Selenium in soil-plant system: Transport, detoxification and bioremediation.

Selenium (Se) is an essential micronutrient for humans and a beneficial element for plants. However, high Se doses always exhibit hazardous effects. Recently, Se toxicity in plant-soil system has received increasing attention. This review will summarize (1) Se concentration in soils and its sources, (2) Se bioavailability in soils and influencing factors, (3) mechanisms on Se uptake and translocation in plants, (4) toxicity and detoxification of Se in plants and (5) strategies to remediate Se pollution. High Se concentration mainly results from wastewater discharge and industrial waste dumping. Selenate (Se [VI]) and selenite (Se [IV]) are the two primary forms absorbed by plants. Soil conditions such as pH, redox potential, organic matter and microorganisms will influence Se bioavailability. In plants, excessive Se will interfere with element uptake, depress photosynthetic pigment biosynthesis, generate oxidative damages and cause genotoxicity. Plants employ a series of strategies to detoxify Se, such as activating antioxidant defense systems and sequestrating excessive Se in the vacuole. In order to alleviate Se toxicity to plants, some strategies can be applied, including phytoremediation, OM remediation, microbial remediation, adsorption technique, chemical reduction technology and exogenous substances (such as Methyl jasmonate, Nitric oxide and Melatonin). This review is expected to expand the knowledge of Se toxicity/detoxicity in soil-plant system and offer valuable insights into soils Se pollution remediation strategies.

Selenium and molybdenum synergistically alleviate chromium toxicity by modulating Cr uptake and subcellular distribution in Nicotiana tabacum L.

Chromium (Cr) is a harmful heavy metal that poses a serious threat to plants and animals. Selenium (Se) and molybdenum (Mo) are two beneficial elements for plant growth and resistance. However, their interactive effects on Cr uptake and distribution are poorly understood. Therefore, a hydroponics experiment was conducted to explore the effects of the use of Se and Mo alone and simultaneously on mitigating Cr toxicity. In this study, Nicotiana tabacum L. seedlings were exposed to control, 50 µM Cr, 50 µM Cr + 2 µM Se, 50 µM Cr + 1 µM Mo, or 50 µM Cr + 2 µM Se + 1 µM Mo in Hoagland solution. After 2 weeks, the plant biomass, Cr, Se and Mo contents, photosynthesis, leaf ultrastructure, antioxidant system, subcellular distribution and associated gene expression in Nicotiana tabacum L. were determined. The results showed that simultaneous use of Se and Mo promoted tobacco growth under Cr stress, as evidenced by reducing reactive oxygen species (ROS) content and reducing Cr translocation factor (TF) and inducing a 51.3% reduction in Cr content in shoots. Additionally, Se-Mo interactions increased the levels of glutathione (GSH) and phytochelatin (PC) and the distribution of Cr in the cell walls and organelles. Furthermore, the relative expression of PCS1 was upregulated, while those of NtST1 and MSN1 were downregulated. The results concluded that the simultaneous use of Se and Mo effectively alleviated Cr toxicity in Nicotiana tabacum L., which not only offers an efficient way for crops to resist Cr toxicity but also provides evidence for the benefit of Se combined with Mo.

Selenium Increased Arsenic Accumulation by Upregulating the Expression of Genes Responsible for Arsenic Reduction, Translocation, and Sequestration in Arsenic Hyperaccumulator Pteris vittata.

Selenate enhances arsenic (As) accumulation in As-hyperaccumulator Pteris vittata, but the associated molecular mechanisms are unclear. Here, we investigated the mechanisms of selenate-induced arsenic accumulation by exposing P. vittata to 50 µM arsenate (AsV50) and 1.25 (Se1.25) or 5 µM (Se5) selenate in hydroponics. After 2 weeks, plant biomass, plant As and Se contents, As speciation in plant and growth media, and important genes related to As detoxification in P. vittata were determined. These genes included P transporters PvPht1;3 and PvPht1;4 (AsV uptake), arsenate reductases PvHAC1 and PvHAC2 (AsV reduction), and arsenite (AsIII) antiporters PvACR3 and PvACR3;2 (AsIII translocation) in the roots, and AsIII antiporters PvACR3;1 and PvACR3;3 (AsIII sequestration) in the fronds. The results show that Se1.25 was more effective than Se5 in increasing As accumulation in both P. vittata roots and fronds, which increased by 27 and 153% to 353 and 506 mg kg-1. The As speciation analyses show that selenate increased the AsIII levels in P. vittata, with 124-282% more AsIII being translocated into the fronds. The qPCR analyses indicate that Se1.25 upregulated the gene expression of PvHAC1 by 1.2-fold, and PvACR3 and PvACR3;2 by 1.0- to 2.5-fold in the roots, and PvACR3;1 and PvACR3;3 by 0.6- to 1.1-fold in the fronds under AsV50 treatment. Though arsenate enhanced gene expression of P transporters PvPht1;3 and PvPht1;4, selenate had little effect. Our results indicate that selenate effectively increased As accumulation in P. vittata, mostly by increasing reduction of AsV to AsIII in the roots, AsIII translocation from the roots to fronds, and AsIII sequestration into the vacuoles in the fronds. The results suggest that selenate may be used to enhance phytoremediation of As-contaminated soils using P. vittata.

Comparison of selenite and selenate in alleviation of drought stress in Nicotiana tabacum L.

