Reactive oxygen species-induced release of intracellular ascorbate in plant cell-suspension cultures and evidence for pulsing of net release rate

The ascorbate biosynthesis pathway in plants is known, but there is a way to go with understanding control and functions

Ascorbate (vitamin C) is one of the most abundant primary metabolites in plants. Its complex chemistry enables it to function as an antioxidant, as a free radical scavenger, and as a reductant for iron and copper. Ascorbate biosynthesis occurs via the mannose/l-galactose pathway in green plants, and the evidence for this pathway being the major route is reviewed. Ascorbate accumulation is leaves is responsive to light, reflecting various roles in photoprotection. GDP-l-galactose phosphorylase (GGP) is the first dedicated step in the pathway and is important in controlling ascorbate synthesis. Its expression is determined by a combination of transcription and translation. Translation is controlled by an upstream open reading frame (uORF) which blocks translation of the main GGP-coding sequence, possibly in an ascorbate-dependent manner. GGP associates with a PAS-LOV protein, inhibiting its activity, and dissociation is induced by blue light. While low ascorbate mutants are susceptible to oxidative stress, they grow nearly normally. In contrast, mutants lacking ascorbate do not grow unless rescued by supplementation. Further research should investigate possible basal functions of ascorbate in severely deficient plants involving prevention of iron overoxidation in 2-oxoglutarate-dependent dioxygenases and iron mobilization during seed development and germination.

Physio-biochemical and metabolomic responses of the woody plant Dalbergia odorifera to salinity and waterlogging

BACKGROUND

Trees have developed a broad spectrum of molecular mechanisms to counteract oxidative stress. Secondary metabolites via phenolic compounds emblematized the hidden bridge among plant kingdom, human health, and oxidative stress. Although studies have demonstrated that abiotic stresses can increase the production of medicinal compounds in plants, research comparing the efficiency of these stresses still needs to be explored. Thus, the present research paper provided an exhaustive comparative metabolomic study in Dalbergia odorifera under salinity (ST) and waterlogging (WL).

RESULTS

High ST reduced D. odorifera's fresh biomass compared to WL. While WL only slightly affected leaf and vein size, ST had a significant negative impact. ST also caused more significant damage to water status and leaflet anatomy than WL. As a result, WL-treated seedlings exhibited better photosynthesis and an up-regulation of nonenzymatic pathways involved in scavenging reactive oxygen species. The metabolomic and physiological responses of D. odorifera under WL and salinity ST stress revealed an accumulation of secondary metabolites by the less aggressive stress (WL) to counterbalance the oxidative stress. Under WL, more metabolites were more regulated compared to ST. ST significantly altered the metabolite profile in D. odorifera leaflets, indicating its sensitivity to salinity. WL synthesized more metabolites involved in phenylpropanoid, flavone, flavonol, flavonoid, and isoflavonoid pathways than ST. Moreover, the down-regulation of L-phenylalanine correlated with increased p-coumarate, caffeate, and ferulate associated with better cell homeostasis and leaf anatomical indexes under WL.

CONCLUSIONS

From a pharmacological and medicinal perspective, WL improved larger phenolics with therapeutic values compared to ST. Therefore, the data showed evidence of the crucial role of medical tree species' adaptability on ROS detoxification under environmental stresses that led to a significant accumulation of secondary metabolites with therapeutic value.

Quantifying determinants of ozone detoxification by apoplastic ascorbate in peach (Prunus persica) leaves using a model of ozone transport and reaction

