Senescence is delayed by selenium in oilseed rape plants

Selenium in plants: A nexus of growth, antioxidants, and phytohormones

Selenium (Se) is an essential micronutrient for both human and animals. Plants serve as the primary source of Se in the food chain. Se concentration and availability in plants is influenced by soil properties and environmental conditions. Optimal Se levels promote plant growth and enhance stress tolerance, while excessive Se concentration can result in toxicity. Se enhances plants ROS scavenging ability by promoting antioxidant compound synthesis. The ability of Se to maintain redox balance depends upon ROS compounds, stress conditions and Se application rate. Furthermore, Se-dependent antioxidant compound synthesis is critically reliant on plant macro and micro nutritional status. As these nutrients are fundamental for different co-factors and amino acid synthesis. Additionally, phytohormones also interact with Se to promote plant growth. Hence, utilization of phytohormones and modified crop nutrition can improve Se-dependent crop growth and plant stress tolerance. This review aims to explore the assimilation of Se into plant proteins, its intricate effect on plant redox status, and the specific interactions between Se and phytohormones. Furthermore, we highlight the proposed physiological and genetic mechanisms underlying Se-mediated phytohormone-dependent plant growth modulation and identified research opportunities that could contribute to sustainable agricultural production in the future.

The importance and status of the micronutrient selenium in South Africa: a review

Selenium (Se) is a vital micronutrient with widespread biological action but leads to toxicity when taken in excessive amounts. The biological benefits of Se are mainly derived from its presence in active sites of selenoproteins such as glutathione peroxidase (GPx). An enzyme whose role is to protect tissues against oxidative stress by catalysing the reduction of peroxidase responsible for various forms of cellular damage. The benefits of Se can be harvested when proper regulations of its intake are used. In South Africa, Se distribution in people's diets and animals are low with socio-economic factors and heterogeneous spread of Se in soil throughout the country playing a significant role. The possible causes of low Se in soils may be influenced by underlying geological material, climatic conditions, and anthropogenic activities. Sedimentary rock formations show higher Se concentrations compared to igneous and metamorphic rock formations. Higher Se concentrations in soils dominates in humid and sub-humid areas of South Africa. Furthermore, atmospheric acid deposition dramatically influences the availability of Se to plants. The studies reviewed in this article have shown that atomic absorption spectroscopy (AAS) is the most utilised analytical technique for total Se concentration determination in environmental samples and there is a lack of speciation data for Se concentrations. Shortcomings in Se studies have been identified, and the future research directions of Se in South Africa have been discussed.

Ethylene Suppresses Abscisic Acid, Modulates Antioxidant System to Counteract Arsenic-Inhibited Photosynthetic Performance in the Presence of Selenium in Mustard

Arsenic (As) stress provokes various toxic effects in plants that disturbs its photosynthetic potential and hampers growth. Ethylene and selenium (Se) have shown regulatory interaction in plants for metal tolerance; however, their synergism in As tolerance through modification of the antioxidant enzymes and hormone biosynthesis needs further elaboration. With this in view, we investigated the impact of ethylene and Se in the protection of photosynthetic performance against As stress in mustard (Brassica juncea L.). Supplementation with ethephon (2-chloroethylphosphonic acid; ethylene source) and/or Se allayed the negative impact of As-induced toxicity by limiting As content in leaves, enhancing the antioxidant defense system, and decreasing the accumulation of abscisic acid (ABA). Ethylene plus Se more prominently regulated stomatal behavior, improved photosynthetic capacity, and mitigated As-induced effects. Ethephon in the presence of Se decreased stress ethylene formation and ABA accumulation under As stress, resulting in improved photosynthesis and growth through enhanced reduced glutathione (GSH) synthesis, which in turn reduced the oxidative stress. In both As-stressed and non-stressed plants treated with ethylene action inhibitor, norbornadiene, resulted in increased ABA and oxidative stress with reduced photosynthetic activity by downregulating expression of ascorbate peroxidase and glutathione reductase, suggesting the involvement of ethylene in the reversal of As-induced toxicity. These findings suggest that ethephon and Se induce regulatory interaction between ethylene, ABA accumulation, and GSH metabolism through regulating the activity and expression of antioxidant enzymes. Thus, in an economically important crop (mustard), the severity of As stress could be reduced through the supplementation of both ethylene and Se that coordinate for maximum stress alleviation.

Effects of Superoxide Dismutase Inhibitors and Glucose on Cell Death and Generation of Reactive Oxygen Species in Pea Leaves

The effects of superoxide dismutase (SOD) inhibitors, diethyldithiocarbamate (DDC), triethylenetetramine (trien), and their combination with glucose on cells of the epidermis from pea leaves of different age (rapidly growing young leaves and slowly growing old leaves) was investigated. DDC and trien caused death of the guard cells as determined by destruction of their nuclei. Glucose did not affect destruction of the nuclei induced by SOD inhibitors in the cells from old leaves, but intensified it in the cells from young leaves. 2-Deoxyglucose, an inhibitor of glycolysis, and propyl gallate, SOD-mimic and antioxidant, suppressed destruction of the nuclei that was caused by SOD inhibitors and glucose in cells of the epidermis from the young, but not from the old leaves. Glucose and trien stimulated, and propyl gallate reduced generation of reactive oxygen species (ROS) in the pea epidermis as determined by the fluorescence of 2',7'-dichlorofluorescein (DCF). Carbonyl cyanide m-chlorophenylhydrazone (CCCP), a protonophoric uncoupler of oxidative and photosynthetic phosphorylation, suppressed the DCF fluorescence in the guard cells. Treatment of the cells with CCCP followed by its removal with washing increased destruction of the nuclei caused by SOD inhibitors and glucose. In young leaves, CCCP was less effective than in old ones. The findings demonstrate the effects of SOD inhibitors and glucose on the cell death and generation of ROS and could indicate glycolysis-dependent ROS production.

Physiological insights into sulfate and selenium interaction to improve drought tolerance in mung bean

The present study involved two pot experiments to investigate the response of mung bean to the individual or combined SO₄ ^2- and selenate application under drought stress. A marked increment in biomass and NPK accumulation was recorded in mung bean seedlings fertilized with various SO₄ ^2- sources, except for CuSO₄. Compared to other SO₄ ^2- fertilizers, ZnSO₄ application resulted in the highest increase in growth attributes and shoot nutrient content. Further, the combined S and Se application (S + Se) significantly enhanced relative water content (16%), SPAD value (72%), photosynthetic rate (80%) and activities of catalase (79%), guaiacol peroxidase (53%) and superoxide dismutase (58%) in the leaves of water-stressed mung bean plants. Consequently, the grain yield of mung bean was markedly increased by 105% under water stress conditions. Furthermore, S + Se application considerably increased the concentrations of P (47%), K (75%), S (80%), Zn (160%), and Fe (15%) in mung bean seeds under drought stress conditions. These findings indicate that S + Se application potentially increases the nutritional quality of grain legumes by stimulating photosynthetic apparatus and antioxidative machinery under water deficit conditions. Our results could provide the basis for further experiments on cross-talk between S and Se regulatory pathways to improve the nutritional quality of food crops.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-00992-6.

Nitric oxide and selenium nanoparticles confer changes in growth, metabolism, antioxidant machinery, gene expression, and flowering in chicory (Cichorium intybus L.): potential benefits and risk assessment

This experiment was conducted to provide a better insight into the plant responses to nitric oxide (NO) and selenium nanoparticle (nSe). Chicory seedlings were sprayed with nSe (0, 4, and 40 mg l-1), and/or NO (0 and 25 μM). NO and/or nSe4 improved shoot and root biomass by an average of 32%. The nSe40 adversely influenced shoot and root biomass (mean = 26%), exhibiting moderate toxicity partly relieved by NO. The nSe and NO treatments transcriptionally stimulated the dehydration response element B1A (DREB1A) gene (mean = 29.6-fold). At the transcriptional level, nSe4 or NO moderately upregulated phenylalanine ammonia-lyase (PAL) and hydroxycinnamoyl-CoA quinate transferase (HCT1) genes (mean = sevenfold). The nSe4 + NO, nSe40, and nSe40 + NO groups drastically induced the expression of PAL and HCT1 genes (mean = 30-fold). With a similar trend, hydroxycinnamoyl-CoA Quinate/shikimate hydroxycinnamoyl transferase (HQT1) gene was also upregulated in response to nSe and/or NO (mean = 25-fold). The activities of nitrate reductase and catalase enzymes were also induced in the nSe- and/or NO-treated seedlings. Likewise, the application of these supplements associated with an increase in ascorbate concentration (mean = 31.5%) reduced glutathione (mean = 35%). NO and/or nSe enhanced the PAL activity (mean = 36.4%) and soluble phenols (mean = 40%). The flowering was also influenced by the supplements in dose and compound dependent manner exhibiting the long-time responses. It appears that the nSe-triggered signaling can associate with a plethora of developmental, physiological, and molecular responses at least in part via the fundamental regulatory roles of transcription factors, like DREB1A as one the most significant genes for conferring tolerance in crops.

