Nitric oxide mediates the hormonal control of Crassulacean acid metabolism expression in young pineapple plants

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

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

The molecular paradigm of reactive oxygen species (ROS) and reactive nitrogen species (RNS) with different phytohormone signaling pathways during drought stress in plants

Drought is undoubtedly a major environmental constraint that negatively affects agricultural yield and productivity throughout the globe. Plants are extremely vulnerable to drought which imposes several physiological, biochemical and molecular perturbations. Increased generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in different plant organs is one of the inevitable consequences of drought. ROS and RNS are toxic byproducts of metabolic reactions and poise oxidative stress and nitrosative stress that are detrimental for plants. In spite of toxic effects, these potentially active radicals also play a beneficial role in mediating several signal transduction events that lead to plant acclimation and enhanced survival under harsh environmental conditions. The precise understanding of ROS and RNS signaling and their molecular paradigm with different phytohormones, such as auxin, gibberellin, cytokinin, abscisic acid, ethylene, brassinosteroids, strigolactones, jasmonic acid, salicylic acid and melatonin play a pivotal role for maintaining plant fitness and resilience to counteract drought toxicity. Therefore, the present review provides an overview of integrated systemic signaling between ROS, RNS and phytohormones during drought stress based on past and recent advancements and their influential role in conferring protection against drought-induced damages in different plant species. Indeed, it would not be presumptuous to hope that the detailed knowledge provided in this review will be helpful for designing drought-tolerant crop cultivars in the forthcoming times.

Comparative transcriptome analysis of the mechanism difference in heat stress response between indica rice cultivar "IR64" and japonica cultivar "Koshihikari" at the seedling stage

Heat stress (HS) has become a major abiotic stress in rice, considering the frequency and intensity of extreme hot weather. There is an urgent need to explore the differences in molecular mechanisms of HS tolerance in different cultivars, especially in indica and japonica. In this study, we investigated the transcriptome information of IR64 (indica, IR) and Koshihikari (japonica, Kos) in response to HS at the seedling stage. From the differentially expressed genes (DEGs) consistently expressed at six time points, 599 DEGs were identified that were co-expressed in both cultivars, as well as 945 and 1,180 DEGs that were specifically expressed in IR and Kos, respectively. The results of GO and KEGG analysis showed two different HS response pathways for IR and Kos. IR specifically expressed DEGs were mainly enriched in chloroplast-related pathways, whereas Kos specifically expressed DEGs were mainly enriched in endoplasmic reticulum and mitochondria-related pathways. Meanwhile, we highlighted the importance of NO biosynthesis genes, especially nitrate reductase genes, in the HS response of IR based on protein-protein interaction networks. In addition, we found that heat shock proteins and heat shock factors play very important roles in both cultivars. This study not only provides new insights into the differences in HS responses between different subspecies of rice, but also lays the foundation for future research on molecular mechanisms and breeding of heat-tolerant cultivars.

A matter of time: regulatory events behind the synchronization of C4 and crassulacean acid metabolism in Portulaca oleracea

Portulaca species can switch between C4 and crassulacean acid metabolism (CAM) depending on environmental conditions. However, the regulatory mechanisms behind this rare photosynthetic adaptation remain elusive. Using Portulaca oleracea as a model system, here we investigated the involvement of the circadian clock, plant hormones, and transcription factors in coordinating C4 and CAM gene expression. Free-running experiments in constant conditions suggested that C4 and CAM gene expression are intrinsically connected to the circadian clock. Detailed time-course, drought, and rewatering experiments revealed distinct time frames for CAM induction and reversion (days versus hours, respectively), which were accompanied by changes in abscisic acid (ABA) and cytokinin metabolism and signaling. Exogenous ABA and cytokinins were shown to promote and repress CAM expression in P. oleracea, respectively. Moreover, the drought-induced decline in C4 transcript levels was completely recovered upon cytokinin treatment. The ABA-regulated transcription factor genes HB7, NFYA7, NFYC9, TT8, and ARR12 were identified as likely candidate regulators of CAM induction following this approach, whereas NFYC4 and ARR9 were connected to C4 expression patterns. Therefore, we provide insights into the signaling events controlling C4-CAM transitions in response to water availability and over the day/night cycle, highlighting candidate genes for future functional studies in the context of facultative C4-CAM photosynthesis.

Changes in crassulacean acid metabolism expression, chloroplast ultrastructure, photochemical and antioxidant activity in the Aloe vera during acclimation to combined drought and salt stress

We determined time course changes of photochemical and antioxidant activity during the induction of strong crassulacean acid metabolism (CAM) in Aloe vera L. plants grown under salt and drought stress. We found that the strong CAM was induced during 25-30days of drought alone treatment. After 25-30days, we showed the withdrawal of strong CAM back to constitutive CAM background under the combination of simultaneous drought and salt stress, which coincided with the accumulation of malondialdehyde, and the decrease in the contents of endogenous nitric oxide (NO) and non-enzymatic antioxidants. At the same time, the chloroplast ultrastructure was damaged with a parallel accumulation of reactive oxygen species, and the whole photosynthetic electron transport flux was impaired by combined stress treatment. In conclusion, the changes in CAM expression parameters was attended by a similar pattern of antioxidant and photochemical change in Aloe plants subjected to only drought or combined stress.

Blue and UV-B light synergistically induce anthocyanin accumulation by co-activating nitrate reductase gene expression in Anthocyanin fruit (Aft) tomato

The tomato accession LA1996, which carries a dominant allele of anthocyanin fruit (Aft) locus, accumulates anthocyanins in the epidermis of fruits when exposed to sunlight. The involvement of blue, UV-A, UV-B and a combination of these wavelengths on anthocyanin accumulation and the molecular mechanism of their regulation was investigated in LA1996. The most effective treatment for inducing anthocyanin biosynthesis in Aft fruits was co-irradiation with blue and UV-B (blue + UV-B) light. Finding the correlated genes is an important approach towards understanding their molecular mechanisms. In the present study, the nitrate reductase (NR) gene SlNIA was isolated using RNA-seq profiling of Aft fruits given different light treatments. The functions of NR-mediated anthocyanin induction by blue + UV-B were confirmed using a series of chemical treatments, followed by assessment of NR activity and nitric oxide (NO) detection. The expression of NR was highly induced by blue + UV-B, and this specificity was also confirmed with the enzyme activity of NR and the NO concentration. The NR inhibitors, which reduce NO generation, the expression levels of anthocyanin related genes and decreased anthocyanin accumulation in LA1996. Our results suggest that NR plays a key role in blue + UV-B-mediated anthocyanin accumulation in LA1996 fruits.

