Antioxidant Metabolites in Primitive, Wild, and Cultivated Citrus and Their Role in Stress Tolerance

Anabolic metabolism of autotoxic substance coumarins in plants

Background

Autotoxicity is an intraspecific manifestation of allelopathy in plant species. The specialized metabolites and their derivatives that cause intraspecific allelopathic inhibition in the plant are known as autotoxic substances. Consequently, autotoxic substances production seriously affects the renewal and stability of ecological communities.

Methods

This article systematically summarizes the types of autotoxic substances present in different plants. They mainly include phenolic compounds, terpenoids, and nitrogenous organic compounds. Phenolic coumarins are the main autotoxic substances in many plants. Therefore, we also discuss differences in coumarin types and content among plant varieties, developmental stages, and tissue parts, as well as their mechanisms of autotoxicity. In addition, we review the metabolic pathways involved in coumarin biosynthesis, the key enzymes, genes, and transcription factors, as well as factors affecting coumarin biosynthesis.

Results

Coumarin biosynthesis involves three stages: (1) the formation of the coumarin nucleus; (2) acylation, hydroxylation, and cyclization; (3) structural modification. The key enzymes involved in the coumarin nuclear formation stage include PAL, C4H, 4CL, HCT, CAOMT, COSY, F6'H, and CCoAOMT1, and the key genes involved include BGA, CYP450 and MDR, among others. Ortho-hydroxylation is a key step in coumarin biosynthesis and PS, COSY and S8H are the key enzymes involved in this process. Finally, UGTs are responsible for the glycosylation modification of coumarins, and the MaUGT gene may therefore be involved in coumarin biosynthesis.

Conclusion

It is important to elucidate the autotoxicity and anabolic mechanisms of coumarins to create new germplasms that produce fewer autotoxic substances.

LC-MS/MS-based metabolomics approach identified novel antioxidant flavonoids associated with drought tolerance in citrus species

Citrus fruits are cultivated around the world, and they face drought stress frequently during their growth and development. Previous studies showed that citrus plants biosynthesized flavonoid compounds in response to abiotic stress. In this study, we have quantified 37 flavonoid compounds from the leaves of three distinct citrus species including sour orange (drought-tolerant), pummelo 'Majia you pummelo' (drought-sensitive), and lemon (drought-sensitive). The 37 flavonoids consisted of 12 flavones, 10 flavonols, 6 flavanones, 5 isoflavanones, and 1 each for chalcone, flavanol, flavanonol, and flavone glycoside. Drought stress differentially altered the flavonoid metabolism in drought-tolerant and drought-sensitive citrus species. The kaempferol 3-neohesperidoside was 17-fold higher in sour orange (124.41 nmol/L) after 18 days of drought stress than lemon (7.33 nmol/L). In sour orange, neohesperidin (69.49 nmol/L) was 1,407- and 37-fold higher than pummelo and lemon, respectively. In sour orange, some flavonoids were significantly increased, such as vitexin, neohesperidin, cynaroside, hyperoside, genistin, kaempferol 3-neohesperidoside, eriocitrin, and luteolin, in response to drought stress, whereas in lemon, these flavonoids were significantly decreased or not altered significantly in response to drought stress. Moreover, the total contents of flavonoids and antioxidant activity were increased in sour orange as compared with pummelo and lemon. The genes associated with flavonoid biosynthesis (PAL, CHI, FLS, GT1, F3H, F3'M, C4H, 4CL, FLS, FG2, FG3, and CYP81E1) were more highly expressed in sour orange leaves than in pummelo and lemon after drought stress. These outcomes showed that pummelo and lemon failed to biosynthesize antioxidant flavonoids to cope with the prolonged drought stress, whereas the sour orange biosynthesized fortified flavonoid compounds with increased antioxidant activity to detoxify the harmful effects of reactive oxygen species produced during drought stress.

