[24] Nitrate reductase from higher plants

Associative Bacteria and Arbuscular Mycorrhizal Fungus Increase Drought Tolerance in Maize (Zea mays L.) through Morphoanatomical, Physiological, and Biochemical Changes

Water deficiency has been recognized as a major abiotic stress that causes losses in maize crops around the world. The maize crop is very important due to the range of products that are derived from this plant. A potential way to reduce the damages caused by water deficiency in maize crops is through the association with plant growth-promoting bacteria (PGPB) and arbuscular mycorrhizal fungi (AMF). To define the mechanisms developed by associative PGPB and AMF in maize that are involved in protection against moderate drought (MD), this study evaluated the biometrical, anatomical, biochemical, and physiological parameters of maize grown under MD and inoculated with different PGPB (Azospirillum brasilense strain Ab-V5 and Bacillus sp. strain ZK) and with AMF. The relative water content did not change in the treatments. The association with ZK increased the shoot:total ratio, total dry weight, maximum quantum yield of photosystem II, vascular cylinder thickness, and vascular cylinder area. The Ab-V5 inoculation led to an increment in root dry weight, the area of metaxylem vessel elements, and nitrate reductase activity. The AMF association did not lead to changes in the measured parameters. The results indicate that the association with PGPB is a relevant alternative to contribute to reducing losses in maize crops under drought. However, AMF is not indicated for this crop under drought.

Exogenous Putrescine Modulates Nitrate Reductase-Dependent NO Production in Cucumber Seedlings Subjected to Salt Stress

Polyamines (PAs) are small aliphatic compounds that participate in the plant response to abiotic stresses. They also participate in nitric oxide (NO) production in plants; however, their role in this process remains unknown. Therefore, the study aimed to investigate the role of putrescine (Put) in NO production in the roots of cucumber seedlings subjected to salt stress (120 mM NaCl) for 1 and 24 h. In salinity, exogenous Put can regulate NO levels by managing NO biosynthesis pathways in a time-dependent manner. In cucumber roots exposed to 1 h of salinity, exogenous Put reduced NO level by decreasing nitrate reductase (NR)-dependent NO production and reduced nitric oxide synthase-like (NOS-like) activity. In contrast, during a 24 h salinity exposure, Put treatment boosted NO levels, counteracting the inhibitory effect of salinity on the NR and plasma membrane nitrate reductase (PM-NR) activity in cucumber roots. The role of endogenous Put in salt-induced NO generation was confirmed using Put biosynthesis inhibitors. Furthermore, the application of Put can modulate the NR activity at the genetic and post-translational levels. After 1 h of salt stress, exogenous Put upregulated CsNR1 and CsNR2 expression and downregulated CsNR3 expression. Put also decreased the NR activation state, indicating a reduction in the level of active dephosphorylated NR (dpNR) in the total enzyme pool. Conversely, in the roots of plants subjected to 24 h of salinity, exogenous Put enhanced the NR activation state, indicating an enhancement of the dpNR form in the total NR pool. These changes were accompanied by a modification of endogenous PA content. Application of exogenous Put led to an increase in the amount of Put in the roots and reduced endogenous spermine (Spm) content in cucumber roots under 24 h salinity. The regulatory role of exogenous Put on NO biosynthesis pathways may link with plant mechanisms of response to salt stress.

Foliar melatonin ameliorates drought-induced alterations in enzyme activities of sugar and nitrogen metabolisms in cotton leaves

Sugar and nitrogen metabolisms help plants maintain cellular homeostasis, stress tolerance, and sustainable growth in drought conditions. Melatonin, a potent antioxidant and signaling molecule, appears to mitigate the negative impacts of drought on plants. This study aimed to investigate the potential role of foliar-applied melatonin in ameliorating drought-induced alterations in leaf sugar and nitrogen metabolisms' enzyme activities during cotton flowering and boll formation. To date, no study has examined drought-induced sugar and nitrogen metabolisms' enzyme activity changes in cotton treated with foliar melatonin. Drought levels (FC1 = 75 ± 5%, FC2 = 60 ± 5%, and FC3 = 45 ± 5%) were maintained between 3 and 35 days after flowering (DAF), and melatonin (M) concentrations (0, 25, 50, and 100 μmol L-1 ) were applied at 3 and 21 DAF in a completely randomized design. M100 concentrations at low FC levels significantly enhanced leaf sugar and N-metabolic enzyme activities, such as sucrose synthase (65.56%) and glutamine synthetase (55.24%), compared to plants not treated with melatonin; peaking between 7 and 21 DAF and declining gradually with crop growth. Moreover, the M100 concentrations at all FC levels, particularly FC3, significantly increased the relative expression of GhSusB, GhSusC, SPS1, and SPS3 genes, indicating that melatonin improves leaf sugar and N-metabolism enzymatic activities under drought stress. Therefore, applying M100 concentrations to cotton foliage under FC3 conditions during reproductive stages improves leaf water status, sugar, and N-metabolism enzyme activities, demonstrating melatonin's potent anti-stress, osmoregulatory, and growth-promoting properties in overcoming drought stress in cotton crops. Future research into the molecular mechanisms of melatonin-mediated sugar and nitrogen metabolism enzyme activities in cotton leaves may lead to biotechnological methods to improve drought resilience in cotton and other crops.

Methyl jasmonate influences ethylene formation, defense systems, nutrient homeostasis and carbohydrate metabolism to alleviate arsenic-induced stress in rice (Oryza sativa)

The plant growth regulator, jasmonic acid (JA) has emerged as important molecule and involved in key processes of plants. In this study, we investigated the role of methyl jasmonate (MeJA) in achieving tolerance mechanisms against arsenic (As) stress in rice (Oryza sativa). Arsenic toxicity is a major global concern that significantly deteriorate rice production. The application of MeJA (20 μM) and ethylene (150 μL L-1) both individually and/or in combination were found significant in protecting against As-induced toxicity in rice, and significantly improved defense systems. The study shown that the positive influence of MeJA in promoting carbohydrate metabolism, photosynthesis and growth under As stress were the result of its interplay with ethylene biosynthesis and reduced oxidative stress-mediated cellular injuries and cell deaths. Interestingly, the use of JA biosynthesis inhibitor, neomycin (Neo) and ethylene biosynthesis inhibitor, aminoethoxyvinylglycine (AVG) overturned the effects of MeJA and ethylene on plant growth under As stress. From the pooled data, it may also be concluded that Neo treatment to MeJA- treated rice plants restricted JA-mediated responses, implying that application of MeJA modulated ethylene- dependent pathways in response to As stress. Thus, the action of MeJA in As tolerance is found to be mediated by ethylene. The study will shed light on the mechanisms that could be used to ensure the sustainability of rice plants under As stress.

Improving yield and quality of rice under acid rain stress by regulating nitrogen assimilation with exogenous Ca2

Acid rain threatens crop yield and nutritional quality, and Ca²⁺ can regulate plant responses to abiotic stresses. To improve the yield and nutritional quality of crops under acid rain stress, we applied exogenous Ca²⁺ to regulate nitrogen assimilation in rice seedlings under simulated acid rain stress (pH 4.5 or 3.0), taking yield and nutritional quality of rice as evaluation criteria. We found that Ca²⁺ (5 mM) maintained the total nitrogen content of rice at the seedling and booting stages to alleviate the inhibitory effect of simulated acid rain on rice yield. Meanwhile, Ca²⁺ improved the activity of glutamate synthase to eliminate the disruption of glutamine synthetase/glutamate synthase balance under simulated acid rain. It decreased the efficiency of nitrogen assimilation, thereby reducing the inhibition of essential amino acid content in rice. The mitigation effect on simulated acid rain at pH 4.5 was better than that of simulated acid rain at pH 3.0. Overall, Ca²⁺ may reduce the negative effect of acid rain on the yield and nutritional quality of crops.