Comparison of CAM expression, photochemistry and antioxidant responses in Sedum album and Portulaca oleracea under combined stress

Previous studies of crassulacean acid metabolism (CAM) pathway during stress have been directed at individual drought and salinity stress, here, we studied the effects of a combination of drought and salt on CAM expression, chlorophyll fluorescence and antioxidant parameters in the C₃ -CAM facultative Sedum album and C₄ -CAM facultative Portulaca oleracea plants. While salinity alone was not able to induce functional CAM expression in P. oleracea leaves, we showed that salinity induced low level of nocturnal acid accumulation in S. album species. After 20 d of exposure to the combination of simultaneous salt and drought stress, P. oleracea plants exhibited more resistance to photoinhibition as compared to S. album plants. The decrease of maximum quantum yield (F_v /F_m ) in S. album leaves under combined stress was in parallel with the largest suppression of CAM expression of >50%, probably displaying the withdrawal of functional CAM back to C₃ pathway. However, under drought treatment alone, S. album plants exhibited higher photosynthetic flexibility, which was associated with the up-regulation of antioxidant enzymes activities and maintenance of glutathione (GSH) pool, and consequently higher photochemical functioning. The levels of nitric oxide (NO) correlated well with CAM expression, which was observed only in S. album, suggesting that NO acts in a different way in C₃ and C₄ species during CAM induction. Additionally, in both species, over the course of CAM induction, the changes in CAM expression parameters exhibited a similar pattern to that of antioxidant capacity and photochemical functioning parameters.

Nitric oxide regulates water status and associated enzymatic pathways to inhibit nutrients imbalance in maize (Zea mays L.) under drought stress

Nitric oxide (NO) is a key signaling molecule that instigates significant changes in plant metabolic processes and promotes tolerance against various environmental stresses including drought. In this study, we focused on NO-mediated physiological mechanisms and enzymatic activities that influence the nutrient concentrations and yield in maize under drought stress. The drought-tolerant (NK-8711) and sensitive (P-1574) maize hybrids were sown in lysimeter tanks and two levels of water stress (well-watered at100% field capacity and drought stress at 60% field capacity) were applied at three-leaves stage of maize. Foliar treatment of sodium nitroprusside (SNP), the donor of NO was applied at the cob development stage. The results showed that the foliar spray of NO regulated water relations by increasing proline content and improved drought tolerance in water stressed maize plants. In addition, it stimulated the activity of antioxidative enzymes which reduced the production of free radicals and lipid peroxidation. The activities of nitrate assimilation enzymes were considerably increased by NO spray which, in turn, increased nutrient accumulation and yield in maize under water deficit conditions. These results acknowledge the importance of NO as a stress-signaling molecule that positively regulates defense mechanisms in maize to withstand water-limited conditions.

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

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

The Role of the Plant Antioxidant System in Drought Tolerance

Water deficiency compromises plant performance and yield in many habitats and in agriculture. In addition to survival of the acute drought stress period which depends on plant-genotype-specific characteristics, stress intensity and duration, also the speed and efficiency of recovery determine plant performance. Drought-induced deregulation of metabolism enhances generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) which in turn affect the redox regulatory state of the cell. Strong correlative and analytical evidence assigns a major role in drought tolerance to the redox regulatory and antioxidant system. This review compiles current knowledge on the response and function of superoxide, hydrogen peroxide and nitric oxide under drought stress in various species and drought stress regimes. The meta-analysis of reported changes in transcript and protein amounts, and activities of components of the antioxidant and redox network support the tentative conclusion that drought tolerance is more tightly linked to up-regulated ascorbate-dependent antioxidant activity than to the response of the thiol-redox regulatory network. The significance of the antioxidant system in surviving severe phases of dehydration is further supported by the strong antioxidant system usually encountered in resurrection plants.

Nitric oxide synthase-mediated early nitric oxide burst alleviates water stress-induced oxidative damage in ammonium-supplied rice roots

BACKGROUND

Nutrition with ammonium (NH4+) can enhance the drought tolerance of rice seedlings in comparison to nutrition with nitrate (NO3-). However, there are still no detailed studies investigating the response of nitric oxide (NO) to the different nitrogen nutrition and water regimes. To study the intrinsic mechanism underpinning this relationship, the time-dependent production of NO and its protective role in the antioxidant defense system of NH4+- or NO3--supplied rice seedlings were studied under water stress.

RESULTS

An early NO burst was induced by 3 h of water stress in the roots of seedlings subjected to NH4+ treatment, but this phenomenon was not observed under NO3- treatment. Root oxidative damage induced by water stress was significantly higher for treatment with NO3- than with NH4+ due to reactive oxygen species (ROS) accumulation in the former. Inducing NO production by applying the NO donor 3 h after NO3- treatment alleviated the oxidative damage, while inhibiting the early NO burst by applying the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO) increased root oxidative damage in NH4+ treatment. Application of the nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester(L-NAME) completely suppressed NO synthesis in roots 3 h after NH4+ treatment and aggravated water stress-induced oxidative damage. Therefore, the aggravation of oxidative damage by L-NAME might have resulted from changes in the NOS-mediated early NO burst. Water stress also increased the activity of root antioxidant enzymes (catalase, superoxide dismutase, and ascorbate peroxidase). These were further induced by the NO donor but repressed by the NO scavenger and NOS inhibitor in NH4+-treated roots.

CONCLUSION

These findings demonstrate that the NOS-mediated early NO burst plays an important role in alleviating oxidative damage induced by water stress by enhancing the antioxidant defenses in roots supplemented with NH4+.

Does oxidative stress determine the thermal limits of the regeneration niche of Vriesea friburgensis and Alcantarea imperialis (Bromeliaceae) seedlings?

The predicted environmental changes may be detrimental to initial seedling growth, particularly the expected increase in air temperature. We therefore investigated the thermal limits for growth and development of Vriesea friburgensis and Alcantarea imperialis seedlings in the context of oxidative stress. The optimal temperatures for the growth of V. friburgensis and A. imperialis were 25 and 25-30 °C, respectively. Extreme temperatures (15, 30, or 35 °C) induced oxidative stress in both species with significant accumulation of hydrogen peroxide (H₂O₂) and nitric oxide (NO). Under oxidative stress, the amount of chlorophyll decreased in both species, more prominently in V. friburgensis, while carotenoid levels dramatically increased in A. imperialis. Notably, the activities of superoxide dismutase, catalase (CAT), and ascorbate peroxidase increased in A. imperialis at extreme temperatures. Similar results were observed for V. friburgensis; however, the activity of CAT remained unaffected regardless of temperature. Seedlings of A. imperialis survived at a wider range of temperatures than V. friburgensis, which had greater than 40% mortality when growing at 30 °C. Overall, precise control of cellular H₂O₂ and NO levels takes place during the establishment of A. imperialis seedlings, allowing the species to cope with relatively high temperatures. The thermal limits of the fundamental niches of the species investigated, determined based on the ability of seedlings to cope with oxidative stress, were distinct from the realized niches of these species. The results suggest that recruitment success is dependent on the ability of seedlings to handle extreme temperature-triggered oxidative stress, which limits the regeneration niche.

