Nitric oxide retards xanthine oxidase-mediated superoxide anion generation in Phalaenopsis flower: an implication of NO in the senescence and oxidative stress regulation

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.

Abiotic Stress and Reactive Oxygen Species: Generation, Signaling, and Defense Mechanisms

Climate change is an invisible, silent killer with calamitous effects on living organisms. As the sessile organism, plants experience a diverse array of abiotic stresses during ontogenesis. The relentless climatic changes amplify the intensity and duration of stresses, making plants dwindle to survive. Plants convert 1-2% of consumed oxygen into reactive oxygen species (ROS), in particular, singlet oxygen (1O2), superoxide radical (O2•-), hydrogen peroxide (H2O2), hydroxyl radical (•OH), etc. as a byproduct of aerobic metabolism in different cell organelles such as chloroplast, mitochondria, etc. The regulatory network comprising enzymatic and non-enzymatic antioxidant systems tends to keep the magnitude of ROS within plant cells to a non-damaging level. However, under stress conditions, the production rate of ROS increases exponentially, exceeding the potential of antioxidant scavengers instigating oxidative burst, which affects biomolecules and disturbs cellular redox homeostasis. ROS are similar to a double-edged sword; and, when present below the threshold level, mediate redox signaling pathways that actuate plant growth, development, and acclimatization against stresses. The production of ROS in plant cells displays both detrimental and beneficial effects. However, exact pathways of ROS mediated stress alleviation are yet to be fully elucidated. Therefore, the review deposits information about the status of known sites of production, signaling mechanisms/pathways, effects, and management of ROS within plant cells under stress. In addition, the role played by advancement in modern techniques such as molecular priming, systems biology, phenomics, and crop modeling in preventing oxidative stress, as well as diverting ROS into signaling pathways has been canvassed.

Revealing on hydrogen sulfide and nitric oxide signals co-ordination for plant growth under stress conditions

In the recent times, plants are facing certain types of environmental stresses, which give rise to formation of reactive oxygen species (ROS) such as hydroxyl radicals, hydrogen peroxides, superoxide anions and so on. These are required by the plants at low concentrations for signal transduction and at high concentrations, they repress plant root growth. Apart from the ROS activities, hydrogen sulfide (H₂ S) and nitric oxide (NO) have major contributions in regulating growth and developmental processes in plants, as they also play key roles as signaling molecules and act as chief plant immune defense mechanisms against various biotic as well as abiotic stresses. H₂ S and NO are the two pivotal gaseous messengers involved in growth, germination and improved tolerance in plants under stressed and non-stress conditions. H₂ S and NO mediate cell signaling in plants as a response to several abiotic stresses like temperature, heavy metal exposure, water and salinity. They alter gene expression levels to induce the synthesis of antioxidant enzymes, osmolytes and also trigger their interactions with each other. However, research has been limited to only cross adaptations and signal transductions. Understanding the change and mechanism of H₂ S and NO mediated cell signaling will broaden our knowledge on the various biochemical changes that occur in plant cells related to different stresses. A clear understanding of these molecules in various environmental stresses would help to confer biotechnological applications to protect plants against abiotic stresses and to improve crop productivity.

Genome-wide identification and transcriptional expression analysis of superoxide dismutase (SOD) family in wheat (Triticum aestivum)

Superoxide dismutases (SODs) are a family of key antioxidant enzymes that play a crucial role in plant growth and development. Previously, this gene family has been investigated in Arabidopsis and rice. In the present study, a genome-wide analysis of the SOD gene family in wheat were performed. Twenty-six SOD genes were identified from the whole genome of wheat, including 17 Cu/Zn-SODs, six Fe-SODs, and three Mn-SODs. The chromosomal location mapping analysis indicated that these three types of SOD genes were only distributed on 2, 4, and 7 chromosomes, respectively. Phylogenetic analyses of wheat SODs and several other species revealed that these SOD proteins can be assigned to two major categories. SOD1 mainly comprises of Cu/Zn-SODs, and SOD2 mainly comprises of Fe-SODs and Mn-SODs. Gene structure and motif analyses indicated that most of the SOD genes showed a relatively conserved exon/intron arrangement and motif composition. Analyses of transcriptional data indicated that most of the wheat SOD genes were expressed in almost all of the examined tissues and had important functions in abiotic stress resistance. Finally, quantitative real-time polymerase chain reaction (qRT-PCR) analysis was used to reveal the regulating roles of wheat SOD gene family in response to NaCl, mannitol, and polyethylene glycol stresses. qRT-PCR showed that eight randomly selected genes with relatively high expression levels responded to all three stresses based on released transcriptome data. However, their degree of response and response patterns were different. Interestingly, among these genes, TaSOD1.7, TaSOD1.9, TaSOD2.1, and TaSOD2.3 feature research value owing to their remarkable expression-fold change in leaves or roots under different stresses. Overall, our results provide a basis of further functional research on the SOD gene family in wheat and facilitate their potential use for applications in the genetic improvement on wheat in drought and salt stress environments.

