A sensitive transgenic plant system to detect toxic inorganic compounds in the environment

A Janus membrane with electro-induced multi-affinity interfaces for high-efficiency water purification

Drinking water with micropollutants is a notable environmental concern worldwide. Membrane separation is one of the few methods capable of removing micropollutants from water. However, existing membranes face challenges in the simultaneous and efficient treatment of small-molecular and ionic contaminants because of their limited permselectivity. Here, we propose a high-efficiency water purification method using a low-pressure Janus membrane with electro-induced multi-affinity. By virtue of hydrophobic and electrostatic interactions between the functional interfaces and contaminants, the Janus membrane achieves simultaneous separation of diverse types of organics and heavy metals from water via single-pass filtration, with an approximately 100% removal efficiency, high water flux (>680 liters m-2 hour-1), and 98% lower energy consumption compared with commercial nanofiltration membranes. The electro-induced switching of interfacial affinity enables 100% regeneration of membrane performance; thus, our work paves a sustainable avenue for drinking water purification by regulating the interfacial affinity of membranes.

Bright Green-Emissive Carbon Nanodots for Sensing and Intracellular Imaging of Cu2+ and Glutathione

Green-emissive carbon nanodots (CDs) with high quantum yield are prepared. The abundant functional groups on the surfaces of CDs can selectively interact with Cu2+. The formed cupric amine complexes induce significant fluorescence quenching. The "on-off" switching can be further adjusted to the fluorescence "on" mode by the introduction of glutathione (GSH), which hinders the interactions between CDs and Cu2+. Based on the fantastic optical behavior of CDs, highly sensitive detection of Cu2+ and GSH can be achieved. Intracellular imaging of the two targets is also validated.

In situ and self-adaptive BOD bioreaction sensing system based on environmentally domesticated microbial populations

Biochemical oxygen demand (BOD) is a water quality parameter of vital importance. Rapid BOD analysis methods have emerged to simplify the five-day BOD (BOD₅) measurement protocol. However, their universal implementations are restricted by the tricky environmental matrix (including environmental microbes, contaminants, ionic compositions, etc.). Here, an in situ and self-adaptive BOD bioreaction sensing system consisting of a "gut-like" microfluidic coil bioreactor with self-renewed biofilm was proposed for the establishment of a rapid, resilient and reliable BOD determination method. With the spontaneous surface adhesion of environmental microbial populations, the biofilm was colonized in situ on the inner surface of the microfluidic coil bioreactor. Exploiting the environmental domestication during every real sample measurement, the biofilm was capable of self-renewal to adapt to the environmental changes and exhibited representative biodegradation behaviors. The aggregated abundant, adequate and adapted microbial populations in the BOD bioreactor rendered a total organic carbon (TOC) removal rate of 67.7% within a short hydraulic retention time of 99 s. As validated by an online BOD prototype, exceptional analytical performance was achieved in terms of reproducibility (relative standard deviation of 3.7%), survivability (inhibition by pH and metal ion interference of <20%) and accuracy (relative error of -5.9% to 9.7%). This work rediscovered the interactive effects of the environmental matrix on BOD assays and demonstrated an instructive attempt by making use of the environment to develop practical online BOD monitoring devices for water quality assessments.

Promising strategies of circular bioeconomy using heavy metal phytoremediated plants - A critical review

Phytoremediation is an excellent method for removing harmful heavy metals from the environment since it is eco-friendly, uses little energy, and is inexpensive. However, as phytoremediated plants can turn into secondary sources for heavy metals, complete heavy metal removal from phytoremediated plants is necessary. Elimination of toxic heavy metals from phytoremediated plants should be considered with foremost care. This review highlights about important sources of heavy metal contamination, health effects caused by heavy metal contamination and technological breakthroughs of phytoremediation. This review critically emphasis about promising strategies to be engaged for absolute reutilization of heavy metals and spectacular approaches of production of commercially imperative products from phytoremediated plants through circular bioeconomy with key barriers. Thus, phytoremediation combined with circular bioeconomy can create a new platform for the eco-friendly life.

Revealing the enhancing mechanisms of Fe-Cu bimetallic catalysts for the Fenton-like degradation of phenol

To develop a heterogeneous Fenton-like catalyst with desirable activity and reusability remains a great challenge for the practical degradation of environmental remediation. Herein, we demonstrate a dendritic Fe-Cu bimetallic catalyst consisted of a Cu/Fe₃O₄ shell and a FeCu core (E100). In comparisons of single Cu, Fe and Fe₃O₄, E100 performs far better performance for the Fenton-like degradation of phenol, and its dominant Fenton-like active centers are Fe species under acidic pH or Cu species under neutral pH. Particularly, Cu-based Fenton-like reactions are greatly accelerated by galvanic micro-cells effects that come from the special co-existence of Cu/Fe₃O₄ shell, and subsequently, owing to the Cu leaching from the shell, the inner FeCu core of E100 is able to be exposed and further strengthen Fe-based Fenton-like reactions. Overall, the appropriate synergistic effects endow E100 with superior catalytic activity and reusability than other catalysts. Our work pushes forward a step for understanding the catalytic mechanism of Fe-Cu bimetallic catalysts and provides new sights for fabricating efficient Fenton-like catalysts for environmental remediation.

Constitutively enhanced genome integrity maintenance and direct stress mitigation characterize transcriptome of extreme stress-adapted Arabidopsis halleri

Heavy metal-rich toxic soils and ordinary soils are both natural habitats of Arabidopsis halleri, a diploid perennial and obligate outcrosser in the sister clade of the genetic model plant Arabidopsis thaliana. The molecular divergence underlying survival in sharply contrasting environments is unknown. Here we comparatively address metal physiology and transcriptomes of A. halleri originating from the most highly heavy metal-contaminated soil in Europe, Ponte Nossa, Italy (Noss), and from non-metalliferous (NM) soils. Plants from Noss exhibit enhanced hypertolerance and attenuated accumulation of cadmium (Cd), and their transcriptomic Cd responsiveness is decreased, compared to plants of NM soil origin. Among the condition-independent transcriptome characteristics of Noss, the most highly overrepresented functional class of 'meiotic cell cycle' comprises 21 transcripts with elevated abundance in vegetative tissues, in particular Argonaute 9 (AGO9) and the synaptonemal complex transverse filament protein-encoding ZYP1a/b. Increased AGO9 transcript levels in Noss are accompanied by decreased long terminal repeat retrotransposon expression. Similar to Noss, plants from other highly metalliferous sites in Poland and Germany share elevated somatic AGO9 transcript levels in comparison to plants originating from NM soils in their respective geographic regions. Transcript levels of Iron-Regulated Transporter 1 (IRT1) are very low and transcript levels of Heavy Metal ATPase 2 (HMA2) are strongly elevated in Noss, which can account for its altered Cd handling. We conclude that in plants adapted to the most extreme abiotic stress, broadly enhanced functions comprise genes with likely roles in somatic genome integrity maintenance, accompanied by few alterations in stress-specific functional networks.