Nitric oxide mitigates the effect of water deficit in Crambe abyssinica

Crambe abyssinica is widely cultivated in the off-season in the Midwest region of Brazil with great potential for biodeisel production. Low precipitation is characteristic of this region, which can drastically affect the productivity of C. abyssinica. Signaling molecules, such as nitric oxide (NO), can potentially alleviate the effects of water stress on plants. Here we test whether nitric oxide, applied by donor sodium nitroprusside (SNP), can alleviate the occurrence of water deficit damages in Crambe plants and maintain physiological and biochemical processes. Crambe plants were sprayed with three doses of SNP (0, 75, and 150 μM) and were submitted to two water levels (100% and 50% of the maximum water holding capacity). After 32 and 136 h, leaves were analyzed to evaluate the concentration of NO, water relations, gas exchange, chlorophyll a fluorescence, chloroplastidic pigments, proline, malondialdehyde, hydrogen peroxide, superoxide anions, and the antioxidant enzymes activity. Application of SNP allowed the maintenance of gas exchange, chlorophyll fluorescence parameters, and activities of antioxidant enzymes in plants exposed to water deficit, as well as increased the concentration of NO, proline, chloroplastidic pigments and osmotic potential. The application of SNP also decreased the concentration of malondialdehyde and reactive oxygen species in plants submitted to water deficit. Thus, the application of SNP prevented the occurrence of symptoms of water deficit in Crambe plants, maintaining the physiological and biochemical responses at reference levels, even under stress conditions.

Trichoderma asperellum T42 Reprograms Tobacco for Enhanced Nitrogen Utilization Efficiency and Plant Growth When Fed with N Nutrients

Trichoderma spp., are saprophytic fungi that can improve plant growth through increased nutrient acquisition and change in the root architecture. In the present study, we demonstrate that Trichoderma asperellum T42 mediate enhancement in host biomass, total nitrogen content, nitric oxide (NO) production and cytosolic Ca2+ accumulation in tobacco. T42 inoculation enhanced lateral root, root hair length, root hair density and root/shoot dry mass in tobacco under deprived nutrients condition. Interestingly, these growth attributes were further elevated in presence of T42 and supplementation of NO3- and NH4+ nutrients to tobacco at 40 and 70 days, particularly in NO3- supplementation, whereas no significant increment was observed in nia30 mutant. In addition, NO production was more in tobacco roots in T42 inoculated plants fed with NO3- nutrient confirming NO generation was dependent on NR pathway. NO3- dependent NO production contributed to increase in lateral root initiation, Ca2+ accumulation and activities of nitrate transporters (NRTs) in tobacco. Higher activities of several NRT genes in response to T42 and N nutrients and suppression of ammonium transporter (AMT1) suggested that induction of high affinity NRTs help NO3- acquisition through roots of tobacco. Among the NRTs NRT2.1 and NRT2.2 were more up-regulated compared to the other NRTs. Addition of sodium nitroprusside (SNP), relative to those supplied with NO3-/NH4+ nutrition and T42 treated plants singly, and with application of NO inhibitor, cPTIO, confirmed the altered NO fluorescence intensity in tobacco roots. Our findings suggest that T42 promoted plant growth significantly ant N content in the tobacco plants grown under N nutrients, notably higher in NO3-, providing insight of the strategy for not only tobacco but probably for other crops as well to adapt to fluctuating nitrate availability in soil.

Nitrate enhancement of CAM activity in two Kalanchoë species is associated with increased vacuolar proton transport capacity

Among species that perform CAM photosynthesis, members of the genus Kalanchoë have been studied frequently to investigate the effect of environmental factors on the magnitude of CAM activity. In particular, different nitrogen sources have been shown to influence the rate of nocturnal CO₂ fixation and organic-acid accumulation in several species of Kalanchoë. However, there has been little investigation of the interrelationship between nitrogen source (nitrate versus ammonium), concentration and the activity of the vacuolar proton pumps responsible for driving nocturnal organic-acid accumulation in these species. In the present study with Kalanchoë laxiflora and Kalanchoë delagoensis cultivated on different nitrogen sources, both species were found to show highest total nocturnal organic-acid accumulation and highest rates of ATP- and PPi-dependent vacuolar proton transport on 2.5 mM nitrate, whereas plants cultivated on 5.0 mM ammonium showed the lowest values. In both species malate was the principal organic-acid accumulated during the night, but the second-most accumulated organic-acid was fumarate for K. laxiflora and citrate for K. delagoensis. Higher ATP- and PPi-dependent vacuolar proton transport rates and greater nocturnal acid accumulation were observed in K. delagoensis compared with K. laxiflora. These results show that the effect of nitrogen source on CAM activity in Kalanchoë species is reflected in corresponding differences in activity of the tonoplast proton pumps responsible for driving sequestration of these acids in the vacuole of CAM-performing cells.

Alternative fluorimetric-based method to detect and compare total S-nitrosothiols in plants

Nitric oxide (NO) is an important signaling molecule occurring in virtually all organisms, whose mechanism of action relies mainly on its interaction with proteins or peptides by nitrosylation, forming compounds such as S-nitrosothiols (SNO). The Saville reaction and the ozone-based chemiluminescence method are the main techniques used for nitrosylated protein quantification. While the Saville assay is not very sensitive, the ozone-based chemiluminescence is expensive and time-consuming. Here we propose a method in which the protein-bound NO groups are exposed to UV light, cleaving the bond and allowing the quantification of the derived NO molecules by diamino-rhodamine (DAR) dyes. The DAR-based method was shown to be specific in plant tissues by pre-treatment of the samples with reducing agents and parallel EPR analysis. Spike-and-recovery assays revealed 72% recovery after a GSNO spike. Moreover, the method was significantly more sensitive than the Saville reaction, and this increase in sensitivity was crucial for detecting the reduced levels of nitrosylated proteins in plant species other than Arabidopsis. The method presented here is a suitable alternative to compare plant samples, allowing simple and fast detection of nitrosylated proteins.

