated with the elicitors prepared from cell walls of Penicillium citrinum. The results showed that the fungal elicitor induced NO burst, SA accumulation and puerarin production of P. thomsonii Benth. cells. The elicitor-induced SA accumulation and puerarin production was suppressed by nitric oxide specific scavenger cPITO, indicating that NO was essential for elicitor-induced SA and puerarin biosynthesis in P. thomsonii Benth. cells. In transgenic NahG P. thomsonii Benth. cells, the fungal elicitor also induced puerarin biosynthesis, NO burst, and JA accumulation, though the SA biosynthesis was impaired. The elicitor-induced JA accumulation in transgenic cells was blocked by cPITO, which suggested that JA acted downstream of NO and its biosynthesis was controlled by NO. External application of NO via its donor sodium nitroprusside (SNP) enhanced puerarin biosynthesis in transgenic NahG P. thomsonii Benth. cells, and the NO-triggered puerarin biosynthesis was suppressed by JA inhibitors IBU and NDGA, which indicated that NO induced puerarin production through a JA-dependent signal pathway in the transgenic cells. Exogenous application of SA suppressed the elicitor-induced JA biosynthesis and reversed the inhibition of IBU and NDGA on elicitor-induced puerarin accumulation in transgenic cells, which indicated that SA inhibited JA biosynthesis in the cells and that SA might be used as a substitute for JA to mediate the elicitor- and NO-induced puerarin biosynthesis. It was, therefore, concluded that NO might mediate the elicitor-induced puerarin biosynthesis through SA- and JA-dependent signal pathways in wildtype P. thomsonii Benth. cells and transgenic NahG cells respectively.

Constitutive arginine-dependent nitric oxide synthase activity in different organs of pea seedlings during plant development

Nitric oxide (NO) is an important signalling molecule in different animal and plant physiological processes. Little is known about its biological function in plants and on the enzymatic source or site of NO production during plant development. The endogenous NO production from L-arginine (NO synthase activity) was analyzed in leaves, stems and roots during plant development, using pea seedlings as a model. NOS activity was analyzed using a novel chemiluminescence-based assay which is more sensitive and specific than previous methods used in plant tissues. In parallel, NO accumulation was analyzed by confocal laser scanning microscopy using as fluorescent probes either DAF-2 DA or DAF-FM DA. A strong increase in NOS activity was detected in stems after 11 days growth, coinciding with the maximum stem elongation. The arginine-dependent NOS activity was constitutive and sensitive to aminoguanidine, a well-known irreversible inhibitor of animal NOS, and this NOS activity was differentially modulated depending on the plant organ and seedling developmental stage. In all tissues studied, NO was localized mainly in the vascular tissue (xylem) and epidermal cells and in root hairs. These loci of NO generation and accumulation suggest novel functions for NO in these cell types.

An iron-induced nitric oxide burst precedes ubiquitin-dependent protein degradation for Arabidopsis AtFer1 ferritin gene expression

Ferritins play an essential role in iron homeostasis by sequestering iron in a bioavailable and non-toxic form. In plants, ferritin mRNAs are highly and quickly accumulated in response to iron overload. Such accumulation leads to a subsequent ferritin protein synthesis and iron storage, thus avoiding oxidative stress to take place. By combining pharmacological and imaging approaches in an Arabidopsis cell culture system, we have identified several elements in the signal transduction pathway leading to the increase of AtFer1 transcript level after iron treatment. Nitric oxide quickly accumulates in the plastids after iron treatment. This compound acts downstream of iron and upstream of a PP2A-type phosphatase to promote an increase of AtFer1 mRNA level. The AtFer1 gene transcription has been previously shown to be repressed under low iron conditions with the involvement of the cis-acting element iron-dependent regulatory sequence identified within the AtFer1 promoter sequence. We show here that the repressor is unlikely a transcription factor directly bound to the iron-dependent regulatory sequence; such a repressor is ubiquitinated upon iron treatment and subsequently degraded through a 26 S proteasome-dependent pathway.