Lemna minor, a hyperaccumulator shows elevated levels of Cd accumulation and genomic template stability in binary application of Cd and Ni: a physiological and genetic approach

In this study, to determine whether having potential to be used as hyperaccumulator for Cd and Ni, numerous experiments were designed for conducting assessments for physiological and genotoxic changes along with defining possible alterations on mineral nutrient status of Lemna minor L. by applying Cd-Ni binary treatments (0, 100, 200 and 400 µM). Our study revealed that there were increases in the concentrations of B, Cr, Fe, K, Mg, and Mn whereas decreases were noticed in the concentrations of Na and Zn and the levels of Ca were inversely proportional to Cd-Ni applications showing tendency to increase at the low concentration and to decrease at the high concentration. Randomly Amplified Polymorphic DNA (RAPD) and Inter Simple Sequence Repeat (ISSR) analyses revealed that rather than band losses and new band formations, mostly intensity changes in the band profiles, and low polymorphism and high genomic template stability (GTS) were observed. Although, to date, L. minor was defined as an efficient hyperaccumulator/potential accumulator or competent phytoremedial agent by researchers. Our research revealed that L. minor showing high accumulation capability for Cd and having low polymorphism rate and high genomic template stability is a versatile hyperaccumulator, especially for Cd; therefore, highly recommended by us for decontamination of water polluted with Cd. NOVELTY STATEMENTMany studies have been focused on the effects of individual metal ions. However, heavy metal contaminants usually exist as their mixtures in natural aquatic environments. Especially, Cd and Ni coexist in industrial wastes.In this study, the accumulation properties of Lemna minor for both Cd and Ni were investigated and the effects of Cd and Ni on the bioaccumulation of B, Ca, Cu, Fe, Mg, K, Mn, Na, Pb and Zn in L. minor were also determined. This study furthermore aimed to assess the genotoxic effects of Cd and Ni found in being extended concentrations on DNA using the Randomly Amplified Polymorphic DNA-Polymerase Chain Reaction (RAPD-PCR) method.

Phosphoproteomic analysis reveals plant DNA damage signalling pathways with a functional role for histone H2AX phosphorylation in plant growth under genotoxic stress

DNA damage responses are crucial for plant growth under genotoxic stress. Accumulating evidence indicates that DNA damage responses differ between plant cell types. Here, quantitative shotgun phosphoproteomics provided high-throughput analysis of the DNA damage response network in callus cells. MS analysis revealed a wide network of highly dynamic changes in the phosphoprotein profile of genotoxin-treated cells, largely mediated by the ATAXIA TELANGIECTASIA MUTATED (ATM) protein kinase, representing candidate factors that modulate plant growth, development and DNA repair. A C-terminal dual serine target motif unique to H2AX in the plant lineage showed 171-fold phosphorylation that was absent in atm mutant lines. The physiological significance of post-translational DNA damage signalling to plant growth and survival was demonstrated using reverse genetics and complementation studies of h2ax mutants, establishing the functional role of ATM-mediated histone modification in plant growth under genotoxic stress. Our findings demonstrate the complexity and functional significance of post-translational DNA damage signalling responses in plants and establish the requirement of H2AX phosphorylation for plant survival under genotoxic stress.

Modulation of the Functional Components of Growth, Photosynthesis, and Anti-Oxidant Stress Markers in Cadmium Exposed Brassica juncea L

Abstract: Heavy metals (including Cadmium) are being entered into the environment through various sources and cause toxicity to plants. Response of Brassica juncea L. var. RLC-1 was evaluated after exposing them to different concentration of cadmium (Cd) for seven days. Seeds of B. juncea were treated with different concentrations of Cd like 0.2-0.6 mM for 7 days, allowing them to grow in Petri-dishes, and seedlings were examined for different physiological responses. Following exposure to Cd, in the seedlings of B. juncea, growth parameters (root and shoot length), stress markers (lipid peroxidation and H2O2 content), secondary metabolites, photosynthetic pigments, and ion analysis, were estimated along with enzymatic and non-enzymatic antioxidants. We observed a significant reduction in root and shoot length after Cd treatment as compared to control seedlings. Malondialdehyde and H2O2 contents were increased accompanied by enhanced Cd uptake. Activities of antioxidative enzymes were also significantly altered following Cd exposure to the seedlings of B. juncea. Conclusively, we suggest that Cd exposure to the seedlings triggered an induction of several defense responses in B. juncea including major metabolites.

Convergent evolution in Arabidopsis halleri and Arabidopsis arenosa on calamine metalliferous soils

It is a plausible hypothesis that parallel adaptation events to the same environmental challenge should result in genetic changes of similar or identical effects, depending on the underlying fitness landscapes. However, systematic testing of this is scarce. Here we examine this hypothesis in two closely related plant species, Arabidopsis halleri and Arabidopsis arenosa, which co-occur at two calamine metalliferous (M) sites harbouring toxic levels of the heavy metals zinc and cadmium. We conduct individual genome resequencing alongside soil elemental analysis for 64 plants from eight populations on M and non-metalliferous (NM) soils, and identify genomic footprints of selection and local adaptation. Selective sweep and environmental association analyses indicate a modest degree of gene as well as functional network convergence, whereby the proximal molecular factors mediating this convergence mostly differ between site pairs and species. Notably, we observe repeated selection on identical single nucleotide polymorphisms in several A. halleri genes at two independently colonized M sites. Our data suggest that species-specific metal handling and other biological features could explain a low degree of convergence between species. The parallel establishment of plant populations on calamine M soils involves convergent evolution, which will probably be more pervasive across sites purposely chosen for maximal similarity in soil composition. This article is part of the theme issue 'Convergent evolution in the genomics era: new insights and directions'.

Convergent evolution in Arabidopsis halleri and Arabidopsis arenosa on calamine metalliferous soils

It is a plausible hypothesis that parallel adaptation events to the same environmental challenge should result in genetic changes of similar or identical effects, depending on the underlying fitness landscapes. However, systematic testing of this is scarce. Here we examine this hypothesis in two closely related plant species, Arabidopsis halleri and Arabidopsis arenosa, which co-occur at two calamine metalliferous (M) sites harbouring toxic levels of the heavy metals zinc and cadmium. We conduct individual genome resequencing alongside soil elemental analysis for 64 plants from eight populations on M and non-metalliferous (NM) soils, and identify genomic footprints of selection and local adaptation. Selective sweep and environmental association analyses indicate a modest degree of gene as well as functional network convergence, whereby the proximal molecular factors mediating this convergence mostly differ between site pairs and species. Notably, we observe repeated selection on identical single nucleotide polymorphisms in several A. halleri genes at two independently colonized M sites. Our data suggest that species-specific metal handling and other biological features could explain a low degree of convergence between species. The parallel establishment of plant populations on calamine M soils involves convergent evolution, which will probably be more pervasive across sites purposely chosen for maximal similarity in soil composition. This article is part of the theme issue 'Convergent evolution in the genomics era: new insights and directions'.

