Hydrogen peroxide contributes to the ultraviolet-B (280-315 nm) induced oxidative stress of plant leaves through multiple pathways

Acclimation of intertidal macroalgae Ulva prolifera to UVB radiation: the important role of alternative oxidase

BACKGROUND

Solar radiation is primarily composed of ultraviolet radiation (UVR, 200 - 400 nm) and photosynthetically active radiation (PAR, 400 - 700 nm). Ultraviolet-B (UVB) radiation accounts for only a small proportion of sunlight, and it is the primary cause of plant photodamage. The use of chlorofluorocarbons (CFCs) as refrigerants caused serious ozone depletion in the 1980s, and this had led to an increase in UVB. Although CFC emissions have significantly decreased in recent years, UVB radiation still remains at a high intensity. UVB radiation increase is an important factor that influences plant physiological processes. Ulva prolifera, a type of macroalga found in the intertidal zone, is intermittently exposed to UVB. Alternative oxidase (AOX) plays an important role in plants under stresses. This research examines the changes in AOX activity and the relationships among AOX, photosynthesis, and reactive oxygen species (ROS) homeostasis in U. prolifera under changes in UVB and photosynthetically active radiation (PAR).

RESULTS

UVB was the main component of solar radiation impacting the typical intertidal green macroalgae U. prolifera. AOX was found to be important during the process of photosynthesis optimization of U. prolifera due to a synergistic effect with non-photochemical quenching (NPQ) under UVB radiation. AOX and glycolate oxidase (GO) worked together to achieve NADPH homeostasis to achieve photosynthesis optimization under changes in PAR + UVB. The synergism of AOX with superoxide dismutase (SOD) and catalase (CAT) was important during the process of ROS homeostasis under PAR + UVB.

CONCLUSIONS

AOX plays an important role in the process of photosynthesis optimization and ROS homeostasis in U. prolifera under UVB radiation. This study provides further insights into the response of intertidal macroalgae to solar light changes.

Dynamics of rhizosphere microbial structure and function associated with the biennial bearing of moso bamboo

Moso bamboo (Phyllostachys edulis) is a valuable nontimber forestry product with a biennial cycle, producing abundant bamboo shoots within one year (on-year) and few shoots within the following year (off-year). Moso bamboo plants undergo clonal reproduction, resulting in similar genetic backgrounds. However, the number of moso bamboo shoots produced each year varies. Despite this variation, the impact of soil nutrients and the root microbiome on the biennial bearing of moso bamboo is poorly understood. We collected 139 soil samples and determined 14 major physicochemical properties of the rhizosphere, rhizoplane, and bulk soil in different seasons (i.e., the growing and deciduous seasons) and different years (i.e., on- and off-years). Based on 16S rRNA and metagenomic sequencing, major variations were found in the rhizospheric microbial composition during different seasons and years in the moso bamboo forest. Environmental driver analysis revealed that essential nutrients (i.e., SOC, TOC, TN, P, and NH4+) were the main drivers of the soil microbial community composition and were correlated with the on- and off-year cycles. Moreover, 19 MAGs were identified as important biomarkers that could distinguish on- and off-years. We found that both season and year influenced both the microbial community structure and functional pathways through the biosynthesis of nutrients that potentially interact with the moso bamboo growth rhythm, especially the on-year root-associated microbiome, which had a greater abundance of specific nutrients such as gibberellins and vitamin B6. This work provides a dynamic perspective of the differential responses of various on- and off-year microbial communities and enhances our understanding of bamboo soil microbiome biodiversity and stability.

Real-Time Mass Spectrometric Detection of Reaction Intermediates Formed during Laser-Induced UV/H2O2 Advanced Oxidation of 2-Methylbenzoisothiazol-3-one

Knowledge on short-lived reaction intermediates is often essential for mechanistic investigations of organic reactions and for reaction optimization. Unfortunately, most conventional analytical methods are too slow to allow the detection of short-lived reaction intermediates. Herein, a direct laser desorption/ionization method coupled with linear quadrupole ion trap/orbitrap high-resolution tandem mass spectrometry was used for the detection and structural characterization of several previously proposed but undetected reaction intermediates formed during laser-induced UV/H2O2 advanced oxidation of 2-methylbenzoisothiazol-3-one. The elemental compositions of most detected (ionized) compounds were determined. Tandem mass spectrometry experiments based on gas-phase collision-activated dissociation (CAD) were conducted to gain information on the ion structures. The mechanisms of the CAD reactions were explored using high-level quantum chemical calculations to support the structures proposed for the neutral reaction intermediates formed during the laser-induced UV/H2O2 advanced oxidation of 2-methylbenzoisothiazol-3-one. In the negative-ion mode experiments, anions corresponding to three reaction intermediates were detected and structurally characterized: 1-hydroxy-2-methyl-1,2-dihydro-3H-1λ4-benzo[d]isothiazol-3-one, 2-(methylcarbamoyl)benzenesulfinic acid, and 2-(dihydroxy(oxo)-λ6-sulfaneyl)-N-methylbenzamide. One of the final products, 2-(methylcarbamoyl)benzenesulfonic acid, was also detected and characterized. In positive-ion mode experiments, cations corresponding to the reactant, 2-methylbenzoisothiazol-3-one, as well as an intermediate reaction product and the two final reaction products, 2-methylbenzo[d]isothiazol-3(2H)-one 1-oxide, N-methylsaccharine, and 2-(methylcarbamoyl)benzenesulfonic acid, respectively, were detected and identified. This research substantially improved the understanding on the reaction intermediates formed during laser-induced UV/H2O2 advanced oxidation of 2-methylbenzoisothiazol-3-one, which facilitates the delineation of the reaction mechanisms occurring in these processes.

