Intrinsic non-symbiotic and truncated haemoglobins and heterologous Vitreoscilla haemoglobin expression in plants

Production of vitamin A and vitamin E: expression of vitreoscilla hemoglobin gene in Erwinia herbicola

Vitamin A prevents eye problems, blindness and skin problems by strengthening the immune system. Vitamin E is a nutrient that has important roles in many areas such as skin health, eye health and hormonal order. Vitreoscilla hemoglobin (VHb) gives an advantage in later phases of grown conditions to cells. In this study, the intracellular and extracellular production of vitamin A and E in E. herbicola and its recombinant strains (vgb- and vgb+) in the three different M9 mediums with supplemented 0.1% glucose, 0.1% fructose and 0.1% sucrose was investigated. Additionally, the viable cell number and total cell mass (OD600) were measured by the host and the recombinant bacteria in these mediums. The VHb gene expression in E. herbicola enhanced vitamin A under different carbon conditionals. Especially, in the vgb + strain (carrying vgb gene) the production of total vitamin in 0.1% glucose medium was recorded as 0.14 µg/ml, while the production in fructose and sucrose media was recorded as 0.07 µg/ml. The production of intracellular vitamin E in the host strain (0.025 µg/ml) was about 13-fold (0.002 µg/ml) higher than vgb + recombinant strain in 0.1% fructose. The vgb + strain showed about 2-fold higher extracellular vitamin E production than the host strain.

Unusually Fast bis-Histidyl Coordination in a Plant Hemoglobin

The recently identified nonsymbiotic hemoglobin gene MtGlb1-2 of the legume Medicago truncatula possesses unique properties as it generates four alternative splice forms encoding proteins with one or two heme domains. Here we investigate the ligand binding kinetics of MtGlb1-2.1 and MtGlb1-2.4, bearing two hemes and one heme, respectively. Unexpectedly, the overall time-course of ligand rebinding was unusually fast. Thus, we complemented nanosecond laser flash photolysis kinetics with data collected with a hybrid femtosecond–nanosecond pump–probe setup. Most photodissociated ligands are rebound geminately within a few nanoseconds, which leads to rates of the bimolecular rebinding to pentacoordinate species in the 10⁸ M⁻¹s⁻¹ range. Binding of the distal histidine to the heme competes with CO rebinding with extremely high rates (k_h ~ 10⁵ s⁻¹). Histidine dissociation from the heme occurs with comparable rates, thus resulting in moderate equilibrium binding constants (K_H ~ 1). The rate constants for ligation and deligation of distal histidine to the heme are the highest reported for any plant or vertebrate globin. The combination of microscopic rates results in unusually high overall ligand binding rate constants, a fact that contributes to explaining at the mechanistic level the extremely high reactivity of these proteins toward the physiological ligands oxygen, nitric oxide and nitrite.

Characterization of Isolated Extracts from Justicia Plant Leaves used as Remedy for Anemia

Indigenous populations use plants as an important healthcare resource or remedy for different diseases. Here, isolated extracts from Justicia (family Acanthanceae) plant leaves used in Africa as remedy for anemia are characterized by different methods to assess composition and potential nutritional or therapeutic value. Extracts from Justicia leaves were obtained by aqueous extraction, with further isolation by centrifuging and high-performance liquid chromatography. Extracts and isolated compounds were characterized by ultraviolet-visible (UV-Vis) spectroscopy and inductively coupled plasma mass spectrometry (ICP-MS). Hemoglobin activity was assessed using different hemoglobin assays (Cayman Chemical, and Sigma-Aldrich), as well as ELISA. In addition, the safety of the isolated samples was assessed in vitro and in vivo in mice. ICP-MS study results revealed many essential metabolites found in blood plasma. The UV-Vis spectroscopy results highlighted the presence of hemoglobin, with assays showing levels over 4 times higher than that of similar mass of lyophilized human hemoglobin. Meanwhile, in vivo studies showed faster recovery from anemia in mice administered with the isolated extracts compared to untreated mice. Moreover, in vitro and in vivo studies highlighted safety of the extracts. This study reveals the presence of high levels of elements essential for blood health in the isolated extracts from Justicia plant leaves. The findings inspire further research with the potential applications in food fortification, and as remedy for blood disorders like anemia, which disproportionally affects cancer patients, pregnant women, and populations in low- and middle-income countries.