Convergent evolution in Arabidopsis halleri and Arabidopsis arenosa on calamine metalliferous soils

It is a plausible hypothesis that parallel adaptation events to the same environmental challenge should result in genetic changes of similar or identical effects, depending on the underlying fitness landscapes. However, systematic testing of this is scarce. Here we examine this hypothesis in two closely related plant species, Arabidopsis halleri and Arabidopsis arenosa, which co-occur at two calamine metalliferous (M) sites harbouring toxic levels of the heavy metals zinc and cadmium. We conduct individual genome resequencing alongside soil elemental analysis for 64 plants from eight populations on M and non-metalliferous (NM) soils, and identify genomic footprints of selection and local adaptation. Selective sweep and environmental association analyses indicate a modest degree of gene as well as functional network convergence, whereby the proximal molecular factors mediating this convergence mostly differ between site pairs and species. Notably, we observe repeated selection on identical single nucleotide polymorphisms in several A. halleri genes at two independently colonized M sites. Our data suggest that species-specific metal handling and other biological features could explain a low degree of convergence between species. The parallel establishment of plant populations on calamine M soils involves convergent evolution, which will probably be more pervasive across sites purposely chosen for maximal similarity in soil composition. This article is part of the theme issue 'Convergent evolution in the genomics era: new insights and directions'.

The role of epicuticular waxes on foliar metal transfer and phytotoxicity in edible vegetables: case of Brassica oleracea species exposed to manufactured particles

The rapid industrialization and urbanization of intra- and peri-urban areas at the world scale are responsible for the degradation of the quality of edible crops, because of their contamination with airborne pollutants. Their consumption could lead to serious health risks. In this work, we aim to investigate the phytotoxicity induced by foliar transfer of atmospheric particles of industrial/urban origin. Leaves of cabbage plants (Brassica oleracea var. Prover) were contaminated with metal-rich particles (PbSO₄ CuO and CdO) of micrometer size. A trichloroacetic acid (TCA) treatment was used to inhibit the synthesis of the epicuticular waxes in order to investigate their protective role against metallic particles toxicity. Besides the location of the particles on/in the leaves by microscopic techniques, photosynthetic activity measurements, genotoxicity assessment, and quantification of the gene expression have been studied for several durations of exposure (5, 10, and 15 days). The results show that the depletion of epicuticular waxes has a limited effect on the particle penetration in the leaf tissues. The stomatal openings appear to be the main pathway of particles entry inside the leaf tissues, as demonstrated by the overexpression of the BolC.CHLI1 gene. The effects of particles on the photosynthetic activity are limited, considering only the photosynthetic F_v/F_m parameter. The genotoxic effects were significant for the contaminated TCA-treated plants, especially after 10 days of exposure. Still, the cabbage plants are able to implement repair mechanisms quickly, and to thwart the physiological effects induced by the particles. Finally, the foliar contamination by metallic particles induces no serious damage to DNA, as observed by monitoring the BolC.OGG1 gene.

From Bacteriophage to Plant Genetics

When first asked to write a review of my life as a scientist, I doubted anyone would be interested in reading it. In addition, I did not really want to compose my own memorial. However, after discussing the idea with other scientists who have written autobiographies, I realized that it might be fun to dig into my past and to reflect on what has been important for me, my life, my family, my friends and colleagues, and my career. My life and research has taken me from bacteriophage to Agrobacterium tumefaciens-mediated DNA transfer to plants to the plant genome and its environmentally induced changes. I went from being a naïve, young student to a postdoc and married mother of two to the leader of an ever-changing group of fantastic coworkers-a journey made rich by many interesting scientific milestones, fascinating exploration of all corners of the world, and marvelous friendships.

Genotoxicity of Zinc Oxide Nanoparticles in Plants Demonstrated Using Transgenic Arabidopsis thaliana

As important members of earth biosphere, higher plants are inevitably exposed to nanoparticles (NP) released into the environment. Therefore, determining NP-induced phytotoxicity is ecologically important. Currently, researches into genotoxic effects of NP on plants are limited. In this study, Arabidopsis thaliana lines transgenic for homologous recombination (HR) and transcriptional gene silencing (TGS) reporter genes were for the first time adopted to assess the genotoxicity of Zinc oxide NP (ZnO-NP). Results showed that the root exposure to ZnO-NP led to increased HR and alleviation of TGS in the aerial tissues, indicative of the genotoxicity of ZnO-NP in plants. The increased Zn content after root exposure to ZnO-NP and the similar induction of HR and TGS alleviation after root exposure to equivalent Zn ions suggested that the genotoxicity of ZnO-NP might be mainly induced by Zn ions in aerial tissues that were transported from decomposed ZnO-NP in either medium or plant roots.

Changes in the Proteome of Medicago sativa Leaves in Response to Long-Term Cadmium Exposure Using a Cell-Wall Targeted Approach

Accumulation of cadmium (Cd) shows a serious problem for the environment and poses a threat to plants. Plants employing various cellular and molecular mechanisms to limit Cd toxicity and alterations of the cell wall structure were observed upon Cd exposure. This study focuses on changes in the cell wall protein-enriched subproteome of alfalfa (Medicago sativa) leaves during long-term Cd exposure. Plants grew on Cd-contaminated soil (10 mg/kg dry weight (DW)) for an entire season. A targeted approach was used to sequentially extract cell wall protein-enriched fractions from the leaves and quantitative analyses were conducted with two-dimensional difference gel electrophoresis (2D DIGE) followed by protein identification with matrix-assisted laser desorption/ionization (MALDI) time-of-flight/time of flight (TOF/TOF) mass spectrometry. In 212 spots that showed a significant change in intensity upon Cd exposure a single protein was identified. Of these, 163 proteins are predicted to be secreted and involved in various physiological processes. Proteins of other subcellular localization were mainly chloroplastic and decreased in response to Cd, which confirms the Cd-induced disturbance of the photosynthesis. The observed changes indicate an active defence response against a Cd-induced oxidative burst and a restructuring of the cell wall, which is, however, different to what is observed in M. sativa stems and will be discussed.