Light-Dependent Regulatory Interactions between the Redox System and miRNAs and Their Biochemical and Physiological Effects in Plants

Light intensity and spectrum play a major role in the regulation of the growth, development, and stress response of plants. Changes in the light conditions affect the formation of reactive oxygen species, the activity of the antioxidants, and, consequently, the redox environment in the plant tissues. Many metabolic processes, thus the biogenesis and function of miRNAs, are redox-responsive. The miRNAs, in turn, can modulate various components of the redox system, and this process is also associated with the alteration in the intensity and spectrum of the light. In this review, we would like to summarise the possible regulatory mechanisms by which the alterations in the light conditions can influence miRNAs in a redox-dependent manner. Daily and seasonal fluctuations in the intensity and spectral composition of the light can affect the expression of miRNAs, which can fine-tune the various physiological and biochemical processes due to their effect on their target genes. The interactions between the redox system and miRNAs may be modulated by light conditions, and the proposed function of this regulatory network and its effect on the various biochemical and physiological processes will be introduced in plants.

The Role of Reactive Oxygen Species in Plant Response to Radiation

Radiation is widespread in nature, including ultraviolet radiation from the sun, cosmic radiation and radiation emitted by natural radionuclides. Over the years, the increasing industrialization of human beings has brought about more radiation, such as enhanced UV-B radiation due to ground ozone decay, and the emission and contamination of nuclear waste due to the increasing nuclear power plants and radioactive material industry. With additional radiation reaching plants, both negative effects including damage to cell membranes, reduction of photosynthetic rate and premature aging and benefits such as growth promotion and stress resistance enhancement have been observed. ROS (Reactive oxygen species) are reactive oxidants in plant cells, including hydrogen peroxide (H₂O₂), superoxide anions (O₂^•-) and hydroxide anion radicals (·OH), which may stimulate the antioxidant system of plants and act as signaling molecules to regulate downstream reactions. A number of studies have observed the change of ROS in plant cells under radiation, and new technology such as RNA-seq has molecularly revealed the regulation of radiative biological effects by ROS. This review summarized recent progress on the role of ROS in plant response to radiations including UV, ion beam and plasma, and may help to reveal the mechanisms of plant responses to radiation.

Molecular Responses of Vegetable, Ornamental Crops, and Model Plants to Salinity Stress

Vegetable and ornamental plants represent a very wide group of heterogeneous plants, both herbaceous and woody, generally without relevant salinity-tolerant mechanisms. The cultivation conditions-almost all are irrigated crops-and characteristics of the products, which must not present visual damage linked to salt stress, determine the necessity for a deep investigation of the response of these crops to salinity stress. Tolerance mechanisms are linked to the capacity of a plant to compartmentalize ions, produce compatible solutes, synthesize specific proteins and metabolites, and induce transcriptional factors. The present review critically evaluates advantages and disadvantages to study the molecular control of salt tolerance mechanisms in vegetable and ornamental plants, with the aim of distinguishing tools for the rapid and effective screening of salt tolerance levels in different plants. This information can not only help in suitable germplasm selection, which is very useful in consideration of the high biodiversity expressed by vegetable and ornamental plants, but also drive the further breeding activities.

Phenolic peroxidases: Dull generalists or purposeful specialists in stress responses?

This study focuses on class III peroxidases (POD) (EC 1.11.1.7) as regulators of cellular H₂O₂ levels in leaves under oxidative stress. The effective regulation of reactive oxygen species (ROS) concentrations in plant tissues is crucial for plant survival, and has been extensively reviewed. However, the majority of studies regard POD as a generalist without substrate specificity. This is partly due to the fact that laboratory protocols assessing POD levels use substrates, which are not contained in plants. Here, we show that both base- and stress-inducible POD activity depends on the choice of substrate. Moreover, the application of diverse substrates, particularly those contained in plants, unmasks POD isoenzymes that are distinguished by substrate preferences. This functional heterogeneity of POD responses is worth studying, especially in parallel with stress-induced changes in the phenolic profiles.

An overview of salinity stress, mechanism of salinity tolerance and strategies for its management in cotton