Targeted expression of Vitreoscilla hemoglobin improves the production of tropane alkaloids in Hyoscyamus niger hairy roots

Under hypoxic conditions, the expression of Vitreoscilla hemoglobin (VHb) in plants is proposed to increase the productivity of certain oxygen-requiring metabolic pathways by promoting the delivery of oxygen. Tropane alkaloids (TAs) are a class of important plant secondary metabolites with significant medicinal value; the final step in their biosynthesis requires oxygen. Whether heterologous expression of VHb, especially in different subcellular compartments, can accelerate the accumulation of TAs is not known. Herein, the effect of heterologous expression of VHb in different subcellular locations on the TA profile of H. niger hairy roots was investigated. The targeted expression of VHb in the plastids (using pVHb-RecA construct), led to the accumulation of 197.68 μg/g hyoscyamine in the transgenic H. niger hairy roots, which was 1.25-fold of the content present in the lines in which VHb expression was not targeted, and 3.66-fold of that present in the wild type (WT) lines. The content of scopolamine was increased by 2.20- and 4.70-fold in the pVHb-RecA transgenic lines compared to that in the VHb transgenic and WT lines. Our results demonstrate that VHb could stimulate the accumulation of TAs in the transgenic H. niger hairy roots. Quantitative RT-PCR analysis revealed that the expression of key genes involved in TA biosynthesis increased significantly in the VHb transgenic lines. We present the first description of a highly efficient strategy to increase TA content in H. niger. Moreover, our results also shed light on how the production of desired metabolites can be efficiently enhanced by using more accurate and appropriate genetic engineering strategies.

Learning To Breathe: Developmental Phase Transitions in Oxygen Status

Plants are developmentally disposed to significant changes in oxygen availability, but our understanding of the importance of hypoxia is almost entirely limited to stress biology. Differential patterns of the abundance of oxygen, nitric oxide (^•NO), and reactive oxygen species (ROS), as well as of redox potential, occur in organs and meristems, and examples are emerging in the literature of mechanistic relationships of these to development. We describe here the convergence of these cues in meristematic and reproductive tissues, and discuss the evidence for regulated hypoxic niches within which oxygen-, ROS-, ^•NO-, and redox-dependent signalling curate developmental transitions in plants.

Evolution, three-dimensional model and localization of truncated hemoglobin PttTrHb of hybrid aspen

Thus far, research on plant hemoglobins (Hbs) has mainly concentrated on symbiotic and non-symbiotic Hbs, and information on truncated Hbs (TrHbs) is scarce. The aim of this study was to examine the origin, structure and localization of the truncated Hb (PttTrHb) of hybrid aspen (Populus tremula L. × tremuloides Michx.), the model system of tree biology. Additionally, we studied the PttTrHb expression in relation to non-symbiotic class1 Hb gene (PttHb1) using RNAi-silenced hybrid aspen lines. Both the phylogenetic analysis and the three-dimensional (3D) model of PttTrHb supported the view that plant TrHbs evolved vertically from a bacterial TrHb. The 3D model suggested that PttTrHb adopts a 2-on-2 sandwich of α-helices and has a Bacillus subtilis -like ligand-binding pocket in which E11Gln and B10Tyr form hydrogen bonds to a ligand. However, due to differences in tunnel cavity and gate residue (E7Ala), it might not show similar ligand-binding kinetics as in Bs-HbO (E7Thr). The immunolocalization showed that PttTrHb protein was present in roots, stems as well as leaves of in vitro -grown hybrid aspens. In mature organs, PttTrHb was predominantly found in the vascular bundles and specifically at the site of lateral root formation, overlapping consistently with areas of nitric oxide (NO) production in plants. Furthermore, the NO donor sodium nitroprusside treatment increased the amount of PttTrHb in stems. The observed PttTrHb localization suggests that PttTrHb plays a role in the NO metabolism.

