Transport and redistribution of selenium in the bean plant (Phaseolus vulgaris)

After incubation for 3 h with (⁷⁵Se) selenate, the selenium distribution in the bean plant (Phaseolus vulgaris L. cv. Contender) through a 29-day period showed an uneven distribution: roots and trifoliate leaves were richer in ⁷⁵Se than stem and primary leaves. The high selenium concentration of roots resulted from the retention of selenate by the root cells: at the end of the 29-day period about 60° of the radioactivity was always ethanol-soluble, and when analysed by paper chromatography, proved to be selenate. By contrast, much of the radioactivity of the leaves was ethanol-insoluble, ⁷⁵Se being quickly captured in metabolic processes which immobilize it. During plant development, a portion of the total selenium remains mobile and is continually mobilized to the younger organs which display a rapid growth rate. This delivery results from a progressive liberation of selenate retained by mature organs, especially the roots, and from turnover in older leaf tissues, especially the trifoliate leaves.     

Translocation of selenium in the bean plant (Phaseolus vulgaris) and the field bean (Vicia faba)

Selenium distribution in the bean plant ( Phaseolus vulgaris L. cv. Contender) was studied using autoradiographs of the whole plant and of sections of organs. A few hours after the incubation of the roots with (⁷⁵Se) selenate, a major part of the selenate accumulates in the roots, while the fraction conveyed towards the aerial organs is unevenly distributed, resulting in accumulation of ⁷⁵Se in the young leaves, the buds and the epicotyl. This distribution results from a general translocation of selenium through the xylem. A secondary process of redistribution is then immediately linked to the transport of ⁷⁵Se labeled products (such as seleno-amino acids) in the phloem from the mature leaves. A similar pattern of translocation of selenium was found in the field bean ( Vicia faba L. cv. Aguadulce) by using aphids that insert their stylets into the sieve tubes. Measurement of the radioactivity of these insects shows that the ⁷⁵Se content of the phloem sap was reduced to low levels when all the mature leaves were excised. The mature leaves thus serve as relaying organs, redistributing the selenium which is carried in by the movement of water through the xylem.     

Selenium Absorption by Excised Astragalus Roots

Absorption of selenate and selenite by excised roots of Astragalus Crotalariae, a selenium accumulator, and of A. lentiginosus, a non-accumulator, was favored by CaCl(2) and a pH of 4.0. The uptake of selenate and possibly selenite, is metabolically linked. Roots of a number of Astragalus species were examined, and in all cases selenate entered the roots much faster than selenite. In these short-term experiments there was no relation between uptake of the 2 ions and classification of a species as selenium-accumulator or non-accumulator.     

Uptake of benzyladenine by excised watermelon cotyledons

The uptake of 8-[¹⁴C]N⁶-benzyladenine (BA) was studied in excised watermelon (Citrullus vulgaris Schrad.) cotyledons 24 hours after the start of imbibition. The passive nature of this uptake is suggested by the following evidence: (a) no sign of saturation on increasing external concentration of BA; (b) no decrease in uptake under conditions that inhibit ATP synthesis; (c) no change in amount of radioactivity absorbed when cotyledons are frozen and thawed before the uptake test. About two-thirds of the radioactivity taken up is released after 12 hours of washing. If the washing is performed at 2 C very little radioactivity is released.     

Selenate and Selenite Uptake and Translocation in Bean Plants (Phaseolus vulgaris)