Nitric oxide in plants: pro- or anti-senescence

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

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.

Proteomic and Biochemical Changes during Senescence of Phalaenopsis 'Red Dragon' Petals

Phalaenopsis flowers are some of the most popular ornamental flowers in the world. For most ornamental plants, petal longevity determines postharvest quality and garden performance. Therefore, it is important to have insight into the senescence mechanism of Phalaenopsis. In the present study, a proteomic approach combined with ultrastructural observation and activity analysis of antioxidant enzymes was used to profile the molecular and biochemical changes during pollination-induced petal senescence in Phalaenopsis “Red Dragon”. Petals appeared to be visibly wilting at 24 h after pollination, accompanied by the mass degradation of macromolecules and organelles during senescence. In addition, 48 protein spots with significant differences in abundance were found by two-dimensional electrophoresis (2-DE) and subjected to matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF/TOF-MS). There were 42 protein spots successfully identified and homologous to known functional protein species involved in key biological processes, including antioxidant pathways, stress response, protein metabolism, cell wall component metabolism, energy metabolism, cell structure, and signal transduction. The activity of all reactive oxygen species (ROS)-scavenging enzymes was increased, keeping the content of ROS at a low level at the early stage of senescence. These results suggest that two processes, a counteraction against increased levels of ROS and the degradation of cellular constituents for maintaining nutrient recycling, are activated during pollination-induced petal senescence in Phalaenopsis. The information provides a basis for understanding the mechanism regulating petal senescence and prolonging the florescence of Phalaenopsis.

Interactive effects of Salicylic acid and nitric oxide in alleviating zinc toxicity of Safflower (Carthamus tinctorius L.)

The purpose of this study was to assess the possible protective role of exogenous salicylic acid (SA), sodium nitroprusside (SNP), a donor of nitric oxide, and their combination on 21-day-old safflower (Carthamus tinctorius L.) seedlings grown under zinc (Zn) stress. The results revealed that exposure to 500 µM ZnSO₄.7H₂O for 10 days markedly reduced the root and shoot dry weights in Zn-treated plants, while the application of SA, SNP and specially SA + SNP significantly increased the root and shoot dry weights in seedlings subjected to Zn stress. Addition of SA, SNP and SA + SNP interestingly reduced root-to-shoot translocation of zinc and increased significantly the level of glutathione (GSH) and ascorbate (ASC) in leaves of Zn-stressed plants. The Zn-treated plants supplemented with SA and SNP revealed an improved activity of ascorbate-glutathione cycle enzymes and those enzymes which are involved in glyoxalase system as compared to the plants treated with Zn only. However, no significant relationship was found between SA or SNP supplementation and glutathione S-transferase activity in Zn-stressed plants. These findings demonstrate that exogenous application of SA or SNP could ameliorate the negative effects of Zn on safflower plants probably by stimulation of antioxidant defense and glyoxalase systems.

Overproduction of stromal ferredoxin:NADPH oxidoreductase in H2O 2-accumulating Brassica napus leaf protoplasts