Ascorbate in leaf apoplast (ASC_apo ) reacts with ozone (O₃ ) and thereby reduces O₃ flux reaching plasmalemma (F_pl ). Some studies have shown significant protection of cells from O₃ by ASC_apo , while others have questioned its efficacy. Hypothesizing that the protection by ASC_apo depends on other variables, we quantified determinants of O₃ detoxification with a model of O₃ transport and reaction in apoplast. The model determines ascorbic acid concentration in apoplast (AA_apo ) using measured values of O₃ concentration (c_o ), leaf tissue ascorbic acid concentration (AA_leaf ), cell wall thickness (L₃ ), apoplastic pH (pH_apo ), and stomatal conductance (G_sw ). We compared the measured and model-estimated AA_apo in leaves of peach (Prunus persica) grown in open-top chambers under non-filtered air (NF) and elevated (EO₃ : NF + 80 ppb) O₃ concentrations. The estimated AA_apo in individual leaves agreed well with the measured values (R²  = .91). Analyses of the simulation results yielded the following findings: (a) The efficacy of O₃ reduction with ASC_apo as quantified by fractional reduction (ϕ₃ ) of O₃ flux at the surface of plasmalemma (F_pl ) was lowered from 70% in NF to 40% in EO₃ due to the reduction of L₃ . The EO₃ reduced AA_apo , but the lower G_sw and L₃ in EO₃ increased AA_apo resulting in no significant change in AA_apo due to EO₃ . ϕ₃ can be calculated with measured values of AA_apo and L₃ , and F_pl can be estimated with the measurement-based ϕ₃ . (b) When c₀ is increased, F_pl increased curvilinearly with the increase of F_st : nominal O₃ flux via stomatal diffusion, exhibiting apparent threshold on F_st . The deviation of F_pl from F_st became greater when L₃ , pH_apo , and AA_leaf were increased. The quantification of ϕ₃ and F_pl using leaf traits shall facilitate the understanding of the mechanisms of differential plant sensitivity to O₃ and improve quantification of the O₃ impacts on plants.

CO2 capture and sequestration in stable Ca-oxalate, via Ca-ascorbate promoted green reaction

The increase in the amount of carbon dioxide (CO₂) emissions related to many anthropic activities is a persistent and growing problem. During the last years, many solutions have been set out, none of them being the ultimate one. Investigators agree on the need of a synergic approach to the problem, in terms of many complementary methods of sequestration that, combined with the reduction of production, will be able to decrease the concentration of the CO₂ in the atmosphere. In this work, we explore the use of a green reaction to trap the CO₂ into a stable crystalline phase (weddellite) resorting to a multidisciplinary approach. CO₂ is reduced and precipitated as calcium oxalate through vitamin C as a sacrificial reductant. Calcium oxalate crystals obtained show a startling good quality that increases their already great stability over a wide chemical and physical conditions' range.

Biotic priming with Pseudomonas fluorescens induce drought stress tolerance in Abelmoschus esculentus (L.) Moench (Okra)

Drought is a major abiotic stress which negatively affects plant growth and yield. The decrease in rainfall and ensuing drought affect crop production to a larger extent. The present investigation was aimed to evaluate the drought stress mitigation potential of Abelmoschus esculentus (L.) Moench (okra) plants primed with Pseudomonas fluorescens (PF). Okra seeds were primed with 10-7 CFU of PF, germinated and the plants were exposed to drought stress for 7 days, and the recovery potential was assessed after re-watering the plants. Physiological and biochemical parameters were evaluated during stress and recovery. PF treated plants mitigated the effect of drought stress by increasing relative water content (RWC), accumulated metabolites such as sugar, free amino acids and enhanced the activity of non enzymatic antioxidants; phenolics, ascorbate (AsA) and glutathione (GSH) and reactive oxygen species scavenging enzyme like superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and guaicol peroxidase (GPX). Drought stress related membrane damage and protein denaturation were also alleviated by PF treatment. During re-watering, PF treated plants regained RWC, total sugar, total amino acid, protein, AsA, GSH, phenolics, SOD, CAT, APX and GPX to appreciable levels. Thus, this study suggests that PF can be used as an agent to effectively mitigate drought stress in okra plants.

