Juris Meija

Overexpression of Selenocysteine Methyltransferase in Arabidopsis and Indian Mustard Increases Selenium Tolerance and Accumulation

A major goal of phytoremediation is to transform fast-growing plants with genes from plant species that hyperaccumulate toxic trace elements. We overexpressed the gene encoding selenocysteine methyltransferase (SMT) from the selenium (Se) hyperaccumulator Astragalus bisulcatus in Arabidopsis and Indian mustard (Brassica juncea). SMT detoxifies selenocysteine by methylating it to methylselenocysteine, a nonprotein amino acid, thereby diminishing the toxic misincorporation of Se into protein. Our Indian mustard transgenic plants accumulated more Se in the form of methylselenocysteine than the wild type. SMT transgenic seedlings tolerated Se, particularly selenite, significantly better than the wild type, producing 3- to 7-fold greater biomass and 3-fold longer root lengths. Moreover, SMT plants had significantly increased Se accumulation and volatilization. This is the first study, to our knowledge, in which a fast-growing plant was genetically engineered to overexpress a gene from a hyperaccumulator in order to increase phytoremediation potential.

Polybrominated diphenyl ethers: Causes for concern and knowledge gaps regarding environmental distribution, fate and toxicity

This manuscript critically considers several areas of study of the polybrominated diphenyl ether compounds. Specifically, a brief review of PBDE toxicity is followed by an in depth discussion of PBDE occurrence in abiotic and biotic environmental matrices. Temporal and geographic trends are examined in conjunction with risk assessment factors. Rather than summarize or tabulate the growing body of literature on PBDEs in the environment, the overall goal of this review paper is to highlight broad patterns that may contribute to a more holistic understanding of PBDE behavior in the environment, as well as to identify critical areas of research that warrant further attention.

Atomic weights of the elements 2013 (IUPAC Technical Report)

Abstract The biennial review of atomic-weight determinations and other cognate data has resulted in changes for the standard atomic weights of 19 elements. The standard atomic weights of four elements have been revised based on recent determinations of isotopic abundances in natural terrestrial materials: cadmium to 112.414(4) from 112.411(8), molybdenum to 95.95(1) from 95.96(2), selenium to 78.971(8) from 78.96(3), and thorium to 232.0377(4) from 232.038 06(2). The Commission on Isotopic Abundances and Atomic Weights (ciaaw.org) also revised the standard atomic weights of fifteen elements based on the 2012 Atomic Mass Evaluation: aluminium (aluminum) to 26.981 5385(7) from 26.981 5386(8), arsenic to 74.921 595(6) from 74.921 60(2), beryllium to 9.012 1831(5) from 9.012 182(3), caesium (cesium) to 132.905 451 96(6) from 132.905 4519(2), cobalt to 58.933 194(4) from 58.933 195(5), fluorine to 18.998 403 163(6) from 18.998 4032(5), gold to 196.966 569(5) from 196.966 569(4), holmium to 164.930 33(2) from 164.930 32(2), manganese to 54.938 044(3) from 54.938 045(5), niobium to 92.906 37(2) from 92.906 38(2), phosphorus to 30.973 761 998(5) from 30.973 762(2), praseodymium to 140.907 66(2) from 140.907 65(2), scandium to 44.955 908(5) from 44.955 912(6), thulium to 168.934 22(2) from 168.934 21(2), and yttrium to 88.905 84(2) from 88.905 85(2). The Commission also recommends the standard value for the natural terrestrial uranium isotope ratio, N(238U)/N(235U)=137.8(1).

Paradigms in isotope dilution mass spectrometry for elemental speciation analysis

Isotope dilution mass spectrometry currently stands out as the method providing results with unchallenged precision and accuracy in elemental speciation. However, recent history of isotope dilution mass spectrometry has shown that the extent to which this primary ratio measurement method can deliver accurate results is still subject of active research. In this review, we will summarize the fundamental prerequisites behind isotope dilution mass spectrometry and discuss their practical limits of validity and effects on the accuracy of the obtained results. This review is not to be viewed as a critique of isotope dilution; rather its purpose is to highlight the lesser studied aspects that will ensure and elevate current supremacy of the results obtained from this method.

Localization and speciation of selenium and mercury in Brassica juncea—implications for Se–Hg antagonism

The occurrence and form of selenium and mercury were investigated in Indian Mustard, Brassica juncea, a selenium accumulating plant, which had been co-exposed to varying concentration levels of these two elements. Plants were grown and exposed in hydroponic solutions. Following exposure, root exudates were collected in fresh solutions and the head-space around the aerial portions of the plants was sampled. These samples and the harvested plant tissues were then processed for determination of Se and Hg-containing compounds. For the plant tissues, roots, stems and leaves were separated and extracted using a sequential procedure that removed water-soluble species, water-soluble proteins, and dodecyl sulfate-soluble proteins. Size exclusion chromatography allowed further fractionation. High molecular-weight selenium/mercury-containing compounds were found primarily in the plant root extract. Evidence suggests that a Se and Hg complex of high molecular weight may be protein associated. For the analysis of exudate solutions, ion-pairing reversed phase chromatography coupled to ICP-MS was used. Multiple selenium and mercury species were detected, with one mercury-containing compound observed eluting near selenocystine. Plant head-space was sampled with solid phase microextraction and analyzed with GC-ICP-MS and GC-TOFMS. Apart from the primary selenium volatiles and elemental mercury, no volatile species simultaneously containing Se and Hg could be detected.