Nitrate Reductase Regulates Plant Nitric Oxide Homeostasis

Nitrate reductase (NR) is a key enzyme for nitrogen acquisition by plants, algae, yeasts, and fungi. Nitrate, its main substrate, is required for signaling and is widely distributed in diverse tissues in plants. In addition, NR has been proposed as an important enzymatic source of nitric oxide (NO). Recently, NR has been shown to play a role in NO homeostasis by supplying electrons from NAD(P)H through its diaphorase/dehydrogenase domain both to a truncated hemoglobin THB1, which scavenges NO by its dioxygenase activity, and to the molybdoenzyme NO-forming nitrite reductase (NOFNiR) that is responsible for NO synthesis from nitrite. We review how NR may play a central role in plant biology by controlling the amounts of NO, a key signaling molecule in plant cells.

Shedding light on NO homeostasis: Light as a key regulator of glutathione and nitric oxide metabolisms during seedling deetiolation

Despite the significant impacts of light on nitric oxide (NO) levels in plants, the mechanism underlying the influence of this environmental factor on NO metabolism remains poorly understood. A critical mechanism controlling NO levels in plant cells relies on the S-nitrosylation of glutathione (GSH), giving rise to S-nitrosoglutathione (GSNO), which can be either stored or degraded depending on the cellular context. Here, we demonstrate that a strict balance is maintained between NO generation and scavenging during tomato (Solanum lycopersicum) seedling deetiolation. Given the absence of accurate methods in the literature to estimate NO scavenging in planta, we first developed a simple, robust system to continuously monitor the global in vivo NO scavenging by plant tissues. Then, using photomorphogenic tomato mutants, we demonstrated that the light-evoked de-etiolation is associated with a dramatic rise in NO content followed by a progressive increment in NO scavenging capacity of the tissues. Light-driven increments in NO scavenging rates coincided with pronounced rises in S-nitrosothiol content and GSNO reductase (GSNOR) activity, thereby suggesting that GSNO formation and subsequent removal via GSNOR might be key for controlling NO levels during seedling deetiolation. Accordingly, treatments with thiol-blocking compounds further indicated that thiol nitrosylation might be critically involved in the NO scavenging mechanism responsible for maintaining NO homeostasis during deetiolation. The impacts of both light and NO on the transcriptional profile of glutathione metabolic genes also revealed an independent but coordinated action of these signals on the regulation of key components of glutathione and GSNO metabolisms. Altogether, these data indicated that GSNO formation and subsequent removal might facilitate maintaining NO homeostasis during light-driven seedling deetiolation.

The bZIP transcription factor MdHY5 regulates anthocyanin accumulation and nitrate assimilation in apple

The basic leucine zipper (bZIP) transcription factor HY5 plays a multifaceted role in plant growth and development. Here the apple MdHY5 gene was cloned based on its homology with Arabidopsis HY5. Expression analysis demonstrated that MdHY5 transcription was induced by light and abscisic acid treatments. Electrophoretic mobility shift assays and transient expression assays subsequently showed that MdHY5 positively regulated both its own transcription and that of MdMYB10 by binding to E-box and G-box motifs, respectively. Furthermore, we obtained transgenic apple calli that overexpressed the MdHY5 gene, and apple calli coloration assays showed that MdHY5 promoted anthocyanin accumulation by regulating expression of the MdMYB10 gene and downstream anthocyanin biosynthesis genes. In addition, the transcript levels of a series of nitrate reductase genes and nitrate uptake genes in both wild-type and transgenic apple calli were detected. In association with increased nitrate reductase activities and nitrate contents, the results indicated that MdHY5 might be an important regulator in nutrient assimilation. Taken together, these results indicate that MdHY5 plays a vital role in anthocyanin accumulation and nitrate assimilation in apple.

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

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

Implications of leaf ontogeny on drought-induced gradients of CAM expression and ABA levels in rosettes of the epiphytic tank bromeliad Guzmania monostachia

Guzmania monostachia is an epiphytic heteroblastic bromeliad that exhibits rosette leaves forming water-holding tanks at maturity. Different portions along its leaf blades can display variable degrees of crassulacean acid metabolism (CAM) up-regulation under drought. Since abscisic acid (ABA) can act as an important long-distance signal, we conducted a joint investigation of ontogenetic and drought impacts on CAM intensity and ABA levels in different leaf groups within the G. monostachia rosette. For this, three groups of leaves were analysed according to their position within the mature-tank rosette (i.e., younger, intermediate, and older leaves) to characterize the general growth patterns and magnitude of drought-modulated CAM expression. CAM activity was evaluated by analysing key molecules in the biochemical machinery of this photosynthetic pathway, while endogenous ABA content was comparatively measured in different portions of each leaf group after seven days under well-watered (control) or drought treatment. The results revealed that G. monostachia shows more uniform morphological characteristics along the leaves when in the atmospheric stage. The drought treatment of mature-tank rosettes generally induced in older leaves a more severe water loss, followed by the lowest CAM activity and a higher increase in ABA levels, while younger leaves showed an opposite response. Therefore, leaf groups at distinct ontogenetic stages within the tank rosette of G. monostachia responded to drought with variable degrees of water loss and CAM expression. ABA seems to participate in this tissue-compartmented response as a long-distance signalling molecule, transmitting the drought-induced signals originated in older leaves towards the younger ones.

Atmospheric emission of nitric oxide and processes involved in its biogeochemical transformation in terrestrial environment

Nitric oxide (NO) is an intra- and intercellular gaseous signaling molecule with a broad spectrum of regulatory functions in biological system. Its emissions are produced by both natural and anthropogenic sources; however, soils are among the most important sources of NO. Nitric oxide plays a decisive role in environmental-atmospheric chemistry by controlling the tropospheric photochemical production of ozone and regulates formation of various oxidizing agents such as hydroxyl radical (OH), which contributes to the formation of acid of precipitates. Consequently, for developing strategies to overcome the deleterious impact of NO on terrestrial ecosystem, it is mandatory to have reliable information about the exact emission mechanism and processes involved in its transformation in soil-atmospheric system. Although the formation process of NO is a complex phenomenon and depends on many physicochemical characteristics, such as organic matter, soil pH, soil moisture, soil temperature, etc., this review provides comprehensive updates about the emission characteristics and biogeochemical transformation mechanism of NO. Moreover, this article will also be helpful to understand the processes involved in the consumption of NO in soils. Further studies describing the functions of NO in biological system are also discussed.