The oxidation of dehydroascorbic acid and 2,3-diketogulonate by distinct reactive oxygen species

l-Ascorbate, dehydro-l-ascorbic acid (DHA), and 2,3-diketo-l-gulonate (DKG) can all quench reactive oxygen species (ROS) in plants and animals. The vitamin C oxidation products thereby formed are investigated here. DHA and DKG were incubated aerobically at pH 4.7 with peroxide (H2O2), 'superoxide' (a ∼50 : 50 mixture of [Formula: see text] and [Formula: see text]), hydroxyl radicals (•OH, formed in Fenton mixtures), and illuminated riboflavin (generating singlet oxygen, 1O2). Products were monitored electrophoretically. DHA quenched H2O2 far more effectively than superoxide, but the main products in both cases were 4-O-oxalyl-l-threonate (4-OxT) and smaller amounts of 3-OxT and OxA + threonate. H2O2, but not superoxide, also yielded cyclic-OxT. Dilute Fenton mixture almost completely oxidised a 50-fold excess of DHA, indicating that it generated oxidant(s) greatly exceeding the theoretical •OH yield; it yielded oxalate, threonate, and OxT. 1O2 had no effect on DHA. DKG was oxidatively decarboxylated by H2O2, Fenton mixture, and 1O2, forming a newly characterised product, 2-oxo-l-threo-pentonate (OTP; '2-keto-l-xylonate'). Superoxide yielded negligible OTP. Prolonged H2O2 treatment oxidatively decarboxylated OTP to threonate. Oxidation of DKG by H2O2, Fenton mixture, or 1O2 also gave traces of 4-OxT but no detectable 3-OxT or cyclic-OxT. In conclusion, DHA and DKG yield different oxidation products when attacked by different ROS. DHA is more readily oxidised by H2O2 and superoxide; DKG more readily by 1O2 The diverse products are potential signals, enabling organisms to respond appropriately to diverse stresses. Also, the reaction-product 'fingerprints' are analytically useful, indicating which ROS are acting in vivo.

Ascorbic acid metabolism and functions: A comparison of plants and mammals

Ascorbic acid is synthesised by eukaryotes, the known exceptions being primates and some other animal groups which have lost functional gulonolactone oxidase. Prokaryotes do not synthesise ascorbate and do not need an ascorbate supply, so the functions that are essential for mammals and plants are not required or are substituted by other compounds. The ability of ascorbate to donate electrons enables it to act as a free radical scavenger and to reduce higher oxidation states of iron to Fe²⁺. These reactions are the basis of its biological activity along with the relative stability of the resulting resonance stabilised monodehydroascorbate radical. The importance of these properties is emphasised by the evolution of at least three biosynthetic pathways and production of an ascorbate analogue, erythroascorbate, by fungi. The iron reducing activity of ascorbate maintains the reactive centre Fe²⁺ of 2-oxoglutarate-dependent dioxygenases (2-ODDs) thus preventing inactivation. These enzymes have diverse functions and, recently, the possibility that ascorbate status in mammals could influence 2-ODDs involved in histone and DNA demethylation thereby influencing stem cell differentiation and cancer has been uncovered. Ascorbate is involved in iron uptake and transport in plants and animals. While the above biochemical functions are shared between mammals and plants, ascorbate peroxidase (APX) is an enzyme family limited to plants and photosynthetic protists. It provides these organisms with increased capacity to remove H₂O₂ produced by photosynthetic electron transport and photorespiration. The Fe reducing activity of ascorbate enables hydroxyl radical production (pro-oxidant effect) and the reactivity of dehydroascorbate (DHA) and reaction of its degradation products with proteins (dehydroascorbylation and glycation) is potentially damaging. Ascorbate status influences gene expression in plants and mammals but at present there is little evidence that it acts as a specific signalling molecule. It most likely acts indirectly by influencing the redox state of thiols and 2-ODD activity. However, the possibility that dehydroascorbylation is a regulatory post-translational protein modification could be explored.

Novel roles of ascorbate in plants: induction of cytosolic Ca2+ signals and efflux from cells via anion channels