Integrating Early Transcriptomic Responses to Rhizotoxins in Rice (Oryza sativa. L.) Reveals Key Regulators and a Potential Early Biomarker of Cadmium Toxicity

As sessile organisms, plants were constantly challenged with biotic and abiotic stresses. Transcriptional activation of stress-responsive genes is a crucial part of the plant adaptation to environmental changes. Here, early response of rice root to eight rhizotoxic stressors: arsenate, copper, cadmium, mercury, chromate, vanadate, ferulic acid and juglone, was analyzed using published microarray data. There were 539 general stress response (GSR) genes up-regulated under all eight treatments, including genes related to carbohydrate metabolism, phytohormone balance, and cell wall structure. Genes related to transcriptional coactivation showed higher K_a/K_s ratio compared to the other GSR genes. Network analysis discovered complicated interaction within GSR genes and the most connected signaling hubs were WRKY53, WRKY71, and MAPK5. Promoter analysis discovers enriched SCGCGCS cis-element in GSR genes. Moreover, GSR genes tend to be intronless and genes with shorter total intron length were induced in a higher level. Among genes uniquely up-regulated by a single stress, a phosphoenolpyruvate carboxylase kinase (PPCK) was identified as a candidate biomarker for detecting cadmium contamination. Our findings provide insights into the transcriptome dynamics of molecular response of rice to different rhizotoxic stress and also demonstrate potential use of comparative transcriptome analysis in identifying a novel potential early biomarker.

Natural history of the narrow endemics Ipomoea cavalcantei and I. marabaensis from Amazon Canga savannahs

Amazon comprises a vast variety of ecosystems, including savannah-like Canga barrens that evolved on iron-lateritic rock plateaus of the Carajás Mountain range. Individual Cangas are enclosed by the rain forest, indicating insular isolation that enables speciation and plant community differentiation. To establish a framework for the research on natural history and conservation management of endemic Canga species, seven chloroplast DNA loci and an ITS2 nuclear DNA locus were used to study natural molecular variation of the red flowered Ipomoea cavalcantei and the lilac flowered I. marabaensis. Partitioning of the nuclear and chloroplast gene alleles strongly suggested that the species share the most recent common ancestor, pointing a new independent event of the red flower origin in the genus. Chloroplast gene allele analysis showed strong genetic differentiation between Canga populations, implying a limited role of seed dispersal in exchange of individuals between Cangas. Closed haplotype network topology indicated a requirement for the paternal inheritance in generation of cytoplasmic genetic variation. Tenfold higher nucleotide diversity in the nuclear ITS2 sequences distinguished I. cavalcantei from I. marabaensis, implying a different pace of evolutionary changes. Thus, Canga ecosystems offer powerful venues for the study of speciation, multitrait adaptation and the origins of genetic variation.

Cadmium-induced genomic instability in Arabidopsis: Molecular toxicological biomarkers for early diagnosis of cadmium stress

Microsatellite instability (MSI) analysis, random-amplified polymorphic DNA (RAPD), and methylation-sensitive arbitrarily primed PCR (MSAP-PCR) are methods to evaluate the toxicity of environmental pollutants in stress-treated plants and human cancer cells. Here, we evaluate these techniques to screen for genetic and epigenetic alterations of Arabidopsis plantlets exposed to 0-5.0 mg L(-1) cadmium (Cd) for 15 d. There was a substantial increase in RAPD polymorphism of 24.5, and in genomic methylation polymorphism of 30.5-34.5 at CpG and of 14.5-20 at CHG sites under Cd stress of 5.0 mg L(-1) by RAPD and of 0.25-5.0 mg L(-1) by MSAP-PCR, respectively. However, only a tiny increase of 1.5 loci by RAPD occurred under Cd stress of 4.0 mg L(-1), and an additional high dose (8.0 mg L(-1)) resulted in one repeat by MSI analysis. MSAP-PCR detected the most significant epigenetic modifications in plantlets exposed to Cd stress, and the patterns of hypermethylation and polymorphisms were consistent with inverted U-shaped dose responses. The presence of genomic methylation polymorphism in Cd-treated seedlings, prior to the onset of RAPD polymorphism, MSI and obvious growth effects, suggests that these altered DNA methylation loci are the most sensitive biomarkers for early diagnosis and risk assessment of genotoxic effects of Cd pollution in ecotoxicology.

Mechanisms of metal toxicity in plants

Metal toxicity in plants is still a global problem for the environment, agriculture and ultimately human health.

The uptake and bioaccumulation of heavy metals by food plants, their effects on plants nutrients, and associated health risk: a review

Heavy metal contamination is a globally recognized environmental issue, threatening human life very seriously. Increasing population and high demand for food resulted in release of various contaminants into environment that finally contaminate the food chain. Edible plants are the major source of diet, and their contamination with toxic metals may result in catastrophic health hazards. Heavy metals affect the human health directly and/or indirectly; one of the indirect effects is the change in plant nutritional values. Previously, a number of review papers have been published on different aspects of heavy metal contamination. However, no related information is available about the effects of heavy metals on the nutritional status of food plants. This review paper is focused upon heavy metal sources, accumulation, transfer, health risk, and effects on protein, amino acids, carbohydrates, fats, and vitamins in plants. The literature about heavy metals in food plants shows that both leafy and nonleafy vegetables are good accumulators of heavy metals. In nonleafy vegetables, the bioaccumulation pattern was leaf > root ≈ stem > tuber. Heavy metals have strong influence on nutritional values; therefore, plants grown on metal-contaminated soil were nutrient deficient and consumption of such vegetables may lead to nutritional deficiency in the population particularly living in developing countries which are already facing the malnutrition problems.

Too much is bad--an appraisal of phytotoxicity of elevated plant-beneficial heavy metal ions

Heavy metal ions such as cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), and zinc (Zn) are considered essential/beneficial for optimal plant growth, development, and productivity. However, these ions readily impact functions of many enzymes and proteins, halt metabolism, and exhibit phytotoxicity at supra-optimum supply. Nevertheless, the concentrations of these heavy metal ions are increasing in agricultural soils worldwide via both natural and anthropogenic sources that need immediate attention. Considering recent breakthroughs on Co, Cu, Fe, Mn, Mo, Ni, and Zn in soil-plant system, the present paper: (a) overviews the status in soils and their uptake, transport, and significance in plants; (b) critically discusses their elevated level-mediated toxicity to both plant growth/development and cell/genome; (c) briefly cross talks on the significance of potential interactions between previous plant-beneficial heavy metal ions in plants; and (d) highlights so far unexplored aspects in the current context.

