Medicinal Plant Active Compounds Produced by UV-B Exposure

Spermidine alleviates oxidative damage and enhances phenolic compounds accumulation in barley seedlings under UV-B stress

BACKGROUND

Ultraviolet B (UV-B) radiation can enhance the accumulation of phenolic compounds (PCs) in barley seedling, although this may result in severe oxidative damage. In the present study, the role of spermidine in alleviating oxidative damage and regulating synthesis of PCs in barley seedlings under UV-B stress was investigated.

RESULTS

Exogenous spermidine increased the length and fresh weight as well as PCs contents of barley seedlings under UV-B stress. Application of dicyclohexylamine, an inhibitor of endogenous spermidine synthesis, significantly inhibited the growth and PC accumulation of barley seedlings under UV-B stress, although this inhibitory effect can be alleviated by exogenous spermidine. Exogenous spermidine increased the contents of vanillic acid, syringic acid, protocatechuic acid and p-coumaric acid in barley seedlings under UV-B stress by 20-200% through enhancing the activities of enzymes related to synthesis of these acids. In addition, exogenous spermidine enhanced activities and gene expression of antioxidant enzymes in barley seedlings under UV-B stress, including peroxidase, glutathione reductase and glutathione S-transferase.

CONCLUSION

Spermidine can alleviate oxidative damage of barley seedlings under UV-B stress and enhance the accumulation of PCs. © 2022 Society of Chemical Industry.

Recent advances on the roles of flavonoids as plant protective molecules after UV and high light exposure

Flavonoids are plant specialized metabolites that consist of one oxygenated and two aromatic rings. Different flavonoids are grouped according to the oxidation degree of the carbon rings; they can later be modified by glycosylations, hydroxylations, acylations, methylations, or prenylations. These modifications generate a wide collection of different molecules which have various functions in plants. All flavonoids absorb in the UV wavelengths, they mostly accumulate in the epidermis of plant cells and their biosynthesis is generally activated after UV exposure. Therefore, they have been assumed to protect plants against exposure to radiation in this range. Some flavonoids also absorb in other wavelengths, for example anthocyanins, which absorb light in the visible part of the solar spectrum. Besides, some flavonoids show antioxidant properties, that is, they act as scavengers of reactive oxygen species that could be produced after high fluence UV exposure. However, to date most reports were based on in vitro studies, and there is very little in vivo evidence of how their roles are carried out. In this review we first summarize the biosynthetic pathway of flavonoids and their characteristics, and we describe recent advances on the investigation of the role of three of the most abundant flavonoids: flavonols, flavones, and anthocyanins, protecting plants against UV exposure and high light exposure. We also present examples of how using UV-B supplementation to increase flavonoid content, is possible to improve plant nutritional and pharmaceutical values.

Increased phenylpropanoids production in UV-B irradiated Salvia verticillata as a consequence of altered genes expression in young leaves

Ultraviolet-B (UV-B) radiation as an environmental potential elicitor induces the synthesis of plant secondary metabolites. The effects of UV-B radiation on photosynthetic pigments and dry weight, biochemical and molecular features of old and young leaves of Salvia verticillata were investigated. Plants were exposed to 10.97 kJ m^-2 day^-1 of biologically effective UV-B radiation for up to 10 days. The sampling process was performed in four steps: 1, 5, 10, and 13 days (recovery time) after the start of irradiation. As a result of plant investment in primary and secondary metabolism, the production of phenolic compounds increased, while chlorophyll levels and leaf dry weight (%) declined. Under long-term UV-B exposure, young leaves exhibited the most significant reduction in chlorophyll a and b content and leaf dry weight. The highest level of total phenol (1.34-fold) and flavonoid concentration (2-fold) relative to the control was observed on the 5th day and recovery time, respectively. Young leaves demonstrated the highest amount of phenolic acids in recovery time. Young leaves on the 5th day of the experiment exerted the highest level of antioxidant activity when compared to the control. A positive correlation was observed between antioxidant activity and the amount of phenolic compounds. Regarding the expression of phenylpropanoid pathway genes, UV-B enhanced the expression of phenylalanine ammonia-lyase, tyrosine aminotransferase, and rosmarinic acid synthase with the highest level in young leaves on the 10th day. Overall, young leaves of S. verticillata indicated higher sensitivity to UV-B radiation and developed more tangible reactions to such radiation.

Effects of Light on Secondary Metabolite Biosynthesis in Medicinal Plants

Secondary metabolites (SMs) found in medicinal plants are one of main sources of drugs, cosmetics, and health products. With the increase in demand for these bioactive compounds, improving the content and yield of SMs in medicinal plants has become increasingly important. The content and distribution of SMs in medicinal plants are closely related to environmental factors, especially light. In recent years, artificial light sources have been used in controlled environments for the production and conservation of medicinal germplasm. Therefore, it is essential to elucidate how light affects the accumulation of SMs in different plant species. Here, we systematically summarize recent advances in our understanding of the regulatory roles of light quality, light intensity, and photoperiod in the biosynthesis of three main types of SMs (polyphenols, alkaloids, and terpenoids), and the underlying mechanisms. This article provides a detailed overview of the role of light signaling pathways in SM biosynthesis, which will further promote the application of artificial light sources in medicinal plant production.

