Identification of a Protein That Inhibits the Phosphorylated Form of Nitrate Reductase from Spinach (Spinacia oleracea) Leaves

Nitrate reductase enzymes in alga Chattonella subsalsa are regulated by environmental cues at the translational and post-translational levels

Nitrate reductase (NR) catalyzes the rate-limiting step in nitrate assimilation. Plant and algal NRs have a highly conserved domain architecture but differ in regulation. In plants, NR activity is regulated by reversible phosphorylation and subsequent binding of 14-3-3 proteins at a conserved serine residue. Algal NRs typically lack 14-3-3 binding motifs, which have only recently been identified in a few algal species. Previous research indicates that the alga, Chattonella subsalsa, possesses a novel NR, NR2-2/2HbN (NR2), which incorporates a 2/2 hemoglobin domain. A second NR (NR3) in C. subsalsa lacks the cytochrome b5 (heme-Fe) domain but includes a putative binding motif for 14-3-3 proteins. The expression of NR2 and NR3 genes indicates that NR2 transcript abundance was regulated by light, nitrogen source, and temperature, while NR3 transcript levels were only regulated by light. Here, we measured total NR activity in C. subsalsa and the potential for regulation of NR activity by putative 14-3-3 binding proteins. Results indicate that NR activity in C. subsalsa was regulated by light, nitrogen source, and temperature at the translational level. NR activity was also regulated by endogenous rhythm and temperature at the post-translational level, supporting the hypothesis that NR3 is regulated by 14-3-3 binding proteins. Together with a previous report describing the regulation of NR gene expression in C. subsalsa, results suggest that C. subsalsa responds to environmental conditions by differential regulation of NRs at transcriptional, translational, and post-translational levels. This flexibility may provide a competitive advantage for this species in the environment. To date, this is the first report which provides evidence for the potential post-translational regulation of NR by 14-3-3 proteins in algal species and suggests that regulatory mechanisms for NR activity may be shared between plants and some algal species.

Role of protein phosphatases in the regulation of nitrogen nutrition in plants

The reversible protein phosphorylation and dephosphorylation mediated by protein kinases and phosphatases regulate different biological processes and their response to environmental cues, including nitrogen (N) availability. Nitrate assimilation is under the strict control of phosphorylation-dephosphorylation mediated post-translational regulation. The protein phosphatase family with approximately 150 members in Arabidopsis and around 130 members in rice is a promising player in N uptake and assimilation pathways. Protein phosphatase 2A (PP2A) enhances the activation of nitrate reductase (NR) by deactivating SnRK1 and reduces the binding of inhibitory 14-3-3 proteins on NR. The functioning of nitrate transporter NPF6.3 is regulated by phosphorylation of CBL9 (Calcineurin B like protein 9) and CIPK23 (CBL interacting protein kinase 23) module. Phosphorylation by CIPK23 inhibits the activity of NPF6.3, whereas protein phosphatases (PP2C) enhance the NPF6.3-dependent nitrate sensing. PP2Cs and CIPK23 also regulate ammonium transporters (AMTs). Under either moderate ammonium supply or high N demand, CIPK23 is bound and inactivated by PP2Cs. Ammonium uptake is mediated by nonphosphorylated and active AMT1s. Whereas, under high ammonium availability, CIPK23 gets activated and phosphorylate AMT1;1 and AMT1;2 rendering them inactive. Recent reports suggest the critical role of protein phosphatases in regulating N use efficiency (NUE). In rice, PP2C9 regulates NUE by improving N uptake and assimilation. Comparative leaf proteome of wild type and PP2C9 over-expressing transgenic rice lines showed 30 differentially expressed proteins under low N level. These proteins are involved in photosynthesis, N metabolism, signalling, and defence.

Study the effect of 24-epibrassinolide application on the Cu/Zn-SOD expression and tolerance to drought stress in common bean

To study the possibility of increasing the drought tolerance of common bean with the exogenous application of 24-epibrassinolide (EBL), an experiment was conducted in 2016 and 2017. In this experiment, two irrigation levels (optimal irrigation and drought stress) were applied to the main plots and two common bean genotypes (Kusha cultivar and COS16 genotype) and four EBL concentrations (0, 2, 4, and 6 μM) were allocated to sub-plots as factorial. In the flowering stage, drought stress was applied and plants were sprayed with EBL. The results showed that drought stress reduced relative water content (RWC) and increased proline content, malondialdehyde (MDA) content, and antioxidant enzymes activity. However, exogenous application of EBL reduced the seed yield loss and increased the drought stress tolerance in both common bean genotypes by decreasing the MDA content and increasing the RWC, proline content, antioxidant enzymes activity, and nitrate reductase activity. It can be concluded that foliar spray of 4 µM EBL as the best concentration may increase the seed yield and enhance the drought stress tolerance of common bean. Also, Cu/Zn-SOD was up-regulated in response to the drought stress and exogenous EBL. The COS16 genotype showed better response to the drought stress and exogenous EBL than the Kusha cultivar, because of the higher up-regulation of Cu/Zn-SOD in this genotype compared to the Kusha cultivar. Therefore, EBL can be used as a plant growth regulator to enhance drought stress tolerance and minimize the seed yield loss of common bean caused by water deficit.

Alternative Pathway is Involved in Nitric Oxide-Enhanced Tolerance to Cadmium Stress in Barley Roots

Alternative pathway (AP) has been widely accepted to be involved in enhancing tolerance to various environmental stresses. In this study, the role of AP in response to cadmium (Cd) stress in two barley varieties, highland barley (Kunlun14) and barley (Ganpi6), was investigated. Results showed that the malondialdehyde (MDA) content and electrolyte leakage (EL) level under Cd stress increased in two barley varieties. The expressions of alternative oxidase (AOX) genes (mainly AOX1a), AP capacity (V_alt), and AOX protein amount were clearly induced more in Kunlun14 under Cd stress, and these parameters were further enhanced by applying sodium nitroprussid (SNP, a NO donor). Moreover, H₂O₂ and O₂⁻ contents were raised in the Cd-treated roots of two barley varieties, but they were markedly relieved by exogenous SNP. However, this mitigating effect was aggravated by salicylhydroxamic acid (SHAM, an AOX inhibitor), suggesting that AP contributes to NO-enhanced Cd stress tolerance. Further study demonstrated that the effect of SHAM application on reactive oxygen species (ROS)-related scavenging enzymes and antioxidants was minimal. These observations showed that AP exerts an indispensable function in NO-enhanced Cd stress tolerance in two barley varieties. AP was mainly responsible for regulating the ROS accumulation to maintain the homeostasis of redox state.

Evolution of TOR-SnRK dynamics in green plants and its integration with phytohormone signaling networks

The target of rapamycin (TOR)-sucrose non-fermenting 1 (SNF1)-related protein kinase 1 (SnRK1) signaling is an ancient regulatory mechanism that originated in eukaryotes to regulate nutrient-dependent growth. Although the TOR-SnRK1 signaling cascade shows highly conserved functions among eukaryotes, studies in the past two decades have identified many important plant-specific innovations in this pathway. Plants also possess SnRK2 and SnRK3 kinases, which originated from the ancient SnRK1-related kinases and have specialized roles in controlling growth, stress responses and nutrient homeostasis in plants. Recently, an integrative picture has started to emerge in which different SnRKs and TOR kinase are highly interconnected to control nutrient and stress responses of plants. Further, these kinases are intimately involved with phytohormone signaling networks that originated at different stages of plant evolution. In this review, we highlight the evolution and divergence of TOR-SnRK signaling components in plants and their communication with each other as well as phytohormone signaling to fine-tune growth and stress responses in plants.

