Selenate-resistant mutants of Arabidopsis thaliana identify Sultr1;2, a sulfate transporter required for efficient transport of sulfate into roots

Genome-wide identification and expression-pattern analysis of sulfate transporter (SULTR) gene family in cotton under multiple abiotic stresses and fiber development

Sulfate transporter (SULTR) proteins are in charge of the transport and absorption on sulfate substances, and have been reported to play vital roles in the biological processes of plant growth and stress response. However, there were few reports of genome-wide identification and expression-pattern analysis of SULTRs in Hibiscus mutabilis. Gossypium genus is a ideal model for studying the allopolyploidy, therefore two diploid species (G. raimondii and G. arboreum) and two tetraploid species (G. hirsutum and G. barbadense) were chosen in this study to perform bioinformatic analyses, identifying 18, 18, 35, and 35 SULTR members, respectively. All the 106 cotton SULTR genes were utilized to construct the phylogenetic tree together with 11 Arabidopsis thaliana, 13 Oryza sativa, and 8 Zea mays ones, which was divided into Group1-Group4. The clustering analyses of gene structures and 10 conserved motifs among the cotton SULTR genes showed the consistent evolutionary relationship with the phylogenetic tree, and the results of gene-duplication identification among the four representative Gossypium species indicated that genome-wide or segment duplication might make main contributions to the expansion of SULTR gene family in cotton. Having conducted the cis-regulatory element analysis in promoter region, we noticed that the existing salicylic acid (SA), jasmonic acid (JA), and abscisic acid (ABA) elements could have influences with expression levels of cotton SULTR genes. The expression patterns of GhSULTR genes were also investigated on the 7 different tissues or organs and the developing ovules and fibers, most of which were highly expressed in root, stem, sepal, receptacel, ovule at 10 DPA, and fiber at 20 and 25 DPA. In addition, more active regulatory were observed in GhSULTR genes responding to multiple abiotic stresses, and 12 highly expressed genes showed the similar expression patterns in the quantitative Real-time PCR experiments under cold, heat, salt, and drought treatments. These findings broaden our insight into the evolutionary relationships and expression patterns of the SULTR gene family in cotton, and provide the valuable information for further screening the vital candidate genes on trait improvement.

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.

Transcriptome and co-expression network revealed molecular mechanism underlying selenium response of foxtail millet (Setaria italica)

Introduction

Selenium-enriched foxtail millet (Setaria italica) represents a functional cereal with significant health benefits for humans. This study endeavors to examine the impact of foliar application of sodium selenite (Na2SeO4) on foxtail millet, specifically focusing on selenium (Se) accumulation and transportation within various plant tissues.

Methods

To unravel the molecular mechanisms governing selenium accumulation and transportation in foxtail millet, we conducted a comprehensive analysis of selenium content and transcriptome responses in foxtail millet spikelets across different days (3, 5, 7, and 12) under Na2SeO4 treatment (200 μmol/L).

Results

Foxtail millet subjected to selenium fertilizer exhibited significantly elevated selenium levels in each tissue compared to the untreated control. Selenate was observed to be transported and accumulated sequentially in the leaf, stem, and spikes. Transcriptome analysis unveiled a substantial upregulation in the transcription levels of genes associated with selenium metabolism and transport, including sulfate, phosphate, and nitrate transporters, ABC transporters, antioxidants, phytohormone signaling, and transcription factors. These genes demonstrated intricate interactions, both synergistic and antagonistic, forming a complex network that regulated selenate transport mechanisms. Gene co-expression network analysis highlighted three transcription factors in the tan module and three transporters in the turquoise module that significantly correlated with selenium accumulation and transportation. Expression of sulfate transporters (SiSULTR1.2b and SiSULTR3.1a), phosphate transporter (PHT1.3), nitrate transporter 1 (NRT1.1B), glutathione S-transferase genes (GSTs), and ABC transporter (ABCC13) increased with SeO4 2- accumulation. Transcription factors MYB, WRKY, and bHLH were also identified as players in selenium accumulation.

Conclusion

This study provides preliminary insights into the mechanisms of selenium accumulation and transportation in foxtail millet. The findings hold theoretical significance for the cultivation of selenium-enriched foxtail millet.

Sulfate transport and metabolism: strategies to improve the seed protein quality

Sulfur-containing amino acids (SAA), namely methionine, and cysteine are crucial essential amino acids (EAA) considering the dietary requirements of humans and animals. However, a few crop plants, especially legumes, are characterized with suboptimal levels of these EAA thereby limiting their nutritive value. Hence, improved comprehension of the mechanistic perspective of sulfur transport and assimilation into storage reserve, seed storage protein (SSP), is imperative. Efforts to augment the level of SAA in seed storage protein form an integral component of strategies to balance nutritive quality and quantity. In this review, we highlight the emerging trends in the sulfur biofortification approaches namely transgenics, genetic and molecular breeding, and proteomic rebalancing with sulfur nutrition. The transgenic 'push and pull strategy' could enhance sulfur capture and storage by expressing genes that function as efficient transporters, sulfate assimilatory enzymes, sulfur-rich foreign protein sinks, or by suppressing catabolic enzymes. Modern molecular breeding approaches that adopt high throughput screening strategies and machine learning algorithms are invaluable in identifying candidate genes and alleles associated with SAA content and developing improved crop varieties. Sulfur is an essential plant nutrient and its optimal uptake is crucial for seed sulfur metabolism, thereby affecting seed quality and yields through proteomic rebalance between sulfur-rich and sulfur-poor seed storage proteins.

Abscisic acid promotes selenium absorption, metabolism and toxicity via stress-related phytohormones regulation in Cyphomandra betacea Sendt. (Solanum betaceum Cav.)

High levels of selenium (Se) uptakes negatively affect plant growth. In this study, the possible molecular mechanism for the effects of abscisic acid (ABA) on Se absorption, metabolism and toxicity in Cyphomandra betacea Sendt. (Solanum betaceum Cav.) young plants were investigated. Se+ABA treatment promoted significant Se absorption in C. betacea while impeding plant growth as compared to Se treatment. The expression levels of sulfate/phosphate transporter protein genes indicated that Se+ABA triggered more S/Se absorption and transportation into chloroplast. Furthermore, Se+ABA promoted higher metabolisms of inorganic sulfur (S)/Se and organic S/Se. The organic Se might be in several forms (SeCysth, SeCys and SeMet) in Se+ABA treatment, whereas SeCysth was the major organic form in Se treatment. More reactive oxygen species production was suggested in Se+ABA treatment from a series of genes involved in antioxidant enzymes and molecules, including superoxide dismutase, peroxiredoxin, glutathione sulfur-transferase and glutathione. Se+ABA further improved the expression levels of genes involved in biosynthesis and signaling transduction genes involved in stress-related phytohormones (jasmonic acid and salicylic acid). Combining with the data in ABA treatment, we hypothesized a model that ABA might first affect the biosynthesis and signaling transduction pathways of stress-related phytohormones, and subsequently altered the metabolic processes responding to Se stress.

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.

Biofortification revisited: Addressing the role of beneficial soil microbes for enhancing trace elements concentration in staple crops

Trace element deficiency is a pervasive issue contributing to malnutrition on a global scale. The primary cause of this hidden hunger is related to low dietary intake of essential trace elements, which is highly prevalent in numerous regions across the world. To address deficiency diseases in humans, fortification of staple crops with vital trace elements has emerged as a viable solution. Current methods for fortifying crops encompass chemical amendments, genetic breeding, and transgenic approaches, yet these approaches possess certain limitations, constraining their agricultural application. In contrast, fortifying staple crops through the utilization of soil-beneficial microbes has emerged as a promising and economically feasible approach to enhance trace element content in crops. A specific subset of these beneficial soil microbes, referred to as plant growth-promoting microbes, have demonstrated their ability to influence the interactions between plants, soil, and minerals. These microbes facilitate the transport of essential soil minerals, such as zinc, iron, and selenium, into plants, offering the potential for the development of tailored bioinoculants that can enhance the nutritional quality of cereals, pulses, and vegetable crops. Nevertheless, further research efforts are necessary to comprehensively understand the molecular mechanisms underlying the uptake, transport, and augmentation of trace element concentrations in staple crops. By delving deeper into these mechanisms, customized bioinoculants of soil-beneficial microbes can be developed to serve as highly effective strategies in combating trace element deficiency and promoting global nutritional well-being.

Selenium hyperaccumulator plant Cardamine enshiensis: from discovery to application

Selenium (Se) is an essential trace element for animals and humans. Se biofortification and Se functional agriculture are emerging strategies to satisfy the needs of people who are deficient in Se. With 200 km2 of Se-excess area, Enshi is known as the "world capital of Se." Cardamine enshiensis (C. enshiensis) is a Se hyperaccumulation plant discovered in the Se mine drainage area of Enshi. It is edible and has been approved by National Health Commission of the People's Republic of China as a new source of food, and the annual output value of the Se-rich industry in Enshi City exceeds 60 billion RMB. This review will mainly focus on the discovery and mechanism underlying Se tolerance and Se hyperaccumulation in C. enshiensis and highlight its potential utilization in Se biofortification agriculture, graziery, and human health.

Influence of exogenous selenomethionine and selenocystine on uptake and accumulation of Se in winter wheat (Triticum aestivum L. cv. Xinong 979)

Soil selenium (Se) is mainly inorganic including selenate and selenite but organic forms such as selenomethionine (SeMet) and selenocystine (SeCys₂) are commonly present. Although organic Se is bioavailable or potentially bioavailable to plants, whether the effects of the organic Se on uptake and accumulation of Se in winter wheat differ in forms is still not clear. Both hydroponic experiments and a pot trial of whole plant growth stage were conducted to investigate the effects of SeMet and L-selenocystine (SeCys₂) on uptake and accumulation of Se in winter wheat (Triticum aestivum L. cv. Xinong 979). Not only metabolic inhibitor (carbonyl cyanide m-chlorophenylhydrazone (CCCP)) inhibited SeMet (44%) influx into wheat roots but also aquaporin inhibitor (AgNO₃) or putative inhibitor (H₂SiO₄ and H₃BO₃) suppressed 83%, 62%, or 64% SeMet influx into the roots. However, these inhibitors had insignificant effects on SeCys₂ influx into the roots. Wheat grain possessed more effective Se accumulation under SeCys₂ treatments than under SeMet treatments, which was contributed to more efficiently translocation of Se from husk to grain, more remobilization of tissue Se to grain, and significantly higher concentration of soluble Se (SOL-Se) and exchangeable and carbonate-bound Se (EXC-Se) in the rhizosphere of winter wheat. The present study indicated that the effects of organic Se on uptake and accumulation of Se in winter wheat differed in forms and that SeCys₂ exhibited the potential to increase grain Se concentration in winter wheat. The results from the present study will replenish information about the effects and related mechanisms of SeMet or SeCys₂ on uptake and accumulation of Se in winter wheat and provide insights of effects of organic Se on wheat grain Se accumulation.

Genome-Wide Characterization of the Sulfate Transporter Gene Family in Oilseed Crops: Camelina sativa and Brassica napus

Sulfate transporters (SULTRs) are responsible for the uptake of sulfate (SO₄^2-) ions in the rhizosphere by roots and their distribution to plant organs. In this study, SULTR family members in the genomes of two oilseed crops (Camelina sativa and Brassica napus) were identified and characterized based on their sequence structures, duplication events, phylogenetic relationships, phosphorylation sites, and expression levels. In total, 36 and 45 putative SULTR genes were recognized in the genomes of C. sativa and B. napus, respectively. SULTR proteins were predicted to be basophilic proteins with low hydrophilicity in both studied species. According to the observed phylogenetic relationships, we divided the SULTRs into five groups, out of which the SULTR 3 group showed the highest variation. Additionally, several duplication events were observed between the SULTRs. The first duplication event occurred approximately five million years ago between three SULTR 3.1 genes in C. sativa. Furthermore, two subunits were identified in the 3D structures of the SULTRs, which demonstrated that the active binding sites differed between C. sativa and B. napus. According to the available RNA-seq data, the SULTRs showed diverse expression levels in tissues and diverse responses to stimuli. SULTR 3 was expressed in all tissues. SULTR 3.1 was more upregulated in response to abiotic stresses in C. sativa, while SULTR 3.3 and SULTR 2.1 were upregulated in B. napus. Furthermore, SULTR 3 and SULTR 4.1 were upregulated in response to biotic stresses in B. napus. Additionally, the qPCR data showed that the SULTRs in C. sativa were involved in the plant's response to salinity. Based on the distribution of cis-regulatory elements in the promoter region, we speculated that SULTRs might be controlled by phytohormones, such as ABA and MeJA. Therefore, it seems likely that SULTR genes in C. sativa have been more heavily influenced by evolutionary processes and have acquired further diversity. The results reveal new insights of the structures and functions of SULTRs in oilseed crops. However, further analyses, related to functional studies, are needed to uncover the role of SULTRs in the plants' development and growth processes, as well as in their response to stimuli.

Insights into the Sulfate Transporter Gene Family and Its Expression Patterns in Durum Wheat Seedlings under Salinity

Sulfate transporters (SULTRs) are an essential plant transporter class responsible for the absorption and distribution of sulfur, an essential plant growth element. SULTRs are also involved in processes related to growth and development and in response to environmental stimuli. In the present study, 22 TdSULTR family members were identified and characterized in the genome of Triticum turgidum L. ssp. durum (Desf.) using available bioinformatics tools. The expression levels of candidate TdSULTR genes were investigated under salt treatments of 150 and 250 mM NaCl after several different exposure times. TdSULTRs showed diversity in terms of physiochemical properties, gene structure, and pocket sites. TdSULTRs and their orthologues were classified into the known five main plant groups of highly diverse subfamilies. In addition, it was noted that segmental duplication events could lengthen TdSULTR family members under evolutionary processes. Based on pocket site analysis, the amino acids leucine (L), valine (V), and serine (S) were most often detected in TdSULTR protein binding sites. Moreover, it was predicted that TdSULTRs have a high potential to be targeted by phosphorylation modifications. According to promoter site analysis, the plant bioregulators ABA and MeJA were predicted to affect TdSULTR expression patterns. Real-time PCR analysis revealed TdSULTR genes are differentially expressed at 150 mM NaCl but show similar expression in response to 250 mM NaCl. TdSULTR reached a maximum level of expression 72 h after the 250 mM salt treatment. Overall, we conclude that TdSULTR genes are involved in the response to salinity in durum wheat. However, additional studies of functionality are needed to determine their precise function and linked-interaction pathways.

Plant Plasma Membrane Proton Pump: One Protein with Multiple Functions

In plants, the plasma membrane proton pump (PM H⁺-ATPase) regulates numerous transport-dependent processes such as growth, development, basic physiology, and adaptation to environmental conditions. This review explores the multifunctionality of this enzyme in plant cells. The abundance of several PM H⁺-ATPase isogenes and their pivotal role in energizing transport in plants have been connected to the phenomena of pleiotropy. The multifunctionality of PM H⁺-ATPase is a focal point of numerous studies unraveling the molecular mechanisms of plant adaptation to adverse environmental conditions. Furthermore, PM H⁺-ATPase is a key element in plant defense mechanisms against pathogen attack; however, it also functions as a target for pathogens that enable plant tissue invasion. Here, we provide an extensive review of the PM H⁺-ATPase as a multitasking protein in plants. We focus on the results of recent studies concerning PM H⁺-ATPase and its role in plant growth, physiology, and pathogenesis.

Unraveling the mechanism of sulfur nutrition in pigeonpea inoculated with sulfur-oxidizing bacteria

An investigation was carried out to understand the mechanism(s) involved in the uptake of sulfur (S) as sulfate in pigeonpea following single inoculation of two sulfur-oxidizing bacteria (SOB), Stenotrophomonas maltophilia and Stenotrophomonas pavanii in the treatments amended with either elemental sulfur (S0) or sulfate (S6). Colonization potential and biofilm formation were analyzed through confocal laser scanning microscope (CLSM) and scanning electron microscope (SEM). Furthermore, the effect of seed inoculation on root architecture, expression of genes involved in sulfur oxidation (sox) in bacterial inoculants, and genes involved in sulfate transport in pigeonpea (PpSULTR) were analyzed to correlate with the higher uptake of S in roots and shoots of pigeonpea. Both the SOB exhibited a good colonization potential and biofilm formation on the roots of pigeonpea. Among the 11 sox genes targeted in rhizosphere of pigeonpea, expression was achieved for seven genes, which showed 2-fold increase in treatments inoculated with S. maltophilia and amended with either S6 or S0. The inoculation of S. maltophilia and amendment of S0 led to increased expression of PpSULTR genes by several folds in roots. The inoculation of SOB had a significant influence on non-enzymatic (osmolytes like proline) and enzymatic (PAL, peroxidase, superoxide dismutase, and catalase) levels. The results revealed a significant increase in sulfur uptake in roots and shoots in treatment inoculated with S. maltophilia and amended with S6. The investigation showed that the SOB-mediated over-expression of PpSULTR genes in roots of pigeonpea and sox genes in the rhizosphere were acting synergistically in facilitating higher uptake and translocation of S in roots and shoots of pigeonpea plants.

Tris(methylthio)methane produced by Mortierella hyalina affects sulfur homeostasis in Arabidopsis

Microbial volatiles are important factors in symbiotic interactions with plants. Mortierella hyalina is a beneficial root-colonizing fungus with a garlic-like smell, and promotes growth of Arabidopsis seedlings. GC–MS analysis of the M. hyalina headspace and NMR analysis of the extracted essential oil identified the sulfur-containing volatile tris(methylthio)methane (TMTM) as the major compound. Incorporation of the sulfur from the fungal volatile into plant metabolism was shown by ³⁴S labeling experiments. Under sulfur deficiency, TMTM down-regulated sulfur deficiency-responsive genes, prevented glucosinolate (GSL) and glutathione (GSH) diminishment, and sustained plant growth. However, excess TMTM led to accumulation of GSH and GSL and reduced plant growth. Since TMTM is not directly incorporated into cysteine, we propose that the volatile from M. hyalina influences the plant sulfur metabolism by interfering with the GSH metabolism, and alleviates sulfur imbalances under sulfur stress.

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.

Crosstalk Between Iron and Sulfur Homeostasis Networks in Arabidopsis

The widespread deficiency of iron (Fe) and sulfur (S) is becoming a global concern. The underlying mechanisms regulating Fe and S sensing and signaling have not been well understood. We investigated the crosstalk between Fe and S using mutants impaired in Fe homeostasis, sulfate assimilation, and glutathione (GSH) biosynthesis. We showed that chlorosis symptoms induced by Fe deficiency were not directly related to the endogenous GSH levels. We found dynamic crosstalk between Fe and S networks and more interestingly observed that the upregulated expression of IRT1 and FRO2 under S deficiency in Col-0 was missing in the cad2-1 mutant background, which suggests that under S deficiency, the expression of IRT1 and FRO2 was directly or indirectly dependent on GSH. Interestingly, the bottleneck in sulfite reduction led to a constitutively higher IRT1 expression in the sir1-1 mutant. While the high-affinity sulfate transporter (Sultr1;2) was upregulated under Fe deficiency in the roots, the low-affinity sulfate transporters (Sultr2;1, and Sultr2;2) were down-regulated in the shoots of Col-0 seedlings. Moreover, the expression analysis of some of the key players in the Fe-S cluster assembly revealed that the expression of the so-called Fe donor in mitochondria (AtFH) and S mobilizer of group II cysteine desulfurase in plastids (AtNFS2) were upregulated under Fe deficiency in Col-0. Our qPCR data and ChIP-qPCR experiments suggested that the expression of AtFH is likely under the transcriptional regulation of the central transcription factor FIT.

Sulfur signaling and starvation response in Arabidopsis

As sessile organisms, plants have developed sophisticated mechanism to sense and utilize nutrients from the environment, and modulate their growth and development according to the nutrient availability. Research in the past two decades revealed that nutrient assimilation is not occurring spontaneously, but nutrient signaling networks are complexly regulated and integrate sensing and signaling, gene expression, and metabolism to ensure homeostasis and coordination with plant energy conversion and other processes. Here, we review the importance of the macronutrient sulfur (S) and compare the knowledge of S signaling with other important macronutrients, such as nitrogen (N) and phosphorus (P). We focus on key advances in understanding sulfur sensing and signaling, uptake and assimilation, and we provide new analysis of published literature, to identify core genes regulated by the key transcriptional factor in S starvation response, SLIM1/EIL3, and compare the impact on other nutrient deficiency and stresses on S-related genes.

The Vacuolar Molybdate Transporter OsMOT1;2 Controls Molybdenum Remobilization in Rice

Molybdenum (Mo) is an essential micronutrient for almost all living organisms. The Mo uptake process in plants has been well investigated. However, the mechanisms controlling Mo translocation and remobilization among different plant tissues are largely unknown, especially the allocation of Mo to rice grains that are the major dietary source of Mo for humans. In this study, we characterized the functions of a molybdate transporter, OsMOT1;2, in the interorgan allocation of Mo in rice. Heterologous expression in yeast established the molybdate transport activity of OsMOT1;2. OsMOT1;2 was highly expressed in the blades of the flag leaf and the second leaf during the grain filling stage. Subcellular localization revealed that OsMOT1;2 localizes to the tonoplast. Knockout of OsMOT1;2 led to more Mo accumulation in roots and less Mo translocation to shoots at the seedling stage and to grains at the maturity stage. The remobilization of Mo from older leaves to young leaves under molybdate-depleted condition was also decreased in the osmot1;2 knockout mutant. In contrast, overexpression of OsMOT1;2 enhanced the translocation of Mo from roots to shoots at the seedling stage. The remobilization of Mo from upper leaves to grains was also enhanced in the overexpression lines during grain filling. Our results suggest that OsMOT1;2 may function as a vacuolar molybdate exporter facilitating the efflux of Mo from the vacuole into the cytoplasm, and thus, it plays an important role in the root-to-shoot translocation of Mo and the remobilization of Mo from leaves to grains.

From leaves to roots: Biophysical models of transport of substances in plants

The transport processes of substances in various plant tissues are extremely diverse. However, models aimed at elucidating the mechanisms of such processes are almost absent in the literature. A unified view of all these transport processes is necessary, considering the laws of statistical physics and thermodynamics. A model of active ion transport was constructed based on the laws of statistical physics. Using this model, we traced the entire pathway of substances and energy in a plant. The pathway included aspects of the production of energy in the process of photosynthesis, consumption of energy to obtain nutrients from the soil, transport of such substances to the main organelles of all types of plant cells, the rise of water with dissolved substances along the trunk to the leaves, and the evaporation of water, accompanied by a change in the percentage of isotopes caused by different rates of evaporation. Models of ion transport in the chloroplasts and mitochondria of plant cells have been constructed. A generalized model comprising plant cells and their vacuoles was analyzed. A model of the transport of substances in the roots of plants was also developed. Based on this model, the problem of transport of substances in tall trees has been considered. The calculated concentrations of ions in the vacuoles of cells and resting potentials agreed well with the experimental data.

Sulfur in Seeds: An Overview

Sulfur is a growth-limiting and secondary macronutrient as well as an indispensable component for several cellular components of crop plants. Over the years various scientists have conducted several experiments on sulfur metabolism based on different aspects of plants. Sulfur metabolism in seeds has immense importance in terms of the different sulfur-containing seed storage proteins, the significance of transporters in seeds, the role of sulfur during the time of seed germination, etc. The present review article is based on an overview of sulfur metabolism in seeds, in respect to source to sink relationships, S transporters present in the seeds, S-regulated seed storage proteins and the importance of sulfur at the time of seed germination. Sulfur is an essential component and a decidable factor for seed yield and the quality of seeds in terms of oil content in oilseeds, storage of qualitative proteins in legumes and has a significant role in carbohydrate metabolism in cereals. In conclusion, a few future perspectives towards a more comprehensive knowledge on S metabolism/mechanism during seed development, storage and germination have also been stated.

Genome-wide identification of SULTR genes in tea plant and analysis of their expression in response to sulfur and selenium

Sulfur (S) is an essential macronutrient required by plants. Plants absorb and transport S through sulfate transporters (SULTRs). In this study, we cloned 8 SULTR genes (CsSULTR1;1/1;2/2;1/3;1/3;2/3;3/3;5/4;1) from tea plant (Camellia sinensis), all of which contain a typical sulfate transporter and antisigma factor antagonist (STAS) conserved domain. Phylogenetic tree analysis further divided the CsSULTRs into four main groups. Many cis-acting elements related to hormones and environmental stresses were found within the promoter sequence of CsSULTRs. Subcellular localization results showed that CsSULTR4;1 localized in the vacuolar membrane and that other CsSULTRs localized to the cellular membrane. The tissue-specific expression of the 8 CsSULTR genes showed different expression patterns during the active growing period and dormancy period. In particular, the expression of CsSULTR1;1 was highest in the roots, but that of CsSULTR1;2 was lowest in the dormancy period. The expression of CsSULTR1;1/1;2/2;1/3;2 was stimulated under different concentrations of selenium (Se) and S; moreover, CsSULTR1;2/2;1/3;3/3;5 was upregulated in response to different valences of Se.

Coordinated bacterial and plant sulfur metabolism in Enterobacter sp. SA187-induced plant salt stress tolerance

Although plant growth–promoting bacteria (PGPB) enhance the performance of plants, only a few mechanisms have been identified so far. We show that the sulfur metabolisms in both PGPB Enterobacter sp. SA187 and Arabidopsis plants play a key role in plant salt stress tolerance. Salt stress induces a sulfur starvation response in plants that is attenuated by SA187. Arabidopsis sulfur metabolic mutants are hypersensitive to salt stress but can be rescued by SA187. Most plant sulfur metabolism occurs in chloroplasts and is linked to stress-induced accumulation of reactive oxygen species that is suppressed by SA187. This work reveals that plant salt stress tolerance requires the coordinated regulation of the sulfur metabolic pathways in both beneficial microbe and host plant.

Enterobacter sp. SA187 is a root endophytic bacterium that maintains growth and yield of plants under abiotic stress conditions. In this work, we compared the metabolic wirings of Arabidopsis and SA187 in the free-living and endophytic interaction states. The interaction of SA187 with Arabidopsis induced massive changes in bacterial gene expression for chemotaxis, flagellar biosynthesis, quorum sensing, and biofilm formation. Besides modification of the bacterial carbon and energy metabolism, various nutrient and metabolite transporters and the entire sulfur pathway were up-regulated. Under salt stress, Arabidopsis resembled plants under sulfate starvation but not when colonized by SA187, which reprogramed the sulfur regulon of Arabidopsis. In accordance, salt hypersensitivity of multiple Arabidopsis sulfur metabolism mutants was partially or completely rescued by SA187 as much as by the addition of sulfate, L-cysteine, or L-methionine. Many components of the sulfur metabolism that are localized in the chloroplast were partially rescued by SA187. Finally, salt-induced accumulation of reactive oxygen species as well as the hypersensitivity of LSU mutants were suppressed by SA187. LSUs encode a central regulator linking sulfur metabolism to chloroplast superoxide dismutase activity. The coordinated regulation of the sulfur metabolic pathways in both the beneficial microorganism and the host plant is required for salt stress tolerance in Arabidopsis and might be a common mechanism utilized by different beneficial microbes to mitigate the harmful effects of different abiotic stresses on plants.

Noncoding-RNA-Mediated Regulation in Response to Macronutrient Stress in Plants

Macronutrient elements including nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) are required in relatively large and steady amounts for plant growth and development. Deficient or excessive supply of macronutrients from external environments may trigger a series of plant responses at phenotypic and molecular levels during the entire life cycle. Among the intertwined molecular networks underlying plant responses to macronutrient stress, noncoding RNAs (ncRNAs), mainly microRNAs (miRNAs) and long ncRNAs (lncRNAs), may serve as pivotal regulators for the coordination between nutrient supply and plant demand, while the responsive ncRNA-target module and the interactive mechanism vary among elements and species. Towards a comprehensive identification and functional characterization of nutrient-responsive ncRNAs and their downstream molecules, high-throughput sequencing has produced massive omics data for comparative expression profiling as a first step. In this review, we highlight the recent findings of ncRNA-mediated regulation in response to macronutrient stress, with special emphasis on the large-scale sequencing efforts for screening out candidate nutrient-responsive ncRNAs in plants, and discuss potential improvements in theoretical study to provide better guidance for crop breeding practices.

Involvement of nitric oxide (NO) in plant responses to metalloids

Plants respond to the limited or excess supply of metalloids, boron (B), silicon (Si), selenium (Se), arsenic (As), and antimony (Sb) via complex signaling pathways that are mainly regulated by nitric oxide (NO). The absorption of metalloids from the soil is facilitated by pathways that involve aquaporins, aquaglyceroporins, phosphate, and sulfate transporters; however, their regulation by NO is poorly understood. Using in silico software, we predicted the S-nitrosation of known metalloid transporters, proposing NO-dependent regulation of metalloid transport systems at the posttranslational level. NO intensifies the stress-mitigating effect of Si, whereas in the case of Se, As, and Sb, the accumulation of NO or reactive nitrogen species contributes to toxicity. NO promotes the beneficial effect of low Se concentrations and mitigates the damage caused by B deficiency. In addition, the exogenous application of NO donor, sodium nitroprusside, reduces B, Se, and As toxicity. The primary role of NO in metalloid stress response is to mitigate oxidative stress by activating antioxidant defense at the level of protein activity and gene expression. This review discusses the role of NO in plant responses to metalloids and suggests future research directions.

Selenium uptake, volatilization, and transformation by the cyanobacterium Microcystis aeruginosa and post-treatment of Se-laden biomass

With a narrow margin between beneficial and toxic effects, selenium (Se) is of great concern due to its increasing level in aquatic environments. The accumulation and transformation of Se by the cyanobacterium Microcystis aeruginosa and effects of nutrients, particularly sulfate, were investigated. The nutrient-deprived cyanobacterium removed water-borne selenate (82.2 ± 0.93%) faster than selenite (58.9 ± 1.77%), with 86.0 ± 1.41% and 77.2 ± 1.00%, respectively, of the Se accumulated in the biomass and the rest volatilized. When supplied with excess nutrients, the Se accumulation and volatilization rates were significantly inhibited, with the removal efficiency dropping to 50.2 ± 2.59% and 7.37 ± 0.93% for selenite and selenate, respectively. When M. aeruginosa was tested with inadequate, appropriate, and adequate levels of sulfate, Se uptake decreased with increasing sulfate concentrations, particularly for selenate (from 34.1 to 4.81%). Using X-ray absorption near-edge structure to speciate biomass Se, selenite and selenate were transformed to organo-Se (87.3-100%), with or without nutrients present, suggesting M. aeruginosa could efficiently reduce Se oxyanions to more bioavailable forms. With increasing sulfate levels (5.0 and 10.0 mg S/L), percentages of SeMet converted from selenite decreased by 28.2-33.0%, with 19.1-33.2% as elemental Se, while organo-Se remained dominant (93.6-95.1%) in selenate-treated M. aeruginosa. Transmission electron microscopy shows structural damage in the cell wall at exposure to selenite (1600 μg Se/L), with the intracellular structure intact. To prevent Se biomagnification along aquatic food chains, the Se-laden biomass was combusted as a post-treatment, leading to a significant reduction in Se content (∼99.2%) and Se bioavailability, with inorganic Se (45.0-70.5%) predominant in the residue.

Tonoplast-Localized OsMOT1;2 Participates in Interorgan Molybdate Distribution in Rice

Molybdenum (Mo) is an essential element for plant growth and is utilized by several key enzymes in biological redox processes. Rice assimilates molybdate ions via OsMOT1;1, a transporter with a high affinity for molybdate. However, other systems involved in the molecular transport of molybdate in rice remain unclear. Here, we characterized OsMOT1;2, which shares amino acid sequence similarity with AtMOT1;2 and functions in vacuolar molybdate export. We isolated a rice mutant harboring a complete deletion of OsMOT1;2. This mutant exhibited a significantly lower grain Mo concentration than the wild type (WT), but its growth was not inhibited. The Mo concentration in grains was restored by the introduction of WT OsMOT1;2. The OsMOT1;2-GFP protein was localized to the vacuolar membrane when transiently expressed in rice protoplasts. At the reproductive growth stage of the WT plant, OsMOT1;2 was highly expressed in the 2nd and lower leaf blades and nodes. The deletion of OsMOT1;2 impaired interorgan Mo allocation in aerial parts: relative to the WT, the mutant exhibited decreased Mo levels in the 1st and 2nd leaf blades and grains but increased Mo levels in the 2nd and lower leaf sheaths, nodes and internodes. When the seedlings were exposed to a solution with a high KNO3 concentration in the absence of Mo, the mutant exhibited significantly lower nitrate reductase activity in the shoots than the WT. Our results suggest that OsMOT1;2 plays an essential role in interorgan Mo distribution and molybdoenzyme activity in rice.

Nitric Oxide Prevents Fe Deficiency-Induced Photosynthetic Disturbance, and Oxidative Stress in Alfalfa by Regulating Fe Acquisition and Antioxidant Defense

Iron (Fe) deficiency impairs photosynthetic efficiency, plant growth and biomass yield. This study aimed to reveal the role of nitric oxide (NO) in restoring Fe-homeostasis and oxidative status in Fe-deficient alfalfa. In alfalfa, a shortage of Fe negatively affected the efficiency of root andshoot length, leaf greenness, maximum quantum yield PSII (Fv/Fm), Fe, S, and Zn accumulation, as well as an increase in H₂O₂ accumulation. In contrast, in the presence of sodium nitroprusside (SNP), a NO donor, these negative effects of Fe deficiency were largely reversed. In response to the SNP, the expression of Fe transporters (IRT1, NRAMP1) and S transporter (SULTR1;2) genes increased in alfalfa. Additionally, the detection of NO generation using fluorescence microscope revealed that SNP treatment increased the level of NO signal, indicating that NO may act as regulatory signal in response to SNP in plants. Interestingly, the increase of antioxidant genes and their related enzymes (Fe-SOD, APX) in response to SNP treatment suggests that Fe-SOD and APX are key contributors to reducing ROS (H₂O₂) accumulation and oxidative stress in alfalfa. Furthermore, the elevation of Ascorbate-glutathione (AsA-GSH) pathway-related genes (GR and MDAR) Fe-deficiency with SNP implies that the presence of NO relates to enhanced antioxidant defense against Fe-deficiency stress.

Physiological and molecular mechanisms of metal accumulation in hyperaccumulator plants

Most of the heavy metals (HMs), and metals/metalloids are released into the nature either by natural phenomenon or anthropogenic activities. Being sessile organisms, plants are constantly exposed to HMs in the environment. The metal non-hyperaccumulating plants are susceptible to excess metal concentrations. They tend to sequester metals in their root vacuoles by forming complexes with metal ligands, as a detoxification strategy. In contrast, the metal-hyperaccumulating plants have adaptive intrinsic regulatory mechanisms to hyperaccumulate or sequester excess amounts of HMs into their above-ground tissues rather than accumulating them in roots. They have unique abilities to successfully carry out normal physiological functions without showing any visible stress symptoms unlike metal non-hyperaccumulators. The unique abilities of accumulating excess metals in hyperaccumulators partly owes to constitutive overexpression of metal transporters and ability to quickly translocate HMs from root to shoot. Various metal ligands also play key roles in metal hyperaccumulating plants. These metal hyperaccumulating plants can be used in metal contaminated sites to clean-up soils. Exploiting the knowledge of natural populations of metal hyperaccumulators complemented with cutting-edge biotechnological tools can be useful in the future. The present review highlights the recent developments in physiological and molecular mechanisms of metal accumulation of hyperaccumulator plants in the lights of metal ligands and transporters. The contrasting mechanisms of metal accumulation between hyperaccumulators and non-hyperaccumulators are thoroughly compared. Moreover, uses of different metal hyperaccumulators for phytoremediation purposes are also discussed in detail.

Genetic architecture of root and shoot ionomes in rice (Oryza sativa L.)

KEY MESSAGE

Association analysis for ionomic concentrations of 20 elements identified independent genetic factors underlying the root and shoot ionomes of rice, providing a platform for selecting and dissecting causal genetic variants. Understanding the genetic basis of mineral nutrient acquisition is key to fully describing how terrestrial organisms interact with the non-living environment. Rice (Oryza sativa L.) serves both as a model organism for genetic studies and as an important component of the global food system. Studies in rice ionomics have primarily focused on above ground tissues evaluated from field-grown plants. Here, we describe a comprehensive study of the genetic basis of the rice ionome in both roots and shoots of 6-week-old rice plants for 20 elements using a controlled hydroponics growth system. Building on the wealth of publicly available rice genomic resources, including a panel of 373 diverse rice lines, 4.8 M genome-wide single-nucleotide polymorphisms, single- and multi-marker analysis pipelines, an extensive tome of 321 candidate genes and legacy QTLs from across 15 years of rice genetics literature, we used genome-wide association analysis and biparental QTL analysis to identify 114 genomic regions associated with ionomic variation. The genetic basis for root and shoot ionomes was highly distinct; 78 loci were associated with roots and 36 loci with shoots, with no overlapping genomic regions for the same element across tissues. We further describe the distribution of phenotypic variation across haplotypes and identify candidate genes within highly significant regions associated with sulfur, manganese, cadmium, and molybdenum. Our analysis provides critical insight into the genetic basis of natural phenotypic variation for both root and shoot ionomes in rice and provides a comprehensive resource for dissecting and testing causal genetic variants.

Structure and function of an Arabidopsis thaliana sulfate transporter

Plant sulfate transporters (SULTR) mediate absorption and distribution of sulfate (SO₄^2-) and are essential for plant growth; however, our understanding of their structures and functions remains inadequate. Here we present the structure of a SULTR from Arabidopsis thaliana, AtSULTR4;1, in complex with SO₄^2- at an overall resolution of 2.8 Å. AtSULTR4;1 forms a homodimer and has a structural fold typical of the SLC26 family of anion transporters. The bound SO₄^2- is coordinated by side-chain hydroxyls and backbone amides, and further stabilized electrostatically by the conserved Arg393 and two helix dipoles. Proton and SO₄^2- are co-transported by AtSULTR4;1 and a proton gradient significantly enhances SO₄^2- transport. Glu347, which is ~7 Å from the bound SO₄^2-, is required for H⁺-driven transport. The cytosolic STAS domain interacts with transmembrane domains, and deletion of the STAS domain or mutations to the interface compromises dimer formation and reduces SO₄^2- transport, suggesting a regulatory function of the STAS domain.

The Relevance of Plant-Derived Se Compounds to Human Health in the SARS-CoV-2 (COVID-19) Pandemic Era

Dietary selenium (Se)-compounds accumulated in plants are essential for human metabolism and normal physiological processes. Inorganic and organic Se species can be readily absorbed by the human body, but are metabolized differently and thus exhibit distinct mechanisms of action. They can act as antioxidants or serve as a source of Se for the synthesis of selenoproteins. Selenocysteine, in particular, is incorporated at the catalytic center of these proteins through a specific insertion mechanism and, due to its electronic features, enhances their catalytic activity against biological oxidants. Selenite and other Se-organic compounds may also act as direct antioxidants in cells due to their strong nucleophilic properties. In addition, Se-amino acids are more easily subjected to oxidation than the corresponding thiols/thioethers and can bind redox-active metal ions. Adequate Se intake aids in preventing several metabolic disorders and affords protection against viral infections. At present, an epidemic caused by a novel coronavirus (SARS-CoV-2) threatens human health across several countries and impacts the global economy. Therefore, Se-supplementation could be a complementary treatment to vaccines and pharmacological drugs to reduce the viral load, mutation frequency, and enhance the immune system of populations with low Se intake in the diet.