The responses of Vitreoscilla hemoglobin-expressing hybrid aspen (Populus tremula × tremuloides) exposed to 24-h herbivory: expression of hemoglobin and stress-related genes in exposed and nonorthostichous leaves

The responses of transcriptome and phenolic compounds were determined with Populus tremula L. × Populus tremuloides Michx. expressing the hemoglobin (Hb) of Vitreoscilla (VHb) and non-transformant (wt) line. After 24-h exposure of leaves to Conistra vaccinii L., the transcript levels of endogenous non-symbiotic class 1 Hb (PttHb1) and truncated Hb (PttTrHb) genes were modestly reduced and increased, respectively, in both wt and VHb-expressing line. Besides the herbivory exposed leaves showing the most significant transcriptome changes, alterations were also detected in the transcriptome of nonorthostichous leaves positioned directly above the exposed leaves. Both wt and VHb-expressing line displayed similar herbivory-induced effects on gene expression, although the extent of responses was more pronounced in the wt than in the VHb-expressing line. The contents of phenolic compounds were not altered due to herbivory and they were alike in the wt and VHb-expressing line. In addition, we determined the relative growth rates (RGRs) of Orthosia gothica L., Ectropis crepuscularia Denis & Schiff. and Orgyia antiqua L. larvae, and found no variation in the RGRs between the lines. Thus, VHb-expressing P. tremula × tremuloides lines showed to be comparable with wt in regards to the food quality of leaves.

Molecular controls of the oxygenation and redox reactions of hemoglobin

SIGNIFICANCE

The broad classes of O(2)-binding proteins known as hemoglobins (Hbs) carry out oxygenation and redox functions that allow organisms with significantly different physiological demands to exist in a wide range of environments. This is aided by allosteric controls that modulate the protein's redox reactions as well as its O(2)-binding functions.

RECENT ADVANCES

The controls of Hb's redox reactions can differ appreciably from the molecular controls for Hb oxygenation and come into play in elegant mechanisms for dealing with nitrosative stress, in the malarial resistance conferred by sickle cell Hb, and in the as-yet unsuccessful designs for safe and effective blood substitutes.

CRITICAL ISSUES

An important basic principle in consideration of Hb's redox reactions is the distinction between kinetic and thermodynamic reaction control. Clarification of these modes of control is critical to gaining an increased understanding of Hb-mediated oxidative processes and oxidative toxicity in vivo.

FUTURE DIRECTIONS

This review addresses emerging concepts and some unresolved questions regarding the interplay between the oxygenation and oxidation reactions of structurally diverse Hbs, both within red blood cells and under acellular conditions. Developing methods that control Hb-mediated oxidative toxicity will be critical to the future development of Hb-based blood substitutes.

Non-symbiotic hemoglobins in the life of seeds

Non-symbiotic hemoglobins (nsHbs), ancestors of symbiotic-Hbs, are hexacoordinated dimeric proteins, for which the crystal structure is well described. According to the extent of hexacoordination, nsHbs are classified as belonging to class-1 (nsHbs1) or class-2 (nsHbs2). The nsHbs1 show weak hexacoordination, moderate rates of O(2)-binding, very small rates of O(2) dissociation, and a remarkably high affinity for O(2), all suggesting a function involving O(2) scavenging. In contrast, the nsHbs2 exhibit strong hexacoordination, low rates of O(2)-binding and moderately low O(2) dissociation and affinity, suggesting a sensing role for sustained low (μM) levels of O(2). The existence of spatial and specific expression of nsHbs1 suggests that nsHbs play tissue-specific rather than housekeeping functions. The permeation of O(2) into seeds is usually prevented during the desiccation phase and early imbibition, generating an internal hypoxic environment that leads to ATP limitation. During evolution, the seed has acquired mechanisms to prevent or reduce this hypoxic stress. The nsHbs1/NO cycle appear to be involved in modulating the redox state in the seed and in maintaining an active metabolism. Under O(2) deficit, NADH and NO are synthesized in the seed and nsHbs1 scavenges O(2), which is used to transform NO into NO(3)(-) with concomitant formation of Fe(3+)-nsHbs1. Expression of nsHbs1 is not detectable in dry viable seeds. However, in the seeds cross-talk occurs between nsHbs1 and NO during germination. This review considers the current status of our knowledge of seed nsHbs and considers key issues of further work to better understand their role in seed physiology.