Light regulation of nitrate reductase by catalytic subunits of protein phosphatase 2A

PP2A catalytic subunit C2 is of special importance for light/dark regulation of nitrate reductase activity. The level of unmethylated PP2A catalytic subunits decreases in darkness. Protein phosphatase 2A (PP2A) dephosphorylates and activates nitrate reductase (NR) in photosynthetically active tissue when plants are transferred from darkness to light. In the present work, investigation of Arabidopsis thaliana PP2A mutant lines revealed that one of the five PP2A catalytic subunit genes, e.g., C2, was of special importance for NR activation. Impairment of NR activation was, especially pronounced in the c2c4 double mutant. Though weaker, NR activation was also impaired in the c2 single mutant, and c1c2 and c2c5 double mutants. On the other hand, NR activation in the c4c5 double mutant was as efficient as in WT. The c4 single mutant had low PP2A activity, whereas the c2 single mutant possessed WT levels of extractable PP2A activity. PP2A activity was low in both c2c4 and c4c5. Differences in extracted PP2A activity among mutants did not strictly correlate with differences in NR activation, but underpinned that C2 has a special function in NR activation in vivo. The terminal leucine in PP2A catalytic subunits is generally methylated to a high degree, but regulation and impact of methylation/demethylation is barely studied. In WT and PP2A mutants, the level of unmethylated PP2A catalytic subunits decreased during 45 min of darkness, but did not change much when light was switched on. In leucine carboxyl methyl transferase1 (LCMT1) knockout plants, which possess mainly unmethylated PP2A, NR was still activated, although not fully as efficient as in WT.

Phenotyping two tomato genotypes with different nitrogen use efficiency

Nitrogen (N) supply usually limits crop production and optimizing N-use efficiency (NUE) to minimize fertilizer loss is important. NUE is a complex trait that can be dissected into crop N uptake from the soil (NUpE) and N utilization (NUtE). We compared NUE in 14 genotypes of three week old tomatoes grown in sand or hydroponic culture supplied with nitrate (NO3(-)). Culture method influenced measured NUE for some cultivars, but Regina Ostuni (RO) and UC82 were consistently identified as high and low NUE genotypes. To identify why these genotypes had contrasting NUE some traits were compared growing under 0.1 and 5 mM NO3(-) supply. UC82 showed greater root (15)NO3(-) influx at low and high supply, and stronger SlNRT2.1/NAR2.1 transporter expression under low supply when compared with RO. Conversely, RO showed a higher total root length and thickness compared to UC82. Compared with UC82, RO showed higher shoot SlNRT2.3 expression and NO3(-) storage at high supply, but similar NO3(-) reductase activity. After N-starvation, root cell electrical potentials of RO were significantly more negative than UC82, but nitrate elicited similar responses in both root types. Overall for UC82 and RO, NUtE may play a greater role than NUpE for improved NUE.

Early generation of nitric oxide contributes to copper tolerance through reducing oxidative stress and cell death in hulless barley roots

The objective of this study was to investigate the specific role of nitric oxide (NO) in the early response of hulless barley roots to copper (Cu) stress. We used the fluorescent probe diaminofluorescein-FM diacetate to establish NO localization, and hydrogen peroxide (H2O2)-special labeling and histochemical procedures for the detection of reactive oxygen species (ROS) in the root apex. An early production of NO was observed in Cu-treated root tips of hulless barley, but the detection of NO levels was decreased by supplementation with a NO scavenger, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO). Application of sodium nitroprusside (a NO donor) relieved Cu-induced root inhibition, ROS accumulation and oxidative damage, while c-PTIO treatment had a synergistic effect with Cu and further enhanced ROS levels and oxidative stress. In addition, the Cu-dependent increase in activities of superoxide dismutase, peroxidase and ascorbate peroxidase were further enhanced by exogenous NO, but application of c-PTIO decreased the activities of catalase and ascorbate peroxidase in Cu-treated roots. Subsequently, cell death was observed in root tips and was identified as a type of programed cell death (PCD) by terminal deoxynucleotidyl transferase dUTP nick end labeling assay. The addition of NO prevented the increase of cell death in root tips, whereas inhibiting NO accumulation further increased the number of cells undergoing PCD. These results revealed that NO production is an early response of hulless barley roots to Cu stress and that NO contributes to Cu tolerance in hulless barley possibly by modulating antioxidant defense, subsequently reducing oxidative stress and PCD in root tips.

Nitrate reductase-mediated nitric oxide production is involved in copper tolerance in shoots of hulless barley

An NR-mediated early NO production in the shoots of hulless barley plays an important role in protecting hulless barley from Cu toxicity through enhanced antioxidant enzyme activities and antioxidant pools. Nitric oxide (NO) has been identified as an important signaling molecule that is involved in multiple plant physiological responses, especially under some abiotic stress. Here, we investigated NO production and its effects on copper (Cu) excess in hulless barley shoots. An early NO burst at 24 h was observed in shoots of hulless barley, and the synthesis of early NO was mediated through nitrate reductase (NR), but not through nitric oxide synthase (NOS). Application of the NO donor sodium nitroprusside (SNP) efficiently alleviated Cu-induced shoot inhibition and decrease in chlorophyll content, as well as oxidative damage and reactive oxygen species (ROS) accumulation, while inhibiting NO accumulation by a specific NO scavenger or a NR inhibitor aggravated shoot inhibition as well as the increase of hydrogen peroxide (H2O2) content, supporting the role of an NR-mediated early NO production in hulless barley responses to Cu toxicity. Furthermore, elevated antioxidant enzyme activities were induced by Cu stress in the shoots of hulless barley and further significantly enhanced by NO donor, whereas suppressed by NO scavenger or NR inhibitor. On the other hand, the application of NO scavenger significantly reduced Cu-induced accumulation of glutathione (GSH) and ascorbate (Asc) in the shoots of hulless barley. Taken together, our results indicate that NO may induce hulless barley seedling tolerance to Cu toxicity through modulating antioxidant enzyme activity and antioxidants accumulation.

Protein phosphatases PP2A, PP4 and PP6: mediators and regulators in development and responses to environmental cues

The three closely related groups of serine/threonine protein phosphatases PP2A, PP4 and PP6 are conserved throughout eukaryotes. The catalytic subunits are present in trimeric and dimeric complexes with scaffolding and regulatory subunits that control activity and confer substrate specificity to the protein phosphatases. In Arabidopsis, three scaffolding (A subunits) and 17 regulatory (B subunits) proteins form complexes with five PP2A catalytic subunits giving up to 255 possible combinations. Three SAP-domain proteins act as regulatory subunits of PP6. Based on sequence similarities with proteins in yeast and mammals, two putative PP4 regulatory subunits are recognized in Arabidopsis. Recent breakthroughs have been made concerning the functions of some of the PP2A and PP6 regulatory subunits, for example the FASS/TON2 in regulation of the cellular skeleton, B' subunits in brassinosteroid signalling and SAL proteins in regulation of auxin transport. Reverse genetics is starting to reveal also many more physiological functions of other subunits. A system with key regulatory proteins (TAP46, TIP41, PTPA, LCMT1, PME-1) is present in all eukaryotes to stabilize, activate and inactivate the catalytic subunits. In this review, we present the status of knowledge concerning physiological functions of PP2A, PP4 and PP6 in Arabidopsis, and relate these to yeast and mammals.