Molybdenum Foliar Fertilization Improves Photosynthetic Metabolism and Grain Yields of Field-Grown Soybean and Maize

Foliar fertilization has been used as a supplemental strategy to plant nutrition especially in crops with high yield potential. Applying nutrients in small doses stimulates photosynthesis and increases yield performance. The aim of this study was to evaluate the efficiency of foliar application of molybdenum (Mo) to soybean and maize. The treatments consisted of the presence (+Mo) and absence (-Mo) of supplementation. Plant nutritional status, nitrate reductase (NR) activity, gas exchange parameters, photosynthetic enzyme activity (Rubisco in soybean and maize and PEPcase in maize), total soluble sugar concentration, leaf protein content, shoot dry matter, shoot nitrogen accumulated, number of grains per plant, mass of 100 grains, and grain yield were evaluated. For soybean and maize, application of Mo increased leaf NR activity, nitrogen and protein content, Rubisco activity, net photosynthesis, and grain yield. These results indicate that foliar fertilization with Mo can efficiently enhance nitrogen metabolism and the plant’s response to carbon fixation, resulting in improved crop yields.

Exogenous Melatonin Modulates Physiological Response to Nitrogen and Improves Yield in Nitrogen-Deficient Soybean (Glycine max L. Merr.)

Melatonin (MT) is a key plant growth regulator. To investigate its effect at different growth stages on the yield of soybean under nitrogen deficiency, 100 μM MT was applied to soybean supplemented with zero nitrogen (0N), low nitrogen (LN), and control nitrogen (CK) levels, during the plant vegetative growth (V3) and filling (R5) stages. This study revealed that the application of MT mainly enhanced the nitrogen fixation of plants by increasing the root nodule number and provided more substrates for glutamine synthetase (GS) under 0N supply. However, under the LN supply, more ammonium was assimilated through the direct promotion of nitrate reductase (NR) activity by MT. MT enhanced the activity of ammonium-assimilation-related enzymes, such as GOGAT and GDH, and the expression of their coding genes, promoted the synthesis of chlorophyll and amino acids, and increased the photosynthetic capacity under nitrogen deficiency. Exogenous MT directly upregulated the expression of genes involved in the photosynthetic system and stimulated dry-matter accumulation. Thus, MT alleviated the inhibitory effect of nitrogen deficiency on soybean yield. This mitigation effect was better when MT was applied at the V3 stage, and the seed weight per plant increased by 16.69 and 12.20% at 0N and LN levels, respectively. The results of this study provide a new theoretical basis to apply MT in agriculture to improve the resilience of soybean plants to low nitrogen availability.

Potential Importance of Molybdenum Priming to Metabolism and Nutritive Value of Canavalia spp. Sprouts

Molybdenum ions (Mo) can improve plants' nutritional value primarily by enhancing nitrogenous metabolism. In this study, the comparative effects of seed priming using Mo were evaluated among sproutings of Canavalia species/cultivars, including Canavalia ensiformis var. gladiata (CA1), Canavalia ensiformis var. truncata Ricker (CA2), and Canavalia gladiata var. alba Hisauc (CA3). Mo impacts on growth, metabolism (e.g., nitrogen and phenolic metabolism, pigment and total nutrient profiles), and biological activities were assayed. Principal component analysis (PCA) was used to correlate Mo-mediated impacts. The results showed that Mo induced photosynthetic pigments that resulted in an improvement in growth and increased biomass. The N content was increased 0.3-fold in CA3 and 0.2-fold in CA1 and CA2. Enhanced nitrogen metabolism by Mo provided the precursors for amino acids, protein, and lipid biosynthesis. At the secondary metabolic level, phenolic metabolism-related precursors and enzyme activities were also differentially increased in Canavalia species/cultivars. The observed increase in metabolism resulted in the enhancement of the antioxidant (2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH) free radical scavenging, 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), ferric reducing antioxidant power (FRAP)) and antidiabetic potential (Glycemic index (GI) and inhibition activity of α-amylase, and α-glucosidase) of species. The antioxidant activity increased 20% in CA3, 14% in CA1, and 8% in CA2. Furthermore, PCA showed significant variations not only between Mo-treated and untreated samples but also among Canavalia species. Overall, this study indicated that the sprouts of Canavalia species have tremendous potential for commercial usage due to their high nutritive value, which can be enhanced further with Mo treatment to accomplish the demand for nutritious feed.

The mechanisms underlying melatonin improved soybean seedling growth at different nitrogen levels

To investigate the function of melatonin (MT) on nitrogen uptake and metabolism in soybean, six groups of treatments, with and without 100μM melatonin were conducted at low, normal, and high nitrogen levels (1.5, 7.5, and 15mM, respectively). The related indexes of nitrogen metabolism and the antioxidant system of seedlings were measured and analysed. Results indicated that MT could enhance the level of nitrogen metabolism by upregulating the coding genes of enzymes related to nitrogen metabolism and increasing total nitrogen content, especially under low nitrogen levels. Under high nitrogen conditions, the addition of MT not only accelerated ammonium assimilation and utilisation by enhancing the activity of glutamine synthetase involved in ammonium assimilation, but also reduced the extent of membrane lipid peroxidation to alleviate the degree of damage by improving the activity of antioxidant enzymes. In addition, MT enhanced soybean growth with positive effects in morphological changes at different nitrogen levels, including significantly increased stem diameter, total leaf area, and root nodule number, and biomass accumulation. Finally, biomass accumulation increased under low, normal, and high nitrogen levels by 9.80%, 14.06%, and 11.44%, respectively. The results suggested that MT could enhance the soybean tolerance to low and excessive N treatments.

Elucidating ROS signaling networks and physiological changes involved in nanoscale zero valent iron primed rice seed germination sensu stricto

Reactive oxygen species (ROS) play pivotal roles during seed dormancy and germination. Metabolically active cells of seeds generate ROS and successful germination is governed by internal ROS contents, maintained within an optimum "oxidative window" by several ROS scavengers. Although ROS was previously considered hazardous, optimum ROS generation in seeds can mediate early seed germination by acting as messengers for cell signaling involved in endosperm weakening, stored food mobilization, etc. Recent reports suggest that nanopriming can expedite seed germination rates and enhance seed quality and crop performances. However, nanoparticle-driven signal cascades involved during seed germination are still unknown. The present study is aimed to explore molecular mechanisms for promoting germination in nanoprimed seeds and to investigate the plausible role of nanoparticle-mediated ROS generation in this process. Here rice seeds were primed with 20 mg L ^-1nanoscale zero valent iron (nZVI) for 72 h and several biochemical and physiological changes were monitored at different time points (5, 10, 20, 40, 60, and 80 h). To gain insight into roles of ROS in germination rate enhancement, intercellular ROS inhibitor, diphenyleneiodonium (DPI) was taken as another priming agent. Seed priming with DPI impaired seed germination percentage, hydrolytic enzyme activities due to ROS imbalance. On the contrary, seeds primed with both DPI and nZVI could recover from deleterious consequences of DPI treatment. Although DPI impaired intercellular ROS generation, nZVI can generate ROS independently which was confirmed from ROS localization assay. In both nZVI and the DPI and nZVI co-primed sets, significant up-regulation in genes like OsGA3Ox2, OsGAMYB were observed which are responsible for regulating the activity of several hydrolases and mediates efficient mobilization of storage food reserves of seeds. Thus, nZVI priming has potential to regulate intracellular ROS levels and orchestrate all the metabolic activities which eventually up-regulates seed germination rate and seed vigour.

