Antioxidant status, peroxidase activity, and PR protein transcript levels in ascorbate-deficient Arabidopsis thaliana vtc mutants

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.

Bio-priming with a Novel Plant Growth-Promoting Acinetobacter indicus Strain Alleviates Arsenic-Fluoride Co-toxicity in Rice by Modulating the Physiome and Micronutrient Homeostasis

Sustainable remediation of arsenic-fluoride from rice fields through efficient bio-extraction is the need of the hour, since these toxicants severely challenge safe cultivation of rice and food biosafety. In the present study, we screened an arsenic-fluoride tolerant strain AB-ARC of Acinetobacter indicus from the soil of a severely polluted region of West Bengal, India, which was capable of efficiently removing extremely high doses of arsenate and fluoride from the media. The strain also behaved as a plant growth-promoting rhizobacterium, since it could produce indole-3-acetic acid and solubilize phosphate, zinc, and starch. Due to these properties of the identified strain, it was used for bio-priming the seeds of the arsenic-fluoride susceptible rice cultivar, Khitish for testing the efficacy of the AB-ARC strain to promote combined arsenic-fluoride tolerance in the rice genotype. Bio-priming with AB-ARC led to accelerated uptake of crucial elements like iron, copper, and nickel which behave as co-factors of physiological and antioxidative enzymes. Thus, the activation of superoxide dismutase, catalase, guaiacol peroxidase, glutathione peroxidase, and glutathione-S-transferase enabled detoxification of reactive oxygen species (ROS) and reduction of the oxidative injuries like malondialdehyde and methylglyoxal generation. Overall, due to ameliorated molecular damages and low uptake of the toxic xenobiotics, the plants were able to maintain improved growth vigor and photosynthesis, as evident from the elevated levels of Hill activity and chlorophyll content. Hence, bio-priming with the A. indicus AB-ARC strain may be advocated for sustainable rice cultivation in arsenic-fluoride co-polluted fields.

Melatonin application differentially modulates the enzymes associated with antioxidative machinery and ascorbate-glutathione cycle during arsenate exposure in indica rice varieties

Arsenic (As) contamination and accumulation in rice is a serious concern causing severe oxidative damage. Melatonin acts as a protective agent in plant defence against multiple abiotic stresses. The mechanism of antioxidant function of melatonin during As stress in rice genotypes is less studied. In this study, hydroponically-grown As-susceptible (Khitish) and As-tolerant (Muktashri) rice cultivars, subjected to 150 µm arsenate stress, were supplemented with exogenously applied melatonin (20 µm) to examine the plant defence mechanism. Melatonin (Mel) increased root and shoot length, fresh and dry weight, chlorophyll a and b content and activated reducing power and free radical scavenging capacity in both rice cultivars. The role of Mel in the sensitive variety appeared to be more prominent with respect to reduced water saturation deficit by reducing endogenous As and H₂ O₂ accumulation, and enhancing overall antioxidant capacity by imposing reduced requirement of catalase for ROS detoxification, and restoring As-inhibited activity of glutathione-S-transferase, glutathione peroxidase and dehydroascorbate reductase. In contrast, melatonin treatment in the tolerant cultivar required reduced involvement of ascorbate peroxidase to deal with As toxicity, and complemented the stress-mediated inhibition of guaiacol peroxidase activity. Isozyme profiling also established extensive varietal differences with regard to induction of new isoform(s) by Mel during As treatment. This study provides clear insights into mechanistic details of the regulation of antioxidative enzymes by melatonin in contrasting rice genotypes, which may prove helpful in generating As tolerance in susceptible rice varieties grown in marginalized soils, thereby improving crop yield and productivity.

Chloroplast immunity illuminated

The chloroplast has recently emerged as pivotal to co-ordinating plant defence responses and as a target of plant pathogens. Beyond its central position in oxygenic photosynthesis and primary metabolism - key targets in the complex virulence strategies of diverse pathogens - the chloroplast integrates, decodes and responds to environmental signals. The capacity of chloroplasts to synthesize phytohormones and a diverse range of secondary metabolites, combined with retrograde and reactive oxygen signalling, provides exquisite flexibility to both perceive and respond to biotic stresses. These processes also represent a plethora of opportunities for pathogens to evolve strategies to directly or indirectly target 'chloroplast immunity'. This review covers the contribution of the chloroplast to pathogen associated molecular pattern and effector triggered immunity as well as systemic acquired immunity. We address phytohormone modulation of immunity and surmise how chloroplast-derived reactive oxygen species underpin chloroplast immunity through indirect evidence inferred from genetic modification of core chloroplast components and direct pathogen targeting of the chloroplast. We assess the impact of transcriptional reprogramming of nuclear-encoded chloroplast genes during disease and defence and look at future research challenges.

Glutaredoxin like protein (RtGRL1) regulates H2O2 and Na+ accumulation by maintaining the glutathione pool during abiotic stress

Reaumuria trigyna, an endangered recretohalophyte, is a small archaic wild shrub endemic to arid and semiarid plateau regions of Inner Mongolia, China. Based on salt-related transcriptomic data, we isolated a GRX family gene, glutaredoxin like protein (RtGRL1), from R. trigyna that is associated with the removal of active oxygen and regulation of redox status. RtGRL1 encodes a plasma membrane and chloroplast-localized protein induced by salt, cold, drought stress, ABA, and H₂O₂. In Arabidopsis thaliana, ectopically expressed RtGRL1 positively regulated biomass accumulation, chlorophyll content, germination rate, and primary root length under salt and drought stress. Overexpression of RtGRL1 induced expression of genes related to antioxidant enzymes and proline biosynthesis, thus increasing glutathione biosynthesis, glutathione-dependent detoxification of reactive oxygen species (ROS), and proline content under stress. Changes in RtGRL1 expression consistently affected glutathione/oxidizedglutathione and ascorbate/dehydroascorbate ratios and H₂O₂ concentrations. Furthermore, RtGRL1 promoted several GSH biosynthesis gene transcripts, decreased leaf Na⁺ content, and maintained lower Na⁺/K⁺ ratios in transgenic A. thaliana compared to wild type plants. These results suggest a critical link between RtGRL1 and ROS modulation, and contribute to a better understanding of the mechanisms governing plant responses to drought and salt stress.

Silicon nanoparticle-pulsing mitigates fluoride stress in rice by fine-tuning the ionomic and metabolomic balance and refining agronomic traits

The present manuscript investigates the roles of silicon nanoparticles (SiNPs) in ameliorating fluoride toxicity in the susceptible rice cultivar, IR-64. Fluoride toxicity reduced overall growth and yield by suppressing grain development. Fluoride stress alarmingly increased the accumulation of cobalt, which together with fluoride triggered electrolyte leakage, malondialdehyde, methylglyoxal and hydrogen peroxide accumulation and NADPH oxidase activity. The overall photosynthesis was compromised due to chlorosis and inhibited Hill activity. Nano-Si-priming efficiently ameliorated molecular injuries and restored yield by reducing fluoride bioaccumulation particularly in the grains. The level of non-enzymatic antioxidants like anthocyanins, flavonoids, phenolics and glutathione was stimulated upon SiNP-priming. Nano-Si-pulsing removed fluoride-mediated inhibition of glutathione synthesis by activating glutathione reductase. Glutathione was utilized to activate glyoxalases and associated enzymes like glutathione-S-transferase and glutathione peroxidase. Uptake of nutrients like silicon, potassium, zinc, copper, iron, nickel, manganese, selenium and vanadium improved seedling health even during prolonged fluoride stress. Nano-Si-pulsing produced a nanozymatic effect, since high level of crucial co-factors like copper, zinc and iron stimulated the activity of superoxide dismutase, catalase, ascorbate peroxidase and guaiacol peroxidase, which synergistically with other enzymatic and non-enzymatic antioxidants scavenged reactive oxygen species and promoted fluoride tolerance. Overall, the study supported by statistical modelling using principal component analysis, t-distributed stochastic neighbour embedding and multidimensional scaling, established the potential of SiNP to promote safe rice cultivation and precision farming even in fluoride-infested environments.