Post-translational control of nitrate reductase activity responding to light and photosynthesis evolved already in the early vascular plants

Regulation of nitrate reductase (NR) by reversible phosphorylation at a conserved motif is well established in higher plants, and enables regulation of NR in response to rapid fluctuations in light intensity. This regulation is not conserved in algae NR, and we wished to test the evolutionary origin of the regulatory mechanism by physiological examination of ancient land plants. Especially a member of the lycophytes is of interest since their NR is candidate for regulation by reversible phosphorylation based on sequence analysis. We compared Selaginella kraussiana, a member of the lycophytes and earliest vascular plants, with the angiosperm Arabidopsis thaliana, and also tested the moss Physcomitrella patens. Interestingly, optimization of assay conditions revealed that S. kraussiana NR used NADH as an electron donor like A. thaliana, whereas P. patens NR activity depended on NADPH. Examination of light/darkness effects showed that S. kraussiana NR was rapidly regulated similar to A. thaliana NR when a differential (Mg(2+) contra EDTA) assay was used to reveal activity state of NR. This implies that already existing NR enzyme was post-translationally activated by light in both species. Light had a positive effect also on de novo synthesis of NR in S. kraussiana, which could be shown after the plants had been exposed to a prolonged dark period (7 days). Daily variations in NR activity were mainly caused by post-translational modifications. As for angiosperms, the post-translational light activation of NR in S. kraussiana was inhibited by 3-(3,4-dichlorophenyl)-1*1-dimethylurea (DCMU), an inhibitor of photosynthesis and stomata opening. Evolutionary, a post-translational control mechanism for NR have occurred before or in parallel with development of vascular tissue in land plants, and appears to be part of a complex mechanisms for coordination of CO2 and nitrogen metabolism in these plants.

Molecular mechanism of 14-3-3 protein-mediated inhibition of plant nitrate reductase

14-3-3 proteins regulate key processes in eukaryotic cells including nitrogen assimilation in plants by tuning the activity of nitrate reductase (NR), the first and rate-limiting enzyme in this pathway. The homodimeric NR harbors three cofactors, each of which is bound to separate domains, thus forming an electron transfer chain. 14-3-3 proteins inhibit NR by binding to a conserved phosphorylation site localized in the linker between the heme and molybdenum cofactor-containing domains. Here, we have investigated the molecular mechanism of 14-3-3-mediated NR inhibition using a fragment of the enzyme lacking the third domain, allowing us to analyze electron transfer from the heme cofactor via the molybdenum center to nitrate. The kinetic behavior of the inhibited Mo-heme fragment indicates that the principal point at which 14-3-3 acts is the electron transfer from the heme to the molybdenum cofactor. We demonstrate that this is not due to a perturbation of the reduction potentials of either the heme or the molybdenum center and conclude that 14-3-3 most likely inhibits nitrate reductase by inducing a conformational change that significantly increases the distance between the two redox-active sites.

Protein phosphatase 2A regulatory subunits are starting to reveal their functions in plant metabolism and development

Canonical protein phosphatase 2A (PP2A) consists of a catalytic subunit (C), a scaffolding subunit (A), and a regulatory subunit (B). The B subunits are believed to confer substrate specificity and cellular localization to the PP2A complex, and are generally divided into three non-related families in plants, i.e., B55, B' and B''. The two Arabidopsis B55 subunits (α and β) interact with nitrate reductase (NR) in the bimolecular fluorescence complementation assay in planta, and are necessary for rapid activation of NR. Interestingly, knockout of all four B55 alleles is probably lethal, because a homozygous double knockout (pp2a-b55αβ) could not be found. The B55 subunits, therefore, appear to have essential functions that cannot be replaced by other regulatory B subunits. A double mutant (pp2a-b'αβ) of two close B' homologs show severely impaired fertility, pointing to the essential role also of B' subunits in plant development.

Protein phosphatase 2A B55 and A regulatory subunits interact with nitrate reductase and are essential for nitrate reductase activation

Posttranslational activation of nitrate reductase (NR) in Arabidopsis (Arabidopsis thaliana) and other higher plants is mediated by dephosphorylation at a specific Ser residue in the hinge between the molybdenum cofactor and heme-binding domains. The activation of NR in green leaves takes place after dark/light shifts, and is dependent on photosynthesis. Previous studies using various inhibitors pointed to protein phosphatases sensitive to okadaic acid, including protein phosphatase 2A (PP2A), as candidates for activation of NR. PP2As are heterotrimeric enzymes consisting of a catalytic (C), structural (A), and regulatory (B) subunit. In Arabidopsis there are five, three, and 18 of these subunits, respectively. By using inducible artificial microRNA to simultaneously knock down the three structural subunits we show that PP2A is necessary for NR activation. The structural subunits revealed overlapping functions in the activation process of NR. Bimolecular fluorescence complementation was used to identify PP2A regulatory subunits interacting with NR, and the two B55 subunits were positive. Interactions of NR and B55 were further confirmed by the yeast two-hybrid assay. In Arabidopsis the B55 group consists of the close homologs B55α and B55β. Interestingly, the homozygous double mutant (b55α × b55β) appeared to be lethal, which shows that the B55 group has essential functions that cannot be replaced by other regulatory subunits. Mutants homozygous for mutation in Bβ and heterozygous for mutation in Bα revealed a slower activation rate for NR than wild-type plants, pointing to these subunits as part of a PP2A complex responsible for NR dephosphorylation.

Kinetic analysis of 14-3-3-inhibited Arabidopsis thaliana nitrate reductase

Eukaryotic assimilatory nitrate reductase (NR) is a dimeric multidomain molybdo-heme-flavo protein that catalyzes the first and rate-limiting step in the nitrate assimilation of plants, algae, and fungi. Nitrate reduction takes place at the N-terminal molybdenum cofactor-containing domain. Reducing equivalents are derived from NADH, which reduce the C-terminal FAD domain followed by single-electron transfer steps via the middle heme domain to the molybdenum center. In plants, nitrate reduction is post-translationally inhibited by phosphorylation and subsequent binding of 14-3-3 protein to a conserved phosphoserine located in the surface-exposed hinge between the catalytic and heme domain. Here we investigated Arabidopsis thaliana NR activity upon phosphorylation and 14-3-3 binding by using a fully defined in vitro system with purified proteins. We demonstrate that among different calcium-dependent protein kinases (CPKs), CPK-17 efficiently phosphorylates Ser534 in NR. Out of eight purified Arabidopsis 14-3-3 proteins, isoforms ω, κ, and λ exhibited the strongest inhibition of NR. The kinetic parameters of noninhibited, phosphorylated NR (pNR) and pNR in a complex with 14-3-3 were investigated. An 18-fold reduction in k(cat) and a decrease in the apparent K(M)(nitrate) (from 280 to 141 μM) were observed upon binding of 14-3-3 to pNR, suggesting a noncompetitive inhibition with a preferential binding to the substrate-bound state of the enzyme. Recording partial activities of NR demonstrated that the transfer of electrons to the heme is not affected by 14-3-3 binding. The Ser534Ala variant of NR was not inhibited by 14-3-3 proteins. We propose that 14-3-3 binding to Ser534 blocks the transfer of electrons from heme to nitrate by arresting the domain movement via hinge 1.

Adjustment of growth and central metabolism to a mild but sustained nitrogen-limitation in Arabidopsis

We have established a simple soil-based experimental system that allows a small and sustained restriction of growth of Arabidopsis by low nitrogen (N). Plants were grown in a large volume of a peat-vermiculite mix that contained very low levels of inorganic N. As a control, inorganic N was added in solid form to the peat-vermiculite mix, or plants were grown in conventional nutrient-rich solids. The low N growth regime led to a sustained 20% decrease of the relative growth rate over a period of 2 weeks, resulting in a two- to threefold decrease in biomass in 35- to 40-day-old plants. Plants in the low N regime contained lower levels of nitrate, lower nitrate reductase activity, lower levels of malate, fumarate and other organic acids and slightly higher levels of starch, as expected from published studies of N-limited plants. However, their rosette protein content was unaltered, and total and many individual amino acid levels increased compared with N-replete plants. This metabolic phenotype reveals that Arabidopsis responds adaptively to low N by decreasing the rate of growth, while maintaining the overall protein content, and maintaining or even increasing the levels of many amino acids.

