Effect of Quercetin and Epicatechin on the Transcript Expression and Activity of Antioxidant Enzymes in Tobacco Seedlings

Leaf necrosis resulting from downregulation of poplar glycosyltransferase UGT72A2

Reactive species (RS) causing oxidative stress are unavoidable by-products of various plant metabolic processes, such as photosynthesis, respiration or photorespiration. In leaves, flavonoids scavenge RS produced during photosynthesis and protect plant cells against deleterious oxidative damages. Their biosynthesis and accumulation are therefore under tight regulation at the cellular level. Glycosylation has emerged as an essential biochemical reaction in the homeostasis of various specialized metabolites such as flavonoids. This article provides a functional characterization of the Populus tremula x P. alba (poplar) UGT72A2 coding for a UDP-glycosyltransferase that is localized in the chloroplasts. Compared with the wild type, transgenic poplar lines with decreased expression of UGT72A2 are characterized by reduced growth and oxidative damages in leaves, as evidenced by necrosis, higher content of glutathione and lipid peroxidation products as well as diminished soluble peroxidase activity and NADPH to NADP+ ratio under standard growing conditions. They furthermore display lower pools of phenolics, anthocyanins and total flavonoids but higher proanthocyanidins content. Promoter analysis revealed the presence of cis-elements involved in photomorphogenesis, chloroplast biogenesis and flavonoid biosynthesis. The UGT72A2 is regulated by the poplar MYB119, a transcription factor known to regulate the flavonoid biosynthesis pathway. Phylogenetic analysis and molecular docking suggest that UGT72A2 could glycosylate flavonoids; however, the actual substrate(s) was not consistently evidenced with either in vitro assays nor analyses of glycosylated products in leaves of transgenic poplar overexpressing or downregulated for UGT72A2. This article provides elements highlighting the importance of flavonoid glycosylation regarding protection against oxidative stress in poplar leaves and raises new questions about the link between this biochemical reaction and regulation of the redox homeostasis system.

The role of quercetin in plants

Flavonoids are a special category of hydroxylated phenolic compounds having an aromatic ring structure. Quercetin is aspecial subclass of flavonoid. It is a bioactive natural compound built upon the flavon structure nC6(ring A)-C3(ring C)-C6(ring B). Quercetin facilitates several plant physiological processes, such as seed germination, pollen growth, antioxidant machinery, and photosynthesis, as well as induces proper plant growth and development. Quercetin is a powerful antioxidant, so it potently provides plant tolerance against several biotic and abiotic stresses. This review highlights quercetin's role in increasing several physiological and biochemical processes under stress and non-stress environments. Additionally, this review briefly assesses quercetin's role in mitigating biotic and abiotic stresses (e.g., salt, heavy metal, and UV stress). The biosynthesis of flavonoids, their signaling pathways, and quercetin's role in plant signaling are also discussed.

Metabolic elicitors of Pseudomonas fluorescens N 21.4 elicit flavonoid metabolism in blackberry fruit

BACKGROUND

The beneficial rhizobacterium, Pseudomonas fluorescens N 21.4, and its metabolic elicitors were inoculated in commercial cultivars of blackberry plants (Rubus cv. Loch Ness). Phenolic compounds present in red and black fruit and the expression of structural marker genes of the phenylpropanoid pathway during fruit ripening were studied.

RESULTS

An inverse relationship between gene expression and accumulation of metabolites was seen, except for the RuDFR gene, which had a direct correlation with cyanidin 3-O-glucoside synthesis, increasing its content 1.3 times when RuDFR was overexpressed in the red fruit of plants inoculated with the metabolic elicitors of P. fluorescens N 21.4, compared with red fruit of plants inoculated with N 21.4. The RuCHS gene also had a fundamental role in the accumulation of metabolites. Both rhizobacterium and metabolic elicitors triggered the flavonoid metabolism, enhancing the catechin and epicatechin content between 1.1 and 1.6 times in the case of red fruit and between 1.1 and 1.8 times in the case of black fruit. Both treatments also boosted the anthocyanin, quercetin, and kaempferol derivative content, highlighting the effects of metabolic elicitors in red fruit and the effects of live rhizobacterium in black fruit.

CONCLUSION

The metabolic elicitors' capacity to modulate gene expression and to increase secondary metabolites content was demonstrated. This work therefore suggests that they are effective, affordable, easily manageable, and ecofriendly plant inoculants that complement, or are alternatives to, beneficial rhizobacteria. © 2020 Society of Chemical Industry.