Differential lead-fluoride and nickel-fluoride uptake in co-polluted soil variably affects the overall physiome in an aromatic rice cultivar

The present study aimed to show that nickel and fluoride exhibited synchronized co-inhibited uptake in the aromatic rice cultivar, Gobindobhog, since bioaccumulation of the two elements was lower than that during individual stress, so that overall growth under combined stress was similar to control seedlings. On the contrary, lead and fluoride stimulated their co-uptake which triggered oxidative damages, NADPH oxidase activity, methylglyoxal accumulation, photosynthetic inhibition, membrane-protein damages, necrosis and genomic template degradation. Accumulation of proline, anthocyanins, non-protein thiols and phytochelatins was stimulated for systemic protection against reactive oxygen species (ROS) and xenobiotic-mediated injuries during lead-fluoride toxicity. ROS accumulation during nickel-fluoride stress was insignificant due to which enhanced accumulation of most antioxidants was not required. Glutathione depletion during combined lead-fluoride toxicity was due to its utilization in the glyoxalase cycle and also inhibition of glutathione reductase. However, the nickel-fluoride-treated sets maintained glutathione reserves and glyoxalase activity similar to those in control. Presence of fluoride 'safeguarded' the glutathione-utilizing enzymes like glutathione reductase, glutathione peroxidase and glutathione-S-transferase during dual lead-fluoride stress. This was because these enzymes showed higher activity compared to that under lead toxicity alone. Enzymatic antioxidants like superoxide dismutase, ascorbate peroxidase and guaiacol peroxidase were activated during lead-fluoride toxicity due to altered iron and copper homeostasis. Catalase activity was strongly inhibited, resulting in the inability to scavenge H₂O₂ and suppression of the fluoride-adaptable phenotype. However, none of the enzymatic antioxidants were inhibited during nickel-fluoride stress, which cumulatively allowed the seedlings to maintain normal physiology. Overall our findings holistically reveal the physiological plasticity of Gobindobhog in response to two different heavy metals under the influence of fluoride.

The effect of low ascorbic acid content on tomato fruit ripening

The oxidant/antioxidant balance affects the ripening time of tomato fruit. Ripening of tomato fruit is associated with several modifications such as loss of cell wall firmness and transformation of chloroplasts to chromoplasts. Besides a peak in H₂O₂, reactive oxygen species (ROS) are observed at the transition stage. However, the role of different components of oxidative stress metabolism in fruit ripening has been scarcely addressed. Two GDP-L-galactose phosphorylase (GGP) Solanum lycopersicum L. cv Micro-Tom mutants which have fruit with low ascorbic acid content (30% of wild type) were used in this work to unravel the participation of ascorbic acid and H₂O₂ in fruit maturation. Both GGP mutants show delayed fruit maturation with no peak of H₂O₂; treatment with ascorbic acid increases its own concentration and accelerates ripening only in mutants to become like wild type plants. Unexpectedly, the treatment with ascorbic acid increases H₂O₂ synthesis in both mutants resembling what is observed in wild type fruit. Exogenous supplementation with H₂O₂ decreases its own synthesis delaying fruit maturation in plants with low ascorbic acid content. The site of ROS production is localized in the chloroplasts of fruit of all genotypes as determined by confocal microscopy analysis. The results presented here demonstrate that both ascorbic acid and H₂O₂ actively participate in tomato fruit ripening.

Deciphering the molecular mechanism behind stimulated co-uptake of arsenic and fluoride from soil, associated toxicity, defence and glyoxalase machineries in arsenic-tolerant rice

The current study elucidates the uncharacterized biohazard associated with rice growth in arsenic and fluoride co-contaminated sites. Analysis of the arsenic-tolerant rice cultivar, Muktashri (known to restrict arsenic uptake) revealed that fluoride largely stimulated arsenic bioaccumulation in the stressed tissues and vice versa. Gene expression studies revealed that high arsenic uptake was facilitated by the fluoride-dependent up regulation of phosphate transporter2 (PT2), PT8 and low silicon rice1 (Lsi1), and elevated fluoride accumulation was stimulated by the arsenic-mediated induction of chloride channels (CLCs). The endogenous accumulation of fluoride and arsenic increased reactive oxygen species (ROS), O₂^-, membrane peroxidation and arsenic localization within tissues. This inhibited plant growth by triggering chlorosis, electrolyte leakage, malondialdehyde production (due to high lipoxygenase activity), protein carbonylation, protease activity and methylglyoxal accumulation due to inhibited glyoxylase activity. Metabolic analysis showed inhibited proline biosynthesis along with increased channelization of glutathione towards phytochelatin synthase and glutathione-S-tranferase-dependent pathways. Inhibition of the antioxidant enzymes like catalase, ascorbate peroxidase and guaiacol peroxidase validated the inefficient scavenging of H₂O₂ during combined stress. In silico analyses predicted the ecotoxicological risks of arsenic-fluoride complex formed during joint stress. Overall, our investigation illustrated the underlying mechanism of arsenic-fluoride co-uptake which resulted in complete suppression of the 'tolerant'-phenotype in Muktashri seedlings.

Spermine ameliorates prolonged fluoride toxicity in soil-grown rice seedlings by activating the antioxidant machinery and glyoxalase system

The current manuscript presents the first report on the ameliorative roles of exogenous spermine (Spm) during prolonged fluoride-induced toxicity and oxidative damages in the susceptible rice cultivar, IR-64. The application of Spm increased the overall growth in the stressed seedlings by significantly restricting fluoride bioaccumulation within the shoots and roots. The Spm-treated stressed seedlings exhibited low chlorosis and induced activity of pyruvate dehydrogenase and nitrate reductase due to reduced accumulation and localization of reactive oxygen species (ROS) in the shoot and root. Spm-supplementation during stress reduced the levels of molecular damages by lowering malondialdehyde, electrolyte leakage and protein carbonylation, and lipoxygenase and protease activity due to effective detoxification of ROS by the antioxidants like proline, glycine-betaine, anthocyanin, flavonoids, phenolics and higher polyamines like Spm and spermidine. Excessive accumulation of the toxic methylglyoxal was reversed due to the activation of the glyoxalase system (comprising of glyoxalase I and II) and the ascorbate-glutathione cycle. Exogenous Spm also triggered the activity of superoxide dismutase, guaiacol peroxidase, glutathione peroxidase and phenylalanine ammonia lyase, which efficiently scavenged ROS in the stressed seedlings. Overall, Spm treatment mitigated the fluoride-induced injuries in IR-64 by reducing fluoride bioaccumulation and elaborately refining the various defence machineries.

Artificial elevation of glutathione contents in salicylic acid-deficient tobacco (Nicotiana tabacum cv. Xanthi NahG) reduces susceptibility to the powdery mildew pathogen Euoidium longipes

The effects of elevated glutathione levels on defence responses to powdery mildew (Euoidium longipes) were investigated in a salicylic acid-deficient tobacco (Nicotiana tabacum cv. Xanthi NahG) and wild-type cv. Xanthi plants, where salicylic acid (SA) contents are normal. Aqueous solutions of reduced glutathione (GSH) and its synthetic precursor R-2-oxothiazolidine-4-carboxylic acid (OTC) were injected into leaves of tobacco plants 3 h before powdery mildew inoculation. SA-deficient NahG tobacco was hyper-susceptible to E. longipes, as judged by significantly more severe powdery mildew symptoms and enhanced pathogen accumulation. Strikingly, elevation of GSH levels in SA-deficient NahG tobacco restored susceptibility to E. longipes to the extent seen in wild-type plants (i.e. enhanced basal resistance). However, expression of the SA-mediated pathogenesis-related gene (NtPR-1a) did not increase significantly in GSH or OTC-pretreated and powdery mildew-inoculated NahG tobacco, suggesting that the induction of this PR gene may not be directly involved in the defence responses induced by GSH. Our results demonstrate that artificial elevation of glutathione content can significantly reduce susceptibility to powdery mildew in SA-deficient tobacco.