14-3-3 and FHA domains mediate phosphoprotein interactions

Many aspects of plant growth and development require specific protein interactions to carry out biochemical and cellular functions. Several proteins mediate these interactions, two of which specifically recognize phosphoproteins: 14-3-3 proteins and proteins with FHA domains. These are the only phosphobinding domains identified in plants. Both domains are present in animals and plants, and are used by plant proteins to regulate metabolic, developmental, and signaling pathways. 14-3-3s regulate sugar metabolism, proton gradients, and control transcription factor localization. FHA domains are modular domains often found in multidomain proteins that are involved in signal transduction and plant development.

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.

Evidence for increased proton dissociation in low-activity forms of dephosphorylated squash-leaf nitrate reductase

The pH dependence of squash-leaf nitrate reductase has been studied. It has been found that high- and low-activity forms of purified nitrate reductase (both forms dephosphorylated) have different optimum pH values. A high-activity form has always a higher pH optimum compared with a low-activity form. Model computations show that the decrease in activity and the corresponding change of the pH optimum is apparently due to a conformation-dependent increase of proton dissociation of the enzyme. As previously shown, this behavior is also observed in leaf extracts during the conversion (and probably phosphorylation of nitrate reductase) from a high-active form to a low-active form when plants are transferred from light to darkness.

Nitrate assimilation in the forage legume Lotus japonicus L

Nitrate assimilation in the model legume, Lotus japonicus, has been investigated using a variety of approaches. A gene encoding a nitrate-inducible nitrate reductase (NR) has been cloned and appears to be the only NR gene present in the genome. Most of the nitrate reductase activity (NRA) is found in the roots and the plant assimilates the bulk of its nitrogen in that tissue. We calculate that the observed rates of nitrate reduction are compatible with the growth requirement for reduced nitrogen. The NR mRNA, NRA and the nitrate content do not show a strong diurnal rhythm in the roots and assimilation continues during the dark period although export of assimilated N to the shoot is lower during this time. In shoots, the previous low NR activity may be further inactivated during the dark either by a phosphorylation mechanism or due to reduced nitrate flux coincident with a decreased delivery through the transpiration stream. From nitrate-sufficient conditions, the removal of nitrate from the external medium causes a rapid drop in hydraulic conductivity and a decline in nitrate and reduced-N export. Root nitrate content, NR and nitrate transporter (NRT2) mRNA decline over a period of 2 days to barely detectable levels. On resupply, a coordinated increase of NR and NRT2 mRNA, and NRA is seen within hours.

Posttranslational regulation of nitrate reductase strongly affects the levels of free amino acids and nitrate, whereas transcriptional regulation has only minor influence

Diurnal variations in nitrate reductase (NR) activity and nitrogen metabolites were examined in wild-type Nicotiana plumbaginifolia and transformants with various degrees of NR deregulation. In the C1 line, NR was only deregulated at the transcriptional level by placing the NR gene under the control of the cauliflower mosaic virus 35S RNA promoter. In the Del8 and S521D lines, NR was additionally deregulated at the posttranslational level either by a deletion mutation in the N-terminal domain or by a mutation of the regulatory phosphorylation site (serine-521). Posttranslational regulation was essential for pronounced diurnal variations in NR activity. Low nitrate content was related to deregulation of NR, whereas the level of total free amino acids was much higher in plants with fully deregulated NR. Abolishing transcriptional and posttranslational regulation (S521D plants) resulted in an increase of glutamine and asparagine by a factor of 9 and 14, respectively, compared with wild type, whereas abolishing transcriptional regulation (C1 plants) only resulted in increases of glutamine and asparagine by factors <2. Among the minor amino acids, isoleucine and threonine, in particular, showed enhanced levels in S521D. Nitrate uptake rates were the same in S521D and wild type as determined with (15)N feeding. Deregulation of NR appears to set the level of certain amino acids, whereas diurnal variations were still determined by light/dark. Generally, deregulation of NR at the transcriptional level did not have much influence on metabolite levels, but additional deregulation at the posttranslational level resulted in profound changes of nitrogen metabolite levels.

Modulation of nitrate reductase activity in rice seedlings under aluminium toxicity and water stress: role of osmolytes as enzyme protectant

Nitrate reductase (NR) activity in the presence of Mg2+ (NR act) representing the non-phosphorylated NR state and the activity in the presence of EDTA (NR max) representing maximum NR activity was measured in roots and shoots of 15 d grown aluminium and water stressed rice seedlings to examine changes in NR activation state due to these stresses. Seedlings subjected to a moderate water stress level of -0.5 MPa for 24 h or grown in presence of 80 microM Al3+showed decreased level of NR max but resulted in higher NR act and NR activation state. However, seedlings grown in presence of a higher level of 160 microM Al3+ showed a decline in NR act as well as NR max. With a higher water stress Level of -2.0MPa a marked decline in the levels of both NR act and NR max was observed, whereas NR activation state remained almost unaltered with severe water stress. NR activity appeared to be sensitive to H2O2, PEG-6000, NaCl and various metal salts. Incorporation of these components in the enzyme assay medium led to decreased affinity of enzyme towards its substrate with increase in Km and decrease in Vmax values. Addition of each of the osmolytes i.e. 1 mol/L proline, 1 mol/L glycine betaine or 1 mol/L sucrose in the enzyme assay medium caused a considerable protection to the enzyme against the damaging effects of stressful components. An enhanced level of proline and glycine betaine was observed in Al-stressed seedlings and sucrose in Al as well as water stressed seedlings. Results suggest that Al toxicity and water stress decrease total amount of functional NR in rice seedlings and the osmolytes proline, glycine betaine and sucrose appear to have a direct protective action on enzyme NR under stressful conditions

Effects of soil drying and subsequent re-watering on the activity of nitrate reductase in roots and leaves of Helianthus annuus

The effects of drought on the activity of nitrate reductase (NR) were studied in Helianthus annuus L. plants subjected to soil drying and subsequent re-watering. Drought did not negatively affect the activation state of NR, but resulted in linearly-correlated decreases in the activity of the unphosphorylated active form and the total activity of NR, in both roots and leaves. The concentration of nitrate in roots, xylem and leaves also decreased in water-stressed plants, whereas the concentration of total amino acids was only transiently depressed at the leaf level. In contrast, soluble sugars accumulated both in roots and leaves of water-stressed plants. Drought-induced decreases in root NR activity were correlated with the observed changes in root nitrate concentration. A higher percentage of the decrease in foliar NR activity could be explained by the decline in nitrate flux to the leaves than by leaf nitrate content. Following re-watering, the extent of recovery of NR activity was higher in roots than in leaves. The delay in the recovery of foliar NR activity did not result from the persistence of reduced flux of nitrate through the xylem. Several hypotheses to explain the after-effect of soil drying on foliar NR activity are discussed.

Light regulation of root and leaf NO3 - uptake and reduction in the floating macrophyte Lemna minor

•  The regulation of NO₃ ^- uptake kinetics and reduction in relation to long- and short-term changes in irradiance was explored in roots and photosynthetic tissues of Lemna minor. •  The NO₃ ^- uptake kinetics, nitrate reductase activity, plant morphology, chlorophyll and tissue NO₃ ^- , organic-N, starch and sugars were measured on roots and fronds of L. minor grown at four combinations of irradiance- and NO₃ ^- availability. •  Long-term acclimatizations paralleled those of terrestrial plants. Short-term changes in irradiance, however, changed frond NO₃ ^- uptake proportionally with frond chlorophyll and N content, indicating a relationship between responsiveness and the metabolic potential of the plants. Root uptake changed to balance the change in frond uptake keeping whole plant uptake varying by < 40%. Nitrate reductase activity was primarily located in the roots and was correlated with frond uptake, indicating a transport of NO₃ ^- from shoot to root before reduction. •  This study shows that irradiance can affect the contribution of root and leaf uptake by aquatic plants and that roots play a major role in NO₃ ^- reduction despite large foliar uptake.