Chemical compositions and biological activities of Serevenia buxifolia essential oil leaves cultivated in Vietnam (Thua Thien Hue)

Serevenia buxifolia is an evergreen citrus plant and has attracted considerable attention due to its bioactive components and biological activities. In the present study, the essential oil (EO) from S. buxifolia cultivated in Vietnam was demonstrated to exhibit the in vitro antioxidant, thrombolytic, anti-hemolysis, anti-inflammatory, and antidiabetic activities. Briefly, the gas chromatography coupled to mass spectrometry analysis revealed that the leaf EO of S. buxifolia was composed of 33 components, with the main constituents being β-carypphyllene (32.5%), and elixene (9.8%). The extracted oil possessed a fairly high free radical scavenging activity against 2, 2-diphenyl-1-picrylhydrazyl (DPPH), with an IC₅₀ value of 190.7 μg/mL compared with positive control, α-tocopherol, IC₅₀ value of 42.6 μg/mL. The EO also exhibited thrombolytic activity: the percentage of inhibition was found to be 70.75% at 100 μL, in comparison with 87.2% for the positive control, streptokinase. For hemolytic activity, the percentage of inhibition of the EO was from 27.4% to 59.6% at concentrations from 10 to 100 μg/mL, respectively. The results of in vitro anti-inflammatory activity indicated that the EO of S. buxifolia leaves effectively protects the heat-induced denaturation, with an IC₅₀ value of 40.25 μg/mL. The EO also exhibited antidiabetic potential, with IC₅₀ values of 87.8 and 134.9 μg/mL against α-amylase and α-glucosidase, respectively. It is noteworthy that the potent biological activities of the obtained S. buxifolia oil increased in a dose-dependent manner. The results achieved show that the EO of S. buxifolia leaves can be a potential source for oxidative stress, inflammatory, and diabetic management.

Green synthesis of ZnFe2O4 nanoparticles using plant extracts and their applications: A review

Magnetic nanoparticles, particularly ZnFe₂O₄ are of enormous significance in biomedical and water treatment fields. However, chemical synthesis of ZnFe₂O₄ nanoparticles endures some major limitations, e.g., the use of toxic substances, unsafe procedure, and cost-ineffectiveness. Biological methods are more preferable approaches since they take advantages of biomolecules available in plant extract serving as reducing, capping, and stabilizing agents. Herein, we review plant-mediated synthesis and properties of ZnFe₂O₄ nanoparticles for multiple applications in catalytic and adsorption performance, biomedical, catalyst, and others. Effect of several factors such as Zn²⁺/Fe³⁺/extract ratio, and calcination temperature on morphology, surface chemistry, particle size, magnetism and bandgap energy of obtained ZnFe₂O₄ nanoparticles was discussed. The photocatalytic activity and adsorption for removal of toxic dyes, antibiotics, and pesticides were also evaluated. Main results of antibacterial, antifungal and anticancer activities for biomedical applications were summarized and compared. Several limitations and prospects of green ZnFe₂O₄ as an alternative to traditional luminescent powders have been proposed.

A review of citrus plants as functional foods and dietary supplements for human health, with an emphasis on meta-analyses, clinical trials, and their chemical composition

Fruits, flowers, leaves, essential oils, hydrosols, and juices of citrus spp. Are utilized to prepare various forms of food products. Along with their nutritional values, in the health industry, different parts of the plants of the citrus genus have been used as supplements or remedies to prevent or control diseases. This review focused on reported meta-analyses and clinical trials on the health benefits of citrus plants as functional foods. Also, chemical compounds of various citrus species were reviewed. The following information sources were used for data collection: Google Scholar, the Web of Science, Scopus, and PubMed. Various keywords, including "citrus AND chemical compounds," "citrus AND phytochemicals," "citrus species," "citrus AND meta-analysis," "nutritional and therapeutical values of citrus spp.," "clinical trials AND citrus," "clinical trials AND Rutaceae," "health benefits of citrus spp.," "citrus edible or non-edible applications," and scientific names of the citrus plants were utilized to collect data for the review. The scientific name and common name of all twenty-eight citrus species, along with any of the above keywords, were also searched in the mentioned databases. Scientific papers and data sources were sought to review and discuss the citrus plant's nutritional and therapeutic importance. Several meta-analyses and clinical trials have reported beneficial effects of citrus spices on a variety of cancer risks, cardiovascular risk factors, neurologic disorders, urinary tract conditions, and gastrointestinal tract conditions. They have shown anxiolytic, antimicrobial, and pain-alleviating effects. Some of them can be helpful in managing obesity and cardiovascular risk factors.