Spermidine application reduces fluoride uptake and ameliorates physiological injuries in a susceptible rice cultivar by activating diverse regulators of the defense machinery

The manuscript illustrates the ameliorative effects of exogenously applied higher polyamine (PA), spermidine (Spd) in the susceptible indica rice cultivar IR-64 subjected to prolonged fluoride stress. The Spd treatment drastically reduced fluoride bioaccumulation by restricting entry of the anions through chloride channels and enabled better maintenance of the proton gradient via accumulation of P-H⁺/ATPase, thereby improving the root and shoot lengths, fresh and dry weights, RWC, chlorophyll content and activities of pyruvate dehydrogenase (PyrDH), α-amylase, and nitrate reductase (NR) in the Spd-treated, stressed plants. Expression of RuBisCo, PyrDH, α-amylase, and NR was stimulated. Spd supplementation reduced the molecular damage indices like malondialdehyde, lipoxygenase, protease activity, electrolyte leakage, protein carbonylation, H₂O₂, and methylglyoxal (detoxified by glyoxalase II). Mitigation of oxidative damage was facilitated by the accumulation and utilization of proline, glycine-betaine, total amino acids, higher PAs, anthocyanin, flavonoids, β-carotene, xanthophyll, and phenolics as verified from the expression of genes like P5CS, BADH1, SAMDC, SPDS, SPMS, DAO, PAO, and PAL. Spd treatment activated the ascorbate-glutathione cycle in the stressed seedlings. Expression and activities of enzymatic antioxidants showed that GPOX, APX, GPX, and GST were the chief ROS scavengers. Exogenous Spd promoted ABA accumulation by upregulating NCED3 and suppressing ABA8ox1 expression. ABA-dependent osmotic stress-responsive genes like Osem, WRKY71, and TRAB1 as well as ABA-independent transcription factor encoding gene DREB2A were induced by Spd. Thus, Spd treatment ameliorated fluoride-mediated injuries in IR-64 by restricting fluoride uptake, refining the defense machinery and activating the ABA-dependent as well as ABA-independent stress-responsive genes.

Melatonin application reduces fluoride uptake and toxicity in rice seedlings by altering abscisic acid, gibberellin, auxin and antioxidant homeostasis

The manuscript presents an elaborate report on the ameliorative effects of exogenous melatonin in soil-grown seedlings of the rice variety, IR-64 subjected to prolonged fluoride stress. Exogenous melatonin stimulated the physiological growth of the stressed seedlings by triggering high accumulation of gibberellic acid (GA) and melatonin via up regulation of the biosynthetic genes like GA3ox, TDC, SNAT and ASMT. The endogenous abscisic acid (ABA) content increased via induction of NCED3 and suppression of ABA8ox1. However, the ABA-dependent genes like TRAB1, WRKY71 and OSBZ8 were down regulated in presence of high endogenous GA and melatonin. High melatonin level led to low indole-3-acetic acid accumulation in the treated seedlings during fluoride stress. Melatonin significantly decreased fluoride bioaccumulation by suppressing its uptake via CLC1 and CLC2, and also restored P-H⁺/ATPase expression. The damage indices like chlorosis (accompanied by low RuBisCo), malondialdehyde, electrolyte leakage, methylglyoxal (detoxified by glyoxalase II) and protein carbonylation were greatly reduced. Increased proline synthesis, activation of the ascorbate-glutathione cycle and enhanced activity of glutathione peroxidase, catalase and guaiacol peroxidase led to low ROS accumulation and localization in the melatonin-treated plants exposed to stress. Overall, melatonin treatment alleviated fluoride-mediated injuries by restricting fluoride uptake, refining the defence machinery and altering the phytohormone homeostasis.

Differential regulation of defence pathways in aromatic and non-aromatic indica rice cultivars towards fluoride toxicity

Excessive bioaccumulation of fluoride in IR-64 caused low abscisic acid level, inhibition of polyamine biosynthesis and ascorbate-glutathione cycle but not in Gobindobhog which had higher antioxidant activity. The current study presents regulation of diverse metabolic and molecular defence pathways during fluoride stress in non-aromatic rice variety, IR-64 and aromatic rice variety, Gobindobhog (GB). Increasing concentration of fluoride affected fresh weight, dry weight, vigour index and relative water content to a lesser extent in GB compared to IR-64. GB exhibited lower methylglyoxal accumulation and lipoxygenase activity compared to IR-64 during stress. The level of osmolytes (proline, amino acids and glycine-betaine) increased in both the stressed varieties. The biosynthesis of higher polyamines was stimulated in stressed GB. IR-64 accumulated higher amount of putrescine due to degradation of higher polyamines as supported by gene expression analysis. Unlike IR-64, GB efficiently maintained the ascorbate-glutathione cycle due to much lower fluoride bioaccumulation, compared to IR-64. GB adapted to fluoride stress by strongly inducing guaiacol peroxidase, phenylalanine ammonia lyase and a novel isozyme of superoxide dismutase. While GB accumulated higher abscisic acid (ABA) level during stress, IR-64 exhibited slow ABA degradation which enabled induction of associated osmotic stress-responsive genes. Unlike GB, ABA-independent DREB2A was downregulated in stressed IR-64. The research illustrates varietal differences in the defence machinery of the susceptible variety, IR-64, and the well adapted cultivar, GB, on prolonged exposure to increasing concentrations of fluoride.

Salicylic acid-induced differential resistance to the Tomato yellow leaf curl virus among resistant and susceptible tomato cultivars

BACKGROUND

In higher plants, salicylic acid (SA) plays important roles in inducing resistance to biotic and abiotic stresses. Tomato yellow leaf curl virus (TYLCV) causes a highly devastating viral disease in plants, particularly in tomato. However, the roles of SA in inducing tomato plant resistance to TYLCV remain unclear.

RESULTS

In this study, we investigated whether the exogenous application of SA can improve the resistance of tomato plants to TYLCV in two tomato cultivars, resistant 'Zhefen-702' and susceptible 'Jinpeng-1'. The impacts of SA on the accumulation of ascorbic acid (AsA) and biosynthetic gene expression, the activity of some important reactive oxygen species (ROS)-scavenging enzymes, and the expression patterns of stress-related genes were also determined. Results indicated that SA can effectively regulate the accumulation of AsA, especially in 'Jinpeng-1'. Similarly, the expression patterns of most of the AsA biosynthetic genes showed a negative relationship with AsA accumulation in the resistant and susceptible tomato cultivars. In the two tomato cultivars, the activities of ascorbate peroxidase (APX) and peroxidase (POD) in the SA + TYLCV treated plants were increased during the experiment period except at 14 days (APX in 'Jinpeng-1' was also at 4 days) post infected (dpi) with TYLCV. Simultaneously, the activity of SOD was reduced in 'Jinpeng-1' and increased in 'Zhefen-702' after treatment with SA + TYLCV. SA can substantially induce the expression of ROS-scavenging genes at different extents. From 2 to 10 dpi, the virus content in the SA + TYLCV treated plants was remarkably lower than those in the TYLCV treated plants in 'Jinpeng-1'and Zhefen-702'.

CONCLUSIONS

The above results suggest that SA can enhance tomato plant resistance by modulating the expression of genes encoding for ROS-scavenging players, altering the activity of resistance-related enzymes, and inducing the expression of pathogenesis-related genes to produce systemic acquired resistance. Simultaneously, these results confirm that SA is a resistance-inducing factor against TYLCV infection that can be effectively applied in tomato plants.

Structure-Guided Mechanisms Behind the Metabolism of 2,4,6-Trinitrotoluene by Glutathione Transferases U25 and U24 That Lead to Alternate Product Distribution

The explosive xenobiotic 2,4,6-trinitrotoluene (TNT) is a major worldwide environmental pollutant and its persistence in the environment presents health and environmental concerns. The chemical structure of TNT dictates that biological detoxification pathways follow predominantly reductive transformation of the nitro groups, and as a result, TNT is notoriously recalcitrant to mineralization in the environment. Plant-based technologies to remediate this toxic pollutant rely on a solid understanding of the biochemical detoxification pathways involved. Toward this, two Arabidopsis Tau class glutathione transferases, GSTU24 and GSTU25, have been identified that catalyze the formation of three TNT-glutathionylated conjugates. These two GSTs share 79% identity yet only GSTU25 catalyzes the substitution of a nitro group for sulfur to form 2-glutathionyl-4,6-dinitrotoluene. The production of this compound is of interest because substitution of a nitro group could lead to destabilization of the aromatic ring, enabling subsequent biodegradation. To identify target amino acids within GSTU25 that might be involved in the formation of 2-glutathionyl-4,6-dinitrotoluene, the structure for GSTU25 was determined, in complex with oxidized glutathione, and used to inform site-directed mutagenesis studies. Replacement of five amino acids in GSTU24 established a conjugate profile and activity similar to that found in GSTU25. These findings contribute to the development of plant-based remediation strategies for the detoxification of TNT in the environment.