Cloning of a nitrate reductase inactivator (NRI) cDNA from Spinacia oleracea L. and expression of mRNA and protein of NRI in cultured spinach cells

The spinach ( Spinacia oleracea L. (cv. Hoyo) nitrate reductase inactivator (NRI) is a novel protein that irreversibly inactivates NR. Using degenerate primers based on an N-terminal amino acid sequence of NRI purified from spinach leaves and a cDNA library, we isolated a full-length NRI cDNA from spinach that contains an open reading frame encoding 479 amino acid residues. This protein shares 67.4% and 51.1-68.3% amino acid sequence similarities with a nucleotide pyrophosphatase (EC 3.6.1.9) from rice and three types of the nucleotide pyrophosphatase-like protein from Arabidopsis thaliana, respectively. Immunoblot analysis revealed that NRI was constitutively expressed in suspension-cultured spinach cells; however, its expression level is quite low in 1-day-subcultured cells. Moreover, northern blot analysis indicated that this expression was regulated at the mRNA level. These results suggest that NRI functions in mature cells.

Purification of a plant nucleotide pyrophosphatase as a protein that interferes with nitrate reductase and glutamine synthetase assays

An activity that inhibited both glutamine synthetase (GS) and nitrate reductase (NR) was highly purified from cauliflower (Brassica oleracea var. botrytis) extracts. The final preparation contained an acyl-CoA oxidase and a second protein of the plant nucleotide pyrophosphatase family. This preparation hydrolysed NADH, ATP and FAD to generate AMP and was inhibited by fluoride, Cu2+, Zn2+ and Ni2+. The purified fraction had no effect on the activity of NR when reduced methylviologen was used as electron donor instead of NADH; and inhibited the oxidation of NADH by both spinach NR and an Escherichia coli extract in a time-dependent manner. The apparent inhibition of GS and NR and the ability of ATP and AMP to relieve the inhibition of NR can therefore be explained by hydrolysis of nucleotide substrates by the nucleotide pyrophosphatase. We have no evidence that the nucleotide pyrophosphatase is a specific physiological regulator of NR and GS, but suggest that nucleotide pyrophosphatase activity may underlie some confusion in the literature about the effects of nucleotides and protein factors on NR and GS in vitro.

A protein kinase activated by darkness phosphorylates nitrate reductase in Komatsuna (Brassica campestris) leaves

Although it has been shown that leaf nitrate reductase (NR: EC 1.6.6.1) is phosphorylated by subjecting plants to darkness, there is no evidence for the existence of dark-activated or dark-induced NR kinase. This study was undertaken to investigate the occurrence of a protein kinase phosphorylating NR in response to dark treatments. Immediately after transferring Komatsuna (Brassica campestris L.) plants to darkness, we observed rapid increases in the phosphorylating activity of the synthetic peptide, which is designed for the amino acid sequence surrounding the regulatory serine residue of the hinge 1 region of Komatsuna NR, in crude extracts from leaves. The activity reached a maximum after 10 min of darkness. Inactivation states of NR estimated from relative activities with or without Mg2+ were correlated to activities of the putative dark-activated protein kinase. Using the synthetic peptide as a substrate, we purified a protein kinase from dark-treated leaves by means of successive chromatographies on Q-Sepharose, Blue Sepharose, FPLC Q-Sepharose, and ATP-gamma-Sepharose columns. The purified kinase had an apparent molecular mass of 150 kDa with a catalytic subunit of 55 kDa, and it was Ca2+-independent. The purified kinase phosphorylated a recombinant cytochrome c reductase protein, a partial protein of NR, and holo NR, and inactivated NR in the presence of both 14-3-3 protein and Mg2+. The kinase also phosphorylated synthetic peptide substrates designed for sucrose phosphate synthase and 3-hydroxy-3-methylglutaryl-Coenzyme A reductase. Among inhibitors tested, only K252a, a potent and specific serine/threonine kinase inhibitor, completely inhibited the activity of the dark-activated kinase. The activity of the purified kinase was also specifically inhibited by K252a. Taken together with these findings, results obtained suggest that the putative dark-activated protein kinase may be the purified kinase itself, and may be responsible for in vivo phosphorylation of NR and its inactivation during darkness.

Nitrate reductase from winter wheat leaves is activated at low temperature via protein dephosphorylation

The effect of short-term low temperature treatment on nitrate reductase (NR, EC 1.6.6.1) activity, NR protein and NR transcript levels in excised leaves of winter wheat (Triticum aestivum L. cv. Sadovo-1) was investigated. NR activity, measured in the presence of Mg2+ (NRact), doubled within 2 h at 4 degrees C, whereas NR activity, measured in the presence of EDTA (NRmax), did not respond to the cold treatment. Such an activation of NR occurred only if leaves were exposed to low temperature in the light but not in the dark. It was not affected by feeding cytoplasmic protein synthesis inhibitor, cycloheximide, or protein kinase inhibitor, staurosporin, but was completely prevented by okadaic acid, an inhibitor of protein phosphatases of the type 1 and 2 A. This inhibitory effect decreased gradually when okadaic acid-concentration in the nutrient solution was lowered below 1 &mgr;M and tended to disappear when leaves were fed with 10 nM okadaic acid. It was demonstrated that the cold-induced NR activation was dependent neither on cold-triggered calcium influx nor on high endogenous abscisic acid levels. The increased NRact in cold-exposed leaves was found to correlate with a higher level of NR transcript but not with an increased NR protein level. Feeding okadaic acid to these leaves prevented the cold-induced accumulation of NR mRNA. These data point to protein phosphatases of the type 2 A being involved in NR protein dephosphorylation and NR transcript accumulation as targets of activation by low temperature treatment.

Modulation of nitrate reductase activity in cucumber (Cucumis sativus) roots

Nitrate reductase (NR) (EC 1.6.6.1) activity and NR activation state, i.e. activity in the presence of Mg(2+) relative to activity in the absence of Mg(2+), in cucumber (Cucumis sativus) leaves increased in the light and decreased in the dark. In contrast to leaves, NR activation state in the roots did not show light/dark-dependent changes. Root NR was activated by anoxia or by addition of uncoupler (CCCP) or mannose. These treatments decreased ATP levels in root tissue. On the contrary, high oxygen supply promoted some NR inactivation. When an extract from anoxic roots was preincubated with ATP, NR was gradually inactivated. Subsequent addition of 5'-AMP resulted in a remarkable reactivation of the enzyme. NR extracted from hyperoxygenated roots was activated by preincubation with 5'-AMP, and the process was reversed by ATP. These results suggest the participation of adenine nucleotides on the in vivo modulation of NR activity in cucumber roots. NR was activated in vivo by cellular acidification and inactivated by alkalinisation. The acid-induced activation of NR was greatly prevented by okadaic acid, a protein phosphatase inhibitor. Our data indicate that, as in barley roots, anoxia, uncouplers, and mannose feeding activate cucumber root NR, at least partly, by enhancing NR dephosphorylation via a decrease in the internal level of ATP and a concomitant cellular acidification.