Salinity stress is one of the primary threats to agricultural crops resulting in impaired crop growth and development. Although cotton is considered as reasonably salt tolerant, it is sensitive to salt stress at some critical stages like germination, flowering, boll formation, resulting in reduced biomass and fiber production. The mechanism of partial ion exclusion (exclusion of Na+ and/or Cl-) in cotton appears to be responsible for the pattern of uptake and accumulation of harmful ions (Na+ and Cl) in tissues of plants exposed to saline conditions. Maintaining high tissue K+/Na+ and Ca2+/Na+ ratios has been proposed as a key selection factor for salt tolerance in cotton. The key adaptation mechanism in cotton under salt stress is excessive sodium exclusion or compartmentation. Among the cultivated species of cotton, Egyptian cotton (Gossypium barbadense L.) exhibit better salt tolerance with good fiber quality traits as compared to most cultivated cotton and it can be used to improve five quality traits and transfer salt tolerance into Upland or American cotton (Gossypium hirsutum L.) by interspecific introgression. Cotton genetic studies on salt tolerance revealed that the majority of growth, yield, and fiber traits are genetically determined, and controlled by quantitative trait loci (QTLs). Molecular markers linked to genes or QTLs affecting key traits have been identified, and they could be utilized as an indirect selection criterion to enhance breeding efficiency through marker-assisted selection (MAS). Transfer of genes for compatible solute, which are an important aspect of ion compartmentation, into salt-sensitive species is, theoretically, a simple strategy to improve tolerance. The expression of particular stress-related genes is involved in plant adaptation to environmental stressors. As a result, enhancing tolerance to salt stress can be achieved by marker assisted selection added with modern gene editing tools can boost the breeding strategies that defend and uphold the structure and function of cellular components. The intent of this review was to recapitulate the advancements in salt screening methods, tolerant germplasm sources and their inheritance, biochemical, morpho-physiological, and molecular characteristics, transgenic approaches, and QTLs for salt tolerance in cotton.

Solar UV-B effects on composition and UV screening efficiency of foliar phenolics in Arabidopsis thaliana are augmented by temperature

The accumulation of foliar phenolics constitutes one strategy of plants against the potentially harmful effects of ultraviolet-B and A (UV-B, UV-A) radiation. These compounds protect photosensitive tissues by shielding and antioxidative function. It is unknown, however, whether seasonal acclimation to natural conditions may modify the UV-B effect on phenylpropanoid composition and localisation, and thus their screening efficiency. To address this debate, a field experiment with the wildtype of Arabidopsis thaliana accession Landsberg erecta (Ler) was implemented over a whole year with plants exposed to different UV-filter treatments. While seasonal increases of UV-B radiation had a slight negative effect on the amount of hydroxycinnamic acids (HCAs), low temperatures increased foliar HCAs. HCAs, however, did not contribute substantially to seasonal changes of in vivo UV absorbance. Kaempferol and quercetin derivatives increased significantly under ambient UV-B radiation, and low temperature interacted with this effect. A shift of epidermal UV-A shielding from kaempferol to quercetin derivatives was elucidated in UV-B presence. Despite this, a substantial 20-fold increase of quercetin derivatives, during periods with high irradiance and low temperature, did not affect UV absorbance leading to the conclusion that quercetin accumulation was not exclusively in epidermal vacuoles. Using confocal microscopy, the potential occurrence of quercetin in mesophyll cells was demonstrated in plants grown with experimental UV-B radiation at low temperature for the first time in A. thaliana. The presented study discusses the idea that cross-talk of UV-B radiation and temperature might adjust the physiological function of quercetin from an (epidermal) screening to an antioxidant substance.

Narrow-Band 311 nm Ultraviolet-B Radiation Evokes Different Antioxidant Responses from Broad-Band Ultraviolet

Supplemental narrow-band 311 nm UV-B radiation was applied in order to study the effect of this specific wavelength on tobacco as a model plant. UV-B at photon fluxes varying between 2.9 and 9.9 μmol m⁻² s⁻¹ was applied to supplement 150 μmol m⁻² s⁻¹ photosynthetically active radiation (PAR) for four hours in the middle of the light period for four days. Narrow-band UV-B increased leaf flavonoid and phenolic acid contents. In leaves exposed to 311 nm radiation, superoxide dismutase activity increased, but phenolic peroxidase activity decreased, and the changes were proportional to the UV flux. Ascorbate peroxidase activities were not significantly affected. Narrow-band UV-B caused a dose-dependent linear decrease in the quantum efficiency of photosystem II, up to approximately 10% loss. A parallel decrease in non-regulated non-photochemical quenching indicates potential electron transfer to oxygen in UV-treated leaves. In addition to a flux-dependent increase in the imbalance between enzymatic H₂O₂ production and neutralization, this resulted in an approximately 50% increase in leaf H₂O₂ content under 2.9–6 μmol m⁻² s⁻¹ UV-B. Leaf H₂O₂ decreased to control levels under higher UV-B fluxes due to the onset of increased non-enzymatic H₂O₂- and superoxide-neutralizing capacities, which were not observed under lower fluxes. These antioxidant responses to 311 nm UV-B were different from our previous findings in plants exposed to broad-band UV-B. The results suggest that signaling pathways activated by 311 nm radiation are distinct from those stimulated by other wavelengths and support the heterogeneous regulation of plant UV responses.

Differential responses of the scavenging systems for reactive oxygen species (ROS) and reactive carbonyl species (RCS) to UV-B irradiation in Arabidopsis thaliana and its high altitude perennial relative Arabis alpina

The present work aimed to compare antioxidant response and lipid peroxide detoxification capacity of an arctic-alpine species Arabis alpina to its close relative model species Arabidopsis thaliana under acute short duration (3 h and 6 h) UV-B stress (4.6 and 8.2 W/m²). After 3 and 6 h exposure to UV-B, A. alpina showed lower lipid peroxidation and H₂O₂ accumulation when compared to A. thaliana. Moreover, F_v/F_m value of A. thaliana dropped to 0.70, while A. alpina dropped to 0.75 indicating better protection of PSII in this species. For elucidation of the antioxidant response, activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), ascorbate peroxidase (APX), glutathione reductase (GR) and dehydroascorbate reductase (DHAR) were measured. SOD induction with 6 h of UV-B was more prominent in A. alpina. Also, A. alpina had higher chloroplastic FeSOD activity when compared to A. thaliana. APX activity was also significantly induced in A. alpina, while its activity decreased at 3 h or did not change at 6 h in A. thaliana. A. alpina was able to maintain constant CAT activity, but drastic decreases were observed in A. thaliana at both time points. Moreover, A. alpina was able to maintain or induce aldehyde dehydrogenase (ALDH), alkenal reductases (AERs) and glutathione-S-transferases (GST) activity, while an opposite trend was observed in A. thaliana. These findings indicate that A. alpina was able to maintain/induce its antioxidant defence and lipid peroxide detoxification conferring better protection against UV-B.

UV-Vis LED-assisted photo-Fenton process for mineralization of losartan and hydrochlorothiazide: optimization using desirability function

This study presents the results obtained for the optimization of the mineralization of losartan (LOS) and hydrochlorothiazide (HCTZ) using the photo-Fenton process with a UV-Vis LED. Experimental design optimization employing a Doehlert matrix and a global desirability function enabled simultaneous evaluation of multiple responses, with factor fitting providing the best conditions that maximized the mineralization efficiency: Fe²⁺ at 10 mg L^-1 and H₂O₂ at 100 mg L^-1. High rates of mineralization of LOS and HCTZ were obtained, with dissolved organic carbon (DOC); removal of almost 75% after 90 min was observed for both pharmaceuticals. The kinetic model showed that the mineralization followed two regimes in the first minutes, with a fast progression followed by slower activity. The energy consumption calculated for mineralization of LOS and HCTZ at a concentration of 20 mg L^-1 using the UV-Vis LED-assisted photo-Fenton process, at 60 min, was 130 kWh m^-3. The desirability function provides a useful tool for finding optimal experimental conditions for the treatment of effluents with different characteristics. The UV-Vis LED was shown to be a good light source in the photo-Fenton process.

Ultraviolet-B acclimation is supported by functionally heterogeneous phenolic peroxidases

Tobacco plants were grown in plant chambers for four weeks, then exposed to one of the following treatments for 4 days: (1) daily supplementary UV-B radiation corresponding to 6.9 kJ m-2 d-1 biologically effective dose (UV-B), (2) daily irrigation with 0.1 mM hydrogen peroxide, or (3) a parallel application of the two treatments (UV-B + H2O2). Neither the H2O2 nor the UV-B treatments were found to be damaging to leaf photosynthesis. Both single factor treatments increased leaf H2O2 contents but had distinct effects on various H2O2 neutralising mechanisms. Non-enzymatic H2O2 antioxidant capacities were increased by direct H2O2 treatment only, but not by UV-B. In contrast, enzymatic H2O2 neutralisation was mostly increased by UV-B, the responses showing an interesting diversity. When class-III peroxidase (POD) activity was assayed using an artificial substrate (ABTS, 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid)), both treatments appeared to have a positive effect. However, only UV-B-treated leaves showed higher POD activities when phenolic compounds naturally occurring in tobacco leaves (chlorogenic acid or quercetin) were used as substrates. These results demonstrate a substrate-dependent, functional heterogeneity in POD and further suggest that the selective activation of specific isoforms in UV-B acclimated leaves is not triggered by excess H2O2 in these leaves.

Ultraviolet-B radiation exposure lowers the antioxidant capacity in the Arabidopsis thaliana pdx1.3-1 mutant and leads to glucosinolate biosynthesis alteration in both wild type and mutant

Pyridoxine (vitamin B6) and its vitamers are used by living organisms both as enzymatic cofactors and as antioxidants. We used Arabidopsis pyridoxine biosynthesis mutant pdx1.3-1 to study the involvement of the PLP-synthase main polypeptide PDX1 in plant responses to ultraviolet radiation of two different qualities, one containing primarily UV-A (315-400 nm) and the other containing both UV-A and UV-B (280-315 nm). The antioxidant capacity and the flavonoid and glucosinolate (GS) profiles were examined. As an indicator of stress, Fv/Fm of photosystem II reaction centers was used. In pdx1.3-1, UV-A + B exposure led to a significant 5% decrease in Fv/Fm on the last day (day 15), indicating mild stress at this time point. The antioxidant capacity of Col-0 wildtype increased significantly (50-73%) after 1 and 3 days of UV-A + B. Instead, in pdx1.3-1, the antioxidant capacity significantly decreased by 44-52% over the same time period, proving the importance of a full complement of functional PDX1 genes for the detoxification of reactive oxygen species. There were no significant changes in the flavonoid glycoside profile under any light condition. However, the GS profile was significantly altered, both with respect to Arabidopsis accession and exposure to UV. The difference in flavonoid and GS profiles reflects that the GS biosynthesis pathway contains at least one pyridoxine-dependent enzyme, whereas no such enzyme is used in flavonoid biosynthesis. Also, there was strong correlation between the antioxidant capacity and the content of some GS compounds. Our results show that vitamin B6 vitamers, functioning both as antioxidants and co-factors, are of importance for the physiological fitness of plants.