Sulfur Regulates the Trade-Off Between Growth and Andrographolide Accumulation via Nitrogen Metabolism in Andrographis paniculata

Nitrogen (N) and sulfur (S) are essential mineral nutrients for plant growth and metabolism. Here, we investigated their interaction in plant growth and andrographolide accumulation in medicinal plant Andrographis paniculata grown at different N (4 and 8 mmol·L^-1) and S concentration levels (0.1 and 2.4 mmol L^-1). We found that increasing the S application rate enhanced the accumulation of andrographolide compounds (AGCs) in A. paniculata. Simultaneously, salicylic acid (SA) and gibberellic acid 4 (GA₄) concentrations were increased but trehalose/trehalose 6-phosphate (Tre/Tre6P) concentrations were decreased by high S, suggesting that they were involved in the S-mediated accumulation of AGCs. However, S affected plant growth differentially at different N levels. Metabolite analysis revealed that high S induced increases in the tricarboxylic acid (TCA) cycle and photorespiration under low N conditions, which promoted N assimilation and S metabolism, and simultaneously increased carbohydrate consumption and inhibited plant growth. In contrast, high S reduced N and S concentrations in plants and promoted plant growth under high N conditions. Taken together, the results indicated that increasing the S application rate is an effective strategy to improve AGC accumulation in A. paniculata. Nevertheless, the interaction of N and S affected the trade-off between plant growth and AGC accumulation, in which N metabolism plays a key role.

Enhanced Nitric Oxide Synthesis Through Nitrate Supply Improves Drought Tolerance of Sugarcane Plants

Nitric oxide (NO) is an important signaling molecule associated with many biochemical and physiological processes in plants under stressful conditions. Nitrate reductase (NR) not only mediates the reduction of NO3 - to NO2 - but also reduces NO2 - to NO, a relevant pathway for NO production in higher plants. Herein, we hypothesized that sugarcane plants supplied with more NO3 - as a source of N would produce more NO under water deficit. Such NO would reduce oxidative damage and favor photosynthetic metabolism and growth under water limiting conditions. Sugarcane plants were grown in nutrient solution and received the same amount of nitrogen, with varying nitrate:ammonium ratios (100:0 and 70:30). Plants were then grown under well-watered or water deficit conditions. Under water deficit, plants exhibited higher root [NO3 -] and [NO2 -] when supplied with 100% NO3 -. Accordingly, the same plants also showed higher root NR activity and root NO production. We also found higher photosynthetic rates and stomatal conductance in plants supplied with more NO3 -, which was associated with increased root growth. ROS accumulation was reduced due to increases in the activity of catalase in leaves and superoxide dismutase and ascorbate peroxidase in roots of plants supplied with 100% NO3 - and facing water deficit. Such positive responses to water deficit were offset when a NO scavenger was supplied to the plants, thus confirming that increases in leaf gas exchange and plant growth were induced by NO. Concluding, NO3 - supply is an interesting strategy for alleviating the negative effects of water deficit on sugarcane plants, increasing drought tolerance through enhanced NO production. Our data also provide insights on how plant nutrition could improve crop tolerance against abiotic stresses, such as drought.

Proteasome Inhibition in Brassica napus Roots Increases Amino Acid Synthesis to Offset Reduced Proteolysis

Cellular homeostasis is maintained by the proteasomal degradation of regulatory and misfolded proteins, which sustains the amino acid pool. Although proteasomes alleviate stress by removing damaged proteins, mounting evidence indicates that severe stress caused by salt, metal(oids), and some pathogens can impair the proteasome. However, the consequences of proteasome inhibition in plants are not well understood and even less is known about how its malfunctioning alters metabolic activities. Lethality causes by proteasome inhibition in non-photosynthetic organisms stem from amino acid depletion, and we hypothesized that plants respond to proteasome inhibition by increasing amino acid biosynthesis. To address these questions, the short-term effects of proteasome inhibition were monitored for 3, 8 and 48 h in the roots of Brassica napus treated with the proteasome inhibitor MG132. Proteasome inhibition did not affect the pool of free amino acids after 48 h, which was attributed to elevated de novo amino acid synthesis; these observations coincided with increased levels of sulfite reductase and nitrate reductase activities at earlier time points. However, elevated amino acid synthesis failed to fully restore protein synthesis. In addition, transcriptome analysis points to perturbed abscisic acid signaling and decreased sugar metabolism after 8 h of proteasome inhibition. Proteasome inhibition increased the levels of alternative oxidase but decreased aconitase activity, most sugars and tricarboxylic acid metabolites in root tissue after 48 h. These metabolic responses occurred before we observed an accumulation of reactive oxygen species. We discuss how the metabolic response to proteasome inhibition and abiotic stress partially overlap in plants.

Influence of nitrate - ammonium ratio on the growth, nutrition, and metabolism of sugarcane

Although ammonium (NH₄⁺) has been claimed as the preferential N source for sugarcane (Saccharum spp.), the intense uptake of this mineral form by plants can impair metabolic processes and crop yield. We aimed to assess the growth, nutrition, and metabolic responses of sugarcane grown under different amounts of nitrate (NO₃^-) and NH₄⁺. Sugarcane setts were grown in nutrient solution at a total concentration of 15 mM N using different NO₃^-/NH₄⁺ ratios (100/0, 75/25, 50/50, 25/75, and 0/100, respectively) for 163 d under controlled conditions. The pH of the medium was daily adjusted to 5.8 ± 0.1, with replacement of the hydroponic solution every 10 d. NH₄⁺-only fed plants showed lower dry biomass yield, nutrient content, leaf surface area, and leaf gas exchange than those under sole NO₃^- supply, in addition to favoring the development of brown rust (Puccinia melanocephala). However, there was no indication that NH₄⁺ is directly related to oxidative stress in sugarcane. On the other hand, the highest N utilization efficiency was obtained with NO₃^--only fed plants, which also resulted in the highest biomass yield, leaf surface area, nutrient content, leaf gas exchange, and root growth. Since NO₃^- was not stored in plant tissues, we therefore suggested that most of this N form is assimilated following its uptake. Despite the well-known preference of the crop for NH₄⁺, the optimal growth response of sugarcane plants to NO₃^-/NH₄⁺ ratios was observed under NO₃^- supply.

Growth, physiology, and phytoextraction potential of poplar and willow established in soils amended with heavy-metal contaminated, dredged river sediments

Phytotechnologies have been used worldwide to remediate and restore damaged ecosystems, especially those caused by industrial byproducts leaching into rivers and other waterways. The objective of this study was to test the growth, physiology, and phytoextraction potential of poplar and willow established in soils amended with heavy-metal contaminated, dredged river sediments from the Great Bačka Canal near Vrbas City, Serbia. The sediments were applied to greenhouse-grown trees of Populus deltoides Bartr. ex Marsh. clone 'Bora' and Salix viminalis L. clone 'SV068'. Individual pots with trees previously grown for two months were amended with 0, 0.5 and 1.0 kg of sediment containing 400 mg Cr kg^-1, 295 mg Cu kg^-1, 465 mg Zn kg^-1, 124 mg Ni kg^-1, 1.87 mg Cd kg^-1, and 61 mg Pb kg^-1. Following amendment, trees were grown for two seasons (i.e., 2014, 2015), with coppicing after the first season. In addition to growth parameters, physiological traits related to the photosynthesis and nitrogen metabolism were assessed during both growing seasons. At the end of the study, trees were harvested for biomass analysis and accumulation of heavy metals in tree tissues and soils. Application of sediment decreased aboveground biomass by 37.3% in 2014, but increased height (16.4%) and leaf area (19.2%) in 2015. Sediment application negatively impacted the content of pigments and nitrate reductase activity, causing them to decrease over time. Generally, the effect of treatments on growth was more pronounced in poplars, while willows had more pronounced physiological activity. Accumulation patterns were similar to previously-published results. In particular, Zn and Cd were mostly accumulated in leaves of both poplar and willow, which indicated successful phytoextraction. In contrast, other metals (e.g., Cr, Ni, Pb, Cu) were mostly phytostabilized in the roots. Differences in metal allocation between poplar and willow were recorded only for Cu, while other metals followed similar distribution patterns in both genera. Results of this study indicated that the composition of heavy metals in the sediments determined the mechanisms of the applied phytoremediation technique.