After 3 h, selenate uptake by roots of Phaseolus vulgaris L.cv. Contender resulted in more than 50% of the Se absorbed beingconveyed to the aerial organs. This distribution was sensitiveto respiratory inhibitors and when roots were soaked in a solutionsupplied with hydroxylamine, the level of Se decreased by about80% in the whole plant, suggesting that selenate uptake requiresenergy. Addition of glucose to the nutrient medium resultedin slightly decreased uptake and distribution. Under the same growth conditions and 3 h incubation with selenite,a major part of the Se had accumulated in the roots, while asmall fraction was conveyed towards the aerial organs. Thispercentage was decreased by about 20% when plants were transferredto a solution supplied with hydroxylamine, suggesting that partof the selenite entered the roots passively. Addition of glucoseto the nutrient solution, resulted in enhanced levels of Sein the whole plant. Application of plant growth substances affected Se transport.When roots were incubated in abscisic acid (ABA), selenate uptakewas affected, while foliar spraying of gibberellin A₃ (GA₃)enhanced selenite uptake and translocation. Key words: Phaseolus vulgaris, selenate, transport, selenite, glucose, harmones     

Selenium uptake, translocation and speciation in wheat supplied with selenate or selenite

Selenite can be a dominant form of selenium (Se) in aerobic soils; however, unlike selenate, the mechanism of selenite uptake by plants remains unclear. Uptake, translocation and Se speciation in wheat (Triticum aestivum) supplied with selenate or selenite, or both, were investigated in hydroponic experiments. The kinetics of selenite influx was determined in short-term (30 min) experiments. Selenium speciation in the water-extractable fraction of roots and shoots was determined by HPLC-ICPMS. Plants absorbed similar amounts of Se within 1 d when supplied with selenite or selenate. Selenate and selenite uptake were enhanced in sulphur-starved and phosphorus-starved plants, respectively. Phosphate markedly increased K(m) of the selenite influx. Selenate and selenite uptake were both metabolically dependent. Selenite was rapidly converted to organic forms in roots, with limited translocation to shoots. Selenomethionine, selenomethionine Se-oxide, Se-methyl-selenocysteine and several other unidentified Se species were detected in the root extracts and xylem sap from selenite-treated plants. Selenate was highly mobile in xylem transport, but little was assimilated to organic forms in 1 d. The presence of selenite decreased selenate uptake and xylem transport. Selenite uptake is an active process likely mediated, at least partly, by phosphate transporters. Selenite and selenate differ greatly in the ease of assimilation and xylem transport.     

On the mechanism of nitrate uptake by plant roots

Summary A study has been made on the influx and outflux of nitrogen compounds by the excised roots of barley, wheat and peas. A two way movement of nitrogen compounds was found to occur between root and external medium. Factors such as initial N content in the roots, species of plant and external concentration of N highly affect the extent to which this two-way movement proceeds. Further investigations are needed for more understanding of the nitrogen balance between plant roots and external medium.     

Variation in selenium tolerance and accumulation among 19 Arabidopsis thaliana accessions

Selenium (Se) is an essential element for many organisms but also toxic at higher levels. The objective of this study was to identify accessions from the model species Arabidopsis thaliana that differ in Se tolerance and accumulation. Nineteen Arabidopsis accessions were grown from seed on agar medium with or without selenate (50 microM) or selenite (20 microM), followed by analysis of Se tolerance and accumulation. Tissue sulfur levels were also compared. The Se Tolerance Index (root length+Se/root length control) varied among the accessions from 0.11 to 0.44 for selenite and from 0.05 to 0.24 for selenate. When treated with selenite, the accessions differed by two-fold in shoot Se concentration (up to 250 mgkg(-1)) and three-fold in root Se concentration (up to 1000 mgkg(-1)). Selenium accumulation from selenate varied 1.7-fold in shoot (up to 1000 mgkg(-1)) and two-fold in root (up to 650 mgkg(-1)). Across all accessions, a strong correlation was observed between Se and S concentration in both shoot and root under selenate treatment, and in roots of selenite-treated plants. Shoot Se accumulation from selenate and selenite were also correlated. There was no correlation between Se tolerance and accumulation, either for selenate or selenite. The F(1) offspring from a cross between the extreme selenate-sensitive Dijon G and the extreme selenate-tolerant Estland accessions showed intermediate selenate tolerance. In contrast, the F(1) offspring from a cross between selenite-sensitive and -tolerant accessions (Dijon GxCol-PRL) were selenite tolerant. The results from this study give new insight into the mechanisms of plant selenium (Se) tolerance and accumulation, which may help develop better plants for selenium phytoremediation or as fortified foods.     