Changes in metabolic profiling of sugarcane leaves induced by endophytic diazotrophic bacteria and humic acids

Plant growth-promoting bacteria (PGPB) and humic acids (HA) have been used as biostimulants in field conditions. The complete genomic and proteomic transcription of Herbaspirillum seropedicae and Gluconacetobacter diazotrophicus is available but interpreting and utilizing this information in the field to increase crop performance is challenging. The identification and characterization of metabolites that are induced by genomic changes may be used to improve plant responses to inoculation. The objective of this study was to describe changes in sugarcane metabolic profile that occur when HA and PGPB are used as biostimulants. Inoculum was applied to soil containing 45-day old sugarcane stalks. One week after inoculation, the methanolic extracts from leaves were obtained and analyzed by gas chromatography coupled to time-of-flight mass spectrometry; a total of 1,880 compounds were observed and 280 were identified in all samples. The application of HA significantly decreased the concentration of 15 metabolites, which generally included amino acids. HA increased the levels of 40 compounds, and these included metabolites linked to the stress response (shikimic, caffeic, hydroxycinnamic acids, putrescine, behenic acid, quinoline xylulose, galactose, lactose proline, oxyproline and valeric acid) and cellular growth (adenine and adenosine derivatives, ribose, ribonic acid and citric acid). Similarly, PGPB enhanced the level of metabolites identified in HA-treated soils; e.g., 48 metabolites were elevated and included amino acids, nucleic acids, organic acids, and lipids. Co-inoculation (HA+PGPB) boosted the level of 110 metabolites with respect to non-inoculated controls; these included amino acids, lipids and nitrogenous compounds. Changes in the metabolic profile induced by HA+PGPB influenced both glucose and pentose pathways and resulted in the accumulation of heptuloses and riboses, which are substrates in the nucleoside biosynthesis and shikimic acid pathways. The mevalonate pathway was also activated, thus increasing phytosterol synthesis. The improvement in cellular metabolism observed with PGPB+HA was compatible with high levels of vitamins. Glucuronate and amino sugars were stimulated in addition to the products and intermediary compounds of tricarboxylic acid metabolism. Lipids and amino acids were the main compounds induced by co-inoculation in addition to antioxidants, stress-related metabolites, and compounds involved in cellular redox. The primary compounds observed in each treatment were identified, and the effect of co-inoculation (HA+PGPB) on metabolite levels was discussed.

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.

RNA sequencing of Brassica napus reveals cellular redox control of Sclerotinia infection

Brassica napus is one of the world's most valuable oilseeds and is under constant pressure by the necrotrophic fungal pathogen, Sclerotinia sclerotiorum, the causal agent of white stem rot. Despite our growing understanding of host pathogen interactions at the molecular level, we have yet to fully understand the biological processes and underlying gene regulatory networks responsible for determining disease outcomes. Using global RNA sequencing, we profiled gene activity at the first point of infection on the leaf surface 24 hours after pathogen exposure in susceptible (B. napus cv. Westar) and tolerant (B. napus cv. Zhongyou 821) plants. We identified a family of ethylene response factors that may contribute to host tolerance to S. sclerotiorum by activating genes associated with fungal recognition, subcellular organization, and redox homeostasis. Physiological investigation of redox homeostasis was further studied by quantifying cellular levels of the glutathione and ascorbate redox pathway and the cycling enzymes associated with host tolerance to S. sclerotiorum. Functional characterization of an Arabidopsis redox mutant challenged with the fungus provides compelling evidence into the role of the ascorbate-glutathione redox hub in the maintenance and enhancement of plant tolerance against fungal pathogens.

Expression of a Drosophila glutathione transferase in Arabidopsis confers the ability to detoxify the environmental pollutant, and explosive, 2,4,6-trinitrotoluene

The explosive 2,4,6-trinitrotoluene (TNT) is a significant, global environmental pollutant that is both toxic and recalcitrant to degradation. Given the sheer scale and inaccessible nature of contaminated areas, phytoremediation may be a viable clean-up approach. Here, we have characterized a Drosophila melanogaster glutathione transferase (DmGSTE6) which has activity towards TNT. Recombinantly expressed, purified DmGSTE6 produces predominantly 2-glutathionyl-4,6-dinitrotoluene, and has a 2.5-fold higher Maximal Velocity (V_max ), and five-fold lower Michaelis Constant (K_m ) than previously characterized TNT-active Arabidopsis thaliana (Arabidopsis) GSTs. Expression of DmGSTE6 in Arabidopsis conferred enhanced resistance to TNT, and increased the ability to remove TNT from contaminated soil relative to wild-type plants. Arabidopsis lines overexpressing TNT-active GSTs AtGST-U24 and AtGST-U25 were compromised in biomass production when grown in the absence of TNT. This yield drag was not observed in the DmGSTE6-expressing Arabidopsis lines. We hypothesize that increased levels of endogenous TNT-active GSTs catalyse excessive glutathionylation of endogenous substrates, depleting glutathione pools, an activity that DmGST may lack. In conclusion, DmGSTE6 has activity towards TNT, producing a compound with potential for further biodegradation. Selecting or manipulating plants to confer DmGSTE6-like activity could contribute towards development of phytoremediation strategies to clean up TNT from polluted military sites.

Analysis of transcripts differentially expressed between fruited and deflowered 'Gala' adult trees: a contribution to biennial bearing understanding in apple

BACKGROUND

The transition from vegetative to floral state in shoot apical meristems (SAM) is a key event in plant development and is of crucial importance for reproductive success. In perennial plants, this event is recurrent during tree life and subject to both within-tree and between-years heterogeneity. In the present study, our goal was to identify candidate processes involved in the repression or induction of flowering in apical buds of adult apple trees.

RESULTS

Genes differentially expressed (GDE) were examined between trees artificially set in either 'ON' or 'OFF' situation, and in which floral induction (FI) was shown to be inhibited or induced in most buds, respectively, using qRT-PCR and microarray analysis. From the period of FI through to flower differentiation, GDE belonged to four main biological processes (i) response to stimuli, including response to oxidative stress; (ii) cellular processes, (iii) cell wall biogenesis, and (iv) metabolic processes including carbohydrate biosynthesis and lipid metabolic process. Several key regulator genes, especially TEMPRANILLO (TEM), FLORAL TRANSITION AT MERISTEM (FTM1) and SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) were found differentially expressed. Moreover, homologs of SPL and Leucine-Rich Repeat proteins were present under QTL zones previously detected for biennial bearing.

CONCLUSIONS

This data set suggests that apical buds of 'ON' and 'OFF' trees were in different physiological states, resulting from different metabolic, hormonal and redox status which are likely to contribute to FI control in adult apple trees. Investigations on carbohydrate and hormonal fluxes from sources to SAM and on cell detoxification process are expected to further contribute to the identification of the underlying physiological mechanisms of FI in adult apple trees.

Ascorbate-Deficient vtc2 Mutants in Arabidopsis Do Not Exhibit Decreased Growth

In higher plants the L-galactose pathway represents the major route for ascorbate biosynthesis. The first committed step of this pathway is catalyzed by the enzyme GDP-L-galactose phosphorylase and is encoded by two paralogs in Arabidopsis - VITAMIN C2 (VTC2) and VTC5. The first mutant of this enzyme, vtc2-1, isolated via an EMS mutagenesis screen, has approximately 20-30% of wildtype ascorbate levels and has been reported to have decreased growth under standard laboratory conditions. Here, we show that a T-DNA insertion into the VTC2 causes a similar reduction in ascorbate levels, but does not greatly affect plant growth. Subsequent segregation analysis revealed the growth defects of vtc2-1 mutants segregate independently of the vtc2-1 mutation. These observations suggest that it is the presence of an independent cryptic mutation that affects growth of vtc2-1 mutants, and not the 70-80% decrease in ascorbate levels that has been assumed in past studies.

The metabolomics of oxidative stress

Oxidative stress resulting from increased availability of reactive oxygen species (ROS) is a key component of many responses of plants to challenging environmental conditions. The consequences for plant metabolism are complex and manifold. We review data on small compounds involved in oxidative stress, including ROS themselves and antioxidants and redox buffers in the membrane and soluble phases, and we discuss the wider consequences for plant primary and secondary metabolism. While metabolomics has been exploited in many studies on stress, there have been relatively few non-targeted studies focused on how metabolite signatures respond specifically to oxidative stress. As part of the discussion, we present results and reanalyze published datasets on metabolite profiles in catalase-deficient plants, which can be considered to be model oxidative stress systems. We emphasize the roles of ROS-triggered changes in metabolites as potential oxidative signals, and discuss responses that might be useful as markers for oxidative stress. Particular attention is paid to lipid-derived compounds, the status of antioxidants and antioxidant breakdown products, altered metabolism of amino acids, and the roles of phytohormone pathways.