Improving Flavonoid Metabolism in Blackberry Leaves and Plant Fitness by Using the Bioeffector Pseudomonas fluorescens N 21.4 and Its Metabolic Elicitors: A Biotechnological Approach for a More Sustainable Crop

Beneficial rhizobacterium Pseudomonas fluorescens N 21.4 and its metabolic elicitors inoculated to cultivars of blackberry (Rubus spp. Var. Loch Ness) reinforced the plants' immune system and improved their fitness by increasing photosynthesis, decreasing oxidative stress, and activating pathogenesis-related proteins. They also triggered the leaves' flavonoid metabolism, enhancing the accumulation of beneficial phenolic compounds such as kaempferols and quercetin derivatives. The elicitation of leaf secondary metabolism allows one to take advantage of the blackberry leaves (a current crop waste), following the premises of the circular economy, to isolate and obtain high added value compounds. The results of this work suggest the use of N 21.4 and/or its metabolic elicitors as plant inoculants as an effective and economically and environmentally friendly agronomic alternative practice in the exploitation of blackberry crops to obtain plants with a better immune system and to revalorize the leaf pruning as a potential source of polyphenols.

Flavonoid Naringenin Alleviates Short-Term Osmotic and Salinity Stresses Through Regulating Photosynthetic Machinery and Chloroplastic Antioxidant Metabolism in Phaseolus vulgaris

The current study was conducted to demonstrate the possible roles of exogenously applied flavonoid naringenin (Nar) on the efficiency of PSII photochemistry and the responses of chloroplastic antioxidant of salt and osmotic-stressed Phaseolus vulgaris (cv. Yunus90). For this aim, plants were grown in a hydroponic culture and were treated with Nar (0.1 mM and 0.4 mM) alone or in a combination with salt (100 mM NaCl) and/or osmotic (10% Polyethylene glycol, -0.54 MPa). Both caused a reduction in water content (RWC), osmotic potential (Ψ_Π), chlorophyll fluorescence (F_v/F_m), and potential photochemical efficiency (F_v/F_o). Nar reversed the changes on these parameters. The phenomenological fluxes (TR_o/CS and ET_o/CS) altered by stress were induced by Nar and Nar led to a notable increase in the performance index (PI_ABS) and the capacity of light reaction [ΦP_o/(1-ΦP_o)]. Besides, Nar-applied plants exhibited higher specific fluxes values [ABS/RC, ET_o/RC, and ΨE_o/(1-ΨE_o)] and decreasing controlled dissipation of energy (DI_o/CS_o and DI_o/RC). The transcripts levels of psbA and psbD were lowered in stress-treated bean but upregulated in Nar-treated plants after stress exposure. Nar also alleviated the changes on gas exchange parameters [carbon assimilation rate (A), stomatal conductance (g_s), intercellular CO₂ concentrations (C_i), transpiration rate (E), and stomatal limitation (L_s)]. By regulating the antioxidant metabolism of the isolated chloroplasts, Nar was able to control the toxic levels of hydrogen peroxide (H₂O₂) and TBARS (lipid peroxidation) produced by stresses. Chloroplastic superoxide dismutase (SOD) activity reduced by stresses was increased by Nar. In response to NaCl, Nar increased the activities of ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), and dehydroascorbate reductase (DHAR), as well as peroxidase (POX). Nar protected the bean chloroplasts by minimizing disturbances caused by NaCl exposure via the ascorbate (AsA) and glutathione (GSH) redox-based systems. Under Nar plus PEG, Nar maintained the AsA regeneration by the induction of MDHAR and DHAR, but not GSH recycling by virtue of no induction in GR activity and the reduction in GSH/GSSG and GSH redox state. Based on these advances, Nar protected in bean chloroplasts by minimizing disturbances caused by NaCl or PEG exposure via the AsA or GSH redox-based systems and POX activity.

Antioxidant and hepato-preventive effect of Citrus aurantium extract against carbon tetrachloride-induced hepatotoxicity in rats and characterisation of its bioactive compounds by HPLC-MS

The objective of this study is to explore the preventive effects of ethyl acetate fraction from Citrus aurantium leaf extract (EACA), associated with its phytochemical content, against the toxic impacts of acute exposure to carbon tetrachloride (CCl₄) in the liver of adult rats. HPLC analysis of ethyl acetate fraction from extract C. aurantium revealed eight compounds. Administration of a single dose of CCl₄ caused hepatoxicity as monitored by an increase in lipid peroxidation (thiobarbituric acid reactive substances) and in protein carbonyl level but a decrease in antioxidant markers in the liver tissue. The pre-treatment with EACA; significantly prevented the increased plasma levels of hepatic markers and lipid levels induced by CCl₄ in rats. Furthermore, this fraction ameliorated biochemical and histological parameters as compared to CCl₄-treated group. Our results suggest that C. aurantium contains promising substances to counteract the CCl₄ intoxication and can be efficient in the prevention of hepatotoxicity complications.