Polyamines induce rapid biosynthesis of nitric oxide (NO) in Arabidopsis thaliana seedlings

In this study, we examined the regulation by putrescine, spermidine and spermine of nitric oxide (NO) biosynthesis in Arabidopsis thaliana seedlings. Using a fluorimetric method employing the cell-impermeable NO-binding dye diaminorhodamine-4M (DAR-4M), we observed that the polyamines (PAs) spermidine and spermine greatly increased NO release in the seedlings, whereas arginine and putrescine had little or no effect. Spermine, the most active PA, stimulated NO release with no apparent lag phase. The response was quenched by addition of 2-aminoethyl-2-thiopseudourea (AET), an inhibitor of the animal nitric oxide synthase (NOS) and plant NO biosynthesis, and by 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-1-oxy-3-oxide (PTIO), an NO scavenger. By fluorescence microscopy, using the cell-permeable NO-binding dye diaminorhodamine-4M acetoxymethyl ester (DAR-4M AM), we observed that PAs induced NO biosynthesis in specific tissues in Arabidopsis seedlings. Spermine and spermidine increased NO biosynthesis in the elongation zone of the Arabidopsis root tip and in primary leaves, especially in the veins and trichomes, while in cotyledons little or no effect of PAs beyond the endogenous levels of NO-induced fluorescence was observed. We conclude that PAs induce NO biosynthesis in plants.

Nitric oxide production in plants: facts and fictions

There is now general agreement that nitric oxide (NO) is an important and almost universal signal in plants. Nevertheless, there are still many controversial observations and opinions on the importance and function of NO in plants. Partly, this may be due to the difficulties in detecting and even more in quantifying NO. Here, we summarize major pathways of NO production in plants, and briefly discuss some methodical problems.

Mechanisms for nitric oxide synthesis in plants

The discovery that nitric oxide (NO) acts as a signal fundamentally shifted our understanding of free radicals from toxic by-products of oxidative metabolism to key regulators of cellular functions. This discovery has led to intense investigation into the synthesis of NO in both animals and plants. Nitric oxide synthases (NOS) are the primary sources of NO in animals and are complex, highly regulated enzymes that oxidize arginine to NO and citrulline. Plant NO synthesis, however, appears more complex and includes both nitrite and arginine-dependent mechanisms. The components of the arginine pathway have been elusive as no known orthologues of animal NOS exist in plants. An Arabidopsis gene (AtNOS1) has been identified that is needed for NO synthesis in vivo and has biochemical properties similar to animal cNOS, yet it has no sequence similarity to any known animal NOS. An Atnos1 insertion mutant has been useful for genetic studies of NO regulation and for uncovering new roles for NO signalling. The elucidation of plant NO synthesis promises to yield novel mechanisms that may be applicable to animal systems.

Nitric oxide modulates the expression of cell cycle regulatory genes during lateral root formation in tomato

Nitric oxide (NO) is a bioactive molecule involved in diverse physiological functions in plants. It has previously been reported that the NO donor sodium nitroprusside (SNP) applied to germinated tomato seeds was able to induce lateral root (LR) formation in the same way that auxin treatment does. In this paper, it is shown that NO modulates the expression of cell cycle regulatory genes in tomato pericycle cells and leads, in turn, to induced LR formation. The addition of the NO scavenger CPTIO at different time points during auxin-mediated LR development indicates that NO is required for LR primordia formation and not for LR emergence. The SNP-mediated LR promotion could be prevented by the cell cycle inhibitor olomoucine, suggesting that NO is involved in cell cycle regulation. A system was developed in which the formation of LRs was synchronized. It was based on the control of NO availability in roots by treatment with the NO scavenger CPTIO. The expression of the cell cycle regulatory genes encoding CYCA2;1, CYCA3;1, CYCD3;1, CDKA1, and the Kip-Related Protein KRP2 was studied using RT-PCR analysis in roots with synchronized and non-synchronized LR formation. NO mediates the induction of the CYCD3;1 gene and the repression of the CDK inhibitor KRP2 gene at the beginning of LR primordia formation. In addition, auxin-dependent cell cycle gene regulation was dependent on NO.