Strigolactones are required for nitric oxide to induce root elongation in response to nitrogen and phosphate deficiencies in rice

The response of the root system architecture to nutrient deficiencies is critical for sustainable agriculture. Nitric oxide (NO) is considered a key regulator of root growth, although the mechanisms remain unknown. Phenotypic, cellular and genetic analyses were undertaken in rice to explore the role of NO in regulating root growth and strigolactone (SL) signalling under nitrogen-deficient and phosphate-deficient conditions (LN and LP). LN-induced and LP-induced seminal root elongation paralleled NO production in root tips. NO played an important role in a shared pathway of LN-induced and LP-induced root elongation via increased meristem activity. Interestingly, no responses of root elongation were observed in SL d mutants compared with wild-type plants, although similar NO accumulation was induced by sodium nitroprusside (SNP) application. Application of abamine (the SL inhibitor) reduced seminal root length and pCYCB1;1::GUS expression induced by SNP application in wild type; furthermore, comparison with wild type showed lower SL-signalling genes in nia2 mutants under control and LN treatments and similar under SNP application. Western blot analysis revealed that NO, similar to SL, triggered proteasome-mediated degradation of D53 protein levels. Therefore, we presented a novel signalling pathway in which NO-activated seminal root elongation under LN and LP conditions, with the involvement of SLs.

When Bad Guys Become Good Ones: The Key Role of Reactive Oxygen Species and Nitric Oxide in the Plant Responses to Abiotic Stress

The natural environment of plants is composed of a complex set of abiotic stresses and their ability to respond to these stresses is highly flexible and finely balanced through the interaction between signaling molecules. In this review, we highlight the integrated action between reactive oxygen species (ROS) and reactive nitrogen species (RNS), particularly nitric oxide (NO), involved in the acclimation to different abiotic stresses. Under stressful conditions, the biosynthesis transport and the metabolism of ROS and NO influence plant response mechanisms. The enzymes involved in ROS and NO synthesis and scavenging can be found in different cells compartments and their temporal and spatial locations are determinant for signaling mechanisms. Both ROS and NO are involved in long distances signaling (ROS wave and GSNO transport), promoting an acquired systemic acclimation to abiotic stresses. The mechanisms of abiotic stresses response triggered by ROS and NO involve some general steps, as the enhancement of antioxidant systems, but also stress-specific mechanisms, according to the stress type (drought, hypoxia, heavy metals, etc.), and demand the interaction with other signaling molecules, such as MAPK, plant hormones, and calcium. The transduction of ROS and NO bioactivity involves post-translational modifications of proteins, particularly S-glutathionylation for ROS, and S-nitrosylation for NO. These changes may alter the activity, stability, and interaction with other molecules or subcellular location of proteins, changing the entire cell dynamics and contributing to the maintenance of homeostasis. However, despite the recent advances about the roles of ROS and NO in signaling cascades, many challenges remain, and future studies focusing on the signaling of these molecules in planta are still necessary.

Nitric Oxide, Ethylene, and Auxin Cross Talk Mediates Greening and Plastid Development in Deetiolating Tomato Seedlings

The transition from etiolated to green seedlings involves the conversion of etioplasts into mature chloroplasts via a multifaceted, light-driven process comprising multiple, tightly coordinated signaling networks. Here, we demonstrate that light-induced greening and chloroplast differentiation in tomato (Solanum lycopersicum) seedlings are mediated by an intricate cross talk among phytochromes, nitric oxide (NO), ethylene, and auxins. Genetic and pharmacological evidence indicated that either endogenously produced or exogenously applied NO promotes seedling greening by repressing ethylene biosynthesis and inducing auxin accumulation in tomato cotyledons. Analysis performed in hormonal tomato mutants also demonstrated that NO production itself is negatively and positively regulated by ethylene and auxins, respectively. Representing a major biosynthetic source of NO in tomato cotyledons, nitrate reductase was shown to be under strict control of both phytochrome and hormonal signals. A close NO-phytochrome interaction was revealed by the almost complete recovery of the etiolated phenotype of red light-grown seedlings of the tomato phytochrome-deficient aurea mutant upon NO fumigation. In this mutant, NO supplementation induced cotyledon greening, chloroplast differentiation, and hormonal and gene expression alterations similar to those detected in light-exposed wild-type seedlings. NO negatively impacted the transcript accumulation of genes encoding phytochromes, photomorphogenesis-repressor factors, and plastid division proteins, revealing that this free radical can mimic transcriptional changes typically triggered by phytochrome-dependent light perception. Therefore, our data indicate that negative and positive regulatory feedback loops orchestrate ethylene-NO and auxin-NO interactions, respectively, during the conversion of colorless etiolated seedlings into green, photosynthetically competent young plants.

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.

Toward systems-level analysis of agricultural production from crassulacean acid metabolism (CAM): scaling from cell to commercial production

Systems-level analyses have become prominent tools for assessing the yield, viability, economic consequences and environmental impacts of agricultural production. Such analyses are well-developed for many commodity crops that are used for food and biofuel, but have not been developed for agricultural production systems based on drought-tolerant plants that use crassulacean acid metabolism (CAM). We review the components of systems-level evaluations, and identify the information available for completing such analyses for CAM cropping systems. Specific needs for developing systems-level evaluations of CAM agricultural production include: improvement of physiological models; assessment of product processing after leaving the farm gate; and application of newly available genetic tools to the optimization of CAM species for commercial production.

A roadmap for research on crassulacean acid metabolism (CAM) to enhance sustainable food and bioenergy production in a hotter, drier world

Crassulacean acid metabolism (CAM) is a specialized mode of photosynthesis that features nocturnal CO2 uptake, facilitates increased water-use efficiency (WUE), and enables CAM plants to inhabit water-limited environments such as semi-arid deserts or seasonally dry forests. Human population growth and global climate change now present challenges for agricultural production systems to increase food, feed, forage, fiber, and fuel production. One approach to meet these challenges is to increase reliance on CAM crops, such as Agave and Opuntia, for biomass production on semi-arid, abandoned, marginal, or degraded agricultural lands. Major research efforts are now underway to assess the productivity of CAM crop species and to harness the WUE of CAM by engineering this pathway into existing food, feed, and bioenergy crops. An improved understanding of CAM has potential for high returns on research investment. To exploit the potential of CAM crops and CAM bioengineering, it will be necessary to elucidate the evolution, genomic features, and regulatory mechanisms of CAM. Field trials and predictive models will be required to assess the productivity of CAM crops, while new synthetic biology approaches need to be developed for CAM engineering. Infrastructure will be needed for CAM model systems, field trials, mutant collections, and data management.