Salinity stress in cotton: effects, mechanism of tolerance and its management strategies

Cotton is classified as moderately salt tolerant crop with salinity threshold level of 7.7 dS m^-1. Salinity is a serious threat for cotton growth, yield and fiber quality. The sensitivity to salt stress depends upon growth stage and type of salt. Understanding of cotton response to salinity, its resistance mechanism and looking into management techniques may assist in formulating strategies to improve cotton performance under saline condition. The studies have showed that germination, emergence and seedling stages are more sensitive to salinity stress as compared to later stages. Salt stress results in delayed flowering, less fruiting positions, fruit shedding and reduced boll weight which ultimately affect seed cotton yield. Depressed activities of metabolic enzymes viz: acidic invertase, alkaline invertase and sucrose phophate synthase lead to fiber quality deterioration in salinity. Excessive sodium exclusion or its compartmentation is the main adaptive mechanism in cotton under salt stress. Up regulation of enzymatic and non-enzymatic antioxidants genes offer important adaptive potential to develop salt tolerant cotton varieties. Seed priming is also an effective approach for improving cotton germination in saline soils. Intra and inter variation in cotton germplasm could be used to develop salt tolerant varieties with the aid of marker assisted selection. Furthermore, transgenic approach could be the promising option for enhancing cotton production under saline condition. It is suggested that future research may be carried out with the combination of conventional and advance molecular technology to develop salt tolerant cultivars.

Changes in grapevine leaf phenolic profiles during the day are temperature rather than irradiance driven

Photosynthesis parameters, adaxial flavonoid index, phenolic profiles and antioxidant capacities of south-facing sun exposed grapevine leaves (Vitis vinifera, Pinot Noir cultivar) were measured hourly between 7 a.m. and 7 p.m. on a clear summer day. Changes in these parameters were statistically compared to changes in environmental conditions, including solar irradiance (photosynthetically active and UV radiations), leaf and air temperature, and relative air humidity. Epidermal UV absorbance, characterised by the flavonoid index, and total extractable phenolic contents were correlated to distinct environmental parameters. The former was positively correlated to irradiance and leaf temperature, while the latter was positively correlated to air temperature. HPLC phenolic profiling identified a positive correlation between air temperature and amounts of the dominant flavonol component, quercetin-3-O-glucuronide. The only phenolic component statistically connected to the flavonoid index was quercetin-3-O-glucoside. This correlation was positive and both parameters decreased during the day, although changes in the amount of this flavonol component showed no correlation to environmental factors. Total antioxidant capacities of leaf extracts were positively correlated to solar UV, and leaf and air temperature, but not to photosynthetically active radiation. Positive correlations of quercetin-3-O-glucoside contents with the flavonoid index, with photosynthesis and with sub-stomatal CO₂ concentration suggest a special protective role of this flavonol. A short-term negative effect of solar UV-A and UV-B on photosynthetic CO₂ uptake was also identified, which was unrelated to changes in stomatal conductance. A hypothesis is presented assuming UV- and photorespiration-derived hydrogen peroxide as the driver of daily changes in leaf antioxidant capacities.

Single-dose β-aminobutyric acid treatment modifies tobacco (Nicotiana tabacum L.) leaf acclimation to consecutive UV-B treatment

β-Aminobutyric acid (BABA) pre-treatment has been shown to alter both biotic and abiotic stress responses. The present study extends this observation to acclimative UV-B-response, which has not been explored in this context so far. A single soil application of 300 ppm BABA modified the non-enzymatic antioxidant capacities and the leaf hydrogen peroxide levels in tobacco (Nicotiana tabacum L.) leaves in response to a 9-day treatment with 5.4 kJ m-2 d-1 biologically effective supplementary UV-B radiation in a model experiment that was performed in a growth chamber. BABA decreased leaf hydrogen peroxide levels both as a single factor and in combination with UV-B, but neither BABA nor UV-B affected leaf photochemistry significantly. The total antioxidant capacities were increased by either BABA or UV-B, and this response was additive in BABA pre-treated leaves. These results together with the observed changes in hydroxyl radical neutralising ability and non-enzymatic hydrogen peroxide antioxidant capacities show that BABA pre-treatment (i) has a long-term effect on leaf antioxidants even in the absence of other factors and (ii) modifies acclimative readjustment of prooxidant-antioxidant balance in response to UV-B. BABA-inducible antioxidants do not include phenolic compounds as a UV-B-induced increase in the adaxial leaf flavonoid index and total leaf extract UV absorption were unaffected by BABA.

Multiple roles for Vitamin B6 in plant acclimation to UV-B

Direct and indirect roles of vitamin B6 in leaf acclimation to supplementary UV-B radiation are shown in vitamin B6 deficient Arabidopsis thaliana mutant rsr4-1 and C24 wild type. Responses to 4 days of 3.9 kJ m-2 d-1 biologically effective UV-B dose were compared in terms of leaf photochemistry, vitamer content, and antioxidant enzyme activities; complemented with a comprehensive study of vitamer ROS scavenging capacities. Under UV-B, rsr4-1 leaves lost more (34%) photochemical yield than C24 plants (24%). In the absence of UV-B, rsr4-1 leaves contained markedly less pyridoxal-5'-phosphate (PLP) than C24 ones, but levels increased up to the C24 contents in response to UV-B. Activities of class-III ascorbate and glutathione peroxidases increased in C24 leaves upon the UV-B treatment but not in the rsr4-1 mutant. SOD activities remained the same in C24 but decreased by more than 50% in rsr4-1 under UV-B. Although PLP was shown to be an excellent antioxidant in vitro, our results suggest that the UV-B protective role of B6 vitamers is realized indirectly, via supporting peroxidase defence rather than by direct ROS scavenging. We hypothesize that the two defence pathways are linked through the PLP-dependent biosynthesis of cystein and heme, affecting peroxidases.