Role of polyphenolic compounds and their nanoformulations: a comprehensive review on cross-talk between chronic kidney and cardiovascular diseases

Chronic kidney disease (CKD) affects a huge portion of the world's population and frequently leads to cardiovascular diseases (CVDs). It might be because of common risk factors between chronic kidney disease and cardiovascular diseases. Renal dysfunction caused by chronic kidney disease creates oxidative stress which in turn leads to cardiovascular diseases. Oxidative stress causes endothelial dysfunction and inflammation in heart which results in atherosclerosis. It ends in clogging of veins and arteries that causes cardiac stroke and myocardial infarction. To develop an innovative therapeutic approach and new drugs to treat these diseases, it is important to understand the pathophysiological mechanism behind the CKD and CVDs and their interrelationship. Natural phytoconstituents of plants such as polyphenolic compounds are well known for their medicinal value. Polyphenols are plant secondary metabolites with immense antioxidant properties, which can protect from free radical damage. Nowadays, polyphenols are generating a lot of buzz in the scientific community because of their potential health benefits especially in the case of heart and kidney diseases. This review provides a detailed account of the pathophysiological link between CKD and CVDs and the pharmacological potential of polyphenols and their nanoformulations in promoting cardiovascular and renal health.

Generation of Highly Antioxidant Submicron Particles from Myrtus communis Leaf Extract by Supercritical Antisolvent Extraction Process

Submicron particles have been produced from an ethanolic extract of Myrtus communnis leaves using supercritical carbon dioxide technology, hereinafter referred to as Supercritical Antisolvent Extraction (SAE). The influence of pressure (9-20 MPa), temperature (308 and 328 K) and injection rate (3 and 8 mL/min) on the particles' precipitation has been investigated, and it has been confirmed that increases in pressure and temperature led to smaller particle sizes. The obtained particles had a quasi-spherical shape with sizes ranging from 0.42 to 1.32 μm. Moreover, the bioactivity of the generated particles was assessed and large contents of phenolic compounds with a high antioxidant activity were measured. The particles were also subjected to in vitro studies against oxidative stress. The myrtle particles demonstrated cytoprotective properties when applied at low concentrations (1 μM) to macrophage cell lines.

An up-to-date review: differential biosynthesis mechanisms and enrichment methods for health-promoting anthocyanins of citrus fruits during processing and storage

Anthocyanins, naturally found in citrus, play key roles in improving the qualities of citrus fruits and products. Dietary consumption of fruit-derived anthocyanins is concerned increasingly owing to health-promoting properties. However, anthocyanins are vulnerable to many physical and chemical factors during processing and storage, affecting fruit qualities and consumer acceptance. Thus, the aim of this review is to focus on main advances in chemical structures, differential biosynthesis mechanisms, enrichment methods, and bioactivities of anthocyanins in pigmented and unpigmented citrus fruits. In this review, anthocyanin species and concentrations display tissue specificity in citrus, and the chemical structures and contents of main anthocyanins are summarized. For differential biosynthesis mechanisms, the reasons why most citrus fruits lose the ability of anthocyanin biosynthesis compared with pigmented fruits, and the molecular differences of biosynthesis mechanisms in pigmented citrus fruits are both discussed in detail. Furthermore, anthocyanins' enrichment methods (low-temperature stimulus, light irradiation, xenobiotics inductions, and ripeness influence) during processing and storage have been summarized, which achieve quality improvement by promoting structural gene expression, reducing anthocyanin-degrading enzyme activities, or altering DNA methylation status. Meantime, the health benefits of extract from pigmented citrus and their waste are mentioned, which provides a new approach for citrus waste recycling. HIGHLIGHTSChemical structures of individual anthocyanins in citrus are reviewed.Differential anthocyanin biosynthesis in citrus depends on mutations of Ruby genes.Anthocyanins are enriched in response to exogenous stimulus during storage.Health benefits of extract in blood oranges and their waste are summarized.