Modulating crossover positioning by introducing large structural changes in chromosomes

BACKGROUND

Crossing over assures the correct segregation of the homologous chromosomes to both poles of the dividing meiocyte. This exchange of DNA creates new allelic combinations thus increasing the genetic variation present in offspring. Crossovers are not uniformly distributed along chromosomes; rather there are preferred locations where they may take place. The positioning of crossovers is known to be influenced by both exogenous and endogenous factors as well as structural features inherent to the chromosome itself. We have introduced large structural changes into Arabidopsis chromosomes and report their effects on crossover positioning.

RESULTS

The introduction of large deletions and putative inversions silenced recombination over the length of the structural change. In the majority of cases analyzed, the total recombination frequency over the chromosomes was unchanged. The loss of crossovers at the sites of structural change was compensated for by increases in recombination frequencies elsewhere on the chromosomes, mostly in single intervals of one to three megabases in size. Interestingly, two independent cases of induced structural changes in the same chromosomal interval were found on both chromosomes 1 and 2. In both cases, compensatory increases in recombination frequencies were of similar strength and took place in the same chromosome region. In contrast, deletions in chromosome arms carrying the nucleolar organizing region did not change recombination frequencies in the remainder of those chromosomes.

CONCLUSIONS

When taken together, these observations show that changes in the physical structure of the chromosome can have large effects on the positioning of COs within that chromosome. Moreover, different reactions to induced structural changes are observed between and within chromosomes. However, the similarity in reaction observed when looking at chromosomes carrying similar changes suggests a direct causal relation between induced change and observed reaction.

A bioassessment of soil nickel genotoxic effect in orchard planted on rehabilitated coalmine overburden

Environmental problems of non-rehabilitated overburden material are present in surrounding of open coal mines worldwide. Ecological restoration of this soil material usually deals with the improvement of its bad physico-chemical properties and its poor nutrient status, sometimes associated with heavy metal problems. Applied overburden restoration by planting orchard (1990) is assumed to be the first of its kind at opencast mines globally, so that present work was aimed at acquiring information about its efficiency of the applied measures concerning their possible use in agriculture. Various physical and chemical properties, together with the pseudo total and DTPA extractable metals (Fe, Mn, Cu, Zn, Co, Ni, Pb, Cr, Cd) as well as sequential Ni extraction analyses, was measured, in order to evaluate the impact of soil's Ni level (76.3-111.7 mg kg⁻¹) on decreasing yields of apples, pears and plums. As a general pattern, reclaimed soil was significantly enriched with organic matter (>2.5 percent) and nutrients compared to the initial (2 m depth) and non-reclaimed adjacent soil, approving this method for overburden restoration. Despite low Ni concentration in organs, Ni accumulation in a fruits' trees qualified these species as suitable for phytostabilization of present heavy metals, with a woody biomass as a large and important sink for Ni, especially in the roots. Applied cytogenetic studies evaluate the lack of genotoxic effect of nickel (Ni) on the gametic cells of investigated species, having no significant effect on meiosis and pollen germination. Most of the found anomalies were in apples, as a kind of aberrations with sticky figures and chromosome lagging, should be ascribed to the environmental and genetic interaction over the aging of trees.

Molecular determination of genotoxic effects of cobalt and nickel on maize (Zea mays L.) by RAPD and protein analyses

Assessment of DNA damages stemming from toxic chemicals is an important issue in terms of genotoxicology. In this study, maize (Zea mays L.) seedlings were used for screening the genotoxic effects of cobalt (Co) and nickel (Ni) treatments at various concentrations (5 mM, 10 mM, 20 mM and 40 mM). For this purpose, randomly amplified polymorphic DNA (RAPD) technique was applied to genomic DNA extracted from metal-exposed and unexposed plant materials. Besides, changes in total protein contents were screened by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis. For RAPD analysis, 16 RAPD primers were found to produce unique polymorphic band profiles on different concentrations of Co-/Ni-treated maize seedlings. Increased polymorphism resulting from the appearance of new bands or disappearance of normal bands was observed with increasing concentration of Co and Ni treatments. Genomic template stability, a qualitative measurement of changes in RAPD patterns of genomic DNA, decreased with increasing metal concentration. In SDS-PAGE analysis, it was observed that the total soluble protein content decreased by Co treatment, while it increased by Ni treatment. The results obtained from this study revealed that RAPD profiles and total soluble protein levels can be applied to detect genotoxicity, and these analyses can offer useful biomarker assays for the evaluation of genotoxic effects on Co- and Ni-polluted plants.

Repairing breaks in the plant genome: the importance of keeping it together

DNA damage threatens the integrity of the genome and has potentially lethal consequences for the organism. Plant DNA is under continuous assault from endogenous and environmental factors and effective detection and repair of DNA damage are essential to ensure the stability of the genome. One of the most cytotoxic forms of DNA damage are DNA double-strand breaks (DSBs) which fragment chromosomes. Failure to repair DSBs results in loss of large amounts of genetic information which, following cell division, severely compromises daughter cells that receive fragmented chromosomes. This review will survey recent advances in our understanding of plant responses to chromosomal breaks, including the sources of DNA damage, the detection and signalling of DSBs, mechanisms of DSB repair, the role of chromatin structure in repair, DNA damage signalling and the link between plant recombination pathways and transgene integration. These mechanisms are of critical importance for maintenance of plant genome stability and integrity under stress conditions and provide potential targets for the improvement of crop plants both for stress resistance and for increased precision in the generation of genetically improved varieties.

Reversion-reporter transgenes to analyze all six base-substitution pathways in Arabidopsis

To expand the repertoire of Arabidopsis (Arabidopsis thaliana) mutation-reporter transgenes, we constructed six mutant alleles in the same codon of the β-glucuronidase-encoding GUS transgene. Each allele reverts to GUS+ only via a particular one of the six transition/transversion pathways. AcV5 epitope tags, fused carboxyl terminal to the inactive GUS- proteins, enabled semiquantitative immunoassays in plant protein extracts. Spontaneous G:C→T:A transversions, previously not measured using reporter transgenes, were quite frequent. This may reflect mispairing of adenine with 8-oxoguanine in DNA attacked by endogenous oxyradicals. Spontaneous G:C→A:T was modest and other reversions were relatively low, as reported previously. Frequencies of ultraviolet C-induced TT→TC and TC→TT reversions were both high. With increased transgene copy number, spontaneous G:C→T:A reversions increased but ultraviolet C-induced reversions decreased. Frequencies of some reversion events were reduced among T4 versus T3 generation plants. Based on these and other analyses of sources of experimental variation, we propose guidelines for the employment of these lines to study genotoxic stress in planta.

Monitoring homologous recombination in rice (Oryza sativa L.)