Ascorbate deficiency influences the leaf cell wall glycoproteome in Arabidopsis thaliana

The cell wall forms the first line of interaction between the plant and the external environment. Based on the observation that ascorbate-deficient vtc mutants of Arabidopsis thaliana have increased cell wall peroxidase activity, the cell wall glycoproteome of vtc2-2 was investigated. Glycoproteins were purified from fully expanded leaves by Concanavalin A affinity chromatography and analysed by liquid chromatography quadrupole time-of-flight mass spectrometry. This procedure identified 63 proteins with predicted glycosylation sites and cell wall localization. Of these, 11 proteins were differentially expressed between vtc2-2 and wild type. In particular, PRX33/34 were identified as contributing to increased peroxidase activity in response to ascorbate deficiency. This is the same peroxidase previously shown to contribute to hydrogen peroxide generation and pathogen resistance. Three fasciclin-like arabinogalactan proteins (FLA1, 2 and 8) had lower abundance in vtc2-2. Inspection of published microarray data shows that these also have lower gene expression in vtc1 and vtc2-1 and are decreased in expression by pathogen challenge and oxidative stresses. Ascorbate deficiency therefore impacts expression of cell wall proteins involved in pathogen responses and these presumably contribute to the increased resistance of vtc mutants to biotrophic pathogens.

Vitamin C deficiency improves somatic embryo development through distinct gene regulatory networks in Arabidopsis

Changes in the endogenous ascorbate redox status through genetic manipulation of cellular ascorbate levels were shown to accelerate cell proliferation during the induction phase and improve maturation of somatic embryos in Arabidopsis. Mutants defective in ascorbate biosynthesis such as vtc2-5 contained ~70 % less cellular ascorbate compared with their wild-type (WT; Columbia-0) counterparts. Depletion of cellular ascorbate accelerated cell division processes and cellular reorganization and improved the number and quality of mature somatic embryos grown in culture by 6-fold compared with WT tissues. To gain insight into the molecular mechanisms underlying somatic embryogenesis (SE), we profiled dynamic changes in the transcriptome and analysed dominant patterns of gene activity in the WT and vtc2-5 lines across the somatic embryo culturing process. Our results provide insight into the gene regulatory networks controlling SE in Arabidopsis based on the association of transcription factors with DNA sequence motifs enriched in biological processes of large co-expressed gene sets. These data provide the first detailed account of temporal changes in the somatic embryo transcriptome starting with the zygotic embryo, through tissue dedifferentiation, and ending with the mature somatic embryo, and impart insight into possible mechanisms for the improved culture of somatic embryos in the vtc2-5 mutant line.

The induction of menadione stress tolerance in the marine microalga, Dunaliella viridis, through cold pretreatment and modulation of the ascorbate and glutathione pools

The effect of cold pretreatment on menadione tolerance was investigated in the cells of the marine microalga, Dunaliella viridis. In addition, the involvement of ascorbate and glutathione in the response to menadione stress was tested by treating cell suspensions with l-galactono-1,4-lactone, an ascorbate precursor, and buthionine sulfoximine, an inhibitor of glutathione synthesis. Menadione was highly toxic to non cold-pretreated cells, and caused a large decrease in cell number. Cold pretreatment alleviated menadione toxicity and cold pretreated cells accumulated lower levels of reactive oxygen species, and had enhanced antioxidant capacity due to increased levels of β-carotene, reduced ascorbate and total glutathione compared to non cold-pretreated cells. Cold pretreatment also altered the response to l-galactono-1,4-lactone and buthionine sulfoximine treatments. Combined l-galactono-1,4-lactone and menadione treatment was lethal in non-cold pretreated cells, but in cold-pretreated cells it had a positive effect on cell numbers compared to menadione alone. Overall, exposure of Dunaliella cells to cold stress enhanced tolerance to subsequent oxidative stress induced by menadione.

Glutathione application affects the transcript profile of genes in Arabidopsis seedling

Glutathione (GSH), a tripeptide thiol compound has multiple functions in plants. Recent works suggested that GSH plays a regulatory role in signaling in plants as part of their adaptation to stress. To better understand the role of GSH as a regulatory molecule, 14 days old Arabidopsis thaliana seedlings were treated with 5mM of GSH for 4h. Changes in gene expression patterns were studied by cDNA microarray analysis. The expression of 453 genes was significantly changed compared to the untreated control, of which 261 genes were up-regulated and 192 genes were down-regulated. Genes from several groups were affected, including those of sulfur metabolism, degradation and synthesis of macromolecules and transcription factors. Up-regulation of genes involved in responses to biotic stresses, or in jasmonate or salicylic acid synthesis and their signaling, suggests that GSH triggers genes that help protect the plants during stresses. In addition, GSH down regulated genes involved in plant growth and development, like those involved in cell wall synthesis and its extension, and genes associated with auxin and cytokinins response, which are related to growth and development of the plants. The results suggest that GSH might have a role in response to biotic stress by initiating defense responses and modifying plants' growth and development in an effort to tune their sessile lifestyle of plants to environmental constraints.

Arabidopsis Glutathione Transferases U24 and U25 Exhibit a Range of Detoxification Activities with the Environmental Pollutant and Explosive, 2,4,6-Trinitrotoluene

The explosive 2,4,6-trinitrotoluene (TNT) is a major worldwide military pollutant. The presence of this toxic and highly persistent pollutant, particularly at military sites and former manufacturing facilities, presents various health and environmental concerns. Due to the chemically resistant structure of TNT, it has proven to be highly recalcitrant to biodegradation in the environment. Here, we demonstrate the importance of two glutathione transferases (GSTs), GST-U24 and GST-U25, from Arabidopsis (Arabidopsis thaliana) that are specifically up-regulated in response to TNT exposure. To assess the role of GST-U24 and GST-U25, we purified and characterized recombinant forms of both enzymes and demonstrated the formation of three TNT glutathionyl products. Importantly, GST-U25 catalyzed the denitration of TNT to form 2-glutathionyl-4,6-dinitrotoluene, a product that is likely to be more amenable to subsequent biodegradation in the environment. Despite the presence of this biochemical detoxification pathway in plants, physiological concentrations of GST-U24 and GST-U25 result in only a limited innate ability to cope with the levels of TNT found at contaminated sites. We demonstrate that Arabidopsis plants overexpressing GST-U24 and GST-U25 exhibit significantly enhanced ability to withstand and detoxify TNT, properties that could be applied for in planta detoxification of TNT in the field. The overexpressing lines removed significantly more TNT from soil and exhibited a corresponding reduction in glutathione levels when compared with wild-type plants. However, in the absence of TNT, overexpression of these GSTs reduces root and shoot biomass, and although glutathione levels are not affected, this effect has implications for xenobiotic detoxification.

Analysis of short-term metabolic alterations in Arabidopsis following changes in the prevailing environmental conditions

Although a considerable increase in our knowledge concerning the importance of metabolic adjustments to unfavorable growth conditions has been recently provided, relatively little is known about the adjustments which occur in response to fluctuation in environmental factors. Evaluating the metabolic adjustments occurring under changing environmental conditions thus offers a good opportunity to increase our current understanding of the crosstalk between the major pathways which are affected by such conditions. To this end, plants growing under normal conditions were transferred to different light and temperature conditions which were anticipated to affect (amongst other processes) the rates of photosynthesis and photorespiration and characterized at the physiological, molecular, and metabolic levels following this transition. Our results revealed similar behavior in response to both treatments and imply a tight connectivity of photorespiration with the major pathways of plant metabolism. They further highlight that the majority of the regulation of these pathways is not mediated at the level of transcription but that leaf metabolism is rather pre-poised to adapt to changes in these input parameters.

The secondary metabolism glycosyltransferases UGT73B3 and UGT73B5 are components of redox status in resistance of Arabidopsis to Pseudomonas syringae pv. tomato

Secondary metabolism plant glycosyltransferases (UGTs) ensure conjugation of sugar moieties to secondary metabolites (SMs) and glycosylation contributes to the great diversity, reactivity and regulation of SMs. UGT73B3 and UGT73B5, two UGTs of Arabidopsis thaliana (Arabidopsis), are involved in the hypersensitive response (HR) to the avirulent bacteria Pseudomonas syringae pv. tomato (Pst-AvrRpm1), but their function in planta is unknown. Here, we report that ugt73b3, ugt73b5 and ugt73b3 ugt73b5 T-DNA insertion mutants exhibited an accumulation of reactive oxygen species (ROS), an enhanced cell death during the HR to Pst-AvrRpm1, whereas glutathione levels increased in the single mutants. In silico analyses indicate that UGT73B3 and UGT73B5 belong to the early salicylic acid (SA)-induced genes whose pathogen-induced expression is co-regulated with genes related to cellular redox homeostasis and general detoxification. Analyses of metabolic alterations in ugt mutants reveal modification of SA and scopoletin contents which correlate with redox perturbation, and indicate quantitative modifications in the pattern of tryptophan-derived SM accumulation after Pst-AvrRpm1 inoculation. Our data suggest that UGT73B3 and UGT73B5 participate in regulation of redox status and general detoxification of ROS-reactive SMs during the HR to Pst-AvrRpm1, and that decreased resistance to Pst-AvrRpm1 in ugt mutants is tightly linked to redox perturbation.

Transcriptomic and proteomic analyses of a pale-green durum wheat mutant shows variations in photosystem components and metabolic deficiencies under drought stress

Background

Leaf pigment content is an important trait involved in environmental interactions. In order to determine its impact on drought tolerance in wheat, we characterized a pale-green durum wheat mutant (Triticum turgidum L. var. durum) under contrasting water availability conditions.

Results

The pale-green mutant was investigated by comparing pigment content and gene/protein expression profiles to wild-type plants at anthesis. Under well-watered (control) conditions the mutant had lower levels of chlorophylls and carotenoids, but higher levels of xanthophyll de-epoxidation compared to wild-type. Transcriptomic analysis under control conditions showed that defense genes (encoding e.g. pathogenesis-related proteins, peroxidases and chitinases) were upregulated in the mutant, suggesting the presence of mild oxidative stress that was compensated without altering the net rate of photosynthesis. Transcriptomic analysis under terminal water stress conditions, revealed the modulation of antioxidant enzymes, photosystem components, and enzymes representing carbohydrate metabolism and the tricarboxylic acid cycle, indicating that the mutant was exposed to greater oxidative stress than the wild-type plants, but had a limited capacity to respond. We also compared the two genotypes under irrigated and rain-fed field conditions over three years, finding that the greater oxidative stress and corresponding molecular changes in the pale-green mutant were associated to a yield reduction.

Conclusions

This study provides insight on the effect of pigment content in the molecular response to drought. Identified genes differentially expressed under terminal water stress may be valuable for further studies addressing drought resistance in wheat.

Assessing the regulation of leaf redox status under water stress conditions in Arabidopsis thaliana: Col-0 ecotype (wild-type and vtc-2), expressing mitochondrial and cytosolic roGFP1

Using Arabidopsis plants Col-0 and vtc2 transformed with a redox sensitive green fluorescent protein, (c-roGFP) and (m-roGFP), we investigated the effects of a progressive water stress and re-watering on the redox status of the cytosol and the mitochondria. Our results establish that water stress affects redox status differently in these two compartments, depending on phenotype and leaf age, furthermore we conclude that ascorbate plays a pivotal role in mediating redox status homeostasis and that Col-0 Arabidopsis subjected to water stress increase the synthesis of ascorbate suggesting that ascorbate may play a role in buffering changes in redox status in the mitochondria and the cytosol, with the presumed buffering capacity of ascorbate being more noticeable in young compared with mature leaves. Re-watering of water-stressed plants was paralleled by a return of both the redox status and ascorbate to the levels of well-watered plants. In contrast to the effects of water stress on ascorbate levels, there were no significant changes in the levels of glutathione, thereby suggesting that the regeneration and increase in ascorbate in water-stressed plants may occur by other processes in addition to the regeneration of ascorbate via the glutathione. Under water stress in vtc2 lines it was observed stronger differences in redox status in relation to leaf age, than due to water stress conditions compared with Col-0 plants. In the vtc2 an increase in DHA was observed in water-stressed plants. Furthermore, this work confirms the accuracy and sensitivity of the roGFP1 biosensor as a reporter for variations in water stress-associated changes in redox potentials.

The physiological roles and metabolism of ascorbate in chloroplasts

Ascorbate is a multifunctional metabolite in plants. It is essential for growth control, involving cell division and cell wall synthesis and also involved in redox signaling, in the modulation of gene expression and regulation of enzymatic activities. Ascorbate also fulfills crucial roles in scavenging reactive oxygen species, both enzymatically and nonenzymatically, a well-established phenomenon in the chloroplasts stroma. We give an overview on these important physiological functions and would like to give emphasis to less well-known roles of ascorbate, in the thylakoid lumen, where it also plays multiple roles. It is essential for photoprotection as a cofactor for violaxanthin de-epoxidase, a key enzyme in the formation of nonphotochemical quenching. Lumenal ascorbate has recently also been shown to act as an alternative electron donor of photosystem II once the oxygen-evolving complex is inactivated and to protect the photosynthetic machinery by slowing down donor-side induced photoinactivation; it is yet to be established if ascorbate has a similar role in the case of other stress effects, such as high light and UV-B stress. In bundle sheath cells, deficient in oxygen evolution, ascorbate provides electrons to photosystem II, thereby poising cyclic electron transport around photosystem I. It has also been shown that, by supporting linear electron transport through photosystem II in sulfur-deprived Chlamydomonas reinhardtii cells, in which oxygen evolution is largely inhibited, externally added ascorbate enhances hydrogen production. For fulfilling its multiple roles, Asc has to be transported into the thylakoid lumen and efficiently regenerated; however, very little is known yet about these processes.

Transgenic expression of Tobacco mosaic virus capsid and movement proteins modulate plant basal defense and biotic stress responses in Nicotiana tabacum

Plant viruses cause metabolic and physiological changes associated with symptomatic disease phenotypes. Symptoms involve direct and indirect effects, which result in disruption of host physiology. We used transgenic tobacco expressing a variant of Tobacco mosaic virus (TMV) coat protein (CP(T42W)) or movement protein (MP), and a hybrid line (MP×CP(T42W)) that coexpresses both proteins, to study the plant response to individual viral proteins. Findings employing microarray analysis of MP×CP(T42W) plants and silenced mp×cp(T42W)* controls revealed that altered transcripts were mostly downregulated, suggesting a persistent shut-off due to MP×CP(T42W) expression. Next, we showed that MP triggered reactive oxygen species (ROS) accumulation, reduction of total ascorbate, and expression of ROS scavenging genes. These effects were enhanced when both proteins were coexpressed. MP and MP×CP(T42W) plants showed increased levels of salicylic acid (SA) and SA-responsive gene expression. Furthermore, these effects were partially reproduced in Nicotiana benthamiana when GMP1 transcript was silenced. CP(T42W) seems to be playing a negative role in the defense response by reducing the expression of PR-1 and RDR-1. MP and MP×CP(T42W) transgenic expression promoted a recovery-like phenotype in TMV RNA infections and enhanced susceptibility to Pseudomonas syringae and Sclerotinia sclerotiorum. The individual effects of viral proteins may reflect the ability of a virus to balance its own virulence.

Low antioxidant concentrations impact on multiple signalling pathways in Arabidopsis thaliana partly through NPR1

Production of reactive oxygen species (ROS) is linked to signalling in both developmental and stress responses. The level of ROS is controlled by both production and removal through various scavengers including ascorbic acid and glutathione. Here, the role of low ascorbic acid or glutathione concentrations was investigated on ozone-induced cell death, defence signalling, and developmental responses. Low ascorbic acid concentrations in vtc1 activated expression of salicylic acid (SA)-regulated genes, a response found to be dependent on the redox-regulated transcriptional co-regulator NPR1. In contrast, low glutathione concentrations in cad2 or pad2 reduced expression of SA-regulated genes. Testing different responses to jasmonic acid (JA) revealed the presence of at least two separate JA signalling pathways. Treatment of the vtc1 mutant with JA led to hyper-induction of MONODEHYDROASCORBATE REDUCTASE3, indicating that low ascorbic acid concentrations prime the response to JA. Furthermore, NPR1 was found to be a positive regulator of JA-induced expression of MDHAR3 and TAT3. The vtc1 and npr1 mutants were sensitive to glucose inhibition of seed germination; an opposite response was found in cad2 and pad2. Overall, low ascorbic acid concentrations mostly led to opposite phenotypes to low glutathione concentrations, and both antioxidants interacted with SA and JA signalling pathways.