Associative bacteria influence maize (Zea mays L.) growth, physiology and root anatomy under different nitrogen levels

Despite the great diversity of plant growth-promoting bacteria (PGPB) with potential to partially replace the use of N fertilisers in agriculture, few PGPB have been explored for the production of commercial inoculants, reinforcing the importance of identifying positive plant-bacteria interactions. Aiming to better understand the influence of PGPB inoculation in plant development, two PGPB species with distant phylogenetic relationship were inoculated in maize. Maize seeds were inoculated with Bacillus sp. or Azospirillum brasilense. After germination, the plants were subjected to two N treatments: full (N+) and limiting (N-) N supply. Then, anatomical, biometric and physiological analyses were performed. Both PGPB species modified the anatomical pattern of roots, as verified by the higher metaxylem vessel element (MVE) number. Bacillus sp. also increased the MVE area in maize roots. Under N+ conditions, both PGPB decreased leaf protein content and led to development of shorter roots; however, Bacillus sp. increased root and shoot dry weight, whereas A. brasilense increased photosynthesis rate and leaf nitrate content. In plants subjected to N limitation (N-), photosynthesis rate and photosystem II efficiency increased in maize inoculated with Bacillus sp., whilst A. brasilense contained higher ammonium, amino acids and total soluble sugars in leaves, compared to the control. Plant developmental and metabolical patterns were switched by the inoculation, regardless of the inoculant bacterium used, producing similar as well as distinct modifications to the parameters studied. These results indicate that even non-diazotrophic inoculant strains can improve the plant N status as result of the morpho-anatomical and physiological modifications produced by the PGPB.

RAPTOR Controls Developmental Growth Transitions by Altering the Hormonal and Metabolic Balance

Vegetative growth requires the systemic coordination of numerous cellular processes, which are controlled by regulatory proteins that monitor extracellular and intracellular cues and translate them into growth decisions. In eukaryotes, one of the central factors regulating growth is the serine/threonine protein kinase Target of Rapamycin (TOR), which forms complexes with regulatory proteins. To understand the function of one such regulatory protein, Regulatory-Associated Protein of TOR 1B (RAPTOR1B), in plants, we analyzed the effect of raptor1b mutations on growth and physiology in Arabidopsis (Arabidopsis thaliana) by detailed phenotyping, metabolomic, lipidomic, and proteomic analyses. Mutation of RAPTOR1B resulted in a strong reduction of TOR kinase activity, leading to massive changes in central carbon and nitrogen metabolism, accumulation of excess starch, and induction of autophagy. These shifts led to a significant reduction of plant growth that occurred nonlinearly during developmental stage transitions. This phenotype was accompanied by changes in cell morphology and tissue anatomy. In contrast to previous studies in rice (Oryza sativa), we found that the Arabidopsis raptor1b mutation did not affect chloroplast development or photosynthetic electron transport efficiency; however, it resulted in decreased CO2 assimilation rate and increased stomatal conductance. The raptor1b mutants also had reduced abscisic acid levels. Surprisingly, abscisic acid feeding experiments resulted in partial complementation of the growth phenotypes, indicating the tight interaction between TOR function and hormone synthesis and signaling in plants.

Diminution of arsenic accumulation in rice seedlings co-cultured with Anabaena sp.: Modulation in the expression of lower silicon transporters, two nitrogen dependent genes and lowering of antioxidants activity

The present study was intended to investigate the role of algae, Anabaena sp. in the amelioration of As toxicity, when co-cultured with rice seedlings. The reduction of growth in rice seedlings against As(III) and As(V) was recovered with Anabaena sp. The Anabaena sp. also reduced the accumulation of As, where it was more efficient against 60µM As(III) (49%) than As(V) (23%) in rice shoot. Similarly, with reduction of As accumulation, lower silicon transporters (Lsi-1 and Lsi-2) was found to be suppressed against As treatments. However, the expression of two nitrogen dependent genes i.e., NR and SAMT were found to be enhanced with the Anabaena sp. Likewise, the activity of antioxidant enzyme, GST, was enhanced, whereas, the activity of other enzymes such as SOD, APX, GPX, GR and DHAR were decreased with As+Algae combinations. Overall, the result suggested that the Anabaena sp. reduces As accumulation, modulates gene expressions and antioxidants to ameliorate the As toxicity in Oryza sativa L.

Nitrogen metabolism correlates with the acclimation of photosynthesis to short-term water stress in rice (Oryza sativa L.)

Nitrogen metabolism is as sensitive to water stress as photosynthesis, but its role in plant under soil drying is not well understood. We hypothesized that the alterations in N metabolism could be related to the acclimation of photosynthesis to water stress. The features of photosynthesis and N metabolism in a japonica rice 'Jiayou 5' and an indica rice 'Zhongzheyou 1' were investigated under mild and moderate soil drying with a pot experiment. Soil drying increased non-photochemical quenching (NPQ) and reduced photon quantum efficiency of PSII and CO₂ fixation in 'Zhongzheyou 1', whereas the effect was much slighter in 'Jiayou 5'. Nevertheless, the photosynthetic rate of the two cultivars showed no significant difference between control and water stress. Soil drying increased nitrate reducing in leaves of 'Zhongzheyou 1', characterized by enhanced nitrate reductase (NR) activity and lowered nitrate content; whereas glutamate dehydrogenase (GDH), glutamic-oxaloacetic transaminase (GOT) and glutamic-pyruvic transaminase (GPT) were relative slightly affected. 'Jiayou 5' plants increased the accumulation of nitrate under soil drying, although its NR activity was increased. In addition, the activities of GDH, GOT and GPT were typically increased under soil drying. Besides, amino acids and soluble sugar were significantly increased under mild and moderate soil drying, respectively. The accumulation of nitrate, amino acid and sugar could serve as osmotica in 'Jiayou 5'. The results reveal that N metabolism plays diverse roles in the photosynthetic acclimation of rice plants to soil drying.

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

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

A protective role for nitric oxide and salicylic acid for arsenite phytotoxicity in rice (Oryza sativa L.)

Nitric oxide (NO) and salicylic acid (SA) are important signaling molecules in plant system. In the present study both NO and SA showed a protective role against arsenite (As^III) stress in rice plants when supplied exogenously. The application of NO and SA alleviated the negative impact of As^III on plant growth. Nitric oxide supplementation to As^III treated plants greatly decreased arsenic (As) accumulation in the roots as well as shoots/roots translocation factor. Arsenite exposure in plants decreased the endogenous levels of NO and SA. Exogenous supplementation of SA not only enhanced endogenous level of SA but also the level of NO through enhanced nitrate reductase (NR) activity, whether As^III was present or not. Exogenously supplied NO decreased the NR activity and level of endogenous NO. Arsenic accumulation was positively correlated with the expression level of OsLsi1, a transporter responsible for As^III uptake. The endogenous level of NO and SA were positively correlated to each other either when As^III was present or not. This close relationship indicates that NO and SA work in harmony to modulate the signaling response in As^III stressed plants.

Nitric Oxide Alleviated Arsenic Toxicity by Modulation of Antioxidants and Thiol Metabolism in Rice (Oryza sativa L.)