Here we describe a system to assay homologous recombination during the complete life cycle of rice (Oryza sativa L.). Rice plants were transformed with two copies of non-functional GUS reporter overlap fragments as recombination substrate. Recombination was observed in all plant organs examined, from the seed stage until the flowering stage of somatic plant development. Embryogenic cells exhibited the highest recombination ability with an average of 3x10(-5) recombination events per genome, which is about 10-fold of that observed in root cells, and two orders of that observed in leaf cells. Histological analysis revealed that recombination events occurred in diverse cell types, but preferentially in cells with small size. Examples of this included embryogenic cells in callus, phloem cells in the leaf vein, and cells located in the root apical meristem. Steady state RNA analysis revealed that the expression levels of rice Rad51 homologs are positively correlated with increased recombination rates in embryogenic calli, roots and anthers. Finally, radiation treatment of plantlets from distinct recombination lines increased the recombination frequency to different extents. These results showed that homologous recombination frequency can be effectively measured in rice using a transgene reporter assay. This system will facilitate the study of DNA damage signaling and homologous recombination in rice, a model monocot.

Genotoxicity/mutagenicity of formaldehyde revealed by the Arabidopsis thaliana plants transgenic for homologous recombination substrates

Formaldehyde (FA) is a major industrial chemical and has been extensively used in the manufacture of synthetic resins and chemicals. The use of FA-containing industrial materials in daily life exposes human to FA extensively. Numerous studies indicate that FA is genotoxic, and can induce various genotoxic effects in vitro and in vivo. The primary DNA lesions induced by FA are DNA-protein crosslinks (DPCs). Recently, it has been reported that the homologous recombination (HR) mechanism is involved in the repair of DPCs, suggesting the homologous recombination could be a potential indicator for the genotoxicity/mutagenicity of FA. However, it has not yet been reported that organisms harboring recombination substrates are used for the detection of genotoxic/mutagenic effects of FA. In this present study, an Arabidopsis thaliana-line transgenic for GUS recombination substrates was used to study the genotoxicity/mutagenicity of FA, and the results showed that FA-exposure significantly increased the induction of HR in growing plants, but not in dormant seeds. We also observed an early up-regulation of expression of HR-related gene, AtRAD54, after FA-exposure. Moreover, the pretreatment with glutathione (GSH) suppressed drastically the induction of HR by FA-exposure.

Acute but not chronic exposure to abiotic stress results in transient reduction of expression levels of the transgene driven by the 35S promoter

The transgenic plant performance depends on the stable expression of the integrated transgene. In this paper, we have analyzed the stability of the most frequently used constitutive promoter, the cauliflower mosaic virus (CaMV) 35S promoter. We used several independent Nicotiana tabacum lines transgenic for the luciferase (LUC) or green fluorescence protein (GFP) coding genes driven by the same 35S promoter. As an indication of the expression level, we measured the steady state RNA level, protein level and protein activity. Exposure of plants to an acute single dose of UVC, UVB or X-ray radiation resulted in a decrease of the transgene expression level, whereas exposure to high temperature increased it. In most of the cases, the expression changed at one to two hours post exposure and returned to normal at four hours. By contrast, plants germinated and grown in the presence of a low dose of either UVB radiation or CuSO(4) for two weeks did not show any changes in expression level. We conclude that although the expression level of the transgenes driven by the 35S promoter can be transiently altered by the acute exposure, no substantial changes occur upon constant low exposure.

Modified accumulation of selected heavy metals in Bt transgenic rice

Safety assessment of genetically modified crops generally does not take into account the potential hazard of altered patterns of heavy metal accumulation in plants. A pot experiment was conducted under greenhouse conditions to evaluate the impact of heavy metal amendments on the accumulation of Cd, Cu, Pb and Zn in a Bt transgenic rice Ke-Ming-Dao (KMD) and its wild-type Xiushui 11 (Xs11). In control soils, significant difference was only found in contents of Cu (p < 0.01) and Pb (p < 0.05) in straw between KMD and Xs11. At three levels of Cd amendments (5, 10, and 20 mg/kg), the Cd contents in grain and straw of KMD were significantlyhigher than those of Xs11, and all grain Cd contents were significantly higher than the International Criteria (0.2-0.4 mg/kg) as specified by the Codex Alimentarius Commission (CAC). These results implied that it may be unsafe for growing Bt transgenic rice in heavily Cd-polluted areas. No significant difference in Zn was found between the two varieties with the exception of roots at Zn amendment level of 600 mg/kg, while Pb contents in KMD were much higher in the straw at the lead amendment level of 1000 mg/kg and inthe root at 250 mg Pb/kg. Data on the heavy metal accumulation patterns for the genetically modified rice may be used for the selection of growing areas as well as for plant residue management for Bt rice.

Phytoremediation—A Novel and Promising Approach for Environmental Clean-up

Phytoremediation is an eco friendly approach for remediation of contaminated soil and water using plants. Phytoremediation is comprised of two components, one by the root colonizing microbes and the other by plants themselves, which degrade the toxic compounds to further non-toxic metabolites. Various compounds, viz. organic compounds, xenobiotics, pesticides and heavy metals, are among the contaminants that can be effectively remediated by plants. Plant cell cultures, hairy roots and algae have been studied for their ability to degrade a number of contaminants. They exhibit various enzymatic activities for degradation of xenobiotics, viz. dehalogenation, denitrification leading to breakdown of complex compounds to simple and non-toxic products. Plants and algae also have the ability to hyper accumulate various heavy metals by the action of phytochelatins and metallothioneins forming complexes with heavy metals and translocate them into vacuoles. Molecular cloning and expression of heavy metal accumulator genes and xenobiotic degrading enzyme coding genes resulted in enhanced remediation rates, which will be helpful in making the process for large-scale application to remediate vast areas of contaminated soils. A few companies worldwide are also working on this aspect of bioremediation, mainly by transgenic plants to replace expensive physical or chemical remediation techniques. Selection and testing multiple hyperaccumulator plants, protein engineering ofphytochelatin and membrane transporter genes and their expression would enhance the rate of phytoremediation, making this process a successful one for bioremediation of environmental contamination. Recent years have seen major investments in the R&D, which have also resulted in competition of filing patents by several companies for economic gains. The details of science & technology related to phytoremediation have been discussed with a focus on future trends and prospects of global relevance.

Signature Optical Cues: Emerging Technologies for Monitoring Plant Health

Optical technologies can be developed as practical tools for monitoring plant health by providing unique spectral signatures that can be related to specific plant stresses. Signatures from thermal and fluorescence imaging have been used successfully to track pathogen invasion before visual symptoms are observed. Another approach for non-invasive plant health monitoring involves elucidating the manner with which light interacts with the plant leaf and being able to identify changes in spectral characteristics in response to specific stresses. To achieve this, an important step is to understand the biochemical and anatomical features governing leaf reflectance, transmission and absorption. Many studies have opened up possibilities that subtle changes in leaf reflectance spectra can be analyzed in a plethora of ways for discriminating nutrient and water stress, but with limited success. There has also been interest in developing transgenic phytosensors to elucidate plant status in relation to environmental conditions. This approach involves unambiguous signal creation whereby genetic modification to generate reporter plants has resulted in distinct optical signals emitted in response to specific stressors. Most of these studies are limited to laboratory or controlled greenhouse environments at leaf level. The practical translation of spectral cues for application under field conditions at canopy and regional levels by remote aerial sensing remains a challenge. The movement towards technology development is well exemplified by the Controlled Ecological Life Support System under development by NASA which brings together technologies for monitoring plant status concomitantly with instrumentation for environmental monitoring and feedback control.

