Regulation of tolerance of Chlamydomonas reinhardtii to heavy metal toxicity by heme oxygenase-1 and carbon monoxide

Competence of algal consortia under municipal wastewater: remediation efficiency, photosynthetic performance, antioxidant defense mechanisms and biofuel production

Utilizing monoalgal species for wastewater treatment is facing tremendous challenges owing to changing wastewater complexity in terms of physico-chemical characteristic, nutrient and metal concentration. The environmental conditions are also fluctuating therefore, the formation of robust system is of utmost importance for concomitant sustainable wastewater treatment and bioenergy production. In the present study, the tolerance and adaptability potential of algal consortia-1 (Chlorococcum humicola and Tetradesmus sp.) and consortia-2 (Chlorococcum humicola, Scenedesmus vacuolatus and Tetradesmus sp.) treated with municipal wastewater were examined under natural environmental conditions. The results exhibited that consortia-2 was more competent in recovering nitrate-nitrogen (82.92%), phosphorus (70.47%), and heavy metals (31-73.70%) from municipal wastewater (100%) than consortia-1. The results further depicted that total chlorophyll, carbohydrate, and protein content decreased significantly in wastewater-treated consortia-1 as compared to consortia-2. However, lipid content was increased by 4.01 and 1.17 folds in algal consortia-1 and consortia-2 compared to their respective controls. Moreover, absorption peak at 1740.6 cm-1 reflected higher biofuel-producing potential of consortia-1 as compared to consortia-2 as confirmed through FTIR spectroscopy. The results also revealed that consortia-2 showed the highest photosynthetic performance which was evident from the increment in the active photosystem-II reaction center (1.724 ± 0.068), quantum efficiency (0.633 ± 0.038), and performance index (3.752 ± 0.356). Further, a significant increase in photosynthetