Application of supplemental UV‐B radiation in pre‐harvest to enhance health‐promoting compounds accumulation in green and red lettuce

Non-destructive real-time analysis of plant metabolite accumulation in radish microgreens under different LED light recipes

Introduction

The future of human space missions relies on the ability to provide adequate food resources for astronauts and also to reduce stress due to the environment (microgravity and cosmic radiation). In this context, microgreens have been proposed for the astronaut diet because of their fast-growing time and their high levels of bioactive compounds and nutrients (vitamins, antioxidants, minerals, etc.), which are even higher than mature plants, and are usually consumed as ready-to-eat vegetables.

Methods

Our study aimed to identify the best light recipe for the soilless cultivation of two cultivars of radish microgreens (Raphanus sativus, green daikon, and rioja improved) harvested eight days after sowing that could be used for space farming. The effects on plant metabolism of three different light emitting diodes (LED) light recipes (L1-20% red, 20% green, 60% blue; L2-40% red, 20% green, 40% blue; L3-60% red, 20% green, 20% blue) were tested on radish microgreens hydroponically grown. A fluorimetric-based technique was used for a real-time non-destructive screening to characterize plant methabolism. The adopted sensors allowed us to quantitatively estimate the fluorescence of flavonols, anthocyanins, and chlorophyll via specific indices verified by standardized spectrophotometric methods. To assess plant growth, morphometric parameters (fresh and dry weight, cotyledon area and weight, hypocotyl length) were analyzed.

Results

We observed a statistically significant positive effect on biomass accumulation and productivity for both cultivars grown under the same light recipe (40% blue, 20% green, 40% red). We further investigated how the addition of UV and/or far-red LED lights could have a positive effect on plant metabolite accumulation (anthocyanins and flavonols).

Discussion

These results can help design plant-based bioregenerative life-support systems for long-duration human space exploration, by integrating fluorescence-based non-destructive techniques to monitor the accumulation of metabolites with nutraceutical properties in soilless cultivated microgreens.

Deciphering transcriptomic signatures explaining the phenotypic plasticity of nonheading lettuce genotypes under artificial light conditions

Elucidating the mechanisms and pathways involved in genotype-environment (G×E) interactions and phenotypic plasticity is critical for improving plant growth. Controlled environment agricultural systems allow growers to modulate the environment for particular genotypes. In this study, we evaluated the effects of interactions among 14 genotypes and four artificial light environments on leaf lettuce phenotypes and dissected the underlying molecular mechanism via transcriptome-based modeling. Variations in morphological traits and phytochemical concentrations in response to artificial light treatments revealed significant G×E interactions. The appropriate genotype and artificial light combinations for maximizing phenotypic expression were determined on the basis of a joint regression analysis and the additive main effect and multiplicative interaction model for these G×E interactions. Transcriptome-based regression modeling explained approximately 50%-90% of the G×E variations. Further analyzes indicated Red Lettuce Leaves 4 (RLL4) regulates UV-B and blue light signaling through the effects of the HY5-MBW pathway on flavonoid biosynthesis and contributes to natural variations in the light-responsive plasticity of lettuce traits. Our study represents an important step toward elucidating the phenotypic variations due to G×E interactions in nonheading lettuce under artificial light conditions.

A comparison of consistent UV treatment versus inconsistent UV treatment in horticultural production of lettuce

UV radiation is an underrated radiation currently missing in many horticultural production systems of vegetables in protected cultivation. It can be added e.g., in LED light sources. Using lettuce as a model plant, this study determined whether the use of UVB LEDs is suitable (1) for use in consistent systems (indoor farming) or (2) inconsistent systems (greenhouse). Blue and red LEDs were selected as additional artificial lighting to UVB LEDs. Both approaches led to a reproducible increase of desired flavonol glycosides, such as quercetin-3-O-(6''-O-malonyl)-glucoside or quercetin-3-O-glucuronide and the anthocyanin cyanidin-3-O-(6''-O-malonyl)-glucoside in lettuce. The impact of the consistent UVB treatment is higher with up to tenfold changes than that of the inconsistent UVB treatment in the greenhouse. Varying natural light and temperature conditions in greenhouses might affect the efficiency of the artificial UVB treatment. Here, UVB LEDs have been tested and can be recommended for further development of lighting systems in indoor farming and greenhouse approaches.

Do changes in Lactuca sativa metabolic performance, induced by mycorrhizal symbionts and leaf UV-B irradiation, play a role towards tolerance to a polyphagous insect pest?

The increased ultraviolet radiation (UV) due to the altered stratospheric ozone leads to multiple plant physiological and biochemical adaptations, likely affecting their interaction with other organisms, such as pests and pathogens. Arbuscular mycorrhizal fungi (AMF) and UV-B treatment can be used as eco-friendly techniques to protect crops from pests by activating plant mechanisms of resistance. In this study, we investigated plant (Lactuca sativa) response to UV-B exposure and Funneliformis mosseae (IMA1) inoculation as well as the role of a major insect pest, Spodoptera littoralis. Lettuce plants exposed to UV-B were heavier and taller than non-irradiated ones. A considerable enrichment in phenolic, flavonoid, anthocyanin, and carotenoid contents and antioxidant capacity, along with redder and more homogenous leaf color, were also observed in UV-B-treated but not in AMF-inoculated plants. Biometric and biochemical data did not differ between AMF and non-AMF plants. AMF-inoculated plants showed hyphae, arbuscules, vesicles, and spores in their roots. AMF colonization levels were not affected by UV-B irradiation. No changes in S. littoralis-feeding behavior towards treated and untreated plants were observed, suggesting the ability of this generalist herbivore to overcome the plant chemical defenses boosted by UV-B exposure. The results of this multi-factorial study shed light on how polyphagous insect pests can cope with multiple plant physiological and biochemical adaptations following biotic and abiotic preconditioning.