Nitric oxide (NO) is a gaseous signaling molecule and has a profound impact on plant growth and development. It is reported to serve as pro oxidant as well as antioxidant in plant system. In the present study, we evaluated the protective role of NO against arsenate (As(V)) toxicity in rice plants. As(V) exposure has hampered the plant growth, reduced the chlorophyll content, and enhanced the oxidative stress, while the exogenous NO supplementation has reverted these symptoms. NO supplementation has reduced the arsenic (As) accumulation in root as well as shoot. NO supplementation to As(V) exposed plants has reduced the gene expression level of OsLsi1 and OsLsi2. As(V) stress significantly impacted thiol metabolism, it reduced GSH content and GSH/GSSG ratio, and enhanced the level of PCs. NO supplementation maintained the GSH/GSSG ratio and reduced the level of PCs. NO supplementation reverted As(V) induced iron deficiency in shoot and had significant impact of gene expression level of various iron transporters (OsYSL2, OsFRDL1, OsIRT1, and OsIRO2). Conclusively, exogenous application of NO could be advantageous against As(V) toxicity and could confer the tolerance to As(V) stress in rice.

Sulfate resupply accentuates protein synthesis in coordination with nitrogen metabolism in sulfur deprived Brassica napus

To investigate the regulatory interactions between S assimilation and N metabolism in Brassica napus, de novo synthesis of amino acids and proteins was quantified by (15)N and (34)S tracing, and the responses of transporter genes, assimilatory enzymes and metabolites pool involving in nitrate and sulfate metabolism were assessed under continuous sulfur supply, sulfur deprivation and sulfate resupply after 3 days of sulfur (S) deprivation. S-deprived plants were characterized by a strong induction of sulfate transporter genes, ATP sulfurylase (ATPS) and adenosine 5'-phosphosulfate reductase (APR), and by a repressed activity of nitrate reductase (NR) and glutamine synthetase (GS). Sulfate resupply to the S-deprived plants strongly increased cysteine, amino acids and proteins concentration. The increase in sulfate and cysteine concentration caused by sulfate resupply was not matched with the expression of sulfate transporters and the activity of ATPS and APR which were rapidly decreased by sulfate resupply. A strong induction of O-acetylserine(thiol)lyase (OASTL), NR and GS upon sulfate resupply was accompanied with the increase in cysteine, amino acids and proteins pool. Sulfate resupply resulted in a strong increase in de novo synthesis of amino acids and proteins, as evidenced by the increases in N and S incorporation into amino acids (1.8- and 2.4-fold increase) and proteins (2.2-and 6.3-fold increase) when compared to S-deprived plants. The results thus indicate that sulfate resupply followed by S-deprivation accelerates nitrate assimilation for protein synthesis.

Salicylic acid modulates arsenic toxicity by reducing its root to shoot translocation in rice (Oryza sativa L.)

Arsenic (As) is posing serious health concerns in South East Asia where rice, an efficient accumulator of As, is prominent crop. Salicylic acid (SA) is an important signaling molecule and plays a crucial role in resistance against biotic and abiotic stress in plants. In present study, ameliorative effect of SA against arsenate (As(V)) toxicity has been investigated in rice (Oryza sativa L.). Arsenate stress hampered the plant growth in terms of root, shoots length, and biomass as well as it enhanced the level of H2O2 and MDA in dose dependent manner in shoot. Exogenous application of SA, reverted the growth, and oxidative stress caused by As(V) and significantly decreased As translocation to the shoots. Level of As in shoot was positively correlated with the expression of OsLsi2, efflux transporter responsible for root to shoot translocation of As in the form of arsenite (As(III)). SA also overcame As(V) induced oxidative stress and modulated the activities of antioxidant enzymes in a differential manner in shoots. As treatment hampered the translocation of Fe in the shoot which was compensated by the SA treatment. The level of Fe in root and shoot was positively correlated with the transcript level of transporters responsible for the accumulation of Fe, OsNRAMP5, and OsFRDL1, in the root and shoot, respectively. Co-application of SA was more effective than pre-treatment for reducing As accumulation as well as imposed toxicity.

2'-Deoxymugineic acid promotes growth of rice (Oryza sativa L.) by orchestrating iron and nitrate uptake processes under high pH conditions

Poaceae plants release 2'-deoxymugineic acid (DMA) and related phytosiderophores to chelate iron (Fe), which often exists as insoluble Fe(III) in the rhizosphere, especially under high pH conditions. Although the molecular mechanisms behind the biosynthesis and secretion of DMA have been studied extensively, little information is known about whether DMA has biological roles other than chelating Fe in vivo. Here, we demonstrate that hydroponic cultures of rice (Oryza sativa) seedlings show almost complete restoration in shoot height and soil-plant analysis development (SPAD) values after treatment with 3-30 μm DMA at high pH (pH 8.0), compared with untreated control seedlings at normal pH (pH 5.8). These changes were accompanied by selective accumulation of Fe over other metals. While this enhanced growth was evident under high pH conditions, DMA application also enhanced seedling growth under normal pH conditions in which Fe was fairly accessible. Microarray and qRT-PCR analyses revealed that exogenous DMA application attenuated the increased expression levels of various genes related to Fe transport and accumulation. Surprisingly, despite the preferential utilization of ammonium over nitrate as a nitrogen source by rice, DMA application also increased nitrate reductase activity and the expression of genes encoding high-affinity nitrate transporters and nitrate reductases, all of which were otherwise considerably lower under high pH conditions. These data suggest that exogenous DMA not only plays an important role in facilitating the uptake of environmental Fe, but also orchestrates Fe and nitrate assimilation for optimal growth under high pH conditions.

Response of nitrate reductase activity and NIA genes expression in roots of Arabidopsis hxk1 mutant treated with selected carbon and nitrogen metabolites

In plants sugar sensing and signal transduction involves pathways dependent or independent on HXK1 as a glucose sensor. Research was conducted to determine which pathway is responsible for regulation of the nitrate reduction. The effect of selected carbon and nitrogen metabolites on nitrate reductase (NR) activity in Arabidopsis thaliana wild type (WT) and hxk1 mutant roots was studied. Exogenously supplied sugar, sucrose (Suc) and organic acid, 2-oxoglutarate (2-OG) led to an increase in the total and actual activity of NR. It was due to both the increase in expression of NIA genes and NR activation state. The stimulatory effect of Suc and 2-OG on nitrate reduction was less pronounced in hxk1 mutant roots with T-DNA insertion in the AtHXK1 gene encoding hexokinase1 (HXK1) and characterized by reduced hexokinase activity and root level of G6P and F6P. On the other hand, it was shown that exogenous glucose did not mimic Suc-mediated NR activation in Arabidopsis roots. Taken together, this data suggest that the Suc signaling pathway might be independent from hexose's sensor dependent mechanism.

24-epibrassinolide restores nitrogen metabolism of pigeon pea under saline stress

BACKGROUND

Several studies have shown that brassinosteroids attenuate the effects of salt stress. However, nothing is known about their effects on amino acid transport, nor the effects of these hormones on nitrate uptake under saline conditions. This study set out to determine the effects of 24-epibrassinolide, at concentrations of 10-7 M and 0.5 × 10-9 M, and clotrimazole (inhibitor of brassinosteroid synthesis), at 10-4 M, on nitrate uptake and metabolism in plants of C. cajan (L.) Millsp, cultivar C11, growing under salinity. The following aspects were analyzed: levels of proteins, amino acids, nitrate, nitrate reductase of roots and the composition of xylem sap amino acids.

RESULTS

Salinity reduced the proportion of N-transport amino acids ASN (the major component), GLU, ASP and GLN. The effect of the hormone in reducing the adverse effects of salt was related to the reestablishment (totally or partially) of the proportions of GLU, ASN and GLN, transported in the xylem and to the small but significant increase in uptake of nitrate. Increased nitrate uptake, induced by 24- epibrassinolide, was associated with a higher activity of nitrate reductase together with greater levels of free amino acids and soluble proteins in roots of plants cultivated under saline conditions.

CONCLUSION

The decline in several components of nitrogen metabolism, induced by salt, was attenuated by 24-epibrassinolide application and accentuated by clotrimazole, indicating the importance of brassinosteroid synthesis for plants growing under salinity.