Expression of the tetrahydrofolate-dependent nitric oxide synthase from the green alga Ostreococcus tauri increases tolerance to abiotic stresses and influences stomatal development in Arabidopsis

Nitric oxide (NO) is a signaling molecule with diverse biological functions in plants. NO plays a crucial role in growth and development, from germination to senescence, and is also involved in plant responses to biotic and abiotic stresses. In animals, NO is synthesized by well-described nitric oxide synthase (NOS) enzymes. NOS activity has also been detected in higher plants, but no gene encoding an NOS protein, or the enzymes required for synthesis of tetrahydrobiopterin, an essential cofactor of mammalian NOS activity, have been identified so far. Recently, an NOS gene from the unicellular marine alga Ostreococcus tauri (OtNOS) has been discovered and characterized. Arabidopsis thaliana plants were transformed with OtNOS under the control of the inducible short promoter fragment (SPF) of the sunflower (Helianthus annuus) Hahb-4 gene, which responds to abiotic stresses and abscisic acid. Transgenic plants expressing OtNOS accumulated higher NO concentrations compared with siblings transformed with the empty vector, and displayed enhanced salt, drought and oxidative stress tolerance. Moreover, transgenic OtNOS lines exhibited increased stomatal development compared with plants transformed with the empty vector. Both in vitro and in vivo experiments indicate that OtNOS, unlike mammalian NOS, efficiently uses tetrahydrofolate as a cofactor in Arabidopsis plants. The modulation of NO production to alleviate abiotic stress disturbances in higher plants highlights the potential of genetic manipulation to influence NO metabolism as a tool to improve plant fitness under adverse growth conditions.

Nitric oxide generated by nitrate reductase increases nitrogen uptake capacity by inducing lateral root formation and inorganic nitrogen uptake under partial nitrate nutrition in rice

Increasing evidence shows that partial nitrate nutrition (PNN) can be attributed to improved plant growth and nitrogen-use efficiency (NUE) in rice. Nitric oxide (NO) is a signalling molecule involved in many physiological processes during plant development and nitrogen (N) assimilation. It remains unclear whether molecular NO improves NUE through PNN. Two rice cultivars (cvs Nanguang and Elio), with high and low NUE, respectively, were used in the analysis of NO production, nitrate reductase (NR) activity, lateral root (LR) density, and (15)N uptake under PNN, with or without NO production donor and inhibitors. PNN increased NO accumulation in cv. Nanguang possibly through the NIA2-dependent NR pathway. PNN-mediated NO increases contributed to LR initiation, (15)NH₄(+)/(15)NO₃(-) influx into the root, and levels of ammonium and nitrate transporters in cv. Nanguang but not cv. Elio. Further results revealed marked and specific induction of LR initiation and (15)NH₄(+)/(15)NO₃(-) influx into the roots of plants supplied with NH₄(+)+sodium nitroprusside (SNP) relative to those supplied with NH₄(+) alone, and considerable inhibition upon the application of cPTIO or tungstate (NR inhibitor) in addition to PNN, which is in agreement with the change in NO fluorescence in the two rice cultivars. The findings suggest that NO generated by the NR pathway plays a pivotal role in improving the N acquisition capacity by increasing LR initiation and the inorganic N uptake rate, which may represent a strategy for rice plants to adapt to a fluctuating nitrate supply and increase NUE.

Genomic analyses of the CAM plant pineapple

The innovation of crassulacean acid metabolism (CAM) photosynthesis in arid and/or low CO2 conditions is a remarkable case of adaptation in flowering plants. As the most important crop that utilizes CAM photosynthesis, the genetic and genomic resources of pineapple have been developed over many years. Genetic diversity studies using various types of DNA markers led to the reclassification of the two genera Ananas and Pseudananas and nine species into one genus Ananas and two species, A. comosus and A. macrodontes with five botanical varieties in A. comosus. Five genetic maps have been constructed using F1 or F2 populations, and high-density genetic maps generated by genotype sequencing are essential resources for sequencing and assembling the pineapple genome and for marker-assisted selection. There are abundant expression sequence tag resources but limited genomic sequences in pineapple. Genes involved in the CAM pathway has been analysed in several CAM plants but only a few of them are from pineapple. A reference genome of pineapple is being generated and will accelerate genetic and genomic research in this major CAM crop. This reference genome of pineapple provides the foundation for studying the origin and regulatory mechanism of CAM photosynthesis, and the opportunity to evaluate the classification of Ananas species and botanical cultivars.

NO homeostasis is a key regulator of early nitrate perception and root elongation in maize

Crop plant development is strongly dependent on nitrogen availability in the soil and on the efficiency of its recruitment by roots. For this reason, the understanding of the molecular events underlying root adaptation to nitrogen fluctuations is a primary goal to develop biotechnological tools for sustainable agriculture. However, knowledge about molecular responses to nitrogen availability is derived mainly from the study of model species. Nitric oxide (NO) has been recently proposed to be implicated in plant responses to environmental stresses, but its exact role in the response of plants to nutritional stress is still under evaluation. In this work, the role of NO production by maize roots after nitrate perception was investigated by focusing on the regulation of transcription of genes involved in NO homeostasis and by measuring NO production in roots. Moreover, its involvement in the root growth response to nitrate was also investigated. The results provide evidence that NO is produced by nitrate reductase as an early response to nitrate supply and that the coordinated induction of non-symbiotic haemoglobins (nsHbs) could finely regulate the NO steady state. This mechanism seems to be implicated on the modulation of the root elongation in response to nitrate perception. Moreover, an improved agar-plate system for growing maize seedlings was developed. This system, which allows localized treatments to be performed on specific root portions, gave the opportunity to discern between localized and systemic effects of nitrate supply to roots.

Nitric oxide and phytohormone interactions: current status and perspectives

Nitric oxide (NO) is currently considered a ubiquitous signal in plant systems, playing significant roles in a wide range of responses to environmental and endogenous cues. During the signaling events leading to these plant responses, NO frequently interacts with plant hormones and other endogenous molecules, at times originating remarkably complex signaling cascades. Accumulating evidence indicates that virtually all major classes of plant hormones may influence, at least to some degree, the endogenous levels of NO. In addition, studies conducted during the induction of diverse plant responses have demonstrated that NO may also affect biosynthesis, catabolism/conjugation, transport, perception, and/or transduction of different phytohormones, such as auxins, gibberellins, cytokinins, abscisic acid, ethylene, salicylic acid, jasmonates, and brassinosteroids. Although still not completely elucidated, the mechanisms underlying the interaction between NO and plant hormones have recently been investigated in a number of species and plant responses. This review specifically focuses on the current knowledge of the mechanisms implicated in NO-phytohormone interactions during the regulation of developmental and metabolic plant events. The modifications triggered by NO on the transcription of genes encoding biosynthetic/degradative enzymes as well as proteins involved in the transport and signal transduction of distinct plant hormones will be contextualized during the control of developmental, metabolic, and defense responses in plants. Moreover, the direct post-translational modification of phytohormone biosynthetic enzymes and receptors through S-nitrosylation will also be discussed as a key mechanism for regulating plant physiological responses. Finally, some future perspectives toward a more complete understanding of NO-phytohormone interactions will also be presented and discussed.

