Accumulation of an organic anticancer selenium compound in a transgenic Solanaceous species shows wider applicability of the selenocysteine methyltransferase transgene from selenium hyperaccumulators

Agronomic and Genetic Strategies to Enhance Selenium Accumulation in Crops and Their Influence on Quality

Selenium (Se) is an essential trace element that plays a crucial role in maintaining the health of humans, animals, and certain plants. It is extensively present throughout the Earth's crust and is absorbed by crops in the form of selenates and selenite, eventually entering the food chain. Se biofortification is an agricultural process that employs agronomic and genetic strategies. Its goal is to enhance the mechanisms of crop uptake and the accumulation of exogenous Se, resulting in the production of crops enriched with Se. This process ultimately contributes to promoting human health. Agronomic strategies in Se biofortification aim to enhance the availability of exogenous Se in crops. Concurrently, genetic strategies focus on improving a crop's capacity to uptake, transport, and accumulate Se. Early research primarily concentrated on optimizing Se biofortification methods, improving Se fertilizer efficiency, and enhancing Se content in crops. In recent years, there has been a growing realization that Se can effectively enhance crop growth and increase crop yield, thereby contributing to alleviating food shortages. Additionally, Se has been found to promote the accumulation of macro-nutrients, antioxidants, and beneficial mineral elements in crops. The supplementation of Se biofortified foods is gradually emerging as an effective approach for promoting human dietary health and alleviating hidden hunger. Therefore, in this paper, we provide a comprehensive summary of the Se biofortification conducted over the past decade, mainly focusing on Se accumulation in crops and its impact on crop quality. We discuss various Se biofortification strategies, with an emphasis on the impact of Se fertilizer strategies on crop Se accumulation and their underlying mechanisms. Furthermore, we highlight Se's role in enhancing crop quality and offer perspective on Se biofortification in crop improvement, guiding future mechanistic explorations and applications of Se biofortification.

Selenium Uptake, Transport, Metabolism, Reutilization, and Biofortification in Rice

Selenium (Se) is an essential trace element for humans and other animals. The human body mainly acquires Se from plant foods, especially cereal grains. Rice is the staple food for more than half of the world's population. Increasing the Se concentration of rice grains can increase the average human dietary Se intake. This review summarizes recent advances in the molecular mechanisms of Se uptake, transport, subcellular distribution, retranslocation, volatilization, and Se-containing protein degradation in plants, especially rice. The strategies for improving Se concentration in rice grains by increasing Se accumulation, reducing Se volatilization, and optimizing Se form were proposed, which provide new insight into Se biofortification in rice by improving the utilization efficiency of Se.

Influence of Selenium Biofortification on the Growth and Bioactive Metabolites of Ganoderma lucidum

Selenium biofortification of edible and medicinal mushrooms is an effective way to produce selenium-enriched food supplements. Ganoderma lucidum is the typical one with excellent biological activity. This study investigated G. lucidum growth and bioactive metabolites alterations during liquid culture with different concentrations of selenite. Low selenium levels did not affect growth and mycelia morphology, whereas high selenium levels negatively influenced growth, dramatically decreased biomass, caused nucleic acid and protein leakage, damaged cell walls and membranes, and resulted in indicators such as degraded cells, a red color, and an unpleasant odor. Through headspace-solid phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) analysis, ten volatile Se compounds were identified in G. lucidum with 200 ppm selenite, which led to an odor change, whereas only three with 50 ppm selenite. SeMet was the major selenoamino acid in the 50 ppm selenite group by high-performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP-MS), but more MeSeCys was produced with 200 ppm selenite. Polysaccharide yields were promoted and inhibited with 50 and 200 ppm selenite, respectively. These results provide comprehensive insights into the effects of selenite on G. lucidum in liquid culture and are beneficial for functional selenium-enriched mushroom production and improving nutritive values.

Exploring the new dimensions of selenium research to understand the underlying mechanism of its uptake, translocation, and accumulation

Selenium (Se) is a vital mineral for both plants and animals. It is widely distributed on the earth's crust and is taken up by the plants as selenite or selenate. Plants substantially vary in their physiological response to Se. The amount of Se in edible plants is genetically controlled. Its availability can be determined by measuring its phytoavailability in soil. The low concentration of Se in plants can help them in combating stress, whereas higher concentrations can be detrimental to plant health and in most cases it is toxic. Thus, solving the double-edged sword problem of nutritional Se deficiency and its elevated concentrations in environment requires a better understanding of Se uptake and metabolism in plants. The studies on Se uptake and metabolism can help in genetic biofortification of Se in plants and also assist in phytoremediation. Moreover, Se uptake and transport, especially biochemical pathways of assimilation and incorporation into proteins, offers striking mechanisms of toxicity and tolerance. These developments have led to a revival of Se research in higher plants with significant break throughs being made in the previous years. This review explores the new dimensions of Se research with major emphasis on key research events related to Se undertaken in last few years. Further, we also discussed future possibilities in Se research for crop improvement.

Identification and functional characterization of a novel selenocysteine methyltransferase from Brassica juncea L

Organic selenium (Se), specifically Se-methylselenocysteine (MeSeCys), has demonstrated potential effects in human disease prevention including cancer and the emerging ameliorating effect on Alzheimer's disease. In plants, selenocysteine methyltransferase (SMT) is the key enzyme responsible for MeSeCys formation. In this study, we first isolated a novel SMT gene, designated as BjSMT, from the genome of a known Se accumulator, Brassica juncea L. BjSMT shows high sequence (amino acid) similarity with its orthologues from Brassica napus and Brassica oleracea var. oleracea, which can use homocysteine (HoCys) and selenocysteine (SeCys) as substrates. Similar to its closest homologues, BjSMT also possesses a conserved Thr187 which is involved in transferring a methyl group to HoCys by donating a hydrogen bond, suggesting that BjSMT can methylate both HoCys and SeCys substrates. Using quantitative real-time PCR (qRT-PCR) technology and BjSMT-transformed tobacco (Nicotiana tabacum) plants, we observed how BjSMT responds to selenite [Se(IV)] and selenate [Se(VI)] stress in B. juncea, and how the phenotypes of BjSMT-overexpressing tobacco cultured under selenite stress are affected. BjSMT expression was nearly undetectable in the B. juncea plant without Se exposure, but in the plant leaves it can be rapidly and significantly up-regulated upon a low level of selenite stress, and enormously up-regulated upon selenate treatment. Overexpression of BjSMT in tobacco substantially enhanced tolerance to selenite stress manifested as significantly higher fresh weight, plant height, and chlorophyll content than control plants. In addition, transgenic plants exhibited low glutathione peroxidase activity in response to a lower dose of selenite stress (with a higher dose of selenite stress resulting in a high activity response) compared with the controls. Importantly, the BjSMT-transformed tobacco plants accumulated a high level of Se upon selenite stress, and the plants also had significantly increased MeSeCys production potential in their leaves. This first study of B. juncea SMT demonstrates its potential applications in crop MeSeCys biofortification and phytoremediation of Se pollution.

Selenium treatment differentially affects sulfur metabolism in high and low glucosinolate producing cultivars of broccoli (Brassica oleracea L.)

The effect of selenium (Se) application on the sulfur (S)-rich glucosinolate (GSL)-containing plant, broccoli (Brassica oleracea L. var. italica) was examined with a view to producing germplasm with increased Se and GSL content for human health, and to understanding the influence of Se on the regulation of GSL production. Two cultivars differing in GSL content were compared. Increased Se application resulted in an increase in Se uptake in planta, but no significant change in total S or total GSL content in either cultivar. Also no significant change was observed in the activity of ATP sulfurylase (ATPS, EC 2.7.7.4) or O-acetylserine(thiol) lyase (OASTL, EC 2.5.1.47) with increased Se application. However, in the first investigation of APS kinase (APSK, EC 2.7.1.25) expression in response to Se fertilisation, an increase in transcript abundance of one variant of APS kinase 1 (BoAPSK1A) was observed in both cultivars, and an increase in BoAPSK2 transcript abundance was observed in the low GSL producing cultivar. A mechanism by which increased APSK transcription may provide a means of controlling the content of S-containing compounds, including GSLs, following Se uptake is proposed.

Mechanisms of Selenium Enrichment and Measurement in Brassicaceous Vegetables, and Their Application to Human Health

Selenium (Se) is an essential micronutrient for human health. Se deficiency affects hundreds of millions of people worldwide, particularly in developing countries, and there is increasing awareness that suboptimal supply of Se can also negatively affect human health. Selenium enters the diet primarily through the ingestion of plant and animal products. Although, plants are not dependent on Se they take it up from the soil through the sulphur (S) uptake and assimilation pathways. Therefore, geographic differences in the availability of soil Se and agricultural practices have a profound influence on the Se content of many foods, and there are increasing efforts to biofortify crop plants with Se. Plants from the Brassicales are of particular interest as they accumulate and synthesize Se into forms with additional health benefits, such as methylselenocysteine (MeSeCys). The Brassicaceae are also well-known to produce the glucosinolates; S-containing compounds with demonstrated human health value. Furthermore, the recent discovery of the selenoglucosinolates in the Brassicaceae raises questions regarding their potential bioefficacy. In this review we focus on Se uptake and metabolism in the Brassicaceae in the context of human health, particularly cancer prevention and immunity. We investigate the close relationship between Se and S metabolism in this plant family, with particular emphasis on the selenoglucosinolates, and consider the methodologies available for identifying and quantifying further novel Se-containing compounds in plants. Finally, we summarize the research of multiple groups investigating biofortification of the Brassicaceae and discuss which approaches might be most successful for supplying Se deficient populations in the future.

Selenium accumulation by plants

BACKGROUND

Selenium (Se) is an essential mineral element for animals and humans, which they acquire largely from plants. The Se concentration in edible plants is determined by the Se phytoavailability in soils. Selenium is not an essential element for plants, but excessive Se can be toxic. Thus, soil Se phytoavailability determines the ecology of plants. Most plants cannot grow on seleniferous soils. Most plants that grow on seleniferous soils accumulate <100 mg Se kg(-1) dry matter and cannot tolerate greater tissue Se concentrations. However, some plant species have evolved tolerance to Se, and commonly accumulate tissue Se concentrations >100 mg Se kg(-1) dry matter. These plants are considered to be Se accumulators. Some species can even accumulate Se concentrations of 1000-15 000 mg Se kg(-1 )dry matter and are called Se hyperaccumulators.

SCOPE

This article provides an overview of Se uptake, translocation and metabolism in plants and highlights the possible genetic basis of differences in these between and within plant species. The review focuses initially on adaptations allowing plants to tolerate large Se concentrations in their tissues and the evolutionary origin of species that hyperaccumulate Se. It then describes the variation in tissue Se concentrations between and within angiosperm species and identifies genes encoding enzymes limiting the rates of incorporation of Se into organic compounds and chromosomal loci that might enable the development of crops with greater Se concentrations in their edible portions. Finally, it discusses transgenic approaches enabling plants to tolerate greater Se concentrations in the rhizosphere and in their tissues.

CONCLUSIONS

The trait of Se hyperaccumulation has evolved several times in separate angiosperm clades. The ability to tolerate large tissue Se concentrations is primarily related to the ability to divert Se away from the accumulation of selenocysteine and selenomethionine, which might be incorporated into non-functional proteins, through the synthesis of less toxic Se metabilites. There is potential to breed or select crops with greater Se concentrations in their edible tissues, which might be used to increase dietary Se intakes of animals and humans.

Selenoglucosinolates and their metabolites produced in Brassica spp. fertilised with sodium selenate

Glucosinolates are sulphur-containing glycosides found in many Brassica spp. that are important because their aglycone hydrolysis products protect the plant from herbivores and exhibit anti-cancer properties in humans. Recently, synthetically produced selenium analogues have been shown to be more effective at suppressing cancers than their sulphur counterparts. Although selenium is incorporated into a number of Brassica amino acids and peptides, firm evidence has yet to be presented for the presence of selenium in the glucosinolates and their aglycones in planta. In this study broccoli and cauliflower florets, and roots of forage rape, all obtained from plants treated with sodium selenate, were analysed for the presence of organoselenides. GC-MS analysis of pentane/ether extracts identified six organoselenium compounds including selenium analogues of known myrosinase-derived Brassica volatiles: 4-(methylseleno)butanenitrile, 5-(methylseleno)pentanenitrile, 3-(methylseleno)propylisothiocyanate, 4-(methylseleno)butylisothiocyanate, and 5-(methylseleno)pentylisothiocyanate. LC-MS analysis of ethanolic extracts identified three selenoglucosinolates: 3-(methylseleno)propylglucosinolate (glucoselenoiberverin), 4-(methylseleno)butylglucosinolate (glucoselenoerucin), and 5-(methylseleno)pentylglucosinolate (glucoselenoberteroin). LC-MS/MS analysis was used to locate the position of the selenium atom in the selenoglucosinolate and indicates preferential incorporation of selenium via selenomethionine into the methylselenyl moiety rather than into the sulphate or β-thioglucose groups. In forage rape, selenoglucosinolates and their aglycones (mainly isothiocyanates), occurred at concentrations up to 10% and 70%, respectively, of their sulphur analogues. In broccoli, concentrations of the selenoglucosinolates and their aglycones (mainly nitriles) were up to 60% and 1300%, respectively of their sulphur analogues. These findings indicate the potential for the incorporation of high levels of selenium into Brassica glucosinolates.