Nitric oxide (NO) as an intermediate in the cryptogein-induced hypersensitive response--a critical re-evaluation

A hypersensitive response (HR) was induced in tobacco leaves and cell suspensions by the fungal elicitor cryptogein, and NO production was followed by chemiluminescence and occasionally by diaminofluorescein (DAF)-fluorescence. Results from both methods were at least partly consistent, but kinetics was different. NO emission was not induced by cryptogein in leaves, whereas in cell suspensions some weak NO emission was observed, which was nitrate reductase (NR)-dependent, but not required for cell death. Nitric oxide synthase (NOS) inhibitors did not prevent cell death, but PR-1 expression was weakened. In conclusion, neither NR nor NOS appear obligatory for the cryptogein-induced HR. However, a role for NO was still suggested by the fact that the NO scavenger cPTIO prevented the HR. Unexpectedly, cPTI, the reaction product of cPTIO and NO, also impaired the HR but without scavenging NO. Thus, prevention of the HR by cPTIO is not necessarily indicative for a role of NO. Further, even a 100-fold NO overproduction (over wild type) by a nitrite reductase-deficient mutant did not interfere with the cryptogein-induced HR. Accordingly, the role of NO in the HR should be reconsidered.

Nitric oxide functions as a positive regulator of root hair development

THE ROOT EPIDERMIS IS COMPOSED OF TWO CELL TYPES: trichoblasts (or hair cells) and atrichoblasts (or non-hair cells). In lettuce (Lactuca sativa cv. Grand Rapids var. Rapidmor oscura) plants grown hydroponically in water, the root epidermis did not form root hairs. The addition of 10 microM sodium nitroprusside (SNP), a nitric oxide (NO) donor, resulted in almost all rhizodermal cells differentiated into root hairs. Treatment with the synthetic auxin 1-naphthyl acetic acid (NAA) displayed a significant increase of root hair formation (RHF) that was prevented by the specific NO scavenger carboxy-PTIO (cPTIO). In Arabidopsis, two mutants have been shown to be defective in NO production and to display altered phenotypes in which NO is implicated. Arabidopsis nos1 has a mutation in an NO synthase structural gene (NOS1), and the nia1 nia2 double mutant is null for nitrate reductase (NR) activity. We observed that both mutants were affected in their capacity of developing root hairs. Root hair elongation was significantly reduced in nos1 and nia1 nia2 mutants as well as in cPTIO-treated wild type plants. A correlation was found between endogenous NO level in roots detected by the fluorescent probe DAF-FM DA and RHF. In Arabidopsis, as well as in lettuce, cPTIO blocked the NAA-induced root hair elongation. Taken together, these results indicate that: (1) NO is a critical molecule in the process leading to RHF and (2) NO is involved in the auxin-signaling cascade leading to RHF.

ABA-induced NO generation and stomatal closure in Arabidopsis are dependent on H2O2 synthesis

Nitric oxide (NO) and hydrogen peroxide (H(2)O(2)) are key signalling molecules produced in response to various stimuli and involved in a diverse range of plant signal transduction processes. Nitric oxide and H(2)O(2) have been identified as essential components of the complex signalling network inducing stomatal closure in response to the phytohormone abscisic acid (ABA). A close inter-relationship exists between ABA and the spatial and temporal production and action of both NO and H(2)O(2) in guard cells. This study shows that, in Arabidopsis thaliana guard cells, ABA-mediated NO generation is in fact dependent on ABA-induced H(2)O(2) production. Stomatal closure induced by H(2)O(2) is inhibited by the removal of NO with NO scavenger, and both ABA and H(2)O(2) stimulate guard cell NO synthesis. Conversely, NO-induced stomatal closure does not require H(2)O(2) synthesis nor does NO treatment induce H(2)O(2) production in guard cells. Tungstate inhibition of the NO-generating enzyme nitrate reductase (NR) attenuates NO production in response to nitrite in vitro and in response to H(2)O(2) and ABA in vivo. Genetic data demonstrate that NR is the major source of NO in guard cells in response to ABA-mediated H(2)O(2) synthesis. In the NR double mutant nia1, nia2 both ABA and H(2)O(2) fail to induce NO production or stomatal closure, but in the nitric oxide synthase deficient Atnos1 mutant, responses to H(2)O(2) are not impaired. Importantly, we show that in the NADPH oxidase deficient double mutant atrbohD/F, NO synthesis and stomatal closure to ABA are severely reduced, indicating that endogenous H(2)O(2) production induced by ABA is required for NO synthesis. In summary, our physiological and genetic data demonstrate a strong inter-relationship between ABA, endogenous H(2)O(2) and NO-induced stomatal closure.