Physiologic and Metabolic Changes in Crepidiastrum denticulatum According to Different Energy Levels of UV-B Radiation

Ultraviolet B (UV-B) light, as a physical elicitor, can promote the secondary metabolites biosynthesis in plants. We investigated effects of different energy levels of UV-B radiation on growth and bioactive compounds of Crepidiastrum denticulatum. Three-week-old seedlings were grown in a plant factory for 5 weeks. Plants were subjected to different levels of UV-B (0, 0.1, 0.25, 0.5, 1.0, and 1.25 W m⁻²), 6 h a day for 6 days. All UV-B treatments had no negative effect on the shoot dry weight; however, relatively high energy treatments (1.0 and 1.25 W m⁻²) inhibited the shoot fresh weight. UV-B light of 0.1, 0.25, and 0.5 W m⁻² did not affect total chlorophyll and H₂O₂ contents; however, they increased total carotenoid content. On 4 days, 0.25 W m⁻² treatment increased antioxidant capacity, total hydroxycinnamic acids (HCAs) content, and several sesquiterpenes. Treatments with 1.0 and 1.25 W m⁻² increased total carotenoid, total HCAs, and H₂O₂ contents, and destroyed chlorophyll pigments, reducing maximum quantum yield of photosystem II and causing visible damage to leaves. Partial least squares discrimination analysis (PLS-DA) showed that secondary metabolites were distinguishably changed according to energy levels of UV-B. The potential of 0.25 W m⁻² UV-B for the efficient production of bioactive compounds without growth inhibition in C. denticulatum was identified.

Ultraviolet B induced bioactive changes of enzymatic and non-enzymatic antioxidants and lipids in Trigonella foenum-graecum L. (Fenugreek)

Ultraviolet radiation (UV) altered plant metabolism. Hence Trigonella foenum-graecum L. (Fenugreek) exposed to UV-B radiation for studying the bioactive changes that may be useful in captive farming. UV-B treatment altered plant growth, and extent of alterations depended on the duration of radiation treatment. Photosynthetic pigments such as chlorophyll and carotenoids decreased after radiation exposure. But bioactive components such as anthocyanin, flavonoids, and phenolics increased after UV-B treatment. Phenylalanine lyase enzyme activity and peroxidase activity also increased with 4.0 hr UV-B exposure even though 8.0 hr exposure decreased the activity of these enzymes. Total lipid content of the plants increased after UV-B exposure. Changes in aromatic oil composition observed due to UV-B exposure, and the changes pointed shifting of plant metabolism towards the synthesis of short chain fatty acid contain lipids and non-enzymatic antioxidants.

Ultraviolet Radiation-Elicited Enhancement of Isoflavonoid Accumulation, Biosynthetic Gene Expression, and Antioxidant Activity in Astragalus membranaceus Hairy Root Cultures

In this work, Astragalus membranaceus hairy root cultures (AMHRCs) were exposed to ultraviolet radiation (UV-A, UV-B, and UV-C) for promoting isoflavonoid accumulation. The optimum enhancement for isoflavonoid production was achieved in 34-day-old AMHRCs elicited by 86.4 kJ/m(2) of UV-B. The resulting isoflavonoid yield was 533.54 ± 13.61 μg/g dry weight (DW), which was 2.29-fold higher relative to control (232.93 ± 3.08 μg/g DW). UV-B up-regulated the transcriptional expressions of all investigated genes involved in isoflavonoid biosynthetic pathway. PAL and C4H were found to be two potential key genes that controlled isoflavonoid biosynthesis. Moreover, a significant increase was noted in antioxidant activity of extracts from UV-B-elicited AMHRCs (IC50 values = 0.85 and 1.08 mg/mL) in comparison with control (1.38 and 1.71 mg/mL). Overall, this study offered a feasible elicitation strategy to enhance isoflavonoid accumulation in AMHRCs and also provided a basis for metabolic engineering of isoflavonoid biosynthesis in the future.

On the history of phyto-photo UV science (not to be left in skoto toto and silence)

This review of the history of ultraviolet photobiology focuses on the effects of UV-B (280-315 nm) radiation on terrestrial plants. It describes the early history of ultraviolet photobiology, the discovery of DNA as a major ultraviolet target and the discovery of photoreactivation and photolyases, and the later identification of Photosystem II as another important target for damage to plants by UV-B radiation. Some experimental techniques are briefly outlined. The insight that the ozone layer was thinning spurred the interest in physiological and ecological effects of UV-B radiation and resulted in an exponential increase over time in the number of publications and citations until 1998, at which time it was realized by the research community that the Montreal Protocol regulating the pollution of the atmosphere with ozone depleting substances was effective. From then on, the publication and citation rate has continued to rise exponentially, but with an abrupt change to lower exponents. We have now entered a phase when more emphasis is put on the "positive" effects of UV-B radiation, and with more emphasis on regulation than on damage and inhibition.

Biochemical traits and proteomic changes in postharvest flowers of medicinal chrysanthemum exposed to enhanced UV-B radiation

The article studied UV-B effects on biochemical traits and proteomic changes in postharvest flowers of medicinal chrysanthemum. The experiment about UV-B effects on biochemical traits in flowers included six levels of UV-B treatments (0 (UV0), 50 (UV50), 200 (UV200), 400 (UV400), 600 (UV600) and 800 (UV800) μWcm(-2)). UV400, UV600 and UV800 treatments significantly increased the contents of hydrogen peroxide, malondialdehyde and UV-B absorbing compounds, and the activity of phenylalanine ammonia lyase enzyme over the control. The contents of chlorogenic acid and flavone in flowers were significantly increased by UV-B treatments (except for UV50 and UV800). Two-dimensional gel electrophoresis was utilized to analyze proteomic changes in flowers with or without UV-B radiation. Results indicated that 43 protein spots (>1.5-fold difference in volume) were detected, including 19 spots with a decreasing trend and 24 spots with an increasing trend, and 19 differentially expressed protein spots were successfully indentified by MALDI-TOF MS. The indentified proteins were classified based on functions, the most of which were involved in photosynthesis, respiration, protein biosynthesis and degradation and defence. An overall assessment using biochemical and differential proteomic data revealed that UV-B radiation could affect biochemical reaction and promote secondary metabolism processes in postharvest flowers.