X-ray absorption spectroscopy study shows that the rapid selenium volatilizer, pickleweed (Salicornia bigelovii Torr.), reduces selenate to organic forms without the aid of microbes

In many plant species, selenium (Se) volatilization is limited by the reduction of selenate and its chemical conversion to organic Se compounds, a process that may be facilitated by rhizosphere microbes. This study was conducted to determine if pickleweed (Salicornia bigelovii Torr.), which is characterized by having high rates of Se volatilization from selenate, is able to reduce selenate into organic forms of Se axenically, or whether it requires the presence of microbes. X-ray absorption spectroscopy analysis showed that shoots and roots of pickleweed plants supplied with 50 microM selenate accumulated Se predominantly in organic Se forms (about 65-75% of the total accumulated Se), regardless of whether the plants were grown axenically or in the presence of microbes. The results suggest that, unlike other species for which selenate reduction appears to be rate limiting. e.g. Indian mustard (Brassica juncea L.) and broccoli (Brassica oleracea L.), pickleweed is unusual in that it has an enhanced capacity to reduce selenate to organic forms that is independent of the presence of rhizosphere microbes.     

Selenium Transport in Root Systems of Tomato

Selenate and selenite transport through tomato root systemswere followed for periods up to 4 h after removal of the planttops, using ⁷⁵Se as a tracer. With selenate, ⁷⁵Se concentrations in the xylem exudate were6 to 13 times higher than in the external solution, and chromatographicanalysis showed that the selenium was transported as inorganicselenate ( ). With selenite, ⁷⁵Se concentrations in the exudate were alwayslower than in the external solution. Analyses of exudate samplesshowed that negligible amounts of selenium were transportedas inorganic selenite ( except at very high external selenite concentrations (500 ?M), when up to 7 percent was transported as selenite. Most of the selenium transportin selenite-fed plants was as selenate or as an unknown seleniumcompound, the relative proportions of these two forms varyingboth with time and with external selenite concentration. Additionof a 5-fold excess of sulphate over selenite had no detectableeffect on the concentrations of selenate in the exudate, butcaused substantial decreases in the maximum concentrations ofboth total selenium (c. 47 per cent decrease) and the unknownselenium compound (c. 69 per cent decrease). Addition of a 5-foldexcess of sulphite decreased the concentration of the unknown(c. 39 per cent) but caused a large (2.7-fold) increase in themaximum total selenium concentration in the exudate and a 7.9-foldincrease in the maximum concentration of selenate. The resultssuggest metabolic involvement in the uptake and long distancetransport of solenium supplied as selenite, despite lower ⁷⁵Seconcentrations in the xylem exudate than in the external solution.An attempt is made to incorporate the new and existing informationinto a selenium transport model.     

Application of X-ray absorption near edge spectroscopy to the study of the effect of sulphur on selenium uptake and assimilation in wheat seedlings

Selenium (Se) is an essential trace element for humans and animals. A hydroponic experiment was performed to study the effects of sulphur (S) on Se uptake, translocation, and assimilation in wheat (Triticum aestivum L.) seedlings. Sulphur starvation had a positive effect on selenate uptake and the form of Se supplied greatly influenced Se speciation in plants. Compared with the control plants, Se uptake by the S-starved plants was enhanced by 4.81-fold in the selenate treatment, and selenate was readily transported from roots to shoots. By contrast, S starvation had no significant effect on selenite uptake, and selenite taken up by roots was rapidly converted to organic forms and tended to accumulate in roots. X-ray absorption near edge spectroscopy (XANES) analysis showed that organic forms of selenium, including selenocystine, Se-methyl-selenocysteine (MeSeCys), and selenomethionine-Se-oxide, were dominant in the plants exposed to selenite and accounted for approximately 90 % of the total Se. Whereas selenate remained as the dominant species in the roots and shoots exposed to selenate, with little selenate converted to selenite and MeSeCys. Besides, sulphur starvation increased the proportion of inorganic Se species in the selenate-supplied plants, but had no significant effects on Se speciation in plants exposed to selenite. The present study provides important knowledge to understand the associated mechanism of Se uptake and metabolism in plants.     