Nitrogen starvation, salt and heat stress in coffee (Coffea arabica L.): identification and validation of new genes for qPCR normalization

Abiotic stresses are among the most important factors that affect food production. One important step to face these environmental challenges is the transcriptional modulation. Quantitative real-time PCR is a rapid, sensitive, and reliable method for the detection of mRNAs and it has become a powerful tool to mitigate plant stress tolerance; however, suitable reference genes are required for data normalization. Reference genes for coffee plants during nitrogen starvation, salinity and heat stress have not yet been reported. We evaluated the expression stability of ten candidate reference genes using geNorm PLUS, NormFinder, and BestKeeper softwares, in plants submitted to nitrogen starvation, salt and heat stress. EF1, EF1α, GAPDH, MDH, and UBQ10 were ranked as the most stable genes in all stresses and software analyses, while RPL39 and RPII were classified as the less reliable references. For reference gene validation, the transcriptional pattern of a Coffea non-symbiotic hemoglobin (CaHb1) was analyzed using the two new recommended and the most unstable gene references for normalization. The most unstable gene may lead to incorrect interpretation of CaHb1 transcriptional analysis. Here, we recommend two new reference genes in Coffea for use in data normalization in abiotic stresses: MDH and EF1.

Abscisic acid and nitric oxide signaling in two different portions of detached leaves of Guzmania monostachia with CAM up-regulated by drought

Guzmania monostachia is an epiphyte tank bromeliad capable of up-regulating crassulacean acid metabolism (CAM) in response to several environmental stimuli, including drought and light stress. In other plant species, abscisic acid (ABA) and nitric oxide (NO) seem to be involved in CAM induction. Because the leaves of tank bromeliads perform different functions along their length, this study attempted to investigate whether ABA and NO are involved in regulation of CAM expression in this species by quantifying these compounds in apical and basal portions of the leaf, and whether there would be differences in this event for each leaf portion. Detached leaves exposed to a 30% polyethylene glycol solution showed a significant upregulation of CAM on the seventh day of treatment only in the apical portion, as indicated by nocturnal acid accumulation and phosphoenolpyruvate carboxylase (PEPC) activity. On the three days prior to CAM induction, ABA, NO and H₂O₂ were quantified. The amounts of ABA were higher in PEG-exposed leaves, along their entire length. NO, however, was higher only in the apical portion, precisely where CAM was up-regulated. H₂O₂ was higher only in the basal portion of PEG-exposed leaves. Our results suggest that ABA might be a systemic signal to drought, occurring in the entire leaf. NO and H₂O₂, however, may be signals restricted only to the apical or basal portions, respectively.

Spatial patterns of photosynthesis in thin- and thick-leaved epiphytic orchids: unravelling C3-CAM plasticity in an organ-compartmented way

BACKGROUND AND AIMS

A positive correlation between tissue thickness and crassulacean acid metabolism (CAM) expression has been frequently suggested. Therefore, this study addressed the question of whether water availability modulates photosynthetic plasticity in different organs of two epiphytic orchids with distinct leaf thickness.

METHODS

Tissue morphology and photosynthetic mode (C3 and/or CAM) were examined in leaves, pseudobulbs and roots of a thick-leaved (Cattleya walkeriana) and a thin-leaved (Oncidium 'Aloha') epiphytic orchid. Morphological features were studied comparing the drought-induced physiological responses observed in each organ after 30 d of either drought or well-watered treatments.

KEY RESULTS

Cattleya walkeriana, which is considered a constitutive CAM orchid, displayed a clear drought-induced up-regulation of CAM in its thick leaves but not in its non-leaf organs (pseudobulbs and roots). The set of morphological traits of Cattleya leaves suggested the drought-inducible CAM up-regulation as a possible mechanism of increasing water-use efficiency and carbon economy. Conversely, although belonging to an orchid genus classically considered as performing C3 photosynthesis, Oncidium 'Aloha' under drought seemed to express facultative CAM in its roots and pseudobulbs but not in its leaves, indicating that such photosynthetic responses might compensate for the lack of capacity to perform CAM in its thin leaves. Morphological features of Oncidium leaves also indicated lower efficiency in preventing water and CO2 losses, while aerenchyma ducts connecting pseudobulbs and leaves suggested a compartmentalized mechanism of nighttime carboxylation via phosphoenolpyruvate carboxylase (PEPC) (pseudobulbs) and daytime carboxylation via Rubisco (leaves) in drought-exposed Oncidium plants.

CONCLUSIONS

Water availability modulated CAM expression in an organ-compartmented manner in both orchids studied. As distinct regions of the same orchid could perform different photosynthetic pathways and variable degrees of CAM expression depending on the water availability, more attention should be addressed to this in future studies concerning the abundance of CAM plants.

Nitrogen metabolism and translocation in soybean plants subjected to root oxygen deficiency

Although nitrate (NO3(-)) but not ammonium (NH4(+)) improves plant tolerance to oxygen deficiency, the mechanisms involved in this phenomenon are just beginning to be understood. By using gas chromatography-mass spectrometry, we investigated the metabolic fate of (15)NO3(-) and (15)NH4(+) in soybean plants (Glycine max L. Merril cv. IAC-23) subjected to root hypoxia. This stress reduced the uptake of (15)NO3(-) and (15)NH4(+) from the medium and decreased the overall assimilation of these nitrogen sources into amino acids in roots and leaves. Root (15)NO3(-) assimilation was more affected by hypoxia than that of (15)NH4(+), resulting in enhanced nitrite and nitric oxide release in the solution. However, (15)NO3(-) was translocated in substantial amounts by xylem sap and considerable (15)NO3(-) assimilation into amino acids also occurred in the leaves, both under hypoxia and normoxia. By contrast, (15)NH4(+) assimilation occurred predominantly in roots, resulting in accumulation of mainly (15)N-alanine in this tissue during hypoxia. Analysis of lactate levels suggested higher fermentation in roots from NH4(+)-treated plants compared to the NO3(-) treatment. Thus, foliar NO3(-) assimilation may be relevant to plant tolerance to oxygen deficiency, since it would economize energy expenditure by hypoxic roots. Additionally, the involvement of nitric oxide synthesis from nitrite in the beneficial effect of NO3(-) is discussed.