Modulation of nitric oxide bioactivity by plant haemoglobins

Nitric oxide (NO) is a highly reactive signalling molecule that has numerous targets in plants. Both enzymatic and non-enzymatic synthesis of NO has been detected in several plant species, and NO functions have been characterized during diverse physiological processes such as plant growth, development, and resistance to biotic and abiotic stresses. This wide variety of effects reflects the basic signalling mechanisms that are utilized by virtually all mammalian and plant cells and suggests the necessity of detoxification mechanisms to control the level and functions of NO. During the last two years an increasing number of reports have implicated non-symbiotic haemoglobins as the key enzymatic system for NO scavenging in plants, indicating that the primordial function of haemoglobins may well be to protect against nitrosative stress and to modulate NO signalling functions. The biological relevance of plant haemoglobins during specific conditions of plant growth and stress, and the existence of further enzymatic and non-enzymatic NO scavenging systems, suggest the existence of precise NO modulation mechanisms in plants, as observed for different NO sources.

Formation and possible roles of nitric oxide in plant roots

Nitric oxide has been reported to act as a signalling molecule in different plant tissues and to participate in a variety of physiological processes. It is produced by different enzymes and sources. The root-specific plasma membrane-bound enzymes forming NO from the substrates nitrate and nitrite are of particular interest because roots serve as interfaces between plants and the soil. The co-ordinated activity of the root-specific plasma membrane-bound nitrate reductase (PM-NR) and nitrite:NO reductase (NI-NOR) suggests that NO might also be involved in root signalling and development. The rate of enzymatic production of this NO depends largely on the environmental conditions, mainly the availability of nitrate and oxygen and it is proposed that this NO plays a role during anoxia as an indicator of the external nitrate availability and in regulating symbiotic interactions at the root surface.

Oxyleghemoglobin scavenges nitrogen monoxide and peroxynitrite: a possible role in functioning nodules?

It has been demonstrated that the NO* produced by nitric oxide synthase or by the reduction of nitrite by nitrate reductase plays an important role in plants' defense against microbial pathogens. The detection of nitrosyl Lb in nodules strongly suggests that NO* is also formed in functional nodules. Moreover, NO* may react with superoxide (which has been shown to be produced in nodules by various processes), leading to the formation of peroxynitrite. We have determined the second-order rate constants of the reactions of soybean oxyleghemoglobin with nitrogen monoxide and peroxynitrite. At pH 7.3 and 20 degrees C, the values are on the order of 10(8) and 10(4) M-1 s-1, respectively. In the presence of physiological amounts of CO2 (1.2 mM), the second-order rate constant of the reaction of oxyleghemoglobin peroxynitrite is even larger (10(5) M-1 s-1). The results presented here clearly show that oxyleghemoglobin is able to scavenge any NO* and peroxynitrite formed in functional nodules. This may help to stop NO* triggering a plant defense reaction.

Nitric oxide reduces aluminum toxicity by preventing oxidative stress in the roots of Cassia tora L

Nitric oxide (NO) as a key signaling molecule has been involved in mediation of various biotic and abiotic stress-induced physiological responses in plants. In the present study, we investigated the effect of NO on Cassia tora L. plants exposed to aluminum (Al). Plants pre-treated for 12 h with 0.4 mM sodium nitroprusside (SNP), an NO donor, and subsequently exposed to 10 microM Al treatment for 24 h exhibited significantly greater root elongation as compared with the plants without SNP treatment. The NO-promoted root elongation was correlated with a decrease in Al accumulation in root apexes. Furthermore, oxidative stress associated with Al treatment increased lipid peroxidation and reactive oxygen species, and the activation of lipoxygenase and antioxidant enzymes was reduced by NO. Such effects were confirmed by the histochemical staining for the detection of peroxidation of lipids and loss of membrane integrity in roots. The ameliorating effect of NO was specific, because the NO scavenger cPTIO [2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethylinidazoline-1-oxyl-3-oxide] completely reversed the effect of NO on root growth in the presence of Al. These results indicate that NO plays an important role in protecting the plant against Al-induced oxidative stress.