Root hairs: the villi of plants

Strikingly, evolution shaped similar tubular structures at the µm to mm scale in roots of sessile plants and in small intestines of mobile mammals to ensure an efficient transfer of essential nutrients from 'dead matter' into biota. These structures, named root hairs (RHs) in plants and villi in mammals, numerously stretch into the environment, and extremely enlarge root and intestine surfaces. They are believed to forage for nutrients, and mediate their uptake. While the conceptional understanding of plant RH function in hydromineral nutrition seems clear, experimental evidence presented in textbooks is restricted to a very limited number of reference-nutrients. Here, we make an element-by-element journey through the periodic table and link individual nutrient availabilities to the development, structure/shape and function of RHs. Based on recent developments in molecular biology and the identification of mutants differing in number, length or other shape-related characteristics of RHs in various plant species, we present comprehensive advances in (i) the physiological role of RHs for the uptake of specific nutrients, (ii) the developmental and morphological responses of RHs to element availability and (iii) RH-localized nutrient transport proteins. Our update identifies crucial roles of RHs for hydromineral nutrition, mostly under nutrient and/or water limiting conditions, and highlights the influence of certain mineral availabilities on early stages of RH development, suggesting that nutritional stimuli, as deficiencies in P, Mn or B, can even dominate over intrinsic developmental programs underlying RH differentiation.

Metalloid hazards: From plant molecular evolution to mitigation strategies

Metalloids such as boron and silicon are key elements for plant growth and crop productivity. However, toxic metalloids such as arsenic are increasing in the environment due to inputs from natural sources and human activities. These hazardous metalloids can cause serious health risks to humans and animals if they enter the food chain. Plants have developed highly regulated mechanisms to alleviate the toxicity of metalloids during their 500 million years of evolution. A better understanding the molecular mechanisms underlying the transport and detoxification of toxic metalloids in plants will shed light on developing mitigation strategies. Key transporters and regulatory proteins responsive to toxic metalloids have been identified through evolutionary and molecular analyses. Moreover, knowledge of the regulatory proteins and their pathways can be used in the breeding of crops with lower accumulation of metalloids. These findings can also assist phytoremediation by the exploration of plants such as fern species that hyperaccumulate metalloids from soils and water, and can be used to engineer plants with elevated uptake and storage capacity of toxic metalloids. In summary, there are solutions to remediate contamination due to toxic metalloids by combining the research advances and industrial technologies with agricultural and environmental practices.

Genome-Wide Identification and Expansion Patterns of SULTR Gene Family in Gramineae Crops and Their Expression Profiles under Abiotic Stress in Oryza sativa

Sulfate transporters (SULTRs), also known as H⁺/SO₄²⁻ symporters, play a key role in sulfate transport, plant growth and stress responses. However, the evolutionary relationships and functional differentiation of SULTRs in Gramineae crops are rarely reported. Here, 111 SULTRs were retrieved from the genomes of 10 Gramineae species, including Brachypodium disachyon, Hordeum vulgare, Setaria italica, Sorghum bicolor, Zea mays, Oryza barthii, Oryza rufipogon, Oryza glabbermia and Oryza sativa (Oryza sativa ssp. indica and Oryza sativa ssp. japonica). The SULTRs were clustered into five clades based on a phylogenetic analysis. Syntheny analysis indicates that whole-genome duplication/segmental duplication and tandem duplication events were essential in the SULTRs family expansion. We further found that different clades and orthologous groups of SULTRs were under a strong purifying selective force. Expression analysis showed that rice SULTRs with high-affinity transporters are associated with the functions of sulfate uptake and transport during rice seedling development. Furthermore, using Oryza sativa ssp. indica as a model species, we found that OsiSULTR10 was significantly upregulated under salt stress, while OsiSULTR3 and OsiSULTR12 showed remarkable upregulation under high temperature, low-selenium and drought stresses. OsiSULTR3 and OsiSULTR9 were upregulated under both low-selenium and high-selenium stresses. This study illustrates the expression and evolutionary patterns of the SULTRs family in Gramineae species, which will facilitate further studies of SULTR in other Gramineae species.

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.

Selenium Biofortification: Roles, Mechanisms, Responses and Prospects

The trace element selenium (Se) is a crucial element for many living organisms, including soil microorganisms, plants and animals, including humans. Generally, in Nature Se is taken up in the living cells of microorganisms, plants, animals and humans in several inorganic forms such as selenate, selenite, elemental Se and selenide. These forms are converted to organic forms by biological process, mostly as the two selenoamino acids selenocysteine (SeCys) and selenomethionine (SeMet). The biological systems of plants, animals and humans can fix these amino acids into Se-containing proteins by a modest replacement of methionine with SeMet. While the form SeCys is usually present in the active site of enzymes, which is essential for catalytic activity. Within human cells, organic forms of Se are significant for the accurate functioning of the immune and reproductive systems, the thyroid and the brain, and to enzyme activity within cells. Humans ingest Se through plant and animal foods rich in the element. The concentration of Se in foodstuffs depends on the presence of available forms of Se in soils and its uptake and accumulation by plants and herbivorous animals. Therefore, improving the availability of Se to plants is, therefore, a potential pathway to overcoming human Se deficiencies. Among these prospective pathways, the Se-biofortification of plants has already been established as a pioneering approach for producing Se-enriched agricultural products. To achieve this desirable aim of Se-biofortification, molecular breeding and genetic engineering in combination with novel agronomic and edaphic management approaches should be combined. This current review summarizes the roles, responses, prospects and mechanisms of Se in human nutrition. It also elaborates how biofortification is a plausible approach to resolving Se-deficiency in humans and other animals.

Specificity and Plasticity of the Functional Ionome of Brassica napus and Triticum aestivum Subjected to Macronutrient Deprivation

The composition of the functional ionome was studied in Brassica napus and Triticum aestivum with respect to the response of 20 elements under macronutrient deprivation. Analysis of relative root contents showed that some nutrients, such as Fe, Ni, Cu, Na, V, and Co, were largely sequestered in roots. After 10 days of deprivation of each one of these 6 macronutrients, plant growth was similar to control plants, and this was probably the result of remobilization from roots (Mg and Ca) or old leaves (N, P, K, S). Some tissue concentrations and net nutrient uptakes into roots were either decreased or increased, revealing multiple interactions (93 in wheat, 66 in oilseed rape) that were common to both species (48) or were species specific. While some interactions have been previously described (increased uptake of Na under K deficiency; or increased uptake of Mo and Se under S deficiency), a number of new interactions were found and some key mechanisms underlying their action have been proposed from analysis of Arabidopsis mutants. For example, nitrate uptake seemed to be functionally linked to Na(influx, while the uptake of vanadium was probably mediated by sulfate transporters whose expression was stimulated during S deprivation.

High-Affinity Sulfate Transporter Sultr1;2 Is a Major Transporter for Cr(VI) Uptake in Plants

Chromate (Cr[VI]) is a highly phytotoxic contaminant that is ubiquitous in soils. However, how Cr(VI) is taken up by plant roots remains largely unknown. Here, we show that the high-affinity sulfate transporter Sultr1;2 is responsible for Cr(VI) uptake by the roots of Arabidopsis thaliana. Sultr1;2 showed a much higher transport activity for Cr(VI) than Sultr1;1 when expressed in yeast cells. Knockdown of Sultr1;2 expression in Arabidopsis markedly reduced the Cr(VI) uptake rate, whereas knockout of Sultr1;1 had no or little effect. A double-knockout mutant (DKO) of the two genes lost the ability of Cr(VI) uptake almost completely. The Sultr1;2 knockdown mutant or DKO plants displayed higher resistance to Cr(VI) under normal sulfate conditions as a consequence of the lower tissue Cr accumulation. Overexpression of Sultr1;2 substantially increased Cr(VI) uptake with shoot Cr concentration being 1.6-2.0 times higher than that in the wild-type. These results indicate that Sultr1;2 is a major transporter responsible for Cr(VI) uptake in Arabidopsis, while Sultr1;1 plays a negligible role. Taken together, our study has identified a major transporter for Cr(VI) uptake in plants, providing potential strategies for engineering plants with low Cr accumulation and consequently enhanced Cr(VI) resistance and also plants with enhanced accumulation of Cr for the purpose of phytoremediation.