Mimosine-inhibited seed germination, seedling growth, and enzymes ofOryza sativa L

The effect of mimosine (50 ppm and 100 ppm concentrations) onOryza sativa (rice) seed germination; root and shoot growth, i.e., length and fresh weight of seedlings; activities of nitrate reductase, peroxidase, catalase, and IAA oxidase were investigated. Significant inhibition in seed germination and shoot length was noted. Root length was inhibited by 100 ppm mimosine; however, the 50 ppm was not significant. Root and shoot fresh weight was not significantly inhibited by the tested concentrations of mimosine. Significant inhibition in activities of nitrate reductase, peroxidase and its isoenzymes, catalase, and IAA oxidase was observed. Ecophysiological implications of mimosine phytotoxicity are discussed.

Regulation of nitrate reduction in Arabidopsis WT and hxk1 mutant under C and N metabolites

As in plants sugar sensing and signal transduction involve pathways dependent or independent on hexokinase 1 (HXK1) as a glucose sensor, research was conducted to determine which pathway is responsible for regulation of the nitrate reduction. An Arabidopsis mutant with T-DNA insertion in the AtHXK1 gene and defects in glucose signaling (hxk1) was used to determine nitrate reductase (NR) activity, NIA genes expression in leaves after 8-h treatment with sugars (glucose and sucrose), organic acids [2-oxoglutarate (2OG)] and amino acids (glutamine and glutamate). Sugars, especially sucrose, caused induction of NR actual activity accompanied by an increase of the NR activation state, indicating the posttranslational nature of the modifications. Those modifications were observed in wild-type (WT) and hxk1 leaves, suggesting that regulation of NR activity by sugars does not involve HXK1 as a glucose sensor. Moreover, sugars enhanced expression of NIA genes. However, a higher level of NIA transcripts did not lead to an increase of total NR activity in sugar-treated plants. This may suggest that posttranslational modification of NR is fundamental regulatory mechanisms controlling NR activity in response to C metabolites. Treatment of plants with 2-OG also modified NR through the posttranslational modifications. Elevation of actual NR activity and the enzyme activation state in WT and hxk1 leaves was observed. Amino acids caused a decrease of NIA gene expression and NR activities in WT and hxk1 leaves indicating that mutation in the hexokinase-dependent glucose signaling pathway did not interrupt the amino acid feedback regulation of NR.

Co-application of selenite and phosphate reduces arsenite uptake in hydroponically grown rice seedlings: toxicity and defence mechanism

The study empirically evaluates the abatement of As(III) uptake in rice seedlings (7d), in presence of Se and phosphate (P) under hydroponic condition. Positive correlation between As(III) translocation to the shoots of As(III) and P treated seedlings, shows P dependent As(III) translocation in rice. Whereas, presence of both P (5 and 10μgml(-1)) and (0.75μgml(-1)) of Se significantly reduces the As(III) uptake in rice seedlings. Application of Se alone also reduces As(III) uptake both in shoots and roots significantly, however, the seedlings suffers from lipid peroxidation. Among all the studied treatments, lower rates of P (5μgml(-1)) and Se (0.75μgml(-1)) when co-applied, significantly reduced As(III) translocation to the shoots without inflicting much toxicity in the seedlings which is manifested as significant increase in biomass with lower thio-barbituric reactive substances (TBARS). Also, significantly lower TBARS in seedlings receiving As(4)+Se(0.75) and higher TBARS in As(4)+Se(1.5), demonstrates that Se applied at lower rates (0.75μgml(-1)), lowers As induced toxicity. Higher SOD, APX and guaiacol peroxidase (POD) activities in As(4)+P(5)+Se(0.75) compared to that of As(4)+P(5) and As(4)+Se(0.75), supports that lower rate of P and Se provides tolerance towards As induced stress. The nitrogen metabolism in As(4)+P+Se treated seedlings is affected adversely at higher rates of Se and P application. Overall study concluded that application of lower rates of P (5μgml(-1)) and Se (0.75μgml(-1)) provides maximum amelioration of As(III) toxicity in rice seedlings.

Effect of oxygen deficiency on nitrogen assimilation and amino acid metabolism of soybean root segments

Plants submitted to O(2) deficiency present a series of biochemical modifications, affecting overall root metabolism. Here, the effect of hypoxia on the metabolic fate of (15)N derived from (15)NO(3)(-), (15)NO(2)(-) and (15)NH(4)(+) in isolated soybean root segments was followed by gas chromatography-mass spectrometry, to provide a detailed analysis of nitrogen assimilation and amino acid biosynthesis under hypoxia. O(2) deficiency decreased the uptake of the nitrogen sources from the solution, as ratified by the lower (15)NO(3)(-) and (15)NH(4)(+) enrichment in the root segments. Moreover, analysis of endogenous NO(2)(-) and (15)NH(4)(+) levels suggested a slower metabolism of these ions under hypoxia. Accordingly, regardless of the nitrogen source, hypoxia reduced total (15)N incorporation into amino acids. Analysis of (15)N enrichment patterns and amino acid levels suggest a redirecting of amino acid metabolism to alanine and γ-aminobutyric acid synthesis under hypoxia and a differential sensitivity of individual amino acid pathways to this stress. Moreover, the role of glutamine synthetase in nitrogen assimilation both under normoxia and hypoxia was ratified. In comparison with (15)NH(4)(+), (15)NO(2)(-) assimilation into amino acids was more strongly affected by hypoxia and NO(2)(-) accumulated in root segments during this stress, indicating that nitrite reductase may be an additional limiting step. NO(2)(-) accumulation was associated with a higher nitric oxide emission. (15)NO(3)(-) led to much lower (15)N incorporation in both O(2) conditions, probably due to the limited nitrate reductase activity of the root segments. Overall, the present work shows that profound alterations of root nitrogen metabolism occur during hypoxic stress.

Involvement of nitrite in the nitrate-mediated modulation of fermentative metabolism and nitric oxide production of soybean roots during hypoxia

It is widely accepted that nitrate but not ammonium improves tolerance of plants to hypoxic stress, although the mechanisms related to this beneficial effect are not well understood. Recently, nitrite derived from nitrate reduction has emerged as the major substrate for the synthesis of nitric oxide (NO), an important signaling molecule in plants. Here, we analyzed the effect of different nitrogen sources (nitrate, nitrite and ammonium) on the metabolic response and NO production of soybean roots under hypoxia. Organic acid analysis showed that root segments isolated from nitrate-cultivated plants presented a lower accumulation of lactate and succinate in response to oxygen deficiency in relation to those from ammonium-cultivated plants. The more pronounced lactate accumulation by root segments of ammonium-grown plants was followed by a higher ethanol release in the medium, evidencing a more intense fermentation under oxygen deficiency than those from nitrate-grown plants. As expected, root segments from nitrate-cultivated plants produced higher amounts of nitrite and NO during hypoxia compared to ammonium cultivation. Exogenous nitrite supplied during hypoxia reduced both ethanol and lactate production and stimulated cyanide-sensitive NO emission by root segments from ammonium-cultivated plants, independent of nitrate. On the other hand, treatments with a NO donor or a NO scavenger did not affect the intensity of fermentation of soybean roots. Overall, these results indicate that nitrite participates in the nitrate-mediated modulation of the fermentative metabolism of soybean roots during oxygen deficiency. The involvement of mitochondrial reduction of nitrite to NO in this mechanism is discussed.