Biological timing and the clock metaphor: oscillatory and hourglass mechanisms

Living organisms have developed a multitude of timing mechanisms--"biological clocks." Their mechanisms are based on either oscillations (oscillatory clocks) or unidirectional processes (hourglass clocks). Oscillatory clocks comprise circatidal, circalunidian, circadian, circalunar, and circannual oscillations--which keep time with environmental periodicities--as well as ultradian oscillations, ovarian cycles, and oscillations in development and in the brain, which keep time with biological timescales. These clocks mainly determine time points at specific phases of their oscillations. Hourglass clocks are predominantly found in development and aging and also in the brain. They determine time intervals (duration). More complex timing systems combine oscillatory and hourglass mechanisms, such as the case for cell cycle, sleep initiation, or brain clocks, whereas others combine external and internal periodicities (photoperiodism, seasonal reproduction). A definition of a biological clock may be derived from its control of functions external to its own processes and its use in determining temporal order (sequences of events) or durations. Biological and chemical oscillators are characterized by positive and negative feedback (or feedforward) mechanisms. During evolution, living organisms made use of the many existing oscillations for signal transmission, movement, and pump mechanisms, as well as for clocks. Some clocks, such as the circadian clock, that time with environmental periodicities are usually compensated (stabilized) against temperature, whereas other clocks, such as the cell cycle, that keep time with an organismic timescale are not compensated. This difference may be related to the predominance of negative feedback in the first class of clocks and a predominance of positive feedback (autocatalytic amplification) in the second class. The present knowledge of a compensated clock (the circadian oscillator) and an uncompensated clock (the cell cycle), as well as relevant models, are briefly re viewed. Hourglass clocks are based on linear or exponential unidirectional processes that trigger events mainly in the course of development and aging. An important hourglass mechanism within the aging process is the limitation of cell division capacity by the length of telomeres. The mechanism of this clock is briefly reviewed. In all clock mechanisms, thresholds at which "dependent variables" are triggered play an important role.

Nitrate reduction in tendrils of semi-leafless pea

The recessive gene af produces a modification of Pisum sativum L. leaf morphology, where leaflets are replaced by tendrils. Previous reports have suggested that tendrils may contribute to plant growth in a similar way to flat leaf structures, but these reports have been restricted to carbon metabolism. In this work, we investigate the nitrate reduction (nitrate reductase activity) capacity of tendrils of a semi-leafless variety, Solara, in relation to other leaf structures. Maximum nitrate reductase activity (EC 1.6.6.1), expressed on a protein basis, was significantly lower in tendrils compared to flat structures. However, the activation state of nitrate reductase was significantly higher in tendrils, reaching 70%, compared to flat leaf structures. According to these results, tendrils contributed up to 25% of the overall plant nitrate reduction in the semi-leafless variety. This figure was even higher when nitrate reduction was calculated from in vivo measurements. The results are discussed in relation to nitrate, magnesium, carbohydrates, amino acids and adenylate levels of tendrils.

Post-translational regulation of cytosolic glutamine synthetase by reversible phosphorylation and 14-3-3 protein interaction

Regulation of the cytosolic isozyme of glutamine synthetase (GS(1); EC 6.3.1.2) was studied in leaves of Brassica napus L. Expression and immunodetection studies showed that GS(1) was the only active GS isozyme in senescing leaves. By use of [gamma-(32)P]ATP followed by immunodetection, it was shown that GS(1) is a phospho-protein. GS(1) is regulated post-translationally by reversible phosphorylation catalysed by protein kinases and microcystin-sensitive serine/threonine protein phosphatases. Dephosphorylated GS(1) is much more susceptible to degradation than the phosphorylated form. The phosphorylation status of GS(1) changes during light/dark transitions and depends in vitro on the ATP/AMP ratio. Phosphorylated GS(1) interacts with 14-3-3 proteins as verified by two different methods: a His-tag 14-3-3 protein column affinity method combined with immunodetection, and a far-Western method with overlay of 14-3-3-GFP. The degree of interaction with 14-3-3-proteins could be modified in vitro by decreasing or increasing the phosphorylation status of GS(1). Thus, the results demonstrate that 14-3-3 protein is an activator molecule of cytosolic GS and provide the first evidence of a protein involved in the activation of plant cytosolic GS. The role of post-translational regulation of cytosolic GS and interactions between phosphorylated cytosolic GS and 14-3-3 proteins in senescing leaves is discussed in relation to nitrogen remobilization.

Binding to 14-3-3 proteins is not sufficient to inhibit nitrate reductase in spinach leaves

To assess the role of 14-3-3 proteins in the magnesium-dependent inhibition of nitrate reductase (NR) we tested the effect of magnesium on NR binding to 14-3-3s by coimmunoprecipitation and gel filtration. The stability of the 14-3-3 complex of NR was, unlike its activity, unaffected by magnesium. We therefore conclude that binding to 14-3-3s per se does not inhibit NR. Magnesium inhibited 14-3-3-bound NR much more strongly than 14-3-3-free NR. 14-3-3s possibly reinforce NR inhibition by magnesium.

Deletion of the nitrate reductase N-terminal domain still allows binding of 14-3-3 proteins but affects their inhibitory properties

Nitrate reductase (NR) is post-translationally regulated by phosphorylation and binding of 14-3-3 proteins. Deletion of 56 amino acids in the amino-terminal domain of NR was previously shown to impair this type of regulation in tobacco (Nicotiana plumbaginifolia) (L. Nussaume, M. Vincentez, C. Meyer, J.-P. Boutin, M. Caboche [1995] Plant Cell 7: 611-621), although both full-length NR and deleted NR (DeltaNR) appeared to be phosphorylated in darkness (C. Lillo, S. Kazazaic, P. Ruoff, C. Meyer [1997] Plant Physiol 114: 1377-1383). We show here that in the presence of Mg(2+) and phosphatase inhibitors, NR and endogenous 14-3-3 proteins copurify through affinity chromatography. Assay of NR activity and western blots showed that endogenous 14-3-3 proteins copurified with both NR and DeltaNR. Electron transport in the heme-binding domain of DeltaNR was inhibited by Mg(2+)/14-3-3, whereas this was not the case for NR. This may indicate a different way of binding for 14-3-3 in the DeltaNR compared with NR. The DeltaNR was more labile than NR, in vitro. Lability was ascribed to the molybdopterin binding domain, and apparently an important function of the 56 amino acids is stabilization of this domain.

14-3-3 proteins: eukaryotic regulatory proteins with many functions

The enigmatically named 14-3-3 proteins have been the subject of considerable attention in recent years since they have been implicated in the regulation of diverse physiological processes, in eukaryotes ranging from slime moulds to higher plants. In plants they have roles in the regulation of the plasma membrane H+-ATPase and nitrate reductase, among others. Regulation of target proteins is achieved through binding of 14-3-3 to short, often phosphorylated motifs in the target, resulting either in its activation (e.g. H+-ATPase), inactivation (e.g. nitrate reductase) or translocation (although this function of 14-3-3 proteins has yet to be demonstrated in plants). The native 14-3-3 proteins are homo- or heterodimers and, as each monomer has a binding site, a dimer can potentially bind two targets, promoting their association. Alternatively, target proteins may have more than one 14-3-3-binding site. In this mini review, we present a synthesis of recent results from plant 14-3-3 research and, with reference to known 14-3-3-binding motifs, suggest further subjects for research.

14-3-3 proteins: eukaryotic regulatory proteins with many functions

The enigmatically named 14-3-3 proteins have been the subject of considerable attention in recent years since they have been implicated in the regulation of diverse physiological processes, in eukaryotes ranging from slime moulds to higher plants. In plants they have roles in the regulation of the plasma membrane H+-ATPase and nitrate reductase, among others. Regulation of target proteins is achieved through binding of 14-3-3 to short, often phosphorylated motifs in the target, resulting either in its activation (e.g. H+-ATPase), inactivation (e.g. nitrate reductase) or translocation (although this function of 14-3-3 proteins has yet to be demonstrated in plants). The native 14-3-3 proteins are homo- or heterodimers and, as each monomer has a binding site, a dimer can potentially bind two targets, promoting their association. Alternatively, target proteins may have more than one 14-3-3-binding site. In this mini review, we present a synthesis of recent results from plant 14-3-3 research and, with reference to known 14-3-3-binding motifs, suggest further subjects for research.