Sulfur triggers glutathione and phytochelatin accumulation causing excess Cd bound to the cell wall of roots in alleviating Cd-toxicity in alfalfa

Although Cd is threatening to the environment, animal, and human, the eco-friendly approach to mitigate the Cd-toxicity in alfalfa was barely studied. Therefore, this study aims at elucidating the role of S, a crucial macroelement, in alleviating Cd toxicity in alfalfa plants. The supplementation of S in Cd-stressed alfalfa reversed the detrimental effect on plant biomass, chlorophyll synthesis, and protein concentration. Interestingly, S surplus restored the photosynthetic kinetics, such as Fv/Fm, Pi_ABS, and Mo values in leaves of Cd-stressed alfalfa. Further, Cd-induced adverse effect on membrane stability, cell viability, and redox status was restored due to S under Cd stress. The exogenous S not only increased S status and the expression of sulfate transporters (MsSULRT1;2 and MsSULTR1;3), but also decreased the Cd concentration in the shoot by retaining elevated Cd in root tissue. Further analysis revealed the upregulation of MsGS (glutathione synthetase) and MsPCS1 (phytochelatin synthase) genes along with the increased concentration of glutathione and phytochelatin, predominantly in roots subjected to S surplus under Cd stress. The subcellular Cd analysis showed elevated Cd in the cell wall but not in the vacuole. It suggests that S-induced elevated glutathione enables the phytochelatin to bind with excess Cd leading to subcellular sequestration in the cell wall of roots. Also, S stimulates the S-metabolites and GR enzyme that coordinately counteracts Cd-induced oxidative damage. These findings can be utilized to popularize the application of S and to perform breeding/transgenic experiments to develop Cd-free forage crops.

Boron-dependent regulation of translation through AUGUAA sequence in yeast

Under high boron (B) conditions, nodulin 26-like intrinsic protein 5;1 (NIP5;1) mRNA, a boric acid channel, is destabilized to avoid excess B entry into roots of Arabidopsis thaliana. In this regulation, the minimum upstream open reading frame (uORF), AUGUAA, in its 5'-untranslated region (5'-UTR) is essential, and high B enhances ribosome stalling at AUGUAA and leads to suppression of translation and mRNA degradation. This B-dependent AUGUAA-mediated regulation occurs also in animal transient expression and reticulocyte lysate translation systems. Thus, uncovering the ubiquitousness of B-dependent unique regulation is important to reveal the evolution of translational regulation. In the present study, we examined the regulation in Saccharomyces cerevisiae. Reporter assay showed that in yeast, carrying ATGTAA in 5'-UTR of NIP5;1 upstream of the reporter gene, the relative reporter activities were reduced significantly under high B conditions compared with control, whereas deletion of ATGTAA abolished such responses. This suggests that AUGUAA mediates B-dependent regulation of translation in Saccharomyces cerevisiae. Moreover, the deletion of ATGTAA resulted in up to 10-fold increase in general reporter activities indicating the suppression effect of AUGUAA on translation of the main ORF. Interestingly, mRNA level of the reporter gene was not affected by B in both yeast cells with and without AUGUAA. This finding reveals that in yeast, unlike the case in plants, mRNA degradation is not associated with AUGUAA regulation. Together, results suggest that B-dependent AUGUAA-mediated translational regulation is common among eukaryotes.

Phosphorus and sulfur in a tropical soil and their effects on growth and selenium accumulation in Leucaena leucocephala (Lam.) de Wit

Selenium (Se) is an essential metalloid element for mammals. Nonetheless, both deficiency and excess of Se in the environment are associated with several diseases in animals and humans. Here, we investigated the interaction of Se, supplied as selenate (Se⁺⁶) and selenite (Se⁺⁴), with phosphorus (P) and sulfur (S) in a weathered tropical soil and their effects on growth and Se accumulation in Leucaena leucocephala (Lam.) de Wit. The P-Se interaction effects on L. leucocephala growth differed between the Se forms (selenate and selenite) supplied in the soil. Selenate was prejudicial to plants grown in the soil with low P dose, while selenite was harmful to plants grown in soil with high P dose. The decreasing soil S dose increased the toxic effect of Se in L. leucocephala plants. Se tissue concentration and total Se accumulation in L. leucocephala shoot were higher with selenate supply in the soil when compared with selenite. Therefore, selenite proved to be less phytoavailable in the weathered tropical soil and, at the same time, more toxic to L. leucocephala plants than selenate. Thus, it is expected that L. leucocephala plants are more efficient to phytoextract and accumulate Se as selenate than Se as selenite from weathered tropical soils, for either strategy of phytoremediation (decontamination of Se-polluted soils) or purposes of biofortification for animal feed (fertilization of Se-poor soils).

Integration analysis of PacBio SMRT- and Illumina RNA-seq reveals candidate genes and pathway involved in selenium metabolism in hyperaccumulator Cardamine violifolia

BACKGROUND

Cardamine violifolia, native to China, is one of the selenium (Se) hyperaccumulators. The mechanism of Se metabolism and tolerance remains unclear, and only limited genetic information is currently available. Therefore, we combined a PacBio single-molecule real-time (SMRT) transcriptome library and the Illumina RNA-seq data of sodium selenate (Na₂SeO₄)-treated C. violifolia to further reveal the molecular mechanism of Se metabolism.

RESULTS

The concentrations of the total, inorganic, and organic Se in C. violifolia seedlings significantly increased as the Na₂SeO₄ treatment concentration increased. From SMRT full-length transcriptome of C. violifolia, we obtained 26,745 annotated nonredundant transcripts, 14,269 simple sequence repeats, 283 alternative splices, and 3407 transcription factors. Fifty-one genes from 134 transcripts were identified to be involved in Se metabolism, including transporter, assimilatory enzyme, and several specific genes. Analysis of Illumina RNA-Seq data showed that a total of 948 differentially expressed genes (DEGs) were filtered from the four groups with Na₂SeO₄ treatment, among which 11 DEGs were related to Se metabolism. The enrichment analysis of KEGG pathways of all the DEGs showed that they were significantly enriched in five pathways, such as hormone signal transduction and plant-pathogen interaction pathways. Four genes related to Se metabolism, adenosine triphosphate sulfurase 1, adenosine 5'-phosphosulfate reductase 3, cysteine (Cys) desulfurase 1, and serine acetyltransferase 2, were regulated by lncRNAs. Twenty potential hub genes (e.g., sulfate transporter 1;1, Cys synthase, methionine gamma-lyase, and Se-binding protein 1) were screened and identified to play important roles in Se accumulation and tolerance in C. violifolia as concluded by weighted gene correlation network analysis. Based on combinative analysis of expression profiling and annotation of genes as well as Se speciation and concentration in C. violifolia under the treatments with different Na₂SeO₄ concentrations, a putative Se metabolism and assimilation pathway in C. violifolia was proposed.

CONCLUSION

Our data provide abundant information on putative gene transcriptions and pathway involved in Se metabolism of C. violifolia. The findings present a genetic resource and provide novel insights into the mechanism of Se hyperaccumulation in C. violifolia.