Identification and characterization of selenate- and selenite-responsive genes in a Se-hyperaccumulator Astragalus racemosus

Plants with capacity to accumulate high levels of selenium (Se) are desired for phytoremediation and biofortification. Plants of genus Astragalus accumulate and tolerate high levels of Se, but their slow growth, low biomass and non-edible properties limit their direct utilization. Genetic engineering may be an alternative way to produce edible or high biomass Se-accumulating plants. The first step towards this goal is to isolate genes that are responsible for Se accumulation and tolerance. Later, these genes can be introduced into other edible and high biomass plants. In the present study, we applied fluorescent differential display to analyze the transcript profile of Se-hyperaccumulator A. racemosus treated with 20 μM selenate (K(2)SeO(4)) for 2 weeks. Among 125 identified Se-responsive candidate genes, the expression levels of nine were induced or suppressed more than twofold by selenate treatment in two independent experiments while 14 showed such changes when treated with selenite (K(2)SeO(3)). Six of them were found to respond to both selenate and selenite treatments. A novel gene CEJ367 was found to be highly induced by both selenate (1,920-fold) and selenite (579-fold). Root- or shoot-preferential expression of nine genes was further investigated. These identified genes may allow us to create Se-enriched transgenic plants.

Volatile metabolites

Volatile organic compounds (volatiles) comprise a chemically diverse class of low molecular weight organic compounds having an appreciable vapor pressure under ambient conditions. Volatiles produced by plants attract pollinators and seed dispersers, and provide defense against pests and pathogens. For insects, volatiles may act as pheromones directing social behavior or as cues for finding hosts or prey. For humans, volatiles are important as flavorants and as possible disease biomarkers. The marine environment is also a major source of halogenated and sulfur-containing volatiles which participate in the global cycling of these elements. While volatile analysis commonly measures a rather restricted set of analytes, the diverse and extreme physical properties of volatiles provide unique analytical challenges. Volatiles constitute only a small proportion of the total number of metabolites produced by living organisms, however, because of their roles as signaling molecules (semiochemicals) both within and between organisms, accurately measuring and determining the roles of these compounds is crucial to an integrated understanding of living systems. This review summarizes recent developments in volatile research from a metabolomics perspective with a focus on the role of recent technical innovation in developing new areas of volatile research and expanding the range of ecological interactions which may be mediated by volatile organic metabolites.

Biofortification of tomato (Solanum lycopersicum) fruit with the anticancer compound methylselenocysteine using a selenocysteine methyltransferase from a selenium hyperaccumulator

Methylselenocysteine (MeSeCys) is an amino acid derivative that possesses potent anticancer activity in animals. Plants that can tolerate growth on soils with high Se content, known as Se hyperaccumulators, do so by converting inorganic Se to MeSeCys by the enzyme selenocysteine methyltransferase (SMT). A cDNA encoding the SMT from a Se hyperaccumulator was overexpressed in tomato (Solanum lycopersicum). Transgenic plants were provided with selenite or selenate to the roots during fruit development, and liquid chromatography-mass spectrometry was used to show that MeSeCys accumulated in the fruit but not in the leaves. Depending on the transgenic line and Se treatment, up to 16% of the total Se in the fruit was present as MeSeCys. MeSeCys was produced more effectively from selenite on a percentage conversion basis, but greater accumulation of MeSeCys could be achieved from selenate due to its better translocation from the roots. MeSeCys was heat stable and survived processing of the fruit to tomato juice.

Organoselenides from Nicotiana tabacum genetically modified to accumulate selenium

Nicotiana tabacum L. (tobacco) plants were transformed to overexpress a selenocysteine methyltransferase gene from the selenium hyperaccumulator Astragalus bisulcatus (Hook.) A. Gray (two-grooved milkvetch), and an ATP-sulfurylase gene from Brassica oleracea L. var. italica (broccoli). Solvent extraction of leaves harvested from plants treated with selenate revealed five selenium-containing compounds, of which four were identified by chemical synthesis as 2-(methylseleno)acetaldehyde, 2,2-bis(methylseleno)acetaldehyde, 4-(methylseleno)-(2E)-nonenal, and 4-(methylseleno)-(2E,6Z)-nonadienal. These four compounds have not previously been reported in nature.