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

Transcriptomic Analysis on the Peel of UV-B-Exposed Peach Fruit Reveals an Upregulation of Phenolic- and UVR8-Related Pathways

UV-B treatment deeply influences plant physiology and biochemistry, especially by activating the expression of responsive genes involved in UV-B acclimation through a UV-B-specific perception mechanism. Although the UV-B-related molecular responses have been widely studied in Arabidopsis, relatively few research reports deepen the knowledge on the influence of post-harvest UV-B treatment on fruit. In this work, a transcriptomic approach is adopted to investigate the transcriptional modifications occurring in the peel of UV-B-treated peach (Prunus persica L., cv Fairtime) fruit after harvest. Our analysis reveals a higher gene regulation after 1 h from the irradiation (88% of the differentially expressed genes-DEGs), compared to 3 h recovery. The overexpression of genes encoding phenylalanine ammonia-lyase (PAL), chalcone syntase (CHS), chalcone isomerase (CHI), and flavonol synthase (FLS) revealed a strong activation of the phenylpropanoid pathway, resulting in the later increase in the concentration of specific flavonoid classes, e.g., anthocyanins, flavones, dihydroflavonols, and flavanones, 36 h after the treatment. Upregulation of UVR8-related genes (HY5, COP1, and RUP) suggests that UV-B-triggered activation of the UVR8 pathway occurs also in post-harvest peach fruit. In addition, a regulation of genes involved in the cell-wall dismantling process (PME) is observed. In conclusion, post-harvest UV-B exposure deeply affects the transcriptome of the peach peel, promoting the activation of genes implicated in the biosynthesis of phenolics, likely via UVR8. Thus, our results might pave the way to a possible use of post-harvest UV-B treatments to enhance the content of health-promoting compounds in peach fruits and extending the knowledge of the UVR8 gene network.

Modulation of defense genes and phenolic compounds in wild blueberry in response to Botrytis cinerea under field conditions

Botrytis blight is an important disease of wild blueberry [(Vaccinium angustifolium (Va) and V. myrtilloides (Vm))] with variable symptoms in the field due to differences in susceptibility among blueberry phenotypes. Representative blueberry plants of varying phenotypes were inoculated with spores of B. cinerea. The relative expression of pathogenesis-related genes (PR3, PR4), flavonoid biosynthesis genes, and estimation of the concentration of ten phenolic compounds between uninoculated and inoculated samples at different time points were analyzed. Representative plants of six phenotypes (brown stem Va, green stem Va, Va f. nigrum, tall, medium, and short stems of Vm) were collected and studied using qRT-PCR. The expression of targeted genes indicated a response of inoculated plants to B. cinerea at either 12, 24, 48 or 96 h post inoculation (hpi). The maximum expression of PR3 occurred at 24 hpi in all the phenotypes except Va f. nigrum and tall stem Vm. Maximum expression of both PR genes occurred at 12 hpi in Va f. nigrum. Chalcone synthase, flavonol synthase and anthocyanin synthase were suppressed at 12 hpi followed by an upregulation at 24 hpi. The expression of flavonoid pathway genes was phenotype-specific with their regulation patterns showing temporal differences among the phenotypes. Phenolic compound accumulation was temporally regulated at different post-inoculation time points. M-coumaric acid and kaempferol-3-glucoside are the compounds that were increased with B. cinerea inoculation. Results from this study suggest that the expression of PR and flavonoid genes, and the accumulation of phenolic compounds associated with B. cinerea infection could be phenotype specific. This study may provide a starting point for understanding and determining the mechanisms governing the wild blueberry-B. cinerea pathosystem.

Hyperspectral Imaging in the UV Range Allows for Differentiation of Sugar Beet Diseases Based on Changes in Secondary Plant Metabolites

Fungal infections trigger defense or signaling responses in plants, leading to various changes in plant metabolites. The changes in metabolites, for example chlorophyll or flavonoids, have long been detectable using time-consuming destructive analytical methods including high-performance liquid chromatography or photometric determination. Recent plant phenotyping studies have revealed that hyperspectral imaging (HSI) in the UV range can be used to link spectral changes with changes in plant metabolites. To compare established destructive analytical methods with new nondestructive hyperspectral measurements, the interaction between sugar beet leaves and the pathogens Cercospora beticola, which causes Cercospora leaf spot disease (CLS), and Uromyces betae, which causes sugar beet rust (BR), was investigated. With the help of destructive analyses, we showed that both diseases have different effects on chlorophylls, carotenoids, flavonoids, and several phenols. Nondestructive hyperspectral measurements in the UV range revealed different effects of CLS and BR on plant metabolites resulting in distinct reflectance patterns. Both diseases resulted in specific spectral changes that allowed differentiation between the two diseases. Machine learning algorithms enabled the differentiation between the symptom classes and recognition of the two sugar beet diseases. Feature importance analysis identified specific wavelengths important to the classification, highlighting the utility of the UV range. The study demonstrates that HSI in the UV range is a promising, nondestructive tool to investigate the influence of plant diseases on plant physiology and biochemistry.