Selenium assimilation and volatilization from dimethylselenoniopropionate by Indian mustard

Earlier work from our laboratory on Indian mustard (Brassica juncea L.) identified the following rate-limiting steps for the assimilation and volatilization of selenate to dimethyl selenide (DMSe): (a) uptake of selenate, (b) activation of selenate by ATP sulfurylase, and (b) conversion of selenomethionine (SeMet) to DMSe. The present study showed that shoots of selenate-treated plants accumulated very low concentrations of dimethylselenoniopropionate (DMSeP). Selenonium compounds such as DMSeP are the most likely precursors of DMSe. DMSeP-supplied plants volatilized Se at a rate 113 times higher than that measured from plants supplied with selenate, 38 times higher than from selenite, and six times higher than from SeMet. The conversion of SeMet to selenonium compounds such as DMSeP is likely to be rate-limiting for DMSe production, but not the formation of DMSe from DMSeP because DMSeP was the rate of Se volatilization from faster than from SeMet and SeMet (but no DMSeP) accumulated in selenite- or SeMet-supplied wild-type plants and in selenate-supplied ATP-sulfurylase transgenic plants. DMSeP-supplied plants absorbed the most Se from the external medium compared with plants supplied with SeMet, selenate, or selenite; they also accumulated more Se in shoots than in roots as an unknown organic compound resembling a mixture of DMSeP and selenocysteine.     

Cation uptake from bentonite suspensions by excised barley roots

Summary The net uptake of Na, K, or Ca by excised barley roots as influenced by the amount and kind of other cation was studied from bi-ionic and tri-ionic bentonite suspensions. The net uptake of Na or K from the Na-Ca or K-Ca system followed the concentration of soluble Na or K in the system. Calcium in both systems was taken up by the excised roots only at the 80 and 100 per cent Ca levels. At lower levels of Ca, the roots lost some of their initial calcium contents to the suspensions. The net uptake of Na or K from the Na-K system was in agreement with the concentrations of soluble Na or K in the system. At the various levels of Na or K, some of the initial calcium of the roots was lost to the suspensions. In the Na-K-Ca system, Na uptake gradually decreased with the increase of Ca level. Calcium was not absorbed by the roots at the various Ca levels. However, calcium greatly enhanced K uptake from this system. Part of a dissertation submitted by the senior author in partial fulfillment of requirements for the Ph. D. degree.     

Tracking Se Assimilation and Speciation through the Rice Plant – Nutrient Competition,Toxicity and Distribution

Up to 1 billion people are affected by low intakes of the essential nutrient selenium (Se) due to low concentrations in crops. Biofortification of this micronutrient in plants is an attractive way of increasing dietary Se levels. We investigated a promising method of Se biofortification of rice seedlings, as rice is the primary staple for 3 billion people, but naturally contains low Se concentrations. We studied hydroponic Se uptake for 0–2500 ppb Se, potential phyto-toxicological effects of Se and the speciation of Se along the shoots and roots as a function of added Se species, concentrations and other nutrients supplied. We found that rice germinating directly in a Se environment increased plant-Se by factor 2–16, but that nutrient supplementation is required to prevent phyto-toxicity. XANES data showed that selenite uptake mainly resulted in the accumulation of organic Se in roots, but that selenate uptake resulted in accumulation of selenate in the higher part of the shoot, which is an essential requirement for Se to be transported to the grain. The amount of organic Se in the plant was positively correlated with applied Se concentration. Our results indicate that biofortification of seedlings with selenate is a successful method to increase Se levels in rice.     