Evaluation of DNA damage and mutagenicity induced by lead in tobacco plants

Tobacco (Nicotiana tabacum L. var. xanthi) seedlings were treated with aqueous solutions of lead nitrate (Pb2+) at concentrations ranging from 0.4 mM to 2.4 mM for 24 h and from 25 microM to 200 microM for 7 days. The DNA damage measured by the comet assay was high in the root nuclei, but in the leaf nuclei a slight but significant increase in DNA damage could be demonstrated only after a 7-day treatment with 200 microM Pb2+. In tobacco plants growing for 6 weeks in soil polluted with Pb2+ severe toxic effects, expressed by the decrease in leaf area, and a slight but significant increase in DNA damage were observed. The tobacco plants with increased levels of DNA damage were severely injured and showed stunted growth, distorted leaves and brown root tips. The frequency of somatic mutations in tobacco plants growing in the Pb2+-polluted soil did not significantly increase. Analytical studies by inductively coupled plasma optical emission spectrometry demonstrate that after a 24-h treatment of tobacco with 2.4 mM Pb2+, the accumulation of the heavy metal is 40-fold higher in the roots than in the above-ground biomass. Low Pb2+ accumulation in the above-ground parts may explain the lower levels or the absence of Pb2+-induced DNA damage in leaves.

Transgenic Plants as Sensors of Environmental Pollution Genotoxicity

Rapid technological development is inevitably associated with manyenvironmental problems which primarily include pollution of soil, water and air. In manycases, the presence of contamination is difficult to assess. It is even more difficult toevaluate its potential danger to the environment and humans. Despite the existence ofseveral whole organism-based and cell-based models of sensing pollution and evaluationof toxicity and mutagenicity, there is no ideal system that allows one to make a quick andcheap assessment. In this respect, transgenic organisms that can be intentionally altered tobe more sensitive to particular pollutants are especially promising. Transgenic plantsrepresent an ideal system, since they can be grown at the site of pollution or potentiallydangerous sites. Plants are ethically more acceptable and esthetically more appealing thananimals as sensors of environmental pollution. In this review, we will discuss varioustransgenic plant-based models that have been successfully used for biomonitoringgenotoxic pollutants. We will also discuss the benefits and potential drawbacks of thesesystems and describe some novel ideas for the future generation of efficient transgenicphytosensors.

Herbicide-induced anthocyanin accumulation in transgenic rice by expression of rice OSB2 under the control of rice CYP72A21 promoter

CYP72A21, a rice cytochrome P450 gene, is induced by chloroacetamide herbicides. OSB2, a rice myc-type transcription factor, induces anthocyanin accumulation in rice leaves. To produce plants for biomonitoring by color change, we combined the CYP72A21 promoter and the OSB2 gene and introduced them into the rice isogenic line Taichung-65 CB A (T65), which contains loci CB and A from the rice cultivar Murasakiine. Leaves of the transgenic plants turned red upon treatment with the chloroacetamide herbicides acetochlor, alachlor, and metolachlor. Seedling shoots reddened upon treatment with alachlor or metolachlor at 10 microM, a concentration slightly higher than that used in the field. Anthocyanin content was increased approximately 200% by the treatment. The color changes were consistent with increased shoot expression of OSB2 and the anthocyanidin synthase gene (ANS). This system promises easy detection of rice plant gene expression. Transgenic plants could be used in the future to biomonitor accumulated herbicides.

Development and application of novel constructs to score C:G-to-T:A transitions and homologous recombination in Arabidopsis

We report on the development of five missense mutants and one recombination substrate of the beta-glucuronidase (GUS)-encoding gene of Escherichia coli and their use for detecting mutation and recombination events in transgenic Arabidopsis (Arabidopsis thaliana) plants by reactivation of GUS activity in clonal sectors. The missense mutants were designed to find C:G-to-T:A transitions in a symmetrical sequence context and are in that respect complementary to previously published GUS point mutants. Small peptide tags (hemagglutinin tag and Strep tag II) and green fluorescent protein were translationally fused to GUS, which offers possibilities to check for mutant GUS production levels. We show that spontaneous mutation and recombination events took place. Mutagenic treatment of the plants with ethyl methanesulfonate and ultraviolet-C increased the number of mutations, validating the use of these constructs to measure mutation and recombination frequencies in plants exposed to biotic or abiotic stress conditions, or in response to different genetic backgrounds. Plants were also subjected to heavy metals, methyl jasmonate, salicylic acid, and heat stress, for which no effect could be seen. Together with an ethyl methanesulfonate mutation induction level much higher than previously described, the need is illustrated for many available scoring systems in parallel. Because all GUS missense mutants were cloned in a bacterial expression vector, they can also be used to score mutation events in E. coli.

Genotoxicity of arsenic evaluated by Allium-root micronucleus assay

Arsenic exposure is associated with various diseases and cancers. By using Allium-root micronucleus (MN) assay, possible genotoxicity of sodium arsenite (0.3-100 mg/l) and arsenic trioxide (0.05-50 mg/l) was evaluated in this study. Our results showed that arsenic compounds induced MN formation concentration-dependently. Exposure to 0.5-20 mg/l arsenic trioxide or to 1-100 mg/l sodium arsenite caused MN significantly in meristematic cells and daughter cells of Allium roots. A time-course study revealed that MN increased significantly after a short term (1 h) exposure to 10 mg/l sodium arsenite, demonstrating an effective rapid response. Arsenic compounds also caused mitotic delay and a concentration-dependent decrease in mitotic index. Results of the present study suggest that Allium-root MN assay is a simple, efficient and reproducible method for the genotoxicity monitoring of arsenic water contamination.