Hemoglobin: a nitric-oxide dioxygenase

Members of the hemoglobin superfamily efficiently catalyze nitric-oxide dioxygenation, and when paired with native electron donors, function as NO dioxygenases (NODs). Indeed, the NOD function has emerged as a more common and ancient function than the well-known role in O2 transport-storage. Novel hemoglobins possessing a NOD function continue to be discovered in diverse life forms. Unique hemoglobin structures evolved, in part, for catalysis with different electron donors. The mechanism of NOD catalysis by representative single domain hemoglobins and multidomain flavohemoglobin occurs through a multistep mechanism involving O2 migration to the heme pocket, O2 binding-reduction, NO migration, radical-radical coupling, O-atom rearrangement, nitrate release, and heme iron re-reduction. Unraveling the physiological functions of multiple NODs with varying expression in organisms and the complexity of NO as both a poison and signaling molecule remain grand challenges for the NO field. NOD knockout organisms and cells expressing recombinant NODs are helping to advance our understanding of NO actions in microbial infection, plant senescence, cancer, mitochondrial function, iron metabolism, and tissue O2 homeostasis. NOD inhibitors are being pursued for therapeutic applications as antibiotics and antitumor agents. Transgenic NOD-expressing plants, fish, algae, and microbes are being developed for agriculture, aquaculture, and industry.

Seed-specific elevation of non-symbiotic hemoglobin AtHb1: beneficial effects and underlying molecular networks in Arabidopsis thaliana

BACKGROUND

Seed metabolism is dynamically adjusted to oxygen availability. Processes underlying this auto-regulatory mechanism control the metabolic efficiency under changing environmental conditions/stress and thus, are of relevance for biotechnology. Non-symbiotic hemoglobins have been shown to be involved in scavenging of nitric oxide (NO) molecules, which play a key role in oxygen sensing/balancing in plants and animals. Steady state levels of NO are suggested to act as an integrator of energy and carbon metabolism and subsequently, influence energy-demanding growth processes in plants.

RESULTS

We aimed to manipulate oxygen stress perception in Arabidopsis seeds by overexpression of the non-symbiotic hemoglobin AtHb1 under the control of the seed-specific LeB4 promoter. Seeds of transgenic AtHb1 plants did not accumulate NO under transient hypoxic stress treatment, showed higher respiratory activity and energy status compared to the wild type. Global transcript profiling of seeds/siliques from wild type and transgenic plants under transient hypoxic and standard conditions using Affymetrix ATH1 chips revealed a rearrangement of transcriptional networks by AtHb1 overexpression under non-stress conditions, which included the induction of transcripts related to ABA synthesis and signaling, receptor-like kinase- and MAP kinase-mediated signaling pathways, WRKY transcription factors and ROS metabolism. Overexpression of AtHb1 shifted seed metabolism to an energy-saving mode with the most prominent alterations occurring in cell wall metabolism. In combination with metabolite and physiological measurements, these data demonstrate that AtHb1 overexpression improves oxidative stress tolerance compared to the wild type where a strong transcriptional and metabolic reconfiguration was observed in the hypoxic response.

CONCLUSIONS

AtHb1 overexpression mediates a pre-adaptation to hypoxic stress. Under transient stress conditions transgenic seeds were able to keep low levels of endogenous NO and to maintain a high energy status, in contrast to wild type. Higher weight of mature transgenic seeds demonstrated the beneficial effects of seed-specific overexpression of AtHb1.