Melatonin Participates in Selenium-Enhanced Cold Tolerance of Cucumber Seedlings

Melatonin is an important and widespread plant hormone. However, the underlying physiological and molecular mechanisms of melatonin as a secondary messenger in improving cold tolerance by selenium are limited. This study investigated the effects of selenite on the cold stress of cucumber seedlings. The results showed that exogenous application of selenite improved the cold tolerance of cucumber seedlings, which was dependent on the concentration effect. In the present experiment, 1 μM of selenite showed the best effect on alleviating cold stress. Interestingly, we found that in the process of alleviating cold stress, selenite increased the content of endogenous melatonin by regulating the expression of melatonin biosynthesis genes (TDC, T5H, SNAT, and COMT). To determine the interrelation between selenite and melatonin in alleviating cold stress, melatonin synthesis inhibitor p-chlorophenylalanine and melatonin were used for in-depth study. This study provides a theoretical basis for cucumber cultivation and breeding.

Nitric oxide alleviates selenium toxicity in rice by regulating antioxidation, selenium uptake, speciation and gene expression

In plants, excess selenium (Se) causes toxicity, while the beneficial effects of nitric oxide (NO) have verified in plants under various abiotic conditions. In order to ensure safely Se-enriched rice production, the objective of the research was to clarify how exogenous NO alleviated high Se toxicity in rice. Under high Se (25 μM) stress, the effects of exogenous NO (by applying sodium nitroprusside, an exogenous NO donor) on growth parameters, Se content, Se speciation, photosynthesis, antioxidant system, expressions of Se transport and metabolism-related genes (phosphate transporter, OsPT2; S-adenosylmethionine synthase 1, OsSAMS1; cysteine synthase, OsCS; Se-binding protein gene, OsSBP1) in rice seedlings were investigated by a hydroponic experiment. The results showed that exogenous NO alleviated high Se-induced irreversible damage to root morphology, growth, photosynthesis, antioxidant capacity and decreased the contents of MDA, H₂O₂ and proline significantly in rice seedlings. Compared with high Se treatment, application of exogenous NO reduced root Se content (10%), and the Se(VI) decreased by 100% in root and shoot. Besides, exogenous NO decreased the accumulation of inorganic Se speciation in rice roots and shoots. Also, the qRT-PCR analysis showed that down-regulated gene expressions of OsPT2, OsSAMS1 and OsCS affected significantly via exogenous NO. So, the exogenous NO could effectively decrease the toxicity of high Se treatment in rice.

Selenium-Ethylene Interplay in Postharvest Life of Cut Flowers

Selenium (Se) is considered a beneficial element in higher plants when provided at low concentrations. Recently, studies have unveiled the interactions between Se and ethylene metabolism throughout plant growth and development. However, despite the evidence that Se may provide longer shelf life in ethylene-sensitive flowers, its primary action on ethylene biosynthesis and cause-effect responses are still understated. In the present review, we discuss the likely action of Se on ethylene biosynthesis and its consequence on postharvest physiology of cut flowers. By combining Se chemical properties with a dissection of ethylene metabolism, we further highlighted both the potential use of Se solutions and their downstream responses. We believe that this report will provide the foundation for the hypothesis that Se plays a key role in the postharvest longevity of ethylene-sensitive flowers.

Selenium biofortification in the 21st century: status and challenges for healthy human nutrition

BACKGROUND

Selenium (Se) is an essential element for mammals and its deficiency in the diet is a global problem. Plants accumulate Se and thus represent a major source of Se to consumers. Agronomic biofortification intends to enrich crops with Se in order to secure its adequate supply by people.

SCOPE

The goal of this review is to report the present knowledge of the distribution and processes of Se in soil and at the plant-soil interface, and of Se behaviour inside the plant in terms of biofortification. It aims to unravel the Se metabolic pathways that affect the nutritional value of edible plant products, various Se biofortification strategies in challenging environments, as well as the impact of Se-enriched food on human health.

CONCLUSIONS

Agronomic biofortification and breeding are prevalent strategies for battling Se deficiency. Future research addresses nanosized Se biofortification, crop enrichment with multiple micronutrients, microbial-integrated agronomic biofortification, and optimization of Se biofortification in adverse conditions. Biofortified food of superior nutritional quality may be created, enriched with healthy Se-compounds, as well as several other valuable phytochemicals. Whether such a food source might be used as nutritional intervention for recently emerged coronavirus infections is a relevant question that deserves investigation.