Effects of cadmium on root apical meristems of Pisum sativum L.: cell viability, cell proliferation and microtubule pattern as suitable markers for assessment of stress pollution

By studying the effects of four concentrations of cadmium (0.25, 2.5, 25, 250microM) on Pisum sativum L. roots, we compared parameters generally used in short-term tests for environmental monitoring - root length, mitotic index, occurrence of mitotic aberrations - with less explored parameters related to meristem activity, such as apex size and viability, percentage of DNA-synthesizing cells and microtubule alterations. The results show that low cadmium concentrations caused a reduction of root growth, which is directly related to reduction of apex length, mitotic activity and percentage of DNA-synthetizing cells. The microtubular cytoskeleton was highly sensitive to cadmium, as microtubule alterations appeared after treatment with the lowest cadmium concentration, pointing to microtubules or microtubule-associated proteins, among the main targets of cadmium. In contrast, cell viability was a less sensitive parameter, as it decreased only upon treatment with the highest cadmium concentrations. The different sensitivities of the parameters examined in this work support the use of different endpoints for assessment of risk from polluted soils and waters.

Impact assessment of cadmium contamination on rice (Oryza sativa L.) seedlings at molecular and population levels using multiple biomarkers

Assessment of environmental contamination on ecology (plant) at molecular and population levels is important in risk quantification and remediation study. Random amplified polymorphic DNA (RAPD) assay and related other fingerprinting techniques have been employed to detect the genotoxin-induced DNA damage and mutations. This research compared the effects occurring at molecular and population levels in rice seedlings exposed to cadmium (Cd) concentrations of 15-60 mg l(-1) for 8 days with quartz sand culture. Inhibition of root growth and increase of total soluble protein content in root tips of rice seedlings were observed with the increase of Cd concentration. For the RAPD analyses, 12 RAPD primers of 50-70% GC content were found to produce unique polymorphic band patterns and subsequently were used to produce a total of 180 bands of 179-3056 bp in molecular size in the control root tips of rice seedlings. Results produced by these RAPD primers indicate that changes in RAPD profiles of root tips after Cd treatment include modifications in band intensity and gain or loss of bands by comparison with control. The effect of changes was dose-dependent. Genomic template stability compares favourably with the traditional indices such as root growth and soluble protein content. The DNA polymorphisms detected by RAPD analysis can be applied as a suitable biomarker assay for the detection of genotoxic effects of Cd contamination on plants.

Heavy metal availability and impact on activity of soil microorganisms along a Cu/Zn contamination gradient

All the regulations that define a maximum concentration of metals in the receiving soil are based on total soil metal concentration. However, the potential toxicity of a heavy metal in the soil depends on its speciation and availability. We studied the effects of heavy metal speciation and availability on soil microorganism activities along a Cu/Zn contamination gradient. Microbial biomass and enzyme activity of soil contaminated with both Cu and Zn were investigated. The results showed that microbial biomass was negatively affected by the elevated metal levels. The microbial biomass-C (C(mic))/organic C (C(org)) ratio was closely correlated to heavy metal stress. There were negative correlations between soil microbial biomass, phosphatase activity and NH4NO3 extractable heavy metals. The soil microorganism activity could be predicted using empirical models with the availability of Cu and Zn. We observed that 72% of the variation in phosphatase activity could be explained by the NH4NO3-extractable and total heavy metal concentration. By considering different monitoring approaches and different viewpoints, this set of methods applied in this study seemed sensitive to site differences and contributed to a better understanding of the effects of heavy metals on the size and activity of microorganisms in soils. The data presented demonstrate the relationship between heavy metals availability and heavy metal toxicity to soil microorganism along a contamination gradient.

Acute exposure to UVB has a more profound effect on plant genome stability than chronic exposure

Environmental factors that damage DNA have various lengths of exposure and intensity levels. Although the results of increasing the intensity of a DNA damaging agent is often predictable, it is not clear whether the stage during development when the exposure is received has any influence on the amount of DNA damage. In this paper we analyzed the influence of UVB on the stability of Arabidopsis thaliana and the Nicotiana tabacum genomes. Our experiments showed that the acute exposure to UVB produces a significantly greater increase in homologous recombination frequency (HRF) and recombination rate (RR) compared with that produced by chronic exposure. The increase in HRF showed a positive correlation with UVB dose and a negative correlation with plant age. In other words, as the UVB dose was increased, there was a concomitant increase in HRF. Conversely, older plants had a lower HRF increase as compared to younger plants. Our experiments suggest that exposure to UVB makes the most significant impact on genome stability during the early stages of plant development.

Toxic metal accumulation, responses to exposure and mechanisms of tolerance in plants

Over the past 200 years emissions of toxic heavy metals have risen tremendously and significantly exceed those from natural sources for practically all metals. Uptake and accumulation by crop plants represents the main entry pathway for potentially health-threatening toxic metals into human and animal food. Of major concern are the metalloids arsenic (As) and selenium (Se), and the metals cadmium (Cd), mercury (Hg), and lead (Pb). This review discusses the molecular mechanisms of toxic metal accumulation in plants and algae, the responses to metal exposure, as well as our understanding of metal tolerance and its evolution. The main emphasis will be on cadmium, which is by far the most widely studied of the non-essential toxic metals/metalloids. Entry via Zn2+, Fe2+, and Ca2+ transporters is the molecular basis of Cd2+ uptake into plant cells. Much less is known about the partitioning of non-essential metals and about the genes underlying the enormous diversity among plants with respect to Cd accumulation in different tissues. Numerous studies have described symptoms and responses of plants upon toxic metal exposure. Mysterious are primary targets of toxicity, the degree of specificity of responses, the sensing and the signaling events that lead to transcriptional activation. All plants apparently possess a basal tolerance of toxic non-essential metals. For Cd and As, this is largely dependent on the phytochelatin pathway. Not understood is the molecular biology of Cd hypertolerance in certain plant species such as the metallophytes Arabidopsis halleri or Thlaspi caerulescens.

Homologous recombination in plants is organ specific

In this paper we analysed the genome stability of various Arabidopsis thaliana plant organs using a transgenic recombination system. The system was based on two copies of non-functional GUS (lines #651 and #11) or LUC (line #15D8) reporter genes serving as a recombination substrate. Both reporter assays showed that recombination in flowers or stems were rare events. Most of the recombination sectors were found in leaves and roots, with leaves having over 2-fold greater number of the recombination events per single cell genome as compared to roots. The recombination events per single genome were 9.7-fold more frequent on the lateral half of the leaves than on the medial halves. This correlated with a 2.5-fold higher metabolic activity in the energy source (lateral) versus energy sink (medial) of leaves. Higher metabolic activity was paralleled by a higher anthocyanin production in lateral halves. The level of double strand break (DSB) occurrence was also different among plant organs; the highest level was observed in roots and the lowest in leaves. High level of DSBs strongly positively correlated with the activity of the key repair enzymes, AtKU70 and AtRAD51. The ratio of AtRAD51 to AtKU70 expression was the highest in leaves, supporting the more active involvement of homologous recombination pathway in the repair of DSBs in this organ. Western blot analysis confirmed the real time PCR expression data for AtKU70 gene.