Regulation of nonsymbiotic and truncated hemoglobin genes of Lotus japonicus in plant organs and in response to nitric oxide and hormones

• In legumes, symbiotic leghemoglobins facilitate oxygen diffusion to the bacteroids, but the roles of nonsymbiotic and truncated hemoglobins are largely unknown. Here the five hemoglobin genes of Lotus japonicus have been functionally characterized to gain insight into their regulatory mechanisms. • Plants were exposed to nitric oxide donors, stressful conditions, and hormones. Gene expression profiling was determined by quantitative PCR, and gene activities were localized using in situ hybridization and promoter-reporter gene fusions. • The LjGLB1-1, LjGLB2, and LjGLB3-1 mRNA expression levels were very high in nodules relative to other plant organs. The expression of these genes was localized in the vascular bundles, cortex, and infected tissue. LjGLB1-1 was the only gene induced by nitric oxide. Cytokinins caused nearly complete inactivation of LjGLB2 and LjGLB3-1 in nodules and induction of LjGLB1-1 in roots. Abscisic acid induced LjGLB1-1 in nodules and LjGLB1-2 and LjGLB2 in roots, whereas polyamines and jasmonic acid induced LjGLB1-1 only in roots. • The enhanced expression of the three types of hemoglobins in nodules, the colocalization of gene activities in nodule and root tissues with high metabolic rates, and their distinct regulatory mechanisms point out complementary roles of hemoglobins and strongly support the hypothesis that LjGLB1-1, LjGLB2, and LjGLB3-1 are required for symbiosis.

The single-domain globin of Vitreoscilla: augmentation of aerobic metabolism for biotechnological applications

Extensive studies have revealed that large-scale, high-cell density bioreactor cultivations have significant impact on metabolic networks of oxygen-requiring production organisms. Oxygen transfer problems associated with fluid dynamics and inefficient mixing efficiencies result in oxygen gradients, which lead to reduced performance of the bioprocess, decreased product yields, and increased production costs. These problems can be partially alleviated by improving bioreactor configuration and setting, but significant improvements have been achieved by metabolic engineering methods, especially by heterologously expressing Vitreoscilla hemoglobin (VHb). Vast numbers of studies have been accumulating during the past 20 years showing the applicability of VHb to improve growth and product yields in a variety of industrially significant prokaryotic and eukaryotic hosts. The global view on the metabolism of globin-expressing Escherichia coli cells depicts increased energy generation, higher oxygen uptake rates, and a decrease in fermentative by-product excretion. Transcriptome and metabolic flux analysis clearly demonstrate the multidimensional influence of heterologous VHb on the expression of stationary phase-specific genes and on the regulation of cellular metabolic networks. The exact biochemical mechanisms by which VHb is able to improve the oxygen-limited growth remain poorly understood. The suggested mechanisms propose either the delivery of oxygen to the respiratory chain or the detoxification of reactive nitrogen species for the protection of cytochrome activity. The expression of VHb in E. coli bioreactor cultures is likely to assist bacterial growth through providing an increase in available intracellular oxygen, although to fully understand the exact role of VHb in vivo, further analysis will be required.

Reactive oxygen species and nitric oxide in plant mitochondria: origin and redundant regulatory systems