Ascorbate is not often considered as a signalling molecule in plants. This study demonstrates that, in Arabidopsis roots, exogenous l-ascorbic acid triggers a transient increase of the cytosolic free calcium activity ([Ca2+]cyt.) that is central to plant signalling. Exogenous copper and iron stimulate the ascorbate-induced [Ca2+]cyt. elevation, while cation channel blockers, free radical scavengers, low extracellular [Ca2+], transition metal chelators, and removal of the cell wall inhibit this reaction. These data show that apoplastic redox-active transition metals are involved in the ascorbate-induced [Ca2+]cyt. elevation. Exogenous ascorbate also induces a moderate increase in programmed cell death symptoms in intact roots, but it does not activate Ca2+ influx currents in patch-clamped root protoplasts. Intriguingly, the replacement of gluconate with ascorbate in the patch-clamp pipette reveals a large ascorbate efflux current, which shows sensitivity to the anion channel blocker, anthracene-9-carboxylic acid (A9C), indicative of the ascorbate release via anion channels. EPR spectroscopy measurements demonstrate that salinity (NaCl) triggers the accumulation of root apoplastic ascorbyl radicals in an A9C-dependent manner, confirming that l-ascorbate leaks through anion channels under depolarization. This mechanism may underlie ascorbate release, signalling phenomena, apoplastic redox reactions, iron acquisition, and control the ionic and electrical equilibrium (together with K+ efflux via GORK channels).

An Anion Conductance, the Essential Component of the Hydroxyl-Radical-Induced Ion Current in Plant Roots

Oxidative stress signaling is essential for plant adaptation to hostile environments. Previous studies revealed the essentiality of hydroxyl radicals (HO•)-induced activation of massive K⁺ efflux and a smaller Ca2+ influx as an important component of plant adaptation to a broad range of abiotic stresses. Such activation would modify membrane potential making it more negative. Contrary to these expectations, here, we provide experimental evidence that HO• induces a strong depolarization, from -130 to -70 mV, which could only be explained by a substantial HO•-induced efflux of intracellular anions. Application of Gd3+ and NPPB, non-specific blockers of cation and anion conductance, respectively, reduced HO•-induced ion fluxes instantaneously, implying a direct block of the dual conductance. The selectivity of an early instantaneous HO•-induced whole cell current fluctuated from more anionic to more cationic and vice versa, developing a higher cation selectivity at later times. The parallel electroneutral efflux of K⁺ and anions should underlie a substantial leak of the cellular electrolyte, which may affect the cell's turgor and metabolic status. The physiological implications of these findings are discussed in the context of cell fate determination, and ROS and cytosolic K⁺ signaling.

AIR12, a b-type cytochrome of the plasma membrane of Arabidopsis thaliana is a negative regulator of resistance against Botrytis cinerea

AIR12 (Auxin Induced in Root culture) is a single gene of Arabidopsis that codes for a mono-heme cytochrome b. Recombinant AIR12 from Arabidopsis accepted electrons from ascorbate or superoxide, and donated electrons to either monodehydroascorbate or oxygen. AIR12 was found associated in vivo to the plasma membrane. Though linked to the membrane by a glycophosphatidylinositol anchor, AIR12 is a hydrophilic and glycosylated protein predicted to be fully exposed to the apoplast. The expression pattern of AIR12 in Arabidopsis is developmentally regulated and correlated to sites of controlled cell separation (e.g. micropilar endosperm during germination, epidermal cells surrounding the emerging lateral root) and cells around wounds. Arabidopsis (Landsberg erecta-0) mutants with altered levels of AIR12 did not show any obvious phenotype. However, AIR12-overexpressing plants accumulated ROS (superoxide, hydrogen peroxide) and lipid peroxides in leaves, indicating that AIR12 may alter the redox state of the apoplast under particular conditions. On the other hand, AIR12-knock out plants displayed a strongly decreased susceptibility to Botrytis cinerea infection, which in turn induced AIR12 expression in susceptible wild type plants. Altogether, the results suggest that AIR12 plays a role in the regulation of the apoplastic redox state and in the response to necrotrophic pathogens. Possible relationships between these functions are discussed.

Plant signalling in acute ozone exposure

Exposure of plants to high ozone concentrations causes lesion formation in sensitive plants. Plant responses to ozone involve fast and massive changes in protein activities, gene expression and metabolism even before any tissue damage can be detected. Degradation of ozone and subsequent accumulation of reactive oxygen species (ROS) in the extracellular space activates several signalling cascades, which are integrated inside the cell into a fine-balanced network of ROS signalling. Reversible protein phosphorylation and degradation plays an important role in the regulation of signalling mechanisms in a complex crosstalk with plant hormones and calcium, an essential second messenger. In this review, we discuss the recent advances in understanding the molecular mechanisms of ozone uptake, perception and signalling pathways activated during the early steps of ozone response, and discuss the use of ozone as a tool to study the function of apoplastic ROS in signalling.