Two SNF1-related protein kinases from spinach leaf phosphorylate and inactivate 3-hydroxy-3-methylglutaryl-coenzyme A reductase, nitrate reductase, and sucrose phosphate synthase in vitro

We resolved from spinach (Spinacia oleracea) leaf extracts four Ca2+-independent protein kinase activities that phosphorylate the AMARAASAAALARRR (AMARA) and HMRSAMSGLHLVKRR (SAMS) peptides, originally designed as specific substrates for mammalian AMP-activated protein kinase and its yeast homolog, SNF1. The two major activities, HRK-A and HRK-C (3-hydroxy-3-methylglutaryl-coenzyme A reductase kinase A and C) were extensively purified and shown to be members of the plant SnRK1 (SNF1-related protein kinase 1) family using the following criteria: (a) They contain 58-kD polypeptides that cross-react with an antibody against a peptide sequence characteristic of the SnRK1 family; (b) they have similar native molecular masses and specificity for peptide substrates to mammalian AMP-activated protein kinase and the cauliflower homolog; (c) they are inactivated by homogeneous protein phosphatases and can be reactivated using the mammalian upstream kinase; and (d) they phosphorylate 3-hydroxy-3-methylglutaryl-coenzyme A reductase from Arabidopsis at the inactivating site, serine (Ser)-577. We propose that HRK-A and HRK-C represent either distinct SnRK1 isoforms or the same catalytic subunit complexed with different regulatory subunits. Both kinases also rapidly phosphorylate nitrate reductase purified from spinach, which is associated with inactivation of the enzyme that is observed only in the presence of 14-3-3 protein, a characteristic of phosphorylation at Ser-543. Both kinases also inactivate spinach sucrose phosphate synthase via phosphorylation at Ser-158. The SNF1-related kinases therefore potentially regulate several major biosynthetic pathways in plants: isoprenoid synthesis, sucrose synthesis, and nitrogen assimilation for the synthesis of amino acids and nucleotides.

Two SNF1-related protein kinases from spinach leaf phosphorylate and inactivate 3-hydroxy-3-methylglutaryl-coenzyme A reductase, nitrate reductase, and sucrose phosphate synthase in vitro

We resolved from spinach (Spinacia oleracea) leaf extracts four Ca2+-independent protein kinase activities that phosphorylate the AMARAASAAALARRR (AMARA) and HMRSAMSGLHLVKRR (SAMS) peptides, originally designed as specific substrates for mammalian AMP-activated protein kinase and its yeast homolog, SNF1. The two major activities, HRK-A and HRK-C (3-hydroxy-3-methylglutaryl-coenzyme A reductase kinase A and C) were extensively purified and shown to be members of the plant SnRK1 (SNF1-related protein kinase 1) family using the following criteria: (a) They contain 58-kD polypeptides that cross-react with an antibody against a peptide sequence characteristic of the SnRK1 family; (b) they have similar native molecular masses and specificity for peptide substrates to mammalian AMP-activated protein kinase and the cauliflower homolog; (c) they are inactivated by homogeneous protein phosphatases and can be reactivated using the mammalian upstream kinase; and (d) they phosphorylate 3-hydroxy-3-methylglutaryl-coenzyme A reductase from Arabidopsis at the inactivating site, serine (Ser)-577. We propose that HRK-A and HRK-C represent either distinct SnRK1 isoforms or the same catalytic subunit complexed with different regulatory subunits. Both kinases also rapidly phosphorylate nitrate reductase purified from spinach, which is associated with inactivation of the enzyme that is observed only in the presence of 14-3-3 protein, a characteristic of phosphorylation at Ser-543. Both kinases also inactivate spinach sucrose phosphate synthase via phosphorylation at Ser-158. The SNF1-related kinases therefore potentially regulate several major biosynthetic pathways in plants: isoprenoid synthesis, sucrose synthesis, and nitrogen assimilation for the synthesis of amino acids and nucleotides.

A conserved acidic motif in the N-terminal domain of nitrate reductase is necessary for the inactivation of the enzyme in the dark by phosphorylation and 14-3-3 binding

It has previously been shown that the N-terminal domain of tobacco (Nicotiana tabacum) nitrate reductase (NR) is involved in the inactivation of the enzyme by phosphorylation, which occurs in the dark (L. Nussaume, M. Vincentz, C. Meyer, J.P. Boutin, and M. Caboche [1995] Plant Cell 7: 611-621). The activity of a mutant NR protein lacking this N-terminal domain was no longer regulated by light-dark transitions. In this study smaller deletions were performed in the N-terminal domain of tobacco NR that removed protein motifs conserved among higher plant NRs. The resulting truncated NR-coding sequences were then fused to the cauliflower mosaic virus 35S RNA promoter and introduced in NR-deficient mutants of the closely related species Nicotiana plumbaginifolia. We found that the deletion of a conserved stretch of acidic residues led to an active NR protein that was more thermosensitive than the wild-type enzyme, but it was relatively insensitive to the inactivation by phosphorylation in the dark. Therefore, the removal of this acidic stretch seems to have the same effects on NR activation state as the deletion of the N-terminal domain. A hypothetical explanation for these observations is that a specific factor that impedes inactivation remains bound to the truncated enzyme. A synthetic peptide derived from this acidic protein motif was also found to be a good substrate for casein kinase II.

Chilling delays circadian pattern of sucrose phosphate synthase and nitrate reductase activity in tomato

Overnight low-temperature exposure inhibits photosynthesis in chilling-sensitive species such as tomato (Lycopersicon esculentum) and cucumber by as much as 60%. In an earlier study we showed that one intriguing effect of low temperature on chilling-sensitive plants is to stall the endogenous rhythm controlling transcription of certain nuclear-encoded genes, causing the synthesis of the corresponding transcripts and proteins to be mistimed when the plant is rewarmed. Here we show that the circadian rhythm controlling the activity of sucrose phosphate synthase (SPS) and nitrate reductase (NR), key control points of carbon and nitrogen metabolism in plant cells, is delayed in tomato by chilling treatments. Using specific protein kinase and phosphatase inhibitors, we further demonstrate that the chilling-induced delay in the circadian control of SPS and NR activity is associated with the activity of critical protein phosphatases. The sensitivity of the pattern of SPS activity to specific inhibitors of transcription and translation indicates that there is a chilling-induced delay in SPS phosphorylation status that is caused by an effect of low temperature on the expression of a gene coding for a phosphoprotein phosphatase, perhaps the SPS phosphatase. In contrast, the chilling-induced delay in NR activity does not appear to arise from effects on NR phosphorylation status, but rather from direct effects on NR expression. It is likely that the mistiming in the regulation of SPS and NR, and perhaps other key metabolic enzymes under circadian regulation, underlies the chilling sensitivity of photosynthesis in these plant species.