Effect of UV-B radiation on morphology, phenolic compound production, gene expression, and subsequent drought stress responses in chili pepper (Capsicum annuum L.)

It has been suggested that accumulation of flavonoids could be a key step in development of plant tolerance to different environmental stresses. Moreover, it has been recognized that abiotic stresses such as drought and UV-B radiation (280-315 nm) induce phenolic compound accumulation, suggesting a role for these compounds in drought tolerance. The aim of the present study was to evaluate the effect of UV-B exposure on chili pepper (Capsicum annuum, cv. 'Coronel') plant performance, phenolic compound production, and gene expression associated with response to subsequent drought stress. Additionally, the phenotypic response to drought stress of these plants was studied. UV-B induced a reduction both in stem length, stem dry weight and number of floral primordia. The largest reduction in these variables was observed when combining UV-B and drought. UV-B-treated well-watered plants displayed fructification approximately 1 week earlier than non-UV-B-treated controls. Flavonoids measured epidermally in leaves significantly increased during UV-B treatment. Specifically, UV-B radiation significantly increased chlorogenic acid and apigenin 8-C-hexoside levels in leaves and a synergistic increase of luteolin 6-C-pentoside-8-C-hexoside was obtained by UV-B and subsequent drought stress. Gene expression of phenylalanine ammonia lyase (PAL) and chalcone synthase (CHS) genes also increased during UV-B treatments. On the other hand, expression of genes related to an oxidative response, such as mitochondrial Mn-superoxide dismutase (Mn-SOD) and peroxidase (POD) was not induced by UV-B. Drought stress in UV-B-treated plants induced mitochondrial Mn-SOD gene expression. Taken together, the UV-B treatment did not induce significant tolerance in plants towards drought stress under the conditions used.

UV-B Pre-treatment Alters Phenolics Response to Monilinia fructicola Infection in a Structure-Dependent Way in Peach Skin

Phenolic compounds represent a large class of secondary metabolites, involved in multiple functions not only in plant life cycle, but also in fruit during post-harvest. phenolics play a key role in the response to biotic and abiotic stresses, thus their accumulation is regulated by the presence of environmental stimuli. The present work aimed to investigate how different pre-UV-B-exposures can modulate the phenolic response of peach fruit infected with Monilinia fructicola. Through HPLC-DAD-MSn, several procyanidins, phenolic acids, flavonols, and anthocyanins were detected. Both UV-B radiation and fungal infection were able to stimulate the accumulation of phenolics, dependent on the chemical structure. Regarding UV-B exposure, inoculated with sterile water, 3 h of UV-B radiation highest concentration of phenolics was found, especially flavonols and cyanidin-3-glucoside far from the wound. However, wounding decreased the phenolics in the region nearby. When peaches were pre-treated with 1 h of UV-B radiation, the fungus had an additive effect in phenolic accumulation far from the infection, while it had a subtractive effect with 3 h of UV-B radiation, especially for flavonols. Canonical discriminant analysis and Pearson correlation revealed that all phenolic compounds, except procyanidin dimer, were highly regulated by UV-B radiation, with particularly strong correlation for quercetin and kaempferol glycosides, while phenolics correlated with the fungus infection were quercetin-3-galactoside, quercetin-3-glucoside, kaempferol-3-galactoside and isorhamnetin-3-glucoside. Modulation of pathogen-induced phenolics also far from inoculation site might suggest a migration of signaling molecules from the infected area to healthy tissues.

Phenolic Compound Diversity Explored in the Context of Photo-Oxidative Stress Protection

INTRODUCTION

Phenolic compounds are a chemically diverse group of plant secondary metabolites with important roles both in plant stress defence and human nutrition.

OBJECTIVE

To explore structure-function relations potentiating phenolic compounds to promote leaf acclimation to light stress by excess photosynthetically active radiation (photoinhibition) and by solar ultraviolet (UV) radiation.

METHODOLOGY

We report singlet oxygen and hydrogen peroxide antioxidant capacities and UV-absorbing properties of 27 flavonoids and 11 phenolic acids. Correlations of these characteristics in the whole data set and related activity-structure relationships in flavonoid data were investigated using simple statistical methods.

RESULTS

In comparison to flavonoids, phenolic acids are relatively ineffective reactive oxygen neutralising antioxidants; and - with the exception of gallic acid - have poor reactivity to hydrogen peroxide. Singlet oxygen and hydrogen peroxide detoxifying capacities of flavonoids are positively correlated, largely due to the strong positive effect of the hydroxylation of the C-ring in position-3. 3-O-Glycosylation halves reactive oxygen species (ROS) reactivities of quercetin and myricetin but eradicates the hydrogen peroxide reactivity of kaemferol. B-ring polyhydroxylation (cathecol structure) increases the hydrogen peroxide antioxidant function but decreases UV-B (280-315 nm) absorption. UV-A (315-400 nm) absorption is increased by the B-ring C2-C3 double bond either in itself or in combination with the C4 oxo-group.