Integrated metabolomics and transcriptomics insights on flavonoid biosynthesis of a medicinal functional forage, Agriophyllum squarrosum (L.), based on a common garden trial covering six ecotypes

Agriophyllum squarrosum (L.) Moq., well known as sandrice, is an important wild forage in sandy areas and a promising edible and medicinal resource plant with great domestication potential. Previous studies showed flavonoids are one of the most abundant medicinal ingredients in sandrice, whereby isorhamnetin and isorhamnetin-3-glycoside were the top two flavonols with multiple health benefits. However, the molecular regulatory mechanisms of flavonoids in sandrice remain largely unclear. Based on a common garden trial, in this study, an integrated transcriptomic and flavonoids-targeted metabolomic analysis was performed on the vegetative and reproductive periods of six sandrice ecotypes, whose original habitats covered a variety of environmental factor gradients. Multiple linear stepwise regression analysis unveiled that flavonoid accumulation in sandrice was positively correlated with temperature and UVB and negatively affected by precipitation and sunshine duration, respectively. Weighted co-expression network analysis (WGCNA) indicated the bHLH and MYB transcription factor (TF) families might play key roles in sandrice flavonoid biosynthesis regulation. A total of 22,778 differentially expressed genes (DEGs) were identified between ecotype DL and ecotype AEX, the two extremes in most environmental factors, whereby 85 DEGs could be related to known flavonoid biosynthesis pathway. A sandrice flavonoid biosynthesis network embracing the detected 23 flavonoids in this research was constructed. Gene families Plant flavonoid O-methyltransferase (AsPFOMT) and UDP-glucuronosyltransferase (AsUGT78D2) were identified and characterized on the transcriptional level and believed to be synthases of isorhamnetin and isorhamnetin-3-glycoside in sandrice, respectively. A trade-off between biosynthesis of rutin and isorhamnetin was found in the DL ecotype, which might be due to the metabolic flux redirection when facing environmental changes. This research provides valuable information for understanding flavonoid biosynthesis in sandrice at the molecular level and laid the foundation for precise development and utilization of this functional resource forage.

Unravelling the consecutive glycosylation and methylation of flavonols in peach in response to UV-B irradiation

Flavonol glycosides are bioactive compounds important for plant defence and human nutrition. Glycosylation and methylation play an important role in enriching the diversity of flavonols in response to the environment. Peach flowers and fruit are rich in flavonol diglycosides such as isorhamnetin 3-O-rutinoside (I3Rut), kaempferol 3-O-rutinoside and quercetin 3-O-rutinoside, and flavonol monoglycosides such as I 3-O-glucoside and Q 3-O-galactoside. UV-B irradiation of fruit significantly induced accumulation of all these flavonol glycosides. Candidate biosynthetic genes induced by UV-B were identified by genome homology searches and the in vitro catalytic activities of purified recombinant proteins determined. PpUGT78T3 and PpUGT78A2 were identified as flavonol 3-O-glucosyltransferase and 3-O-galactosyltransferase, respectively. PpUGT91AK6 was identified as flavonol 1,6-rhamnosyl trasferase catalysing the formation of flavonol rutinosides and PpFOMT1 was identified as a flavonol O-methyltransferase that methylated Q at the 3'-OH-OH to form isorhamnetin derivatives. Transient expression in Nicotiana benthamiana confirmed the specificity of PpUGT78T3 as a flavonol 3-O-glucosyltransferase, PpUGT78A2 as a 3-O-galactosyltransferase, PpUGT91AK6 as a 1,6-rhamnosyltrasferase and PpFOMT1 as an O-methyltransferase. This study provides new insights into the mechanisms of glycosylation and methylation of flavonols, especially the formation of flavonol diglycosides such as I3Rut, and will also be useful for future potential metabolic engineering of complex flavonols.

Beyond the Visible and Below the Peel: How UV-B Radiation Influences the Phenolic Profile in the Pulp of Peach Fruit. A Biochemical and Molecular Study