The isolation of Brassica napus leaf protoplasts induces reactive oxygen species generation and accumulation in the chloroplasts. An activated isoform of NADPH oxidase-like protein was detected in the protoplasts and the protoplast chloroplasts. The purpose of this study is to define the NADH oxidase-like activities in the H2O2-accumulating protoplast chloroplasts. Proteomic analysis of this protein revealed an isoform of ferredoxin:NADPH oxidoreductase (FNR1). While leaves highly expressed the LFNR1 transcript, protoplasts decreased the expression significantly. The protoplast chloroplasts predominantly expressed soluble FNR1 proteins. While the albino leaves of white kale (Brassica oleracea var. acephala f. tricolor cv. white pigeon) expressed FNR1 protein at the same level as B. napus leaves, the protoplasts of albino leaves displayed reduced FNR1 expression. The albino leaf protoplasts of white kale generated and accumulated H2O2 in the cytoplasm and on the plasma membrane. Intracellular pH showed that the chloroplasts were acidic, which suggest that excess H(+) was generated in chloroplast stroma. NADPH content of the protoplast chloroplasts increased by over sixfold during the isolation of protoplasts. This study reports a possibility of mediating electrons to oxygen by an overproduced soluble FNR, and suggests that the FNR has a function in utilizing any excess reducing power of NADPH.

Nitric oxide-triggered remodeling of chloroplast bioenergetics and thylakoid proteins upon nitrogen starvation in Chlamydomonas reinhardtii

Starving microalgae for nitrogen sources is commonly used as a biotechnological tool to boost storage of reduced carbon into starch granules or lipid droplets, but the accompanying changes in bioenergetics have been little studied so far. Here, we report that the selective depletion of Rubisco and cytochrome b6f complex that occurs when Chlamydomonas reinhardtii is starved for nitrogen in the presence of acetate and under normoxic conditions is accompanied by a marked increase in chlororespiratory enzymes, which converts the photosynthetic thylakoid membrane into an intracellular matrix for oxidative catabolism of reductants. Cytochrome b6f subunits and most proteins specifically involved in their biogenesis are selectively degraded, mainly by the FtsH and Clp chloroplast proteases. This regulated degradation pathway does not require light, active photosynthesis, or state transitions but is prevented when respiration is impaired or under phototrophic conditions. We provide genetic and pharmacological evidence that NO production from intracellular nitrite governs this degradation pathway: Addition of a NO scavenger and of two distinct NO producers decrease and increase, respectively, the rate of cytochrome b6f degradation; NO-sensitive fluorescence probes, visualized by confocal microscopy, demonstrate that nitrogen-starved cells produce NO only when the cytochrome b6f degradation pathway is activated.

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.

NOS-like-mediated nitric oxide is involved in Pinus thunbergii response to the invasion of Bursaphelenchus xylophilus

The content of NO and H(2)O(2) as well as the activities of nitric oxide synthase (NOS)-like and nitrate reductase (NR) were monitored in the needles of Pinus thunbergii infected by Bursaphelenchus xylophilus. The results showed that the content of NO increased significantly only 8 h after the invasion of B. xylophilus, while H(2)O(2) increased 12 h after invasion. NO donor SNP could promote and NO scavenger cPTIO could prevent the production of NO and H(2)O(2). The content of NO changed earlier than that of H(2)O(2). In addition, the symptoms appeared 9, 5 and 12 days, respectively, after the inoculation with B. xylophilus, SNP pre-treatment and cPTIO pre-treatment followed by B. xylophilus infection. After B. xylophilus infection, the content of NO in P. thunbergii changed fiercely more earlier than the appearance of external symptoms, which indicated that the content of NO was related with the appearance and the development of the symptoms. The treatment with L-NNA (NOS inhibitor) inhibited the content of NO significantly, whereas, Na(2)WO(4) (NR inhibitor) had no effect. The further analysis of NOS revealed that NO changed in consistent with cNOS activity. To sum up, NO, as the upstream signal molecule of H(2)O(2), was involved in the pine early response to the invasion of B. xylophilus and influenced the accumulation of the content of H(2)O(2). Moreover, NOS-like rather than NR was responsible for the endogenous NO generation, which was modulated by cNOS during the interaction between P. thunbergii and B. xylophilus. Key message NO is involved in early response of P. thunbergii to the invasion of B. xylophilus and NOS is the key enzyme responsible for NO generation in P. thunbergii.

Is there an important role for reactive oxygen species and redox regulation during floral senescence?