Inhibition of aconitase by nitric oxide leads to induction of the alternative oxidase and to a shift of metabolism towards biosynthesis of amino acids

Nitric oxide (NO) is a free radical molecule involved in signalling and in hypoxic metabolism. This work used the nitrate reductase double mutant of Arabidopsis thaliana (nia) and studied metabolic profiles, aconitase activity, and alternative oxidase (AOX) capacity and expression under normoxia and hypoxia (1% oxygen) in wild-type and nia plants. The roots of nia plants accumulated very little NO as compared to wild-type plants which exhibited ∼20-fold increase in NO emission under low oxygen conditions. These data suggest that nitrate reductase is involved in NO production either directly or by supplying nitrite to other sites of NO production (e.g. mitochondria). Various studies revealed that NO can induce AOX in mitochondria, but the mechanism has not been established yet. This study demonstrates that the NO produced in roots of wild-type plants inhibits aconitase which in turn leads to a marked increase in citrate levels. The accumulating citrate enhances AOX capacity, expression, and protein abundance. In contrast to wild-type plants, the nia double mutant failed to show AOX induction. The overall induction of AOX in wild-type roots correlated with accumulation of glycine, serine, leucine, lysine, and other amino acids. The findings show that NO inhibits aconitase under hypoxia which results in accumulation of citrate, the latter in turn inducing AOX and causing a shift of metabolism towards amino acid biosynthesis.

Fractionation and availability of heavy metals in tannery sludge-amended soil and toxicity assessment on the fully-grown Phaseolus vulgaris cultivars

This study was conducted to assess the effect of tannery sludge on the bush bean (Phaseolus vulgaris) cultivars fully-grown on a culture sandy soil, as tannery sludge is valuable to improve soil fertility but long term studies evaluating the effect on fully grown plants are scarce. Tannery sludge amendments (0, 0.77, 1.54, 3.08 and 6.16 g tannery sludge kg(-1) soil) were characterized and the main heavy metals identified (Cr, Mn, Fe, K, and Zn) later on sequentially and singly extracted, for soil fractionation and availability determination, respectively. Metals showed different fractionation and availability patterns, being the most toxic metal (Cr) found to primarily bind to the carbonate fraction in soil, while almost 10% of the total Cr was available for plant uptake. In the green house experiments, bush bean cultivars exposed to increasing tannery sludge amendments were evaluated at different plant stages. Metal accumulation and physiological parameters (chlorophyll, carotenoids, nitrate reductase activity and dry weight) were determined. Toxicity was primarily due to Cr, stimulating or affecting the response of physiological parameters and suppressing seed formation at the highest tannery sludge ratio. Metals were mainly accumulated in the roots of bush beans, diminishing in the upper part of the plants with minimal translocation to seeds, supposing little risk for human consumption. Additionally, important correlations, antagonistic and synergistic relationships were observed between the extracted metals and metal accumulation in plant tissues.

Effect of short-term salinity on the nitrate reductase activity in cucumber roots

In short-term experiments, the effect of high salinity on cucumber (Cucumis sativus) nitrate reductase activity was studied. The 60-min exposure of cucumber roots to 200 mM NaCl resulted in significant increase of the actual NR activity (measured in the presence of Mg²+), whereas the total enzyme activity (measured with EDTA) was not affected. NaCl-induced stimulation of the actual NR activity was rapidly reversed upon transfer of roots to salt-free solution. The increase in actual activity was completely prevented by microcystin-LR and cantharidin, protein phosphatases inhibitors. In addition, a significant decrease in ATP level was also observed in roots incubated with NaCl. These data suggest that the reversible protein phosphorylation is involved in the induction of NR activity during the first hour of salt stress. The effect of short-term salinity on the expression of genes encoding for nitrate reductase in cucumber roots was also studied. 200 mM NaCl diminished the increase in CsNR1 expression observed in control roots. During the same time period, the expression of CsNR2 was not affected, whereas the expression of CsNR3 decreased significantly after 1h incubation of the excised roots in both, control and salt-containing nutrient solutions. Incubation of roots in the presence of iso-osmotic concentration of PEG had no effect on both, NR activity and expression. This indicates that only the ionic component of salt stress was involved in the salt-induced modifications of nitrate reductase activity.

The PP2A regulatory subunit Tap46, a component of the TOR signaling pathway, modulates growth and metabolism in plants

Tap42/α4, a regulatory subunit of protein phosphatase 2A, is a downstream effector of the target of rapamycin (TOR) protein kinase, which regulates cell growth in coordination with nutrient and environmental conditions in yeast and mammals. In this study, we characterized the functions and phosphatase regulation of plant Tap46. Depletion of Tap46 resulted in growth arrest and acute plant death with morphological markers of programmed cell death. Tap46 interacted with PP2A and PP2A-like phosphatases PP4 and PP6. Tap46 silencing modulated cellular PP2A activities in a time-dependent fashion similar to TOR silencing. Immunoprecipitated full-length and deletion forms of Arabidopsis thaliana TOR phosphorylated recombinant Tap46 protein in vitro, supporting a functional link between Tap46 and TOR. Tap46 depletion reproduced the signature phenotypes of TOR inactivation, such as dramatic repression of global translation and activation of autophagy and nitrogen mobilization, indicating that Tap46 may act as a positive effector of TOR signaling in controlling those processes. Additionally, Tap46 silencing in tobacco (Nicotiana tabacum) BY-2 cells caused chromatin bridge formation at anaphase, indicating its role in sister chromatid segregation. These findings suggest that Tap46, in conjunction with associated phosphatases, plays an essential role in plant growth and development as a component of the TOR signaling pathway.

Production and scavenging of nitric oxide by barley root mitochondria

We examined whether root mitochondria and mitochondrial membranes produce nitric oxide (NO) exclusively by reduction of nitrite or also via a nitric oxide synthase (NOS), and to what extent direct NO measurements could become undetectable due to NO oxidation. Chemiluminescence detection of NO in the gas phase was used to monitor NO emission from suspensions (i.e. direct chemiluminescence). For comparison, diaminofluorescein (DAF) and diaminorhodamine (DAR) were used as NO indicators. NO oxidation to nitrite and nitrate was quantified after reduction of nitrite + nitrate to NO by vandium (III) with subsequent chemiluminescence detection (i.e. indirect chemiluminescence). Nitrite and NADH consumption were also measured. Anaerobic nitrite-dependent NO emission was exclusively associated with the membrane fraction, without participation of matrix components. Rates of nitrite and NADH consumption matched, whereas the rate of NO emission was lower. In air, mitochondria apparently produced no nitrite-dependent NO, and no NOS activity was detected by direct or indirect chemiluminescence. In contrast, with DAF-2 or DAR-4M, an l-arginine-dependent fluorescence increase took place. However, the response of this apparent low NOS activity to inhibitors, substrates and cofactors was untypical when compared with commercial inducible NOS (iNOS), and the existence of NOS in root mitochondria is therefore doubtful. In a solution of commercial iNOS, about two-thirds of the NO (measured as NADPH consumption) were oxidized to nitrite + nitrate. Addition of mitochondria to iNOS decreased the apparent NO emission, but without a concomitant increase in nitrite + nitrate formation. Thus, mitochondria appeared to accelerate oxidation of NO to volatile intermediates.

Effects of arsenic on nitrate metabolism in arsenic hyperaccumulating and non-hyperaccumulating ferns

This study investigated the effects of arsenic on the in vitro activities of the enzymes (nitrate reductase and nitrite reductase) involved in nitrate metabolism in the roots, rhizomes, and fronds of four-month old Pteris vittata (arsenic - hyperaccumulator) and Pteris ensiformis (non-arsenic-hyperaccumulator) plants. The arsenic treatments (0, 150, and 300 microM as sodium arsenate) in hydroponics had adverse effects on the root and frond dry weights, and this effect was more evident in P. ensiformis than in P. vittata. Nitrate reductase and nitrite reductase activities of arsenate-treated plants were reduced more in P. ensiformis than in P. vittata. This effect was accompanied by similar decreases in tissue NO(3)(-) concentrations. Therefore, this decrease is interpreted as being indirect, i.e., the consequence of the reduced NO(3)(-) uptake and translocation in the plants. The study shows the difference in the tolerance level of the two Pteris species with varying sensitivity to arsenic.