Flavonoid compounds from the red marine alga Alsidium corallinum protect against potassium bromate-induced nephrotoxicity in adult mice

Potassium bromate (KBrO₃ ), an environmental pollutant, is a well-known human carcinogen and a potent nephrotoxic agent. Currently, natural products have built a well-recognized role in the management of many diseases induced by pollutants. As potent natural sources of bioactive compounds, marine algae have been demonstrated to be rich in novel secondary metabolites with a broad range of biological functions. In this study, adults male mice were orally treated for 15 days with KBrO₃ (0.5 g/L) associated or not with extract of Alsidium corallinum, a red Mediterranean alga. In vitro study demonstrated that algal extract has antioxidant efficacy attributable to the presence of flavonoids and polyphenols. Among these, Liquid chromatography-mass spectrometry analysis showed A. corallinum is rich in kaempferol, apigenin, catechin, and quercetin flavonoids. In vivo study showed that supplementation with the alga significantly prevented KBrO₃ -induced nephrotoxicity as indicated by plasma biomarkers (urea, uric acid, and creatinin levels) and oxidative stress related parameters (malondialdehyde, superoxide dismutase, catalase, glutathione peroxidase, reduced glutathione, vitamin C, hydrogen peroxide, protein oxidation products) in kidney tissue. The corrective effect of A. corallinum on KBrO₃ -induced kidney injury was also supported by molecular and histopathological observations. In conclusion, it was established that the red alga, thanks to its bioactive compounds, effectively counteracts toxic effects of KBrO₃ and could be a useful coadjuvant agent for treatment of this pollutant poisonings. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 1475-1486, 2017.

Aloin Protects Skin Fibroblasts from Heat Stress-Induced Oxidative Stress Damage by Regulating the Oxidative Defense System

Oxidative stress is commonly involved in the pathogenesis of skin damage induced by environmental factors, such as heat stress. Skin fibroblasts are responsible for the connective tissue regeneration and the skin recovery from injury. Aloin, a bioactive compound in Aloe vera, has been reported to have various pharmacological activities, such as anti-inflammatory effects. The aim of this study was to investigate the protective effect of aloin against heat stress-mediated oxidative stress in human skin fibroblast Hs68 cells. Hs68 cells were first incubated at 43°C for 30 min to mimic heat stress. The study was further examined if aloin has any effect on heat stress-induced oxidative stress. We found that aloin protected Hs68 cells against heat stress-induced damage, as assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide and lactate dehydrogenase assay. Aloin protected Hs68 cells by regulating reactive oxygen species production and increasing the levels of glutathione, cytosolic and mitochondrial superoxide dismutase. Aloin also prevented the elevation of thiobarbituric acid reactive substances and the reduction of 8-OH-dG induced by heat stress. These results indicated that aloin protected human skin fibroblasts from heat stress-induced oxidative stress damage by regulating the oxidative defense system.

Cellular and molecular mechanisms activating the cell death processes by chalcones: Critical structural effects

Chalcones are naturally occurring compounds with diverse pharmacological activities. Chalcones derive from the common structure: 1,3-diphenylpropenone. The present study aims to better understand the mechanistic pathways triggering chalcones anticancer effects and providing evidences that minor structural difference could lead to important difference in mechanistic effect. We selected two recently investigated chalcones (A and B) and investigated them on glioblastoma cell lines. It was found that chalcone A induced an apoptotic process (type I PCD), via the activation of caspase-3, -8 and -9. Chalcone A also increased CDK1/cyclin B ratios and decreased the mitochondrial transmembrane potential (ΔΨm). Chalcone B induced an autophagic cell death process (type II PCD), ROS-related but independent of both caspases and protein synthesis. Both chalcones increased Bax/Bcl2 ratios and decreased Ki67 and CD71 antigen expressions. The present investigation reveals that despite the close structure of chalcones A and B, significant differences in mechanism of effect were found.