Selenium Biofortification of Crop Food by Beneficial Microorganisms

Selenium (Se) is essential for human health, however, Se is deficient in soil in many places all around the world, resulting in human diseases, such as notorious Keshan disease and Keshin–Beck disease. Therefore, Se biofortification is a popular approach to improve Se uptake and maintain human health. Beneficial microorganisms, including mycorrhizal and root endophytic fungi, dark septate fungi, and plant growth-promoting rhizobacteria (PGPRs), show multiple functions, especially increased plant nutrition uptake, growth and yield, and resistance to abiotic stresses. Such functions can be used for Se biofortification and increased growth and yield under drought and salt stress. The present review summarizes the use of mycorrhizal fungi and PGPRs in Se biofortification, aiming to improving their practical use.

Potential Implications of Interactions between Fe and S on Cereal Fe Biofortification

Iron (Fe) and sulfur (S) are two essential elements for plants, whose interrelation is indispensable for numerous physiological processes. In particular, Fe homeostasis in cereal species is profoundly connected to S nutrition because phytosiderophores, which are the metal chelators required for Fe uptake and translocation in cereals, are derived from a S-containing amino acid, methionine. To date, various biotechnological cereal Fe biofortification strategies involving modulation of genes underlying Fe homeostasis have been reported. Meanwhile, the resultant Fe-biofortified crops have been minimally characterized from the perspective of interaction between Fe and S, in spite of the significance of the crosstalk between the two elements in cereals. Here, we intend to highlight the relevance of Fe and S interrelation in cereal Fe homeostasis and illustrate the potential implications it has to offer for future cereal Fe biofortification studies.

Agronomic biofortification with selenium impacts storage proteins in grains of upland rice

BACKGROUND

Selenium (Se) is an essential element for humans and animals. Rice is one of the most commonly consumed cereals in the world, so the agronomic biofortification of cereals with Se may be a good strategy to increase the levels of daily intake of Se by the population. This study evaluated the agronomic biofortification of rice genotypes with Se and its effects on grain nutritional quality. Five rates of Se (0, 10, 25, 50, and 100 g ha ^-1 ) were applied as selenate via the soil to three rice genotypes under field conditions.

RESULTS

Selenium concentrations in the leaves and polished grains increased linearly in response to Se application rates. A highly significant correlation was observed between the Se rates and the Se concentration in the leaves and grains, indicating high translocation of Se. The application of Se also increased the concentration of albumin, globulin, prolamin, and glutelin in polished grains.

CONCLUSION

Biofortifying rice genotypes using 25 g Se ha ^-1 could increase the average daily Se intake from 4.64 to 66 μg day^-1 . Considering that the recommended daily intake of Se by adults is 55 μg day^-1 , this agronomic strategy could contribute to alleviating widespread Se malnutrition. © 2019 Society of Chemical Industry.

SLIM1 Transcription Factor Promotes Sulfate Uptake and Distribution to Shoot, Along with Phytochelatin Accumulation, Under Cadmium Stress in Arabidopsis thaliana

Sulfur (S) assimilation, which is initiated by sulfate uptake, generates cysteine, the substrate for glutathione (GSH) and phytochelatin (PC) synthesis. GSH and PC contribute to cadmium (Cd) detoxification by capturing it for sequestration. Although Cd exposure is known to induce the expression of S-assimilating enzyme genes, including sulfate transporters (SULTRs), mechanisms of their transcriptional regulation are not well understood. Transcription factor SLIM1 controls transcriptional changes during S deficiency (-S) in Arabidopsis thaliana. We examined the potential involvement of SLIM1 in inducing the S assimilation pathway and PC accumulation. Cd treatment reduced the shoot fresh weight in the sulfur limitation1 (slim1) mutant but not in the parental line (1;2PGN). Cd-induced increases of sulfate uptake and SULTR1;2 expressions were diminished in the slim1 mutant, suggesting that SLIM1 is involved in inducing sulfate uptake during Cd exposure. The GSH and PC levels were lower in slim1 than in the parental line, indicating that SLIM1 was required for increasing PC during Cd treatment. Hence, SLIM1 indirectly contributes to Cd tolerance of plants by inducing -S responses in the cell caused by depleting the GSH pool, which is consumed by enhanced PC synthesis and sequestration to the vacuole.

Comparative de novo transcriptomics and untargeted metabolomic analyses elucidate complicated mechanisms regulating celery (Apium graveolens L.) responses to selenium stimuli

Presently, concern regarding the effects of selenium (Se) on the environment and organisms worldwide is increasing. Too much Se in the soil is harmful to plants. In this study, Illumina RNA sequencing and the untargeted metabolome of control and Se-treated celery seedlings were analyzed. In total, 297,911,046 clean reads were obtained and assembled into 150,218 transcripts (50,876 unigenes). A total of 36,287 unigenes were annotated using different databases. Additionally, 8,907 differentially expressed genes, including 5,319 up- and 3,588 downregulated genes, were identified between mock and Se-treated plants. “Phenylpropanoid biosynthesis” was the most enriched KEGG pathway. A total of 24 sulfur and selenocompound metabolic unigenes were differentially expressed. Furthermore, 1,774 metabolites and 237 significant differentially accumulated metabolites were identified using the untargeted metabolomic approach. We conducted correlation analyses of enriched KEGG pathways of differentially expressed genes and accumulated metabolites. Our findings suggested that candidate genes and metabolites involved in important biological pathways may regulate Se tolerance in celery. The results increase our understanding of the molecular mechanism responsible for celery’s adaptation to Se stress.

Quick selenium accumulation in the selenium-rich rice and its physiological responses in changing selenium environments

BACKGROUND

The element selenium (Se) deficiency is thought to be a global human health problem, which could disperse by daily-supplement from Se-rich food. Increasing the accumulation of Se in rice grain is an approach matched to these nutrient demands. Nonetheless, Se is shown to be essential but also toxic to plants, with a narrow margin between deficiency and toxicity. Notably, the regulatory mechanism balancing the accumulation and tolerance of Se in Se-rich rice plants remains unknown.

RESULTS

In this study, we investigated the phenotypical, physiological, and biochemical alterations of Se-rich rice in the exposure to a variety of Se applications. Results showed that the Se-rich rice was able to accumulate more abundance of Se from the root under a low Se environment comparing to the Se-free rice. Besides, excessive Se led to phytotoxic effects on Se-rich rice plants by inducing chlorosis and dwarfness, decreasing the contents of antioxidant, and exacerbating oxidative stresses. Furthermore, both phosphate transporter OsPT2 and sulfate transporters OsSultr1;2 may contribute to the uptake of selenate in rice.

CONCLUSIONS

Se-rich red rice is more sensitive to exogenous application of Se, while and the most effective application of Se in roots of Se-rich rice was reached in 20 μM. Our findings present a direct way to evaluate the toxic effects of Se-rich rice in the Se contaminated field. Conclusively, some long-term field trial strategies are suggested to be included in the evaluation of risks and benefits within various field managements.

Comparative proteomic analysis of pepper (Capsicum annuum L.) seedlings under selenium stress

Selenium (Se) is an essential trace element for human and animal health. Se fertilizer has been used to increase the Se content in crops to meet the Se requirements in humans and animals. To address the challenge of Se poisoning in plants, the mechanisms underlying Se-induced stress in plants must be understood. Here, to elucidate the effects of Se stress on the protein levels in pepper, we used an integrated approach involving tandem mass tag labeling, high performance liquid chromatography fractionation, and mass spectrometry-based analysis. A total of 4,693 proteins were identified, 3,938 of which yielded quantitative information. Among them, the expression of 172 proteins was up-regulated, and the expression of 28 proteins was down-regulated in the Se/mock treatment comparison. According to the above data, we performed a systematic bioinformatics analysis of all identified proteins and differentially expressed proteins (DEPs). The DEPs were most strongly associated with the terms “metabolic process,” “posttranslational modification, protein turnover, chaperones,” and “protein processing in endoplasmic reticulum” according to Gene Ontology, eukaryotic orthologous groups classification, and Kyoto Encyclopedia of Genes and Genomes enrichment analysis, respectively. Furthermore, several heat shock proteins were identified as DEPs. These results provide insights that may facilitate further studies on the pepper proteome expressed downstream of the Se stress response. Our data revealed that the responses of pepper to Se stress involve various pathways.

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.