Reactive oxygen species and nitric oxide affect cell wall metabolism in tobacco BY-2 cells

The effects of hydrogen peroxide (H2O2), nitric oxide (NO), and a combination of both on the metabolism of cell wall polysaccharides were studied in tobacco (Nicotiana tabacum L.) cv Bright Yellow 2 (BY-2) suspension cultured cells in the presence of D-[U-14C]glucose or D-[U-14C]galactose as radioactive tracers. We found that the radiolabelling of newly synthesised total cell wall polysaccharides (pectins, hemicelluloses and alpha-cellulose), buffer-soluble polysaccharides, and membrane-associated polysaccharides decreased under the influence of exogenous systems generating H2O2 and NO. However, when the total amount of newly synthesised cell wall polysaccharides was calculated as a percentage of the total cellular radioactivity (ethanol-soluble pool plus the homogenate of ethanol-insoluble material), all treatments showed negligible effects in the presence of D-[U-14C]glucose or D-[U-14C]galactose as tracers. This occurred because the treatments generating H2O2, NO and H2O2 plus NO caused a marked decrease in the concentration of the ethanol-soluble pool as well as in the total radioactivity found in the homogenate of the ethanol-insoluble material. Most of the radioactivity taken up by the cells was evolved as 14CO2 during the respiratory processes. A qualitative and quantitative characterisation of the ethanol-soluble pool showed that radioactive UDP-sugars in BY-2 suspension cultured cells were differentially reduced by all treatments. Therefore, the decrease of the newly synthesised cell wall polysaccharides seems to be strictly dependent on the reduction of the UDP-sugars pool.     

The absorption of potassium and several organic compounds by barley roots: Effect of siduron

Absorption of ⁴²K by excised roots of barley (Hordeum vulgareL.) grown in 0 or 5 ppm siduron (l-(2-methylcyclohexyl)-3-phenylurea)was a linear function of time for at least 60 minutes with transportbeing unidirectional. Absorption of siduron was a function ofthe external concentration to the limits of its solubility (0.09mM). However, the siduron- ¹⁴C absorbed by roots grown in either0 or 5 ppm siduron was in a readily exchangeable form and desorptionfor 4 hr exchanged 80 % of the label. Glucose-¹⁴C, adenine-8¹⁴Cand leucine¹⁴C were actively absorbed with 70 to 85 % of thelabel being absorbed in 24 hr. Although roots grown in iduronabsorbed less ⁴²K, glucose-¹⁴C, adenine-¹⁴C and siduron-¹⁴C,and more leucine-¹⁴C than similar roots grown in water culture,it is probable that these differences were not large enoughto account for the noted reduction (60%) in root growth. (Received January 9, 1968; )     

Effect of byproduct,nitrogen fertilizer,and zeolite on phosphate rock dissolution and extractable phosphorus in acid soil

The relative plant availability of selenate versus selenite depends on the concentrations of competing ions, specifically sulfate and phosphate, respectively. In solution culture, the concentration of phosphate is typically 100- to 1000-fold greater than in soil solution, an artifact that could lead to underestimation of the phytoavailability of selenite. A nutrient solution study was conducted to compare the availability of selenite and selenate to perennial ryegrass (Lolium perenne L. cv. Evening Shade) and strawberry clover (Trifolium fragiferrum L. cv. O'Conner) at basal concentrations of SO₄ (0.5 mM) and PO₄ (2 μM) similar to those found in soil solution. Concentrations up to 5 mM SO₄ and 200 μM PO₄ allowed quantitative comparison of the efficacy of the competing ions. In both species, selenite was more phytotoxic than selenate, especially for shoot growth. Selenate was less toxic, and tended to preferentially inhibit root growth. Translocation percentages were much higher with selenate (≥84%) than with selenite (≤47%). A 10-fold increase in sulfate decreased uptake from selenate by >90% in both species. In ryegrass, 10-fold increases in phosphate caused 30% to 50% decreases in Se accumulation from selenite, but in clover such decreases only occurred in the roots. Sulfate-selenate antagonisms were thus stronger than phosphate-selenite antagonisms. Nonetheless, conventional nutrient solutions with millimolar phosphate will significantly underestimate Se availability from selenite, and moderate levels of sulfate salinity can inhibit selenate uptake sufficiently to reverse the apparent relative availability of the two forms of Se. This revised version was published online in June 2006 with corrections to the Cover Date.     