CONCLUSION

Among the studied compounds, anthocyanins and flavonols were the strongest singlet oxygen and hydrogen peroxide scavengers, and are thus capable of supporting defence against both photoinhibition by visible light and UV stress in leaves, while flavanols may only be effective against the latter. Copyright © 2017 John Wiley & Sons, Ltd.

Selective responses of class III plant peroxidase isoforms to environmentally relevant UV-B doses

Efficient hydrogen peroxide detoxification is an essential aspect of plant defence against a large variety of stressors. Among others, class III peroxidase (POD, EC 1.11.1.7) enzymes provide this function. Previous studies have shown that PODs are present in several isoforms and have in general low substrate specificities. The aim of our work was to study how various assays based on using various substrates reflect differences in peroxidase activities of tobacco leaves due to either developmental or environmental factors. The former factor was studied comparing fully developed leaves of the 3rd and 5th nodes; and the latter was achieved using plants acclimated to low doses of supplementary UV-B (280-315 nm) in growth chambers. To investigate the above, POD activities were measured using three different, commonly used chromophore substrates: ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid)), guaiacol (2-methoxyphenol), OPD (o-phenylenediamine) and a fourth substrate, the secondary metabolite quercetin. All substrates registered a UV-B induced increase in leaf peroxidases as compared to untreated controls, although to different extents. However, age-related differences between upper and lower leaves were only detectable when either ABTS or quercetin were used as substrates. Additionally, native PAGE separation of POD isoforms followed by visualisation using one of the substrates showed that leaf acclimation to supplementary UV-B is realized via a selective activation of POD isoforms.

Difference in the action spectra for UVR8 monomerisation and HY5 transcript accumulation in Arabidopsis

The action spectrum for monomerisation of the plant UV-B photoreceptor UVR8 peaks at a shorter wavelength than that for HY5 transcript accumulation, mediated by UVR8, in the same tissue.

Environmental effects of ozone depletion and its interactions with climate change: Progress report, 2016

The Parties to the Montreal Protocol are informed by three Panels of experts. One of these is the Environmental Effects Assessment Panel (EEAP), which deals with two focal issues. The first focus is the effects of UV radiation on human health, animals, plants, biogeochemistry, air quality, and materials. The second focus is on interactions between UV radiation and global climate change and how these may affect humans and the environment. When considering the effects of climate change, it has become clear that processes resulting in changes in stratospheric ozone are more complex than previously believed. As a result of this, human health and environmental issues will be longer-lasting and more regionally variable. Like the other Panels, the EEAP produces a detailed report every four years; the most recent was published as a series of seven papers in 2015 (Photochem. Photobiol. Sci., 2015, 14, 1-184). In the years in between, the EEAP produces less detailed and shorter Progress Reports of the relevant scientific findings. The most recent of these was for 2015 (Photochem. Photobiol. Sci., 2016, 15, 141-147). The present Progress Report for 2016 assesses some of the highlights and new insights with regard to the interactive nature of the direct and indirect effects of UV radiation, atmospheric processes, and climate change. The more detailed Quadrennial Assessment will be made available in 2018.

Membrane Proteomics of Arabidopsis Glucosinolate Mutants cyp79B2/B3 and myb28/29

Glucosinolates (Gls) constitute a major group of natural metabolites represented by three major classes (aliphatic, indolic and aromatic) of more than 120 chemical structures. In our previous work, soluble proteins and metabolites in Arabidopsis mutants deficient of aliphatic (myb28/29) and indolic Gls (cyp79B2B3) were analyzed. Here we focus on investigating the changes at the level of membrane proteins in these mutants. Our LC/MS-MS analyses of tandem mass tag (TMT) labeled peptides derived from the cyp79B2/B3 and myb28/29 relative to wild type resulted in the identification of 4,673 proteins, from which 2,171 are membrane proteins. Fold changes and statistical analysis showed 64 increased and 74 decreased in cyp79B2/B3, while 28 increased and 17 decreased in myb28/29. As to the shared protein changes between the mutants, one protein was increased and eight were decreased. Bioinformatics analysis of the changed proteins led to the discovery of three cytochromes in glucosinolate molecular network (GMN): cytochrome P450 86A7 (At1g63710), cytochrome P450 71B26 (At3g26290), and probable cytochrome c (At1g22840). CYP86A7 and CYP71B26 may play a role in hydroxyl-indolic Gls production. In addition, flavone 3'-O-methyltransferase 1 represents an interesting finding as it is likely to participate in the methylation process of the hydroxyl-indolic Gls to form methoxy-indolic Gls. The analysis also revealed additional new nodes in the GMN related to stress and defense activity, transport, photosynthesis, and translation processes. Gene expression and protein levels were found to be correlated in the cyp79B2/B3, but not in the myb28/29.