The hydroxyl radical in plants: from seed to seed

The hydroxyl radical (OH(•)) is the most potent yet short-lived of the reactive oxygen species (ROS) radicals. Just as hydrogen peroxide was once considered to be simply a deleterious by-product of oxidative metabolism but is now acknowledged to have signalling roles in plant cells, so evidence is mounting for the hydroxyl radical as being more than merely an agent of destruction. Its oxidative power is harnessed to facilitate germination, growth, stomatal closure, reproduction, the immune response, and adaptation to stress. It features in plant cell death and is a key tool in microbial degradation of plant matter for recycling. Production of the hydroxyl radical in the wall, at the plasma membrane, and intracellularly is facilitated by a range of peroxidases, superoxide dismutases, NADPH oxidases, and transition metal catalysts. The spatio-temporal activity of these must be tightly regulated to target substrates precisely to the site of radical production, both to prevent damage and to accommodate the short half life and diffusive capacity of the hydroxyl radical. Whilst research has focussed mainly on the hydroxyl radical's mode of action in wall loosening, studies now extend to elucidating which proteins are targets in signalling systems. Despite the difficulties in detecting and manipulating this ROS, there is sufficient evidence now to acknowledge the hydroxyl radical as a potent regulator in plant cell biology.

A role for oxalic acid generation in ozone-induced signallization in Arabidopis cells

Ozone (O(3) ) is an air pollutant with an impact increasingly important in our industrialized world. It affects human health and productivity in various crops. We provide the evidences that treatment of Arabidopsis thaliana with O(3) results in ascorbate-derived oxalic acid production. Using cultured cells of A. thaliana as a model, here we further showed that oxalic acid induces activation of anion channels that trigger depolarization of the cell, increase in cytosolic Ca(2+) concentration, generation of reactive oxygen species and cell death. We confirmed that O(3) reacts with ascorbate in the culture, thus resulting in production of oxalic acid and this could be part of the O(3) -induced signalling pathways that trigger programmed cell death.

Re-evaluating the role of ascorbic acid and phenolic glycosides in ozone scavenging in the leaf apoplast of Arabidopsis thaliana L

Phenolic glycosides are effective reactive oxygen scavengers and peroxidase substrates, suggesting that compounds in addition to ascorbate may have functional importance in defence responses against ozone (O(3)), especially in the leaf apoplast. The apoplastic concentrations of ascorbic acid (AA) and phenolic glycosides in Arabidopsis thaliana L. Col-0 wild-type plants were determined following exposure to a range of O(3) concentrations (5, 125 or 175 nL L(-1)) in controlled environment chambers. AA in leaf apoplast extracts was almost entirely oxidized in all treatments, suggesting that O(3) scavenging by direct reactions with reduced AA was very limited. In regard to phenolics, O(3) stimulated transcription of numerous phenylpropanoid pathway genes and increased the apoplastic concentration of sinapoyl malate. However, modelling of O(3) scavenging in the apoplast indicated that sinapoyl malate concentrations were too low to be effective protectants. Furthermore, null mutants for sinapoyl esters (fah1-7), kaempferol glycosides (tt4-1) and the double mutant (tt4-1/fah1-7) were equally sensitive to chronic O(3) as Ler-0 wild-type plants. These results indicate that current understanding of O(3) defence schemes deserves reassessment as mechanisms other than direct scavenging of O(3) by extracellular AA and antioxidant activity of some phenolics may predominate in some plant species.