Cytokinins can act as suppressors of nitric oxide in Arabidopsis

Maintaining nitric oxide (NO) homeostasis is essential for normal plant physiological processes. However, very little is known about the mechanisms of NO modulation in plants. Here, we report a unique mechanism for the catabolism of NO based on the reaction with the plant hormone cytokinin. We screened for NO-insensitive mutants in Arabidopsis and isolated two allelic lines, cnu1-1 and 1-2 (continuous NO-unstressed 1), that were identified as the previously reported altered meristem program 1 (amp1) and as having elevated levels of cytokinins. A double mutant of cnu1-2 and nitric oxide overexpression 1 (nox1) reduced the severity of the phenotypes ascribed to excess NO levels as did treating the nox1 line with trans-zeatin, the predominant form of cytokinin in Arabidopsis. We further showed that peroxinitrite, an active NO derivative, can react with zeatin in vitro, which together with the results in vivo suggests that cytokinins suppress the action of NO most likely through direct interaction between them, leading to the reduction of endogenous NO levels. These results provide insights into NO signaling and regulation of its bioactivity in plants.

Nitric oxide in plants: an assessment of the current state of knowledge

BACKGROUND AND AIMS

After a series of seminal works during the last decade of the 20th century, nitric oxide (NO) is now firmly placed in the pantheon of plant signals. Nitric oxide acts in plant-microbe interactions, responses to abiotic stress, stomatal regulation and a range of developmental processes. By considering the recent advances in plant NO biology, this review will highlight certain key aspects that require further attention.

SCOPE AND CONCLUSIONS

The following questions will be considered. While cytosolic nitrate reductase is an important source of NO, the contributions of other mechanisms, including a poorly defined arginine oxidizing activity, need to be characterized at the molecular level. Other oxidative pathways utilizing polyamine and hydroxylamine also need further attention. Nitric oxide action is dependent on its concentration and spatial generation patterns. However, no single technology currently available is able to provide accurate in planta measurements of spatio-temporal patterns of NO production. It is also the case that pharmaceutical NO donors are used in studies, sometimes with little consideration of the kinetics of NO production. We here include in planta assessments of NO production from diethylamine nitric oxide, S-nitrosoglutathione and sodium nitroprusside following infiltration of tobacco leaves, which could aid workers in their experiments. Further, based on current data it is difficult to define a bespoke plant NO signalling pathway, but rather NO appears to act as a modifier of other signalling pathways. Thus, early reports that NO signalling involves cGMP-as in animal systems-require revisiting. Finally, as plants are exposed to NO from a number of external sources, investigations into the control of NO scavenging by such as non-symbiotic haemoglobins and other sinks for NO should feature more highly. By crystallizing these questions the authors encourage their resolution through the concerted efforts of the plant NO community.

Oxidative and nitrosative-based signaling and associated post-translational modifications orchestrate the acclimation of citrus plants to salinity stress

Reactive oxygen and nitrogen species are involved in a plethora of cellular responses in plants; however, our knowledge on the outcomes of oxidative and nitrosative signaling is still unclear. To better understand how oxidative and nitrosative signals are integrated to regulate cellular adjustments to external conditions, local and systemic responses were investigated in the roots and leaves of sour orange plants (Citrus aurantium L.) after root treatment with hydrogen peroxide (H(2) O(2) ) or sodium nitroprusside (a nitric oxide donor), followed by NaCl stress for 8 days. Phenotypic and physiological data showed that pre-exposure to these treatments induced an acclimation to subsequent salinity stress that was accompanied by both local and systemic H(2) O(2) and nitric oxide (NO) accumulation. Combined histochemical and fluorescent probe approaches showed the existence of a vascular-driven long-distance reactive oxygen species and NO signaling pathway. Transcriptional analysis of genes diagnostic for H(2) O(2) and NO signaling just after treatments or after 8 days of salt stress revealed tissue- and time-specific mechanisms controlling internal H(2) O(2) and NO homeostasis. Furthermore, evidence is presented showing that protein carbonylation, nitration and S-nitrosylation are involved in acclimation to salinity stress. In addition, this work enabled characterization of potential carbonylated, nitrated and nitrosylated proteins with distinct or overlapping signatures. This work provides a framework to better understand the oxidative and nitrosative priming network in citrus plants subjected to salinity conditions.

Tungstate: is it really a specific nitrate reductase inhibitor in plant nitric oxide research?

Nitrate reductase (NR) is an enzymatic source of nitric oxide (NO) in plants, and it needs Mo for the Mo-cofactor to be activated. Because NR-deficient mutants are not always available in some species, a cheap and simple pharmacological application of tungstate, which substitutes for Mo in the Mo-cofactor as a competitive antagonist, is widely used as a NR inhibitor in plant NO research. However, evidence indicates that tungstate not only inactivates NR but also inhibits other molybdate-dependent enzymes in plants. In addition, a number of investigations have shown that tungstate also inhibits root growth, affects cortical microtubule formation, and induces programmed cell death (PCD) in plants, just like other heavy metals do. Therefore, tungstate has been shown to exert many other effects that are not connected with the inhibition of NR activity. The origin and mechanism of using tungstate as a NR inhibitor in plants is reviewed here and the progress regarding tungstate toxicity to plants and the possible problems involved in using tungstate as a NR inhibitor in plant NO research are analysed. In summary, the use of tungstate as a NR inhibitor in plant NO research must be treated with caution, keeping in mind that it is not completely specific. It is necessary to search for more NR-deficient mutants and new, specific NR inhibitors. A combination of pharmacological and biochemical analysis with a genetic approach will be necessary in order to investigate the roles of NO in plants.

Nitric oxide enhances desiccation tolerance of recalcitrant Antiaris toxicaria seeds via protein S-nitrosylation and carbonylation

The viability of recalcitrant seeds is lost following stress from either drying or freezing. Reactive oxygen species (ROS) resulting from uncontrolled metabolic activity are likely responsible for seed sensitivity to drying. Nitric oxide (NO) and the ascorbate-glutathione cycle can be used for the detoxification of ROS, but their roles in the seed response to desiccation remain poorly understood. Here, we report that desiccation induces rapid accumulation of H₂O₂, which blocks recalcitrant Antiaris toxicaria seed germination; however, pretreatment with NO increases the activity of antioxidant ascorbate-glutathione pathway enzymes and metabolites, diminishes H₂O₂ production and assuages the inhibitory effects of desiccation on seed germination. Desiccation increases the protein carbonylation levels and reduces protein S-nitrosylation of these antioxidant enzymes; these effects can be reversed with NO treatment. Antioxidant protein S-nitrosylation levels can be further increased by the application of S-nitrosoglutathione reductase inhibitors, which further enhances NO-induced seed germination rates after desiccation and reduces desiccation-induced H₂O₂ accumulation. These findings suggest that NO reinforces recalcitrant seed desiccation tolerance by regulating antioxidant enzyme activities to stabilize H₂O₂ accumulation at an appropriate concentration. During this process, protein carbonylation and S-nitrosylation patterns are used as a specific molecular switch to control antioxidant enzyme activities.