Exogenous selenium (Se) improves the tolerance of plants to abiotic stress. However, the effects and mechanisms of different Se species on drought stress alleviation are poorly understood. This study aims to evaluate and compare the different effects and mechanisms of sodium selenate (Na2SeO4) and sodium selenite (Na2SeO3) on the growth, photosynthesis, antioxidant system, osmotic substances and stress-responsive gene expression of Nicotiana tabacum L. under drought stress. The results revealed that drought stress could significantly inhibit growth, whereas both Na2SeO4 and Na2SeO3 could significantly facilitate the growth of N. tabacum under drought stress. However, compared to Na2SeO3, Se application as Na2SeO4 induced a significant increase in the root tip number and number of bifurcations under drought stress. Furthermore, both Na2SeO4 and Na2SeO3 displayed higher levels of photosynthetic pigments, better photosynthesis, and higher concentrations of osmotic substances, antioxidant enzymes, and stress-responsive gene (NtCDPK2, NtP5CS, NtAREB and NtLEA5) expression than drought stress alone. However, the application of Na2SeO4 showed higher expression levels of the NtP5CS and NtAREB genes than Na2SeO3. Both Na2SeO4 and Na2SeO3 alleviated many of the deleterious effects of drought in leaves, which was achieved by reducing stress-induced lipid peroxidation (MDA) and H2O2 content by enhancing the activity of antioxidant enzymes, while Na2SeO4 application showed lower H2O2 and MDA content than Na2SeO3 application. Overall, the results confirm the positive effects of Se application, especially Na2SeO4 application, which is markedly superior to Na2SeO3 in the role of resistance towards abiotic stress in N. tabacum.

Synergistic effect of silicon and selenium on the alleviation of cadmium toxicity in rice plants.

Silicon (Si) and selenium (Se), two beneficial elements that alleviate cadmium (Cd) toxicity, are important for agricultural production and human health. However, the effects and related mechanisms of Si-Se interaction on Cd toxicity alleviation are still poorly understood. Herein, a hydroponic experiment was employed to evaluate the effects of Si and Se alone and together, on the growth, Cd content, and biochemical parameters of Cd-treated rice plants. The results revealed that both Si and Se can effectively alleviate Cd toxicity, and a strong synergistic effect of Si and Se was observed. Simultaneous use of Si and Se significantly promoted rice plant growth, decreased malondialdehyde (MDA) content in both the roots and shoots, and reduced Cd translocation factor leading to a significant 73.2 % decrease in shoot Cd content. Additionally, Si-Se interaction increased glutathione (GSH) content, phytochelatin (PC) content and Cd distribution in root cell walls and organelles. Furthermore, the relative expression of OsHMA2 was down-regulated, while those of OsNramp1 and OsMHA3 were up-regulated. The above findings suggest that synergistic effect of Si and Se on Cd toxicity amelioration occurs mainly via regulating gene expression, sequestering Cd in the root cell walls and organelles, and reducing Cd transfer to the shoots.

Methyl jasmonate mitigates high selenium damage of rice via altering antioxidant capacity, selenium transportation and gene expression.

Beneficial effects of methyl jasmonate (MeJA) on plants under different abiotic conditions have long been demonstrated. This study aimed to figure out how exogenous MeJA mitigated high-Se toxicity in rice from plant physiology and gene express perspective to provide the theory and technique for safe production of Se-rich rice. The results showed that low concentrations of MeJA at 0.1-1.0 µM inhibited high-Se induced nonreversible toxicity by enhancing antioxidant-system and reducing H2O2 and MDA content in rice seedlings. In comparison with control, addition of low concentrations of MeJA at 0.1-1.0 µM reduced the Se content in roots by 13.6-48.8% and in shoots by 52.6-59.9%. Besides, lower concentrations of MeJA decreased the Se(IV) transformation to SeCys and SeMet. The qRT-PCR analysis showed that application of low concentration of MeJA down-regulated the gene expression of OsNIP2;1, and OsPT2 in roots and OsNIP2;1, OsPT2, OsSBP1, and OsCS in shoots, which inhibited Se absorption. However, high concentrations of MeJA at 2.5-5.0 µM decreased antioxidant capacity and increased H2O2 and MDA content in rice seedlings. The results suggested that MeJA at 0.1-1.0 µM can be used to mitigate high-Se toxicity in rice production. This research augments the knowledge for future utilization of MeJA in down-regulating Se levels in crops.

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

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

Dynamics of Selenium uptake, speciation, and antioxidant response in rice at different panicle initiation stages.

Selenium (Se) is an essential element in animals and humans, and its deficiency may cause conditions such as cardiac disease. The production of Se-enriched rice is one of the most important ways to supply Se in the human body, and thus, understanding of the mechanisms of Se-enriched rice is of great significance. A pot experiment was conducted to study the effects of Se addition on the growth, antioxidation, Se uptake and distribution, and Se speciation in three different stages of panicle initiation stage (i.e., pistil and stamen formation stage, pollen mother cell formation stage, pollen mother cell meiosis stage) and the maturity stage. The results showed that soil Se application significantly increased Se uptake in rice. Low rates of Se (<5 mg kg-1) application enhanced the plant growth and rice yield. Se speciation assays showed that SeCys and SeMet were the two main forms found in rice, of which SeMet accounted for 65.5%-100% in the ears and leaves, while SeCys accounted for 61.4%-75.6% in brown rice. SeMet was also the main Se-species found in different subcellular parts at the panicle initiation stage. However, inorganic Se was present in brown rice, mainly as Se(VI), when the soil Se addition exceeded 5 mg kg-1. Lower rates of Se (<5 mg kg-1) promoted the antioxidant capacity, while high levels of Se (≥5 mg kg-1) reduced the antioxidant capacity of rice. The results indicate that Se effects are dose dependent, and the suitable amount of soil Se application for Se-enriched rice production would be <5 mg kg-1.