Nitric oxide is involved in abscisic acid-induced antioxidant activities in Stylosanthes guianensis

Previous studies suggest that abscisic acid (ABA) stimulates the activities of antioxidant enzymes under normal and chilling temperature and enhanced chilling resistance in Stylosanthes guianensis. The objective of this study was to test whether nitric oxide (NO) is involved in the ABA-induced activities of the antioxidant enzymes in Stylosanthes guianensis due to its nature as a second messenger in stress responses. Plants were treated with NO donors, ABA, ABA in combination with NO scavengers or the nitric oxide synthase (NOS) inhibitor and their effects on the activity of antioxidant enzymes and NO production were compared. The results showed that ABA increased the activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX). The effect of ABA on antioxidant enzyme activities was suppressed by the NOS inhibitor, N(omega)-nitro-L-arginine (L-NNA), and the NO scavenger, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl3-oxide (PTIO). NO content increased after 5 h of ABA treatment. The NO-scavenger, PTIO, and the NOS-inhibitor, L-NNA, inhibited the accumulation of NO in ABA-treated Stylosanthes guianensis. NO donor treatment enhanced the activities of SOD, CAT, and APX. The results suggested that NO was involved in the ABA-induced activities of SOD, CAT, and APX in Stylosanthes guianensis. ABA triggered NO production that may lead to the stimulation of antioxidant enzyme activities.

Atmospheric nitrogen dioxide gas is a plant vitalization signal to increase plant size and the contents of cell constituents

We report the unexpected novel finding that exogenously supplied atmospheric NO2 at an ambient concentration is a plant vitalization signal to double shoot size and the contents of cell constituents. When seedlings of Nicotiana plumbaginifolia were grown for 10 wk under natural light and irrigation with 10 mm KNO3 in air containing (+NO2 plants) or not containing (-NO2 plants) 15NO2 (150 +/- 50 ppb), shoot biomass, total leaf area, and contents per shoot of carbon (C), nitrogen (N), sulphur (S), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), free amino acids and crude proteins were all approximately 2 times greater in +NO2 plants than in -NO2 plants. In mass spectrometric analysis of the 15N/14N ratio, it was found that NO2-derived N (NO2-N) comprised < 3% of total plant N, indicating that the contribution of NO2-N to total N was very minor. It thus seems very likely that the primary role of NO2 is as a multifunctional signal to stimulate plant growth, nutrient uptake and metabolism.

In higher plants, only root mitochondria, but not leaf mitochondria reduce nitrite to NO, in vitro and in situ

At oxygen concentrations of < or =1%, even completely nitrate reductase (NR)-free root tissues reduced added nitrite to NO, indicating that, in roots, NR was not the only source for nitrite-dependent NO formation. By contrast, NR-free leaf slices were not able to reduce nitrite to NO. Root NO formation was blocked by inhibitors of mitochondrial electron transport (Myxothiazol and SHAM), whereas NO formation by NR-containing leaf slices was insensitive to the inhibitors. Consistent with that, mitochondria purified from roots, but not those from leaves, reduced nitrite to NO at the expense of NADH. The inhibitor studies suggest that, in root mitochondria, both terminal oxidases participate in NO formation, and they also suggest that even in NR-containing roots, a large part of the reduction of nitrite to NO was catalysed by mitochondria, and less by NR. The differential capacity of root and leaf mitochondria to reduce nitrite to NO appears to be common among higher plants, since it has been observed with Arabidopsis, barley, pea, and tobacco. A specific role for nitrite to NO reduction in roots under anoxia is discussed.