Drought-induced effects on nitrate reductase activity and mRNA and on the coordination of nitrogen and carbon metabolism in maize leaves

Maize (Zea mays L.) plants were grown to the nine-leaf stage. Despite a saturating N supply, the youngest mature leaves (seventh position on the stem) contained little NO3- reserve. Droughted plants (deprived of nutrient solution) showed changes in foliar enzyme activities, mRNA accumulation, photosynthesis, and carbohydrate and amino acid contents. Total leaf water potential and CO2 assimilation rates, measured 3 h into the photoperiod, decreased 3 d after the onset of drought. Starch, glucose, fructose, and amino acids, but not sucrose (Suc), accumulated in the leaves of droughted plants. Maximal extractable phosphoenolpyruvate carboxylase activities increased slightly during water deficit, whereas the sensitivity of this enzyme to the inhibitor malate decreased. Maximal extractable Suc phosphate synthase activities decreased as a result of water stress, and there was an increase in the sensitivity to the inhibitor orthophosphate. A correlation between maximal extractable foliar nitrate reductase (NR) activity and the rate of CO2 assimilation was observed. The NR activation state and maximal extractable NR activity declined rapidly in response to drought. Photosynthesis and NR activity recovered rapidly when nutrient solution was restored at this point. The decrease in maximal extractable NR activity was accompanied by a decrease in NR transcripts, whereas Suc phosphate synthase and phosphoenolpyruvate carboxylase mRNAs were much less affected. The coordination of N and C metabolism is retained during drought conditions via modulation of the activities of Suc phosphate synthase and NR commensurate with the prevailing rate of photosynthesis.

Overexpression of nitrate reductase in tobacco delays drought-induced decreases in nitrate reductase activity and mRNA

Transformed (cauliflower mosaic virus 35S promoter [35S]) tobacco (Nicotiana plumbaginifolia L.) plants constitutively expressing nitrate reductase (NR) and untransformed controls were subjected to drought for 5 d. Drought-induced changes in biomass accumulation and photosynthesis were comparable in both lines of plants. After 4 d of water deprivation, a large increase in the ratio of shoot dry weight to fresh weight was observed, together with a decrease in the rate of photosynthetic CO2 assimilation. Foliar sucrose increased in both lines during water stress, but hexoses increased only in leaves from untransformed controls. Foliar NO3- decreased rapidly in both lines and was halved within 2 d of the onset of water deprivation. Total foliar amino acids decreased in leaves of both lines following water deprivation. After 4 d of water deprivation no NR activity could be detected in leaves of untransformed plants, whereas about 50% of the original activity remained in the leaves of the 35S-NR transformants. NR mRNA was much more stable than NR activity. NR mRNA abundance increased in the leaves of the 35S-NR plants and remained constant in controls for the first 3 d of drought. On the 4th d, however, NR mRNA suddenly decreased in both lines. Rehydration at d 3 caused rapid recovery (within 24 h) of 35S-NR transcripts, but no recovery was observed in the controls. The phosphorylation state of the protein was unchanged by long-term drought. There was a strong correlation between maximal extractable NR activity and ambient photosynthesis in both lines. We conclude that drought first causes increased NR protein turnover and then accelerates NR mRNA turnover. Constitutive NR expression temporarily delayed drought-induced losses in NR activity. 35S-NR expression may therefore allow more rapid recovery of N assimilation following short-term water deficit.

Abolition of Posttranscriptional Regulation of Nitrate Reductase Partially Prevents the Decrease in Leaf NO3- Reduction when Photosynthesis Is Inhibited by CO2 Deprivation, but Not in Darkness

The activity of nitrate reductase (NR) in leaves is regulated by light and photosynthesis at transcriptional and posttranscriptional levels. To understand the physiological role of these controls, we have investigated the effects of light and CO2 on in vivo NO3- reduction in transgenic plants of Nicotiana plumbaginifolia lacking either transcriptional regulation alone or transcriptional and posttranscriptional regulation of NR. The abolition of both levels of NR regulation did not modify the light/dark changes in exogenous 15NO3- reduction in either intact plants or detached leaves. The same result was obtained for 15N incorporation into free amino acids in leaves after 15NO3- was supplied to the roots, and for reduction of endogenous NO3- after transfer of the plants to an N-deprived solution. In the light, however, deregulation of NR at the posttranscriptional level partially prevented the inhibition of leaf 15NO3- reduction resulting from the removal of CO2 from the atmosphere We concluded from these observations that in our conditions deregulation of NR in the transformants investigated had little impact on the adverse effect of darkness on leaf NO3- reduction, and that posttranscriptional regulation of NR is one of the mechanisms responsible for the short-term coupling between photosynthesis and leaf NO3- reduction in the light.

Characterization of Nitrate Reductase from Light- and Dark-Exposed Leaves (Comparison of Different Species and Effects of 14-3-3 Inhibitor Proteins)

Nitrate reductase (NR) was extracted and partially purified from leaves of squash (Curcurbita maxima), spinach (Spinacia oleracea), and three transgenic Nicotiana plumbaginifolia leaves in the presence of phosphatase inhibitors to preserve its phosphorylation state. Purified squash NR showed activation by substrates (hysteresis) when prepared from leaves in the light as well as in darkness. A 14-3-3 protein known to inhibit phosphorylated spinach NR in the presence of Mg2+ decreased by 70 to 85% the activity of purified NR from dark-exposed leaves, whereas NR from light-exposed leaves decreased by 10 to 25%. Apparent lack of posttranslational NR regulation in a transgenic N. plumbaginifolia expressing an NR construct with an N-terminal deletion ([delta]NR) may be explained by more easy dissociation of 14-3-3 proteins from [delta]NR. Partially purified [delta]NR was, however, inhibited by 14-3-3 protein, and the binding constant of 14-3-3 protein (4 x 108 M-1) and the NR-inhibiting protein concentration that results in a 50% reduction of free NR (2.5 nM) were the same for NR and [delta]NR. Regulation of NR activity by phosphorylation and binding of 14-3-3 protein was a general feature for all plants tested, whereas activation by substrates as a possible regulation mechanism was verified only for squash.

Three spinach leaf nitrate reductase-3-hydroxy-3-methylglutaryl-CoA reductase kinases that are regulated by reversible phosphorylation and/or Ca2+ ions

In spinach (Spinacea oleracea L.) leaf extracts, three protein kinases (PKI, PKII and PKIII) were identified each of which phosphorylated spinach nitrate reductase on serine-543, and inactivated the enzyme in the presence of nitrate reductase inhibitor, 14-3-3. PKIII was also very active in phosphorylating and inactivating Arabidopsis (Landsberg erecta) 3-hydroxy-3-methylglutaryl-coenzyme A reductase 1 (HMGR1). PKI and PKII phosphorylated HMGR1 more slowly than PKIII, compared with their relative rates of phosphorylation of nitrate reductase. HMGR1 identical with those that are seen after phosphorylation of serine-577 by the sucrose non-fermenting (SNF1)-like PK, 3-hydroxy-3-methylglutaryl-Co A reductase kinase A (HRK-A), from cauliflower [Dale, Arró, Becerra, Morrice, Boronat, Hardie and Ferrer (1995) Eur. J. Biochem. 233, 506-513]. PKI was Ca2+-dependent when prepared in the absence of protein phosphatase (PP) inhibitors, and largely Ca2+-dependent when prepared in the presence of PP inhibitors (NaF and EGTA). The Ca2+-independent portion of PKI was inactivated by either PP2A or PP2C, while the Ca2+-dependent portion of PKI became increasingly activated during storage, which we presume was mimicking the effect of an unidentified PP. These findings indicate that PK1 is regulated by two functionally distinct phosphorylations. PKI had a molecular mass of 45 kDa on gel filtration and was active towards substrate peptides that terminated at the +2 residue from the phosphorylation site, whereas PKIII was inactive towards these peptides. PKII was Ca2+-stimulated under all conditions tested. PKIII was Ca2+-indepdented, inactivated by PP2A or PP2C, had a requirement for a hydrophobic residue in the +4 position of peptide substrates, had a molecular mass by gel filtration of approximately 140 kDa, and an antibody against the rye SNF1-related PK (RKIN1) recognized a 58 kDa subunit in fractions containing PKIII. These properties of PKIII are identical with those reported previously for the SNF1-like enzyme, HRK-A. Our results indicate a considerable complexity of kinase cascades mediating the regulation of assimilatory and biosynthetic pathways in response to environmental stimuli in plants.