The influence of ascorbate on anthocyanin accumulation during high light acclimation in Arabidopsis thaliana: further evidence for redox control of anthocyanin synthesis

Ascorbate and anthocyanins act as photoprotectants during exposure to high light (HL). They accumulate in Arabidopsis leaves in response to HL on a similar timescale, suggesting a potential relationship between them. Flavonoids and related metabolites were identified and profiled by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The ascorbate-deficient mutants vtc1, vtc2 and vtc3 accumulated less anthocyanin than wild-type (WT) during HL acclimation. In contrast, kaempferol glycoside accumulation was less affected by light and not decreased by ascorbate deficiency, while sinapoyl malate levels decreased during HL acclimation. Comparison of six Arabidopsis ecotypes showed a positive correlation between ascorbate and anthocyanin accumulation in HL. mRNA-Seq analysis showed that all flavonoid biosynthesis transcripts were increased by HL acclimation in WT. RT-PCR analysis showed that vtc1 and vtc2 were impaired in HL induction of transcripts of anthocyanin biosynthesis enzymes, and the transcription factors PAP1, GL3 and EGL3 that activate the pathway. Abscisic acid (ABA) and jasmonic acid (JA), hormones that could affect anthocyanin accumulation, were unaffected in vtc mutants. It is concluded that HL induction of anthocyanin synthesis involves a redox-sensitive process upstream of the known transcription factors. Because anthocyanins accumulate in preference to kaempferol glycosides and sinapoyl malate in HL, they might have specific properties that make them useful in HL acclimation.

The transcription factor ABI4 Is required for the ascorbic acid-dependent regulation of growth and regulation of jasmonate-dependent defense signaling pathways in Arabidopsis

Cellular redox homeostasis is a hub for signal integration. Interactions between redox metabolism and the ABSCISIC ACID-INSENSITIVE-4 (ABI4) transcription factor were characterized in the Arabidopsis thaliana vitamin c defective1 (vtc1) and vtc2 mutants, which are defective in ascorbic acid synthesis and show a slow growth phenotype together with enhanced abscisic acid (ABA) levels relative to the wild type (Columbia-0). The 75% decrease in the leaf ascorbate pool in the vtc2 mutants was not sufficient to adversely affect GA metabolism. The transcriptome signatures of the abi4, vtc1, and vtc2 mutants showed significant overlap, with a large number of transcription factors or signaling components similarly repressed or induced. Moreover, lincomycin-dependent changes in LIGHT HARVESTING CHLOROPHYLL A/B BINDING PROTEIN 1.1 expression were comparable in these mutants, suggesting overlapping participation in chloroplast to nucleus signaling. The slow growth phenotype of vtc2 was absent in the abi4 vtc2 double mutant, as was the sugar-insensitive phenotype of the abi4 mutant. Octadecanoid derivative-responsive AP2/ERF-domain transcription factor 47 (ORA47) and AP3 (an ABI5 binding factor) transcripts were enhanced in vtc2 but repressed in abi4 vtc2, suggesting that ABI4 and ascorbate modulate growth and defense gene expression through jasmonate signaling. We conclude that low ascorbate triggers ABA- and jasmonate-dependent signaling pathways that together regulate growth through ABI4. Moreover, cellular redox homeostasis exerts a strong influence on sugar-dependent growth regulation.

Comparative physiology and transcriptional networks underlying the heat shock response in Populus trichocarpa, Arabidopsis thaliana and Glycine max

The heat shock response continues to be layered with additional complexity as interactions and crosstalk among heat shock proteins (HSPs), the reactive oxygen network and hormonal signalling are discovered. However, comparative analyses exploring variation in each of these processes among species remain relatively unexplored. In controlled environment experiments, photosynthetic response curves were conducted from 22 to 42 °C and indicated that temperature optimum of light-saturated photosynthesis was greater for Glycine max relative to Arabidopsis thaliana or Populus trichocarpa. Transcript profiles were taken at defined states along the temperature response curves, and inferred pathway analysis revealed species-specific variation in the abiotic stress and the minor carbohydrate raffinose/galactinol pathways. A weighted gene co-expression network approach was used to group individual genes into network modules linking biochemical measures of the antioxidant system to leaf-level photosynthesis among P. trichocarpa, G. max and A. thaliana. Network-enabled results revealed an expansion in the G. max HSP17 protein family and divergence in the regulation of the antioxidant and heat shock modules relative to P. trichocarpa and A. thaliana. These results indicate that although the heat shock response is highly conserved, there is considerable species-specific variation in its regulation.

The effect of ascorbic acid and dehydroascorbic acid on the root gravitropic response in Arabidopsis thaliana

The effects of ascorbic acid (AA) and dehydroascorbic acid (DHA), one of products of the disproportionation of monodehydroascorbate (MDHA) by AA oxidase (AAO, EC 1.10.3.3), on the gravitropic curvature of Arabidopsis roots were characterized by biochemical and genetic approaches. Exogenously applied AA and DHA both stimulated root gravitropic responses in a concentration-dependent fashion. AA also changed the Indole-3-acetic acid (IAA) distribution in the roots after gravistimulation. In an effort to determine the relationship between AA and DHA in the gravitropic response, changes in the amount of reduced AA were evaluated in Arabidopsis under a variety of conditions. The expression level of an AAO gene (AAO1) was increased upon gravistimulation. Brassinolide (BL), indole-3-acetic acid (IAA), and AA also increased the transcript levels of this gene. Root elongation and the gravitropic response were both suppressed in the AA biosynthesis mutant, vtc1, which has a greatly reduced level of total AA. Furthermore, the line of AAO double mutants (aao1-1 X aao3-1, 41-21) showed a reduced gravitropic response and reduced root elongation. Taken together, the results of this study imply that both AA and DHA help to determine the redox environment for the root gravitropic response, but DHA, rather than AA, is a major player in the regulation of the gravitropic response mediated by AA in the roots of Arabidopsis thaliana.

Biochemical and genetic requirements for function of the immune response regulator BOTRYTIS-INDUCED KINASE1 in plant growth, ethylene signaling, and PAMP-triggered immunity in Arabidopsis

Arabidopsis thaliana BOTRYTIS-INDUCED KINASE1 (BIK1) regulates immune responses to a distinct class of pathogens. Here, mechanisms underlying BIK1 function and its interactions with other immune response regulators were determined. We describe BIK1 function as a component of ethylene (ET) signaling and PAMP-triggered immunity (PTI) to fungal pathogens. BIK1 in vivo kinase activity increases in response to flagellin peptide (flg22) and the ET precursor 1-aminocyclopropane-1-carboxylic acid (ACC) but is blocked by inhibition of ET perception. BIK1 induction by flg22, ACC, and pathogens is strictly dependent on EIN3, and the bik1 mutation results in altered expression of ET-regulated genes. BIK1 site-directed mutants were used to determine residues essential for phosphorylation and biological functions in planta, including PTI, ET signaling, and plant growth. Genetic analysis revealed flg22-induced PTI to Botrytis cinerea requires BIK1, EIN2, and HUB1 but not genes involved in salicylate (SA) functions. BIK1-mediated PTI to Pseudomonas syringae is modulated by SA, ET, and jasmonate signaling. The coi1 mutation suppressed several bik1 phenotypes, suggesting that COI1 may act as a repressor of BIK1 function. Thus, common and distinct mechanisms underlying BIK1 function in mediating responses to distinct pathogens are uncovered. In sum, the critical role of BIK1 in plant immune responses hinges upon phosphorylation, its function in ET signaling, and complex interactions with other immune response regulators.