NO3-/NO2- assimilation in halophilic archaea: physiological analysis, nasA and nasD expressions

The haloarchaeon Haloferax mediterranei is able to assimilate nitrate or nitrite using the assimilatory nitrate pathway. An assimilatory nitrate reductase (Nas) and an assimilatory nitrite reductase (NiR) catalyze the first and second reactions, respectively. The genes involved in this process are transcribed as two messengers, one polycistronic (nasABC; nasA encodes Nas) and one monocistronic (nasD; codes for NiR). Here we report the Hfx mediterranei growth as well as the Nas and NiR activities in presence of high nitrate, nitrite and salt concentrations, using different approaches such as physiological experiments and enzymatic activities assays. The nasA and nasD expression profiles are also analysed by real-time quantitative PCR. The results presented reveal that the assimilatory nitrate/nitrite pathway in Hfx mediterranei takes place even if the salt concentration is higher than those usually present in the environments where this microorganism inhabits. This haloarchaeon grows in presence of 2 M nitrate or 50 mM nitrite, which are the highest nitrate and nitrite concentrations described from a prokaryotic microorganism. Therefore, it could be attractive for bioremediation applications in sewage plants where high salt, nitrate and nitrite concentrations are detected in wastewaters and brines.

Characterization of a mutant of Chlamydomonas reinhardtii deficient in the molybdenum cofactor

Molybdenum (Mo) is an essential micronutrient for almost all organisms. In eukaryotes, it forms a part of the molybdenum cofactor (Moco), which is required for numerous enzymes involved in carbon, nitrogen and sulfur metabolism. Mo is taken up by cells in the form of molybdate and recently molybdate transporters have been identified in Arabidopsis thaliana and Chlamydomonas reinhardtii. Here, we report the characterization of a novel mutant (DB6) of C. reinhardtii generated by random insertional mutagenesis that is unable to assimilate nitrate as a nitrogen source because it lacks functional nitrate reductase (NR). Besides lacking NR, DB6 also lacks xanthine dehydrogenase activity; a common requirement of both enzymes is Moco. DB6 displays a 'molybdate-repairable' phenotype--growth on nitrate is partially restored by supplementing media with high levels of molybdate. This phenotype is typically associated with mutants defective in either molybdate transport or insertion of Mo into the pterin precursor of Moco. Mo content was found to be significantly lower in DB6 than in the wild-type strain, AOXR1, which suggests that DB6 is defective in Mo uptake. Genetic complementation with a variety of candidate genes that include the known molybdate transporter MOT1 and DNA that spans the site of mutation was unable to recover the wild-type phenotype. Taken together, our results indicate that DB6 is a novel molybdate transport-deficient mutant.

Effect of foliar feeding on nitrogen assimilation in alfalfa plants at insufficient molybdenum supply

The influence of foliar feeding on the nitrogen assimilation in alfalfa plants under conditions of Mo shortage was studied. It was established that foliar fertilization with 0.3% solution of Agroleaf® resulted in increase of nitrogen fixation and nitrogen assimilation in the absence of Mo. Insufficient molybdenum supply leads to significant reduction of plant Mo content and nitrogen-fixing activity, while stress induced amino acids as alanine, GABA, threonine, proline and serine increased repeatedly. The negative effect of Mo deficiency on the enzyme activities related to the primary nitrogen assimilation (NR, GS, GOGAT) and plant growth diminished due to the foliar absorbed nutrients.

Nitrate uptake and metabolism by roots of soybean plants under oxygen deficiency

Nitrate is reported to improve tolerance of plants towards oxygen deficiency enabled by waterlogging of the root system, but the mechanism underlying the phenomenon remains poorly understood. We studied the metabolism of nitrate in roots exposed to hypoxia, using soybean plants growing in a hydroponic system after suspending aeration and covering the surface of the nutrient solution with mineral oil. Nitrate depletion from the medium was more intense under hypoxia than normoxia, but in the presence of chloramphenicol, consumption under hypoxia was significantly reduced. Nitrite accumulated in the medium in the state of hypoxia and this effect was partially eliminated by chloramphenicol. Nitrate consumption sensitive to chloramphenicol was attributed to bacterial activity. Endogenous root nitrate was strongly reduced under hypoxia indicating mobilization. Although the transport of nitrate to the shoot via the xylem was also reduced under hypoxia, the severity of this reduction was dependent on the concentration of nitrate in the medium, suggesting that at least some of the nitrate in the xylem came from the medium. Root nitrate reductase was also strongly reduced under hypoxia, but recovered rapidly on return to normoxia. Overall, the data are consistent with two main metabolic fates for chloramphenicol-insensitive nitrate depletion under hypoxia: the reduction of some nitrate to nitrite (despite the reduced nitrate reductase activity) followed by its release to the medium (at least one-third of the nitrate consumed followed this route), and the transport of nitrate to the shoot. Nevertheless, it is highly unlikely that these metabolic routes account for all the nitrate consumed.

Virus-induced gene silencing of 14-3-3 genes abrogates dark repression of nitrate reductase activity in Nicotiana benthamiana

In order to study the effect of repression of 14-3-3 genes on actual activity of the nitrate reductase (NR) in Nicotiana benthamiana leaves, Nb14-3-3a gene was silenced by virus-induced gene silencing (VIGS) method using potato virus X (PVX). Expression of Nb14-3-3a as well as Nb14-3-3b genes was altogether repressed in the leaves of PVX-14-3a-infected plants. Furthermore, two-dimensional gel electrophoresis and immunoblot analysis with anti-14-3-3 antiserum suggested that the expressions of Nb14-3-3a and Nb14-3-3b proteins are accordingly repressed in PVX-14-3a-infected plants. It is well known that binding of 14-3-3 proteins to phosphorylated NR leads to substantial decrease in NR activity of leaves under darkness. Therefore, we studied the changes in NR activity in response to light/dark transitions in the leaves of PVX-14-3a-infected plants. NR activation state was kept at a high level under darkness in PVX-14-3a-infected plants, but not in PVX-green fluorescent protein (GFP)-infected and control plants. This result suggests that Nb14-3-3a and/or Nb14-3-3b proteins are indeed involved in the inactivation of NR activity under darkness in N. benthamiana.

A nitrite transporter associated with nitrite uptake by higher plant chloroplasts

Chloroplasts take up cytosolic nitrite during nitrate assimilation. In this study we identified a nitrite transporter located in the chloroplasts of higher plants. The transporter, CsNitr1-L, a member of the proton-dependent oligopeptide transporter (POT) family, was detected during light-induced chloroplast development in de-etiolating cucumber seedlings. We detected a CsNitr1-L-green fluorescent protein (GFP) fusion protein in the chloroplasts of leaf cells and found that an immunoreactive 51 kDa protein was present in the isolated inner envelope membrane of chloroplasts. CsNitr1-L has an isoform, CsNitr1-S, with an identical 484 amino acid core sequence; however, in CsNitr1-S the 120 amino acid N-terminal extension is missing. Saccharomyces cerevisiae cells expressing CsNitr1-S absorbed nitrite from an acidic medium at a slower rate than mock-transformed control cells, and accumulated nitrite to only one-sixth the concentration of the control cells, suggesting that CsNitr1-S enhances the efflux of nitrite from the cell. Insertion of T-DNA in a single CsNitr1-L homolog (At1g68570) in Arabidopsis resulted in nitrite accumulation in leaves to more than five times the concentration found in the wild type. These results show that it is possible that both CsNitr1-L and CsNitr1-S encode efflux-type nitrite transporters, but with different subcellular localizations. CsNitr1-L may possibly load cytosolic nitrite into chloroplast stroma in the chloroplast envelope during nitrate assimilation. The presence of genes homologous to CsNitr1-L in the genomes of Arabidopsis and rice indicates that facilitated nitrite transport is of general physiological importance in plant nutrition.