Nitric oxide mediates the fungal elicitor-induced hypericin production of Hypericum perforatum cell suspension cultures through a jasmonic-acid-dependent signal pathway

Fungal elicitor prepared from the cell walls of Aspergillum niger induces multiple responses of Hypericum perforatum cells, including nitric oxide (NO) generation, jasmonic acid (JA) biosynthesis, and hypericin production. To determine the role of NO and JA in elicitor-induced hypericin production, we study the effects of NO scavenger 2- to 4-carboxyphenyl-4,4, 5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPITO), nitric oxide synthase inhibitor S,S'-1,3-phenylene-bis(1,2-ethanediyl)-bis-isothiourea, and inhibitors of the octadecanoid pathway on elicitor-induced NO generation, JA biosynthesis, and hypericin production. Pretreatment of the cells with cPITO and JA biosynthesis inhibitors suppresses not only the elicitor-induced NO generation and JA accumulation but also the elicitor-induced hypericin production, which suggests that both NO and JA are involved in elicitor-induced hypericin biosynthesis. S,S'-1,3-phenylene-bis(1,2-ethanediyl)-bis-isothiourea and cPITO inhibit both elicitor-induced NO generation and JA biosynthesis, while JA biosynthesis inhibitors do not affect the elicitor-induced NO generation, indicating that JA acts downstream of NO generation and that its biosynthesis is regulated by NO. External application of NO via its donor sodium nitroprusside induces hypericin production in the absence of fungal elicitor. Sodium-nitroprusside-induced hypericin production is blocked by JA biosynthesis inhibitors, showing that JA biosynthesis is essential for NO-induced hypericin production. The results demonstrate a causal relationship between elicitor-induced NO generation, JA biosynthesis, and hypericin production in H. perforatum cells and indicate a sequence of signaling events from NO to hypericin production, within which NO mediates the elicitor-induced hypericin biosynthesis at least partially via a JA-dependent signaling pathway.

The Drosophila nuclear receptor e75 contains heme and is gas responsive

Nuclear receptors are a family of transcription factors with structurally conserved ligand binding domains that regulate their activity. Despite intensive efforts to identify ligands, most nuclear receptors are still "orphans." Here, we demonstrate that the ligand binding pocket of the Drosophila nuclear receptor E75 contains a heme prosthetic group. E75 absorption spectra, resistance to denaturants, and effects of site-directed mutagenesis indicate a single, coordinately bound heme molecule. A correlation between the levels of E75 expression and the levels of available heme suggest a possible role as a heme sensor. The oxidation state of the heme iron also determines whether E75 can interact with its heterodimer partner DHR3, suggesting an additional role as a redox sensor. Further, the E75-DHR3 interaction is also regulated by the binding of NO or CO to the heme center, suggesting that E75 may also function as a diatomic gas sensor. Possible mechanisms and roles for these interactions are discussed.

The haemoglobin/nitric oxide cycle: involvement in flooding stress and effects on hormone signalling

BACKGROUND

Class 1 haemoglobins (Hbs) are induced in plant cells under hypoxic conditions. They have a high affinity for oxygen, which is two orders of magnitude lower than that of cytochrome oxidase, permitting the utilization of oxygen by the molecule at extremely low oxygen concentrations. Their presence reduces the levels of nitric oxide (NO) that is produced from nitrate ion during hypoxia and improves the redox and energy status of the hypoxic cell.

SCOPE

The mechanism by which Hb interacts with NO under hypoxic conditions in plants is examined, and the effects of Hb expression on metabolism and signal transduction are discussed.

CONCLUSIONS

The accumulated evidence suggests that a metabolic pathway involving NO and Hb provides an alternative type of respiration to mitochondrial electron transport under limited oxygen. Hb in hypoxic plants acts as part of a soluble, terminal, NO dioxygenase system, yielding nitrate ion from the reaction of oxyHb with NO. NO is mainly formed due to anaerobic accumulation of nitrite. The overall reaction sequence, referred to as the Hb/NO cycle, consumes NADH and maintains ATP levels via an as yet unknown mechanism. Hb gene expression appears to influence signal transduction pathways, possibly through its effect on NO, as evidenced by phenotypic changes in normoxic Hb-varying transgenic plants. Ethylene levels are elevated when Hb gene expression is suppressed, which could be a factor leading to root aerenchyma formation during hypoxic stress.