Oxidation of dehydroascorbic acid and 2,3-diketogulonate under plant apoplastic conditions

The rate of L-ascorbate catabolism in plants often correlates positively with the rate of cell expansion. The reason for this correlation is difficult to explore because of our incomplete knowledge of ascorbate catabolism pathways. These involve enzymic and/or non-enzymic oxidation to dehydroascorbic acid (DHA), which may then be hydrolysed to 2,3-diketogulonate (DKG). Both DHA and DKG were susceptible to further oxidation under conditions of pH and H₂O₂ concentration comparable with the plant apoplast. The kinetics of their oxidation and the identity of some of the products have been investigated here. DHA, whether added in pure form or generated in situ by ascorbate oxidation, was oxidised non-enzymically to yield, almost simultaneously, a monoanion (cyclic-oxalyl-threonate; cOxT) and a dianion (oxalyl-threonate; OxT). The monoanion was resistant to periodate oxidation, showing that it was not oxalic threonic anhydride. The OxT population was shown to be an interconverting mixture of 3-OxT and 4-OxT, differing in pK(a). The 3-OxT appeared to be formed earlier than 4-OxT, but the latter predominated at equilibrium. DKG was oxidised by H₂O₂ to two partially characterised products, one of which was itself further oxidised by H₂O₂ to yield threonate. The possible occurrence of these reactions in the apoplast in vivo and the biological roles of vitamin C catabolites are discussed.

Alternative pathways of dehydroascorbic acid degradation in vitro and in plant cell cultures: novel insights into vitamin C catabolism

L-Ascorbate catabolism involves reversible oxidation to DHA (dehydroascorbic acid), then irreversible oxidation or hydrolysis. The precursor–product relationships and the identity of several major DHA breakdown products remained unclear. In the presence of added H2O2, DHA underwent little hydrolysis to DKG (2,3-dioxo-L-gulonate). Instead, it yielded OxT (oxalyl L-threonate), cOxT (cyclic oxalyl L-threonate) and free oxalate (~6:1:1), essentially simultaneously, suggesting that all three product classes independently arose from one reactive intermediate, proposed to be cyclic-2,3-O-oxalyl-L-threonolactone. Only with plant apoplastic esterases present were the esters significant precursors of free oxalate. Without added H2O2, DHA was slowly hydrolysed to DKG. Downstream of DKG was a singly ionized dicarboxy compound (suggested to be 2-carboxy-L-xylonolactone plus 2-carboxy-L-lyxonolactone), which reversibly de-lactonized to a dianionic carboxypentonate. Formation of these lactones and acid was minimized by the presence of residual unreacted ascorbate. In vivo, the putative 2-carboxy-L-pentonolactones were relatively stable. We propose that DHA is a branch-point in ascorbate catabolism, being either oxidized to oxalate and its esters or hydrolysed to DKG and downstream carboxypentonates. The oxidation/hydrolysis ratio is governed by reactive oxygen species status. In vivo, oxalyl esters are enzymatically hydrolysed, but the carboxypentonates are stable. The biological roles of these ascorbate metabolites invite future exploration.

Changes in cinnamic acid derivatives associated with the habituation of maize cells to dichlobenil

The habituation of cell cultures to cellulose biosynthesis inhibitors such as dichlobenil (DCB) represents a valuable tool to improve our knowledge of the mechanisms involved in plant cell wall structural plasticity. Maize cell lines habituated to lethal concentrations of DCB were able to grow through the acquisition of a modified cell wall in which cellulose was partially replaced by a more extensive network of arabinoxylans. The aim of this work was to investigate the phenolic metabolism of non-habituated and DCB-habituated maize cell cultures. Maize cell cultures were fed [(14)C]cinnamate and the fate of the radioactivity in different intra-protoplasmic and wall-localized fractions throughout the culture cycle was analyzed by autoradiography and scintillation counting. Non-habituated and habituated cultures did not markedly differ in their ability to uptake exogenous [(14)C]cinnamic acid. However, interesting differences were found in the radiolabeling of low- and high-M(r) metabolites. Habituated cultures displayed a higher number and amount of radiolabeled low-M(r) compounds, which could act as reserves later used for polysaccharide feruloylation. DCB-habituated cultures were highly enriched in esterified [(14)C]dehydrodiferulates and larger coupling products. In conclusion, an extensive and early cross-linking of hydroxycinnamates was observed in DCB-habituated cultures, probably strengthening their cellulose-deficient walls.