Senescence is a highly regulated process terminating with programmed cell death (PCD). Floral senescence, and in particular petal senescence, forms an interesting model to study this process in that floral lifespan is species specific and linked to biological function. A feature of petal senescence is a rise in reactive oxygen species (ROS) and a change in redox balance. A key question is whether this is merely a consequence of de-regulation of antioxidant systems as cells enter PCD, or whether the rise in ROS may have a regulatory or signalling function. An important division in the physiology of floral senescence is between species in which ethylene is a key regulator, and those in which it appears not to perform an important regulatory role. Another important question we can therefore ask is whether the redox and ROS changes have the same significance in species with different physiologies. Transcriptomic studies in ethylene-sensitive and -insensitive species allow us to further determine whether changes in the activity of ROS-scavenging enzymes are transcriptionally regulated during floral senescence. Finally, it is important to assess how a signalling role for ROS or redox status would fit with known plant growth regulator (PGR) control of floral senescence.

Chloroplastic NADPH oxidase-like activity-mediated perpetual hydrogen peroxide generation in the chloroplast induces apoptotic-like death of Brassica napus leaf protoplasts

Despite extensive research over the past years, regeneration from protoplasts has been observed in only a limited number of plant species. Protoplasts undergo complex metabolic modification during their isolation. The isolation of protoplasts induces reactive oxygen species (ROS) generation in Brassica napus leaf protoplasts. The present study was conducted to provide new insight into the mechanism of ROS generation in B. napus leaf protoplasts. In vivo localization of H(2)O(2) and enzymes involved in H(2)O(2) generation and detoxification, molecular antioxidant-ascorbate and its redox state and lipid peroxidation were investigated in the leaf and isolated protoplasts. Incubating leaf strips in the macerating enzyme (ME) for different duration (3, 6, and 12 h) induced accumulation of H(2)O(2) and malondialdehyde (lipid peroxidation, an index of membrane damage) in protoplasts. The level of H(2)O(2) was highest just after protoplast isolation and subsequently decreased during culture. Superoxide generating NADPH oxidase (NOX)-like activity was enhanced, whereas superoxide dismutase (SOD) and ascorbate peroxidase (APX) decreased in the protoplasts compared to leaves. Diaminobenzidine peroxidase (DAB-POD) activity was also lower in the protoplasts compared to leaves. Total ascorbate content, ascorbate to dehydroascorbate ratio (redox state), were enhanced in the protoplasts compared to leaves. Higher activity of NOX-like enzyme and weakening in the activity of antioxidant enzymes (SOD, APX, and DAB-POD) in protoplasts resulted in excessive accumulation of H(2)O(2) in chloroplasts of protoplasts. Chloroplastic NADPH oxidase-like activity mediated perpetual H(2)O(2) generation probably induced apoptotic-like cell death of B. napus leaf protoplasts as indicated by parallel DNA laddering and decreased mitochondrial membrane potential.

Sodium nitroprusside-mediated alleviation of iron deficiency and modulation of antioxidant responses in maize plants

BACKGROUND AND AIMS

Nitric oxide (NO) has been reported to alleviate Fe-deficiency effects, possibly by enhancing the functional Fe status of plants. This study examines changes in tissue Fe status and oxidative metabolism in Fe-deficient maize (Zea mays L.) plants enriched with NO using sodium nitroprusside (SNP) as a source.

METHODOLOGY

Measurements included changes in concentrations of H(2)O(2), non-protein thiols, levels of lipid peroxidation and activities of superoxide dismutase (SOD) and of the Fe-requiring antioxidant haem enzymes catalase, peroxidase and ascorbate peroxidases. Internal NO in Fe-deficient maize plants was manipulated with SNP and the NO scavenger, methylene blue (MB). A key control was treatment with sodium ferrocyanide (SF), a non-NO-supplying analogue of SNP.

PRINCIPAL RESULTS

SNP but not SF caused re-greening of leaves in Fe-deficient maize plants over 10-20 days, increased in vivo NO content, raised chlorophyll and carotenoid concentrations, promoted growth in dry weight, increased the activities of H(2)O(2)-scavenging haem enzymes and enhanced lipid peroxidation, while decreasing SOD activity and H(2)O(2) concentrations. The NO scavenger, MB, blocked the effects of the SNP. Although SNP and SF each donated Fe and increased active Fe, only SNP increased leaf chlorophyll.

CONCLUSIONS

NO plays a role in Fe nutrition, independently of its effect on total or active Fe status. The most probable mechanism of NO involvement is to increase the intracellular availability of Fe by means of modulating redox. This is likely to be achieved by enhancing the chemical reduction of foliar Fe(III) to Fe(II).