Plant mitochondria differ from their mammalian counterparts in many respects, which are due to the unique and variable surroundings of plant mitochondria. In green leaves, plant mitochondria are surrounded by ample respiratory substrates and abundant molecular oxygen, both resulting from active photosynthesis, while in roots and bulky rhizomes and fruit carbohydrates may be plenty, whereas oxygen levels are falling. Several enzymatic complexes in mitochondrial electron transport chain (ETC) are capable of reactive oxygen species (ROS) formation under physiological and pathological conditions. Inherently connected parameters such as the redox state of electron carriers in the ETC, ATP synthase activity and inner mitochondrial membrane potential, when affected by external stimuli, can give rise to ROS formation via complexes I and III, and by reverse electron transport (RET) from complex II. Superoxide radicals produced are quickly scavenged by superoxide dismutase (MnSOD), and the resulting H(2)O(2) is detoxified by peroxiredoxin-thioredoxin system or by the enzymes of ascorbate-glutathione cycle, found in the mitochondrial matrix. Arginine-dependent nitric oxide (NO)-releasing activity of enzymatic origin has been detected in plant mitochondria. The molecular identity of the enzyme is not clear but the involvement of mitochondria-localized enzymes responsible for arginine catabolism, arginase and ornithine aminotransferase has been shown in the regulation of NO efflux. Besides direct control by antioxidants, mitochondrial ROS production is tightly controlled by multiple redundant systems affecting inner membrane potential: NAD(P)H-dependent dehydrogenases, alternative oxidase (AOX), uncoupling proteins, ATP-sensitive K(+) channel and a number of matrix and intermembrane enzymes capable of direct electron donation to ETC. NO removal, on the other hand, takes place either by reactions with molecular oxygen or superoxide resulting in peroxynitrite, nitrite or nitrate ions or through interaction with non-symbiotic hemoglobins or glutathione. Mitochondrial ROS and NO production is tightly controlled by multiple redundant systems providing the regulatory mechanism for redox homeostasis and specific ROS/NO signaling.

Induction of Pseudomonas aeruginosa fhp and fhpR by reactive oxygen species

In Pseudomonas aeruginosa, flavohemoglobin (Fhp) and its cognate regulator FhpR (PA2665) form a protective regulatory circuit, which responds to reactive nitrogen species and is also capable of protecting cells against nitrosative stress. Recently, it has been shown that the expression of the fhp promoter is regulated not only by FhpR, but also by two new regulators, PA0779 and PA3697. It has also been suggested that the bacterial flavohemoglobins (flavoHbs) could play a crucial role in the protection of cells against reactive oxygen species (ROS). Therefore, the role and function of the Fhp/FhpR system during oxidative stress were studied by assessing the viability and membrane integrity of P. aeruginosa cells and by analyzing the promoter activities of fhp and fhpR upon exposure to paraquat, hydrogen peroxide, and tert-butyl hydroperoxide, under both aerobic and low-oxygen conditions. The results showed that under aerobic conditions, both fhp and fhpR promoters are induced by ROS generated by the stressors. Thus, the Fhp/FhpR system is implicated in the oxidative stress response. ROS-induced fhp promoter activity was dependent on FhpR, PA0779, and PA3697 regulators. Tert-butyl hydroperoxide-induced fhpR promoter activity was found to be highly repressed by PA0779, and FhpR showed negative autoregulation of its own promoter. Under low-oxygen conditions, the activity of the fhp promoter was not inducible by ROS, but fhpR promoter activity was induced by paraquat, and hydrogen peroxide was repressed in both cases by the regulators PA0779 and PA3697.

Production of L-DOPA and dopamine in recombinant bacteria bearing the Vitreoscilla hemoglobin gene

Given the well-established beneficial effects of Vitreoscilla hemoglobin (VHb) on heterologous organisms, the potential of this protein for the production of L-DOPA and dopamine in two bacteria, Citrobacter freundii and Erwinia herbicola, was investigated. The constructed recombinants bearing the VHb gene (vgb(+)) had substantially higher levels of cytoplasmic L-DOPA (112 mg/L for C. freundii and 97 mg/L for E. herbicola) than their respective hosts (30.4 and 33.8 mg/L) and the vgb(-) control strains (35.6 and 35.8 mg/L). Further, the vgb(+) recombinants of C. freundii and E. herbicola had 20-fold and about two orders of magnitude higher dopamine levels than their hosts, repectively. The activity of tyrosine phenol-lyase, the enzyme converting L-tyrosine to L-DOPA, was well-correlated to cytoplasmic L-DOPA levels. As cultures aged, higher tyrosine phenol-lyase activity of the vgb(+) strains was more apparent.