Regulation of carotenoid and flavonoid biosynthetic pathways in Lactuca sativa var capitate L. in protected cultivation

In the face of a growing world population and limited land, there is an urgent demand for higher productivity of food crops, and cultivation systems must be adapted to future needs. Sustainable crop production should aim for not only high yields, but also high nutritional values. In particular, the consumption of bioactive compounds such as carotenoids and flavonoids is associated with a reduced incidence of non-transmissible diseases. Modulating environmental conditions by improving cultivation systems can lead to the adaption of plant metabolisms and the accumulation of bioactive compounds. The present study investigates the regulation of carotenoid and flavonoid metabolisms in lettuce (Lactuca sativa var capitate L.) grown in a protected environment (polytunnels) compared to plants grown without polytunnels. Carotenoid, flavonoid and phytohormone (ABA) contents were determined using HPLC-MS and transcript levels of key metabolic genes were analyzed by RT-qPCR. In this study, we observed inverse contents of flavonoids and carotenoids in lettuce grown without or under polytunnels. Flavonoid contents on a total and individual level were significantly lower, while total carotenoid content was higher in lettuce plants grown under polytunnels compared to without. However, the adaptation was specific to the level of individual carotenoids. For instance, the accumulation of the main carotenoids lutein and neoxanthin was induced while the β-carotene content remained unchanged. In addition, our findings suggest that the flavonoid content of lettuce depends on transcript levels of the key biosynthetic enzyme, which is modulated by UV light. A regulatory influence can be assumed based on the relation between the concentration of the phytohormone ABA and the flavonoid content in lettuce. In contrast, the carotenoid content is not reflected in transcript levels of the key enzyme of either the biosynthetic or the degradation pathway. Nevertheless, the carotenoid metabolic flux determined using norflurazon was higher in lettuce grown under polytunnels, suggesting posttranscriptional regulation of carotenoid accumulation, which should be an integral part of future studies. Therefore, a balance needs to be found between the individual environmental factors, including light and temperature, in order to optimize the carotenoid or flavonoid contents and to obtain nutritionally highly valuable crops in protected cultivation.

Supplemental UV-B Exposure Influences the Biomass and the Content of Bioactive Compounds in Linum usitatissimum L. Sprouts and Microgreens

The interest in the pre-harvest ultraviolet-B (UV-B) exposure of crops in indoor cultivation has grown consistently, though very little is known about its influence on the nutraceutical quality of microgreens. Flaxseeds constitute a valuable oilseed species, mostly appreciated for their nutritional properties and the presence of health-promoting compounds. Therefore, although scarcely studied, flaxseed sprouts and microgreens might constitute a high-quality food product to be included in a healthy diet. This study aims to unravel the effects of pre-harvest ultraviolet-B irradiation on the nutritional and nutraceutical quality of flaxseed sprouts and microgreens grown under artificial conditions. The UV-B irradiation decreased the biomass and stem length of microgreens. However, the content of total phenolics and flavonoids and the antioxidant capacity were strongly enhanced by the UV-B treatment in both sprouts and microgreens. Among photosynthetic pigments, chlorophyll a, violaxanthin, antheraxanthin, and lutein in sprouts were reduced by the treatment, while chlorophyll b increased in microgreens. In conclusion, our results showed that growing flaxseed sprouts and microgreens in controlled conditions with supplemental UV-B exposure might increase their nutritional and nutraceutical quality, as well as their antioxidant capacity, making them high-quality functional foods.

Carotenoids from Persimmon (Diospyros kaki Thunb.) Byproducts Exert Photoprotective, Antioxidative and Microbial Anti-Adhesive Effects on HaCaT

Persimmon (Diospyros kaki Thunb.) fruits are a remarkable source of carotenoids, which have shown protective effects against UV radiation in bacteria, fungi, algae, and plants. The aim of this study was to analyze the photoprotection provided by an acetone extract, rich in carotenoids and obtained from byproducts derived from the persimmon juice industry, against UV-induced cell death in the keratinocyte HaCaT cell line. For this purpose, the cytotoxicity and phototoxicity of carotenoid extract, as well as its intracellular reactive oxygen species (ROS) scavenging and anti-adhesive activities towards HaCaT cells, were evaluated. The in vitro permeation test provided information about the permeability of the carotenoid extract. Persimmon extracts, rich in carotenoids (PEC), were absorbed by HaCaT keratinocyte cells, which reduced the UV-induced intracellular ROS production in treated cells. Thus, PEC exerted a photoprotective and regenerative effect on UV-irradiated HaCaT cells, and this protection was UV dose-dependent. No cytotoxic effect was observed in HaCaT cultures at the concentration tested. PEC treatment also stimulated the adhesion capacity of skin microbiome to HaCaT cells, while exhibiting a significant anti-adhesive activity against all tested pathogens. In conclusion, PEC showed potential for use as a functional ingredient in skin-care products.

Anything New under the Sun? An Update on Modulation of Bioactive Compounds by Different Wavelengths in Agricultural Plants

Plants continuously rely on light as an energy source and as the driver of many processes in their lifetimes. The ability to perceive different light radiations involves several photoreceptors, which in turn activate complex signalling cascades that ultimately lead to a rearrangement in plant metabolism as an adaptation strategy towards specific light conditions. This review, after a brief summary of the structure and mode of action of the different photoreceptors, introduces the main classes of secondary metabolites and specifically focuses on the influence played by the different wavelengths on the content of these compounds in agricultural plants, because of their recognised roles as nutraceuticals.