The roles of ascorbic acid and glutathione in symptom alleviation to SA-deficient plants infected with RNA viruses

Salicylic acid (SA) is required for plant systemic acquired resistance (SAR) to viruses. However, SA-deficient plants adapt to RNA virus infections better, which show a lighter symptom and have less reactive oxygen species (ROS) accumulation. The virus replication levels are higher in the SA-deficient plants during the first 10 days, but lower than the wild-type seedlings after 20 dpi. The higher level of glutathione and ascorbic acid (AsA) in SA-deficient plants may contribute to their alleviated symptoms. Solo virus-control method for mortal viruses results in necrosis and chlorosis, no matter what level of virus RNAs would accumulate. Contrastingly, early and high-dose AsA treatment alleviates the symptom, and eventually inhibits virus replication after 20 days. ROS eliminators could not imitate the effect of AsA, and could neither alleviate symptom nor inhibit virus replication. It suggests that both symptom alleviation and virus replication control should be considered for plant virus cures.

Expression of aspartyl protease and C3HC4-type RING zinc finger genes are responsive to ascorbic acid in Arabidopsis thaliana

Ascorbate (AsA) is a redox buffer and enzyme cofactor with various proposed functions in stress responses and growth. The aim was to identify genes whose transcript levels respond to changes in leaf AsA. The AsA-deficient Arabidopsis mutant vtc2-1 was incubated with the AsA precursor L-galactono-1,4-lactone (L-GalL) to increase leaf AsA concentration. Differentially expressed genes screened by DNA microarray were further characterized for AsA responsiveness in wild-type plants. The analysis of 14 candidates by real-time PCR identified an aspartyl protease gene (ASP, At1g66180) and a C3HC4-type RING zinc finger gene (AtATL15, At1g22500) whose transcripts were rapidly responsive to increases in AsA pool size caused by L-GalL and AsA supplementation and light. Transgenic Arabidopsis plants expressing an AtATL15 promoter::luciferase reporter confirmed that the promoter is L-GalL, AsA, and light responsive. The expression patterns of ASP and AtATL15 suggest they have roles in growth regulation. The promoter of AtATL15 is responsive to AsA status and will provide a tool to investigate the functions of AsA in plants further.

Comparative proteomic analysis of proteins in response to simulated acid rain in Arabidopsis

A proteomic study using 2-D gel electrophoresis and MALDI-TOF MS was performed to characterize the responses of Arabidopsis thaliana plants to simulated acid rain (SiAR) stress, which is a global environmental problem and has become a serious issue in China in recent years. The emphasis of the present study was to investigate the overall protein expression changes when exposed to SiAR. Out of over 1000 protein spots reproducibly resolved, 50 of them changed their abundance by at least 2-fold. Analysis of protein expression patterns revealed that a set of proteins associated with energy production, metabolism, cell rescue, cell defense and protein folding, etc., could play important roles in mediating plant response to SiAR. In addition to this, some proteins involved in stress responses and jasmonic acid pathway are also involved in plant response to SiAR. More interestingly, the expression of several ubiquitination-related proteins changed dramatically after 32-h SiAR treatment, suggesting that they may act as a molecular marker for the injury phenotype caused by SiAR. Based on our results, we proposed a schematic model to explain the mechanisms associated with the systematic response of Arabidopsis plants to SiAR.

Plasma membrane Ca²+ transporters mediate virus-induced acquired resistance to oxidative stress

This paper reports the phenomenon of acquired cross-tolerance to oxidative stress in plants and investigates the activity of specific Ca²+ transport systems mediating this phenomenon. Nicotiana benthamiana plants were infected with Potato virus X (PVX) and exposed to oxidative [either ultraviolet (UV-C) or H₂O₂] stress. Plant adaptive responses were assessed by the combined application of a range of electrophysiological (non-invasive microelectrode ion flux measurements), biochemical (Ca²+- and H+-ATPase activity), imaging (fluorescence lifetime imaging measurements of changes in intracellular Ca²+ concentrations), pharmacological and cytological transmission electrone microscopy techniques. Virus-infected plants had a better ability to control UV-induced elevations in cytosolic-free Ca²+ and prevent structural and functional damage of chloroplasts. Taken together, our results suggest a high degree of crosstalk between UV and pathogen-induced oxidative stresses, and highlight the crucial role of Ca²+ efflux systems in acquired resistance to oxidative stress in plants.

Ascorbic acid deficiency in arabidopsis induces constitutive priming that is dependent on hydrogen peroxide, salicylic acid, and the NPR1 gene

The ascorbic acid (AA)-deficient Arabidopsis thaliana vtc1-1 mutant exhibits increased resistance to the virulent bacterial pathogen Pseudomonas syringae. This response correlates with heightened levels of salicylic acid (SA), which induces antimicrobial pathogenesis-related (PR) proteins. To determine if SA-mediated, enhanced disease resistance is a general phenomenon of AA deficiency, to elucidate the signal that stimulates SA synthesis, and to identify the biosynthetic pathway through which SA accumulates, we studied the four AA-deficient vtc1-1, vtc2-1, vtc3-1, and vtc4-1 mutants. We also studied double mutants defective in the AA-biosynthetic gene VTC1 and the SA signaling pathway genes PAD4, EDS5, and NPR1, respectively. All vtc mutants were more resistant to P. syringae than the wild type. With the exception of vtc4-1, this correlated with constitutively upregulated H(2)O(2), SA, and messenger RNA levels of PR genes. Double mutants exhibited decreased SA levels and enhanced susceptibility to P. syringae compared with the wild type, suggesting that vtc1-1 requires functional PAD4, EDS5, and NPR1 for SA biosynthesis and pathogen resistance. We suggest that AA deficiency causes constitutive priming through a buildup of H(2)O(2) that stimulates SA accumulation, conferring enhanced disease resistance in vtc1-1, vtc2-1, and vtc3-1, whereas vtc4-1 might be sensitized to H(2)O(2) and SA production after infection.

Decreased L-ascorbate content mediating bolting is mainly regulated by the galacturonate pathway in Oncidium

We investigated the alteration in l-ascorbate (AsA, reduced form) content and the expression pattern of its related genes during the phase transition in Oncidium orchid. During the vegetative growth, a high H2O2 level was associated with a high content of the reduced form of AsA. During the bolting period, the AsA content and H2O2 level were greatly reduced in parallel with increased expression of OgLEAFY, the gene encoding a key transcription factor integrating different flowering-inducing pathways. This observation suggests that reduced AsA content, due to it having been consumed in scavenging H2O2, is a prerequisite for mediating the phase transition in Oncidium. A survey of the AsA biosynthetic pathway revealed that the gene expression and enzymatic activities of the products of relevant genes of the galacturonate (GalUA) pathway, such as polygalacturonase (OgPG), pectin methylesterase (OgPME) and galacturonate reductase (OgGalUAR), were markedly decreased during the bolting period, as compared with during the vegetative stage. However, the genes whose products were involved in the Smirnoff-Wheeler pathway retained a similar expression level in the two growth stages. The data suggested that OgPME of the GalUA pathway was the pivotal gene in regulating AsA biosynthesis during the bolting period. Further elucidation by overexpressing OgPME in Arabidopsis demonstrated a considerable increase in AsA content, as well as a resulting delayed-flowering phenotype. Our results strongly imply that the reduced level of AsA, regulating bolting for phase transition, resulting in part from its consumption by scavenging H2O2, was mainly caused by the down-regulation of the GalUA pathway, not the Smirnoff-Wheeler pathway.

Alterations in the endogenous ascorbic acid content affect flowering time in Arabidopsis

Ascorbic acid (AA) protects plants against abiotic stress. Previous studies suggested that this antioxidant is also involved in the control of flowering. To decipher how AA influences flowering time, we studied the four AA-deficient Arabidopsis (Arabidopsis thaliana) mutants vtc1-1, vtc2-1, vtc3-1, and vtc4-1 when grown under short and long days. These mutants flowered and senesced before the wild type irrespective of the photoperiod, a response that cannot simply be attributed to slightly elevated oxidative stress in the mutants. Transcript profiling of various flowering pathway genes revealed a correlation of altered mRNA levels and flowering time. For example, circadian clock and photoperiodic pathway genes were significantly higher in the vtc mutants than in the wild type under both short and long days, a result that is consistent with the early-flowering phenotype of the mutants. In contrast, when the AA content was artificially increased, flowering was delayed, which correlated with lower mRNA levels of circadian clock and photoperiodic pathway genes compared with plants treated with water. Similar observations were made for the autonomous pathway. Genetic analyses demonstrated that various photoperiodic and autonomous pathway mutants are epistatic to the vtc1-1 mutant. In conclusion, our transcript and genetic analyses suggest that AA acts upstream of the photoperiodic and autonomous pathways.