LC-MS/MS-based metabolomics approach revealed novel phytocompounds from sugarcane rind with promising pharmacological value

BACKGROUND

Sugarcane provides many secondary metabolites for the pharmacological and cosmetic industries. Secondary metabolites, such as phenolic compounds, flavonoids, and anthocyanins, have been studied, but few reports focus on the identification of alkaloid and non-alkaloid phytocompounds in sugarcane.

RESULTS

In this study, we identified 40 compounds in total from the rinds of cultivated sugarcane varieties (including eight alkaloids, 24 non-alkaloids, and eight others) by using the liquid chromatography-tandem mass spectrometry (LC-MS/MS) approach. Among these compounds, 31 were novel and are reported for the first time in sugarcane. Some alkaloids such as 3-indoleacrylic acid, N,N-dimethyl-5-methoxytryptamine, tryptamine, 6-hydroxynicotinic acid, and 6-deoxyfagomine are identified the first time in sugarcane rind. Four alkaloids such as trigonelline, piperidine, 3-indoleacrylic acid, and 6-deoxyfagomine are found abundantly in sugarcane rind and these compounds have promising pharmaceutical value. Some phytocompounds such as choline and acetylcholine (non-alkaloid compounds) were most common in the rind of ROC22 and Yuetang93/159 (YT93/159). Hierarchical cluster analysis and principal component analysis revealed that the ROC22, Taitang172 (F172), and Yuetang71/210 (YT71/210) varieties were quite similar in alkaloid composition when compared with other sugarcane varieties. We have also characterized the biosynthesis pathway of sugarcane alkaloids. The rind of F172, ROC22, and YT71/210 showed the highest total alkaloid content, whereas the rind of ROC16 revealed a minimum level. Interestingly, the rind extract from YT71/210 and F172 showed maximum antioxidant activity, followed by ROC22.

CONCLUSION

Our results showed the diversity of alkaloid and non-alkaloid compounds in the rind of six cultivated sugarcanes and highlighted the promising phytocompounds that can be extracted, isolated, and utilized by the pharmacological industry. © 2022 Society of Chemical Industry.

Phytochemical Profile, Antioxidant Potential and Toxicity Evaluation of the Essential Oils from Duguetia and Xylopia Species (Annonaceae) from the Brazilian Amazon

The essential oils (EOs) of Duguetia echinophora, D. riparia, Xylopia emarginata and X. frutescens (Annonaceae) were obtained by hydrodistillation and the chemical composition was analyzed by GC-MS. An antioxidant assay using the ABTS and DPPH radicals scavenging method and cytotoxic assays against Artemia salina were also performed. We evaluated the interaction of the major compounds of the most toxic EO (X. emarginata) with the binding pocket of the enzyme Acetylcholinesterase, a molecular target related to toxicity in models of Artemia salina. The chemical composition of the EO of D. echinophora was characterized by β-phellandrene (39.12%), sabinene (17.08%) and terpinolene (11.17%). Spathulenol (22.22%), caryophyllene oxide (12.21%), humulene epoxide II (11.86%) and allo-aromadendrene epoxide (10.20%) were the major constituents of the EO from D. riparia. Spathulenol (5.65%) and caryophyllene oxide (5.63%) were the major compounds of the EO from X. emarginata. The EO of X. frutescens was characterized by α-pinene (20.84%) and byciclogermacrene (7.85%). The results of the radical scavenger DPPH assays ranged from 15.87 to 69.38% and the highest percentage of inhibition was observed for the EO of X. emarginata, while for ABTS radical scavenging, the antioxidant capacity of EOs varied from 14.61 to 63.67%, and the highest percentage of inhibition was observed for the EO of X. frutescens. The EOs obtained from D. echinophora, X. emarginata and X. frutescens showed high toxicity, while the EO of D. riparia was non-toxic. Because the EO of X. emarginata is the most toxic, we evaluated how its major constituents were able to interact with the Acetylcholinesterase enzyme. The docking results show that the compounds are able to bind to the binding pocket through non-covalent interactions with the residues of the binding pocket. The species X. emarginata and X. frutescens are the most promising sources of antioxidant compounds; in addition, the results obtained for preliminary cytotoxicity of the EOs of these species may also indicate a potential biological activity.