In the last decades, UV-B radiation has attracted attention due to its potential to increase nutraceutical values of fruit and vegetables, especially by inducing the accumulation of phenolics in a structure-dependent way. However, most current studies have investigated the UV-B-driven changes only in the peel or focusing on individual phenolic classes. Adopting an "-omics" approach, this work aimed to deepen the knowledge about the effects of UV-B radiation on the phenolic profile in the pulp of peach fruit. Based on these considerations, melting flesh yellow peaches (Prunus persica L., cv. Fairtime) were subjected to either a 10- or 60-min UV-B treatment (1.39 and 8.33 kJ m-2, respectively), and sampled at different time points from the exposure. A UHPLC-ESI/QTOF-MS analysis coupled with a phenolics-specific database for the annotation of compounds and a multivariate discriminant analysis revealed a marked effect of UV-B radiation on the phenolic profiles of peach pulp. Particularly, a general, transient increase was observed after 24 h from the irradiation, especially for flavanols, flavonols, and flavones. Such behavior diverges from what was observed in the peel, where an overall increase of phenolics was observed after 36 h from the irradiation. Concerning the flavonols in the pulp, UV-B exposure stimulated a specific accumulation of isorhamnetin and kaempferol derivatives, with variations imposed by the different sugar moiety bound. Anthocyanins, which were the second most abundant flavonoid group after flavonols, displayed a general decrease after 36 h that was not attributable to specific molecules. The UV-B treatments also increased the glycoside/aglycone ratio of flavonols and anthocyanins after 24 h, by increasing the glycoside concentration of both, flavonols and anthocyanins, and decreasing the aglycone concentration of anthocyanins. In support of the biochemical results, targeted gene expression analysis by RT-qPCR revealed an UV-B-induced activation of many genes involved in the flavonoid pathway, e.g., CHS, F3H, F3'H, DFR, as well as some MYB transcription factors and few genes involved in the UV-B perception. Generally, all the flavonoid-related and MYB genes showed a transient UV-B dose-dependent activation after 6 h from the irradiation, similarly to what was observed in the peel.

Zn(II) Complex of Plant Phenolic Chlorogenic Acid: Antioxidant, Antimicrobial and Structural Studies

The structure of the Zn(II) complex of 5-caffeoylquinic acid (chlorogenic acid, 5-CQA) and the type of interaction between the Zn(II) cation and the ligand were studied by means of various experimental and theoretical methods, i.e., electronic absorption spectroscopy UV/Vis, infrared spectroscopy FT-IR, elemental, thermogravimetric and density functional theory (DFT) calculations at B3LYP/6-31G(d) level. DPPH (2,2-diphenyl-1-picrylhydrazyl), ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), FRAP (ferric reducing antioxidant power), CUPRAC (cupric reducing antioxidant power) and trolox oxidation assays were applied in study of the anti-/pro-oxidant properties of Zn(II) 5-CQA and 5-CQA. The antimicrobial activity of these compounds against Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis, Staphylococcus aureus, Salmonella enteritidis and Candida albicans was tested. An effect of Zn(II) chelation by chlorogenic acid on the anti-/pro-oxidant and antimicrobial activities of the ligand was discussed. Moreover, the mechanism of the antioxidant properties of Zn(II) 5-CQA and 5-CQA were studied on the basis of the theoretical energy descriptors and thermochemical parameters. Zn(II) chlorogenate showed better antioxidant activity than chlorogenic acid and commonly applied natural (L-ascorbic acid) and synthetic antioxidants (butylated hydroxyanisol (BHA) and butylated hydroxytoluene (BHT)). The pro-oxidant activity of Zn(II) 5-CQA was higher than the ligand and increased with the rise of the compound concentration The type of Zn(II) coordination by the chlorogenate ligand strongly affected the antioxidant activity of the complex.

Accessions of Brazilian ginseng (Pfaffia glomerata) with contrasting anthocyanin content behave differently in growth, antioxidative defense, and 20-hydroxyecdysone levels under UV-B radiation

Ultraviolet-B (UV-B) radiation is an elicitor of secondary metabolites in plant tissue culture, but the effects on 20-hydroxyecdysone (20E) are still unclear. The 20E may show biotechnological, pharmacological, medical, and agrochemical applicability. Here, we use Pfaffia glomerata, a medically important species, to understand the impacts of UV-B radiation on their physiological performance, the expression of key genes involved in the 20E biosynthesis, and the 20E content. Two accessions (A22 and A43) of plants 20 days old grown in vitro were exposed to 0 (control), 2 (6.84 kJ m^-2), and 4 (13.84 kJ m^-2) h UV-B radiation for 20 consecutive days. Our data showed that UV-B reduced glucose concentration in A22 and A43 under 4 h of exposure (29 and 30%, respectively), while sucrose concentration increased (32 and 57%, respectively). UV-B also differentially impacted the accessions (A22 and A43), where the A22 under 4 h of UV-B had reduced total dry weight (8%) and electron transport rate (31%); in contrast, A43 did not change. Also, only A22 had increased POD activity under 4 h of UV-B (66%), as well as increased gene expression of the 20E pathway and the 20E content under 2 and 4 h of UV-B in leaves (28 and 21%, respectively) and roots (16 and 13%, respectively). This differential performance to UV-B can be explained by the contrasting anthocyanin contents. Notably, A43 displayed 56% more anthocyanin to the former, a possible defense against UV-B. In conclusion, UV-B radiation is a potential elicitor for increasing 20E content in P. glomerata grown in vitro.