Selenium in plants by mass spectrometric techniques: developments in bio-analytical methods

Various plants can accumulate Se up to the thousands of ppm. These are called accumulators and they have potential to remediate areas contaminated with this metalloid. Some of them, like Arabidopsis thaliana or Brassica juncea (Indian mustard) have been investigated in terms of the Se metabolic pathway. To date few studies have been done for selenium speciation with most studies reporting total selenium concentration in various parts of the plant. This present report summarizes some of the studies carried out in terms of: extraction of Se species, cleaning procedures, separation methodologies and mass spectrometric techniques employed. The use of inductively coupled plasma mass spectrometry (ICP-MS) in conjunction with liquid or gas chromatographic techniques (HPLC and GC) as separation techniques provide an attractive methodology for determining Se species in plants. Some of the species produced by the plant, such as Se-methylselenocysteine or Se-methylselenomethionine can be identified at ppb levels by RP-HPLC-ICP-MS, while others needed to be further characterized by ES-MS. The coupling of GC-ICP-MS using solid phase microextraction (SPME) as sample preparation system is also evaluated for the determination of the Se/S volatile species in the head-space of Brassica juncea seedlings. Detection limits on the ppt level (7–300 ppt depending on the species) and adequate precision for Se and S species suggest GC-ICP-MS as an excellent methodology for determination of volatiles with minimum sample preparation.

Use of optional gas and collision cell for enhanced sensitivity of the organophosphorus pesticides by GC-ICP-MS

Organophosphorus pesticides (OP) are used widely in agricultural and residential applications as insecticides, herbicides and fungicides. This family of chemicals is one that replaced the organochlorine pesticides banned for use in the United States in the 1970s. In this work, inductively coupled plasma mass spectrometry (ICP-MS) is explored as an option for the detection of OPs, monitoring the phosphorus present in the compounds previously separated by GC. Historically, phosphorus has been recognized as one of the elements that is difficult to analyze in an argon plasma. This is due to its relatively high ionization potential (10.5 eV) as well as the inherent presence of the polyatomic interferences 14N16O1H+ and 15N16O+, overlapping its only isotope at m/z = 31. In this work, the use of a commercially available GC-ICP-MS interface, in conjunction with the addition of small amounts of optional gases, has been investigated to obtain enhanced sensitivity and it is found that nitrogen yields an increase in sensitivity of over one order of magnitude. Accompanying this increase in sensitivity is an observed increase in the background due to the above detailed interferences formed in the gas phase. A collision cell is employed to reduce this background without affecting analyte signal using He as collision gas. Instrument detection limits in the high ng L−1 range are reported and the developed methodology is applied to the analysis of tap water samples from the City of Cincinnati.

Calculations of double spike isotope dilution results revisited

The use of isotope pattern deconvolution analysis is proposed to simplify the double spiking species-specific isotope dilution result calculations when inter-conversion might occur. A detailed example is given for the determination of Cr(III)/Cr(VI) in yeast by isotope dilution-HPLC-ICP-MS. The results are in exact agreeent with the conventional isotope dilution calculations.

Mass Bias Fractionation Laws for Multi-Collector ICPMS: Assumptions and Their Experimental Verification

Although mass bias fractionation correction is among the most studied theoretical aspects of analytical mass spectrometry, several assumptions imbedded in the conventional correction models remain largely untested. Experimental evidence is given herein highlighting parts-per-thousand deviations from the conventional mass bias correction models which can occur with use of multicollector inductively coupled plasma mass spectrometry. Furthermore, current mass spectrometric approaches are not capable of elucidating the fractionation exponent.

HPLC-ICP-MS and ESI-Q-TOF analysis of biomolecules induced in Brassica juncea during arsenic accumulation

Arsenic (As) bioaccumulation by plants can be used as a strategy to detoxify arsenic polluted sites. Genetic engineering may provide a means of optimizing this natural process to increase its efficiency. However, this approach requires a thorough understanding of As metabolism and detoxification in plants. Identifying As-containing metabolites in plants is an important first step in elucidating As metabolism. Brassica juncea (Indian mustard) is studied here as a model for As accumulation in terms of total metalloid accumulation and its elemental speciation. A study on extraction conditions using 25 mM ammonium acetate buffer at increasing pH of 4.4, 5.6 and 7.8 has been performed. Those extracting solutions were also employed as mobile phases for the separation of the As species formed by size exclusion chromatography with inductively coupled plasma mass spectrometry (ICP-MS) as a selective As detector. Two main As containing species have been found in Brassica tissues (one of them at about 2 kDa and the other below 1.2 kDa). The first As species was found to be associated to thiol groups (monitoring 32S with double focusing ICP-MS). This can be ascribed to the presence of As-phytochelatin complexes. Electrospray-quadrupole-time of flight (ESI-Q-TOF) results indicated the presence of phytochelatins (apo-forms), the main metal bioligands in plants, which have also been shown to be induced by As. Oligomers of two, three and four sub-units, respectively (PC2, PC3 and PC4), with internal oxidation of the SH groups, have been extracted from Brassica leaves as well as a potential As–PC4 complex. These species have been further identified by collisional induced dissociation (CID).