Uptake and transport of selenite and selenate by soybean seedlings of two genotypes

In an attempt to address the role of biological behavior on Se uptake by soybean crop and the genotype effects, experiments with time and concentration sequences of Se uptake by seedlings in Hoagland solution are conducted using selenite and selenate respectively. Two soybean cultivars Tong-ai 405 (TA) and Qidong Green-skin (QG) are used as different genotypes. In presence of selenite, Se uptake by both roots and shoots exhibited a linear increase with the growing time at 5 M and with the solution Se concentrations. However, in presence of selenate, the linear response to growing time is only valid before 24 h of growing. While root Se uptake is much slower under selenate than under selenite in the time sequence experiment, shoot Se levels are similar between the two different Se form treatments. Nevertheless, in the experiment of concentration sequence, either root Se or shoot Se responses linearly to solution Se concentration regardless of the Se forms supplied. A big discrepancy of root Se level with a similarity of shoot Se between the two cultivars is observed in the concentration sequence experiment. This supports a much faster passive uptake of selenite but more or less an active uptake of selenate by soybean seedlings. Comparatively, cultivars TA have a consistently higher Se concentration than QG both in roots and shoots under selenate, while no difference of concentration ratio of shoot to root is recognized between them. The higher Se level in seed grains, therefore, may be accounted for not by Se transport form root to shoot but by greater ability of Se uptake and retention under selenate by the former cultivar. Therefore, not only forms of Se supply but also genotype difference affects the Se bioavailability by different soybean cultivars. This should be taken into account for screening the high Se-efficiency plants or cultivars to improve the Se supply of the food chain.     

Distinct uptake of tellurate from selenate in a selenium accumulator, Indian mustard (Brassica juncea)

Tellurium (Te) is widely used in industry because of its unique chemical and physical properties, and has recently become a part of everyday life as a component of phase-change optical magnetic disks. However, the recovery of Te from the environment has not been discussed yet. In this regard, we evaluated the potential use of Indian mustard (Brassica juncea), a selenium (Se) accumulator, for the phytoremediation of Te. The Indian mustard plant was exposed to selenate and tellurate and the concentrations of Se and Te and the chemical species in the plant were determined. The Indian mustard plant accumulated less Te than Se, and the amount of Te accumulated in the plant was approximately 1/69 of that of Se. Although the incorporation of selenate was reduced by increasing sulfate concentration in the medium, the incorporation of Te was not affected by it, suggesting that this plant was able to discriminate tellurate from selenate in the roots. Three Te species were detected in the plant. The major species was tellurate. The other two species were not identical to available Te standards and thus could not be identified. Consequently, the Indian mustard plant is inappropriate for the phytoremediation of Te because it can strictly distinguish tellurate from selenate.     

Relationship of ion absorption to growth rate in taiga trees

Summary Rates of nutrient absorption were measured on excised roots of taiga tree seedlings grown in the laboratory. Phosphate and to a lesser extent ammonium (relatively immobile ions in the soil) were absorbed most rapidly by poplar and aspen, two species with rapid growth rates and most slowly by alder and/or black spruce, species with slow growth rates. In contrast, potassium (which is more mobile in soil) was absorbed most rapidly by slowly growing species. All species had low rates of nitrate and chloride absorption. Absorption rate of each ion was most temperature sensitive in those species that typically occupy the warmest soils (i.e. poplar and aspen). We suggest that in infertile soils a high capacity for uptake is an important component of root competition only in the case of mobile ions (e.g. potassium, nitrate), because only for these ions do diffusion shells of adjacent roots overlap; in contrast plants compete for immobile ions (e.g. phosphate) only by increasing absorptive surface via root growth or mycorrhizal association.