Elevated ROS-scavenging enzymes contribute to acclimation to UV-B exposure in transplastomic tobacco plants, reducing the role of plastid peroxidases

Leaf peroxidases play a key role in the successful acclimation of plants to low UV-B doses. The aim of the present study was to examine whether selective enhancement of alternative chloroplast antioxidant pathways achieved by chloroplast transformation affected the need for peroxidase defense. Transplastomic tobacco lines expressing glutathione reductase in combination with either dehydroascorbate reductase or glutathione-S-transferase in their plastids exhibited better tolerance to supplemental UV-B than wild type plants. After 10days UV treatment, both the maximum and effective quantum yields of PSII decreased in the wild type by 10% but were unaffected in either of the transformed lines. Activities of total peroxidase and ascorbate peroxidase, in addition to dehydroascorbate reductase and gluthatione-S-transferase, were increased by UV in all lines. Gluthatione reductase activity was unaffected by UV in the transplastomic line engineered to have a higher constitutive level of this enzyme, but increased in the two other genotypes. However, the observed more successful acclimation required less activation of peroxidases in the doubly transformed plants than in the wild type and less increase in non-enzymatic hydroxyl radical neutralization in the dehydroascorbate reductase plus glutathione reductase fortified plants than in either of the other lines. These results highlight the fundamental role of efficient glutathione, and especially ascorbate, recycling in the chloroplast in response to exposure of plants to UV-B. They also identify chloroplast localized peroxidases among the large variety of leaf peroxidases as essential elements of defense, supporting our earlier hypothesis on hydrogen peroxide UV-B photo-cleavage as the primary mechanism behind damage.

UV-B effects on leaves-Oxidative stress and acclimation in controlled environments

As the steady decline in the Earth's stratospheric ozone layer and parallel increase in solar ultraviolet-B (UV-B: 280-315nm) has come to an end, the focus of plant UV research has been shifted from regarding UV-B as threatening plant life to recognizing it as a regulatory factor. While UV-B photoreceptor mediated signaling is increasingly understood, the role of UV-B inducible reactive oxygen species is still to be explored. Earlier experiments with high UV-B irradiation doses and isolated thylakoid membranes demonstrated the potential of UV-B to trigger oxidative stress. However, under realistic UV conditions pro-oxidants cannot be reliably traced in more complex biological samples possessing an array of antioxidant defenses. In the absence of direct experimental evidence we must rely on indications and propose hypotheses on how and whether pro-oxidants, such as reactive oxygen species contribute to acclimative responses. Here we briefly review how a balance between pro-oxidants and antioxidants is affected by UV-B in whole plant experiments performed in controlled environments. A working hypothesis is proposed in which the extents of UV-induced peroxidase and superoxide dismutase activations affect the success of acclimation to UV-B.

Ultraviolet-B protection of ascorbate and tocopherol in plants related with their function on the stability on carotenoid and phenylpropanoid compounds

Ascorbate and tocopherol are important hydrophilic or lipophilic antioxidants in plants, while their crucial roles in the antioxidant defense system under ultraviolet B radiation were not well understood. The mutants of Arabidopsis thaliana deficient in ascorbate (vtc1 and vtc2) or tocopherol (vte1) were used to analyze their physiological, biochemical and metabolic change in responses to Ultraviolet B radiation. Results showed that loss of either ascorbate or tocopherol caused reduction in phenylpropanoid and flavonol glycosides compounds, as well as reduction in superoxide dismutase activity and total cellular antioxidant capacity. This ultimately led to higher oxidative stress as well as lower levels of photosynthetic pigments (carotenoid and chlorophyll) and CO2 assimilation rate in the vtc1, vtc2, and vte1 mutants than the wild type under UV-B radiation, besides unstable early light-induced protein (ELIP1) in those mutants. On the other hand, the loss of tocopherol in vte1 mutants was compensated by the increase of zeaxanthin and anthocyanin contents, which armed vte1 mutants with higher heat dissipation capacity in PS II and higher antioxidative capacity than vtc mutants. Consequently the tolerance to UV-B radiation were much higher in vte1 mutant than in vtc mutants, furthermore, PS II function and light harvesting protein (LHCb1) abundance were reduced only in ascorbate-deficient mutant relative to wild type. Our results suggested that the ascorbate and tocopherol provided not only direct protective function against UV-B radiation but also indirect effects by influencing other protective system, in particular by affecting the stability of various carotenoid and phenylpropanoid compounds.

Antioxidant defence in UV-irradiated tobacco leaves is centred on hydrogen-peroxide neutralization

Greenhouse grown tobacco (Nicotiana tabacum L. cv. Petit Havana) plants were exposed to supplemental UV centred at 318 nm and corresponding to 13.6 kJ m(-2) d(-1) biologically effective UV-B (280-315 nm) radiation. After 6 days this treatment decreased photosynthesis by 30%. Leaves responded by a large increase in UV-absorbing pigment content and antioxidant capacities. UV-stimulated defence against ROS was strongest in chloroplasts, since activities of plastid enzymes FeSOD and APX had larger relative increases than other, non-plastid specific SODs or peroxidases. In addition, non-enzymatic defence against hydroxyl radicals was doubled in UV treated leaves as compared to controls. In UV treated leaves, the extent of activation of ROS neutralizing capacities followed a peroxidases > hydroxyl-radical neutralization > SOD order. These results suggest that highly effective hydrogen peroxide neutralization is the focal point of surviving UV-inducible oxidative stress and argue against a direct signalling role of hydrogen peroxide in maintaining adaptation to UV, at least in laboratory experiments.