Selenium is a micronutrient with a wide range of functions in animals, including humans, and in microorganisms such as microalgae. However, its role in plant metabolism remains ambiguous. Recent studies of Se supplementation showed that not only does it increase the content of the element itself, but also affects the accumulation of secondary metabolites in plants. The purpose of this review is to analyze and summarize the available data on the place of selenium in the secondary metabolism of plants and its effect on the accumulation of some plant metabolites (S- and N-containing secondary metabolites, terpenes, and phenolic compounds). In addition, possible molecular mechanisms and metabolic pathways underlying these effects are discussed. It should be noted that available data on the effect of Se on the accumulation of secondary metabolites are inconsistent and contradictory. According to some studies, selenium has a positive effect on the accumulation of certain metabolites, while other similar studies show a negative effect or no effect at all. The following aspects were identified as possible ways of regulating plant secondary metabolism by Se-supplementation: changes occurring in primary S/N metabolism, hormonal regulation, redox metabolism, as well as at the transcriptomic level of secondary metabolite biosynthesis. In all likelihood, the confusion in the results can be explained by other, more complex regulatory mechanisms in which selenium is involved and which affect the production of metabolites. Further study on the involvement of various forms of selenium in metabolic and signaling pathways is crucial for a deeper understanding of its role in growth, development, and health of plants, as well as the regulatory mechanisms behind them.

Hyssop (Hyssopus officinalis L.) is an aromatic plant that is rich in various biologically active compounds and is widely used as a natural preservative and flavoring agent in the food and cosmetic industry and as a folk medicine against certain respiratory diseases. Despite the fact that selenium is not an essential trace element for plants, in a certain range of concentrations it can not only improve plant growth, but also increase the content of nutrients and biologically active compounds in plants. In the present study, the effect of treatment type (in a nutrient solution (NS) or by foliar application (FA)) and selenium (Se) concentration (2.0, 5.0, 10.0, 20.0, 40.0 μM) on the biomass, phenolic compounds, essential oil content, and antioxidant properties of hyssop grown in hydroponics was studied. Neither a toxic nor a beneficial effect of Se addition on the plants was observed. Selenium treatment significantly increased Se concentration in hyssop up to 38.35 μg g⁻¹ (NS) and 34.86 μg g⁻¹ (FA). The effect of selenium on the content of phenolic compounds, essential oil, and the antioxidant activity of hyssop depended on the applied selenium concentration. Hyssop treated with 10 μM Se (NS) and with 20 μM Se (FA) had a higher total content of hydroxycinnamic acids and higher contents of rosmarinic and chlorogenic acids, as well as a higher total content of phenolic compounds, higher content of essential oil, and higher antioxidant activity compared to other experimental plants. The results confirm the feasibility of selenium treatment of hyssop without reducing its production in order to enhance its nutritional and pharmaceutical properties.

Drought is one of the most important abiotic factors limiting plant productivity. Although the aromatic plants of the Lamiaceae family often grow in arid regions, drought tolerance varies greatly among the different species of this family. The effect of induced drought stress can be reduced by the application of selenium. The current study aims to compare the growth and biochemical responses of three species of the Lamiaceae family (hyssop, salvia, and oregano) to drought stress and the possibility of reducing the effect of stress in these plants by foliar treatment with selenium. Drought stress reduced the fresh and dry biomass of hyssop (by 35% and 15%), salvia (by 45% and 41%), and oregano (by 51% and 32%). Se treatment did not affect the growth of plants under drought stress, but it improved relative water content in hyssop and salvia under moderate drought conditions. A reduction in the content of chlorophyll a and chlorophyll b (in hyssop and salvia). In addition, an increase in the content of hydrogen peroxide (in oregano and salvia), malondialdehyde, and proline in plants cultivated under drought conditions was observed. Se treatment led to reduced levels of hydrogen peroxide and malondialdehyde, along with an increase in chlorophyll a content (in hyssop and oregano) and proline content. The response of the antioxidant system depended on the plant species. Hyssop exhibited a significant increase in glutathione peroxidase, superoxide dismutase, and peroxidase activities. Oregano showed enhanced catalase activity. Salvia experienced a sharp increase in ascorbic acid content. Se treatment stimulated the accumulation of phenolic compounds and increased glutathione peroxidase activity in all studied species.

Oregano (Origanum vulgare L.) is one of the most commercially popular aromatic spices which has long been used in folk medicine as a medicinal plant due to the presence of a wide group of bioactive secondary metabolites. The experimental data available to date indicate that plants of individual species within the Lamiaceae family implement different strategies for the absorption and distribution of selenium (Se) and differ in their metabolic response to Se treatment. In this regard, the present study investigated the effect of Se application (in the form of sodium selenate) at various concentrations (2.0, 5.0, 10.0, 20.0, and 40.0 μM) on the growth and accumulation of Se in oregano plant shoots and roots, as well as on nutritional components and secondary metabolites in shoots. The addition of Se to the nutrient solution at concentrations of 2–20 μM did not adversely affect the growth and accumulation of biomass in the oregano plants, which were characterized by a significant ability to transfer Se from roots to shoots (the translocation factor was >2). The Se treatment enhanced the nutritional proprieties of the oregano and, particularly, it stimulated the accumulation of carbohydrates and proteins (by 30 and 17%) and minerals (calcium and manganese). The total contents of phenolic compounds, flavonoids, hydroxycinnamic acids, luteolin-7-glucoside and its derivatives, catechin, 3,4-dihydroxybenzoic acid, rosmarinic acid and oleanolic and ursolic acids, as well as the percentage of essential oil, showed a dose-dependent increase in the oregano under the Se treatment. Changes in the relative content of the four main components of oregano essential oil (sabinene, germacrene D, β-caryophyllene, and (Z)-β-ocimene) under the Se treatment were not significant. The relative proportions of β-caryophyllene oxide and terpinen-4-ol increased with Se concentration augmentation in the nutrient solution. The obtained results indicate the prospect of using Se at 5–20 μM concentrations in nutrient solution in hydroponically grown oregano to produce Se-biofortified plants with higher levels of health beneficial compounds and antioxidant activity without yield reduction.