Transcriptomic and Widely Targeted Metabolomic Approach Identified Diverse Group of Bioactive Compounds, Antiradical Activities, and Their Associated Genes in Six Sugarcane Varieties

Sugarcane is cultivated mainly for its high sucrose content but it can also produce many metabolites with promising antioxidant potential. However, very few studies have been reported on the biosynthesis of metabolites in sugarcane to date. In this study, we have identified a wide range of amino acids and organic acids in the rind of six sugarcane varieties by the LC-MS/MS method. A total number of 72 amino acids and 55 organic acid compounds were characterized; among these, 100 were reported for the first time. Moreover, 13 amino acids and seven organic acids were abundantly distributed in all varieties tested and considered major amino acids and organic acids in sugarcane. The variety Taitang134 (F134) showed the highest content of total amino acids, whereas the varieties ROC16 and Yuetang93/159 (YT93/159) had maximum content of organic acids. The amino acids of the rind extract presented higher antioxidant capacity than the organic acids of the rind extract. In addition, the transcriptomic and metabolic integrated analysis highlighted some candidate genes associated with amino acid biosynthesis in sugarcane. We selected a transcription factor gene, MYB(t), and over-expressed it in Arabidopsis. The transgenic plants showed a higher accumulation of amino acids with higher antiradical activity compared with the wild-type Arabidopsis plants. Thus, we characterize a wide range of amino acids and organic acids and their antiradical activities in different sugarcane varieties and present candidate genes that can be potentially valuable for the genetic improvement of metabolites in sugarcane bagasse

Comparative Analysis of the Chalcone-Flavanone Isomerase Genes in Six Citrus Species and Their Expression Analysis in Sweet Orange (Citrus sinensis)

Citrus fruit contains rich nutrients which is edible and of officinal value. Citrus flavanones are widely used in the treatment of cardiovascular and other diseases, and they are a foundational material of Chinese medicine. The chalcone-flavanone isomerase (CHI) plays a key role in flavanone synthesis. Therefore, we comprehensively analyzed CHI genes in Citrus species. Here, thirty CHI genes were identified for the first time in six Citrus species, which were divided into CHI and FAP groups. Evolutionary analysis showed that CHI gene members were highly conserved and were an ancient family. All CsCHI genes showed the highest expression level after the second physiological fruit-falling period in C. sinensis. CsCHI1 and CsCHI3 were highly expressed at 50 days after the flowering (DAF) stage in albedo. The expression of CsFAP2 and CsCHI3 genes at the 50 DAF stage was 16.5 and 24.3 times higher than that at the 220 DAF stage, respectively. The expression of CsCHI1, CsCHI3, and CsFAP2 genes in the peel was higher than that in the pulp, especially in common sweet orange. The CsCHI3 gene maintained a high expression level in the epicarp and juice sac at all periods. The members of CHIs interacted with chalcone synthase (CHS), flavonol synthase/flavanone 3-hydroxylase (FLS) and naringenin, and 2-oxoglutarate 3-dioxygenase (F3H) to form heterodimers, which might together play a regulatory role and participate in the flavonoid pathway. This study will provide the basis for the selection of flavonoids in plant tissues and periods and fundamental information for further functional studies.

Novel Insights into Anthocyanin Metabolism and Molecular Characterization of Associated Genes in Sugarcane Rinds Using the Metabolome and Transcriptome

Saccharum officinarum (sugarcane) is the fifth major cultivated crop around the world. Sugarcane rind is a promising source for anthocyanin pigments; however, limited information is available on the anthocyanin and its biosynthesis in sugarcane rinds. In this study, we have quantified 49 compounds including 6 flavonoids and 43 anthocyanins in the rind of 6 sugarcane cultivars by using LCMS/MS approach. Thirty of them were quantified for the first time in sugarcane. The 43 anthocyanins included 10 cyanidin (Cya), 11 pelargonidin (Pel), 9 peonidin (Peo), 5 malvidin (Mal), 4 delphinidin (Del), and 4 petunidin (Pet) metabolites. High contents of Cya derivatives were observed in the rind of YT71/210 (dark purple rind), such as cya-3-O-(6-O-malonyl)-glu 1283.3 µg/g and cya-3-O-glu 482.67 µg/g followed by ROC22 (red rind) 821.3 µg/g and 409 µg/g, respectively, whereas the YT93/159 (green rind) showed a minimum level of these compounds. Among six cultivars, ROC22 rind has high levels of Peo derivatives such as peo-3-O-glu (197 µg/g), peo-3-O-(6-O-malonyl)-glu (69 µg/g) and peo-3-O-(6-O-p-coumaryl)-glu (55.17 µg/g). The gene expression analysis revealed that some genes, including a MYB(t) gene, were highly associated with the color phenotype. Thus, we cloned and overexpressed the gene in Arabidopsis and found the pinkish brown color in the hypocotyl of all transgenic lines compared with the wild type. Hence, we have quantified a wide range of anthocyanins in major sugarcane cultivars, reported many new anthocyanins for the first time, and concluded that Cya and Peo derivatives are the major contributing factor of dissimilar colors in sugarcane. The finding and the verification of a novel MYB gene involved in anthocyanin biosynthesis have demonstrated that our study was very valuable for gene discovery and genetic improvement of sugarcane cultivars to harvest high anthocyanin contents.

An Overview of Bioactive Flavonoids from Citrus Fruits

Citrus species are one of the world’s popular fruit crops, cultivated all over the world for their economic and nutritional values. Citrus, like other fruits and vegetables, are an important source of several antioxidant molecules (polyphenols, ascorbic acid, and carotenoids) that can inhibit the harmful effects of free radicals on the human body; due to their functional values and health-promoting properties, Citrus species are considered valuable fruits not only in agri-food industry, but also in pharmaceutical industry. Flavonoids are among the major constituents of polyphenols found in different parts of Citrus fruits (skin, peels, seed, pulp membrane, and juice). Flavonoids have different biological properties (antiviral, antifungal, and antibacterial activities). Several studies have also shown the health-related properties of Citrus flavonoids, especially antioxidant, anticancer, anti-inflammation, anti-aging, and cardiovascular protection activities. In the present review, attempts are made to discuss the current trends of research on flavonoids in different Citrus species.

Molecular signatures between citrus and Candidatus Liberibacter asiaticus

Citrus Huanglongbing (HLB), also known as citrus greening, is one of the most devastating citrus diseases worldwide. Candidatus Liberibacter asiaticus (CLas) is the most prevalent strain associated with HLB, which is yet to be cultured in vitro. None of the commercial citrus cultivars are resistant to HLB. The pathosystem of Ca. Liberibacter is complex and remains a mystery. In this review, we focus on the recent progress in genomic research on the pathogen, the interaction of host and CLas, and the influence of CLas infection on the transcripts, proteins, and metabolism of the host. We have also focused on the identification of candidate genes for CLas pathogenicity or the improvements of HLB tolerance in citrus. In the end, we propose potentially promising areas for mechanistic studies of CLas pathogenicity, defense regulators, and genetic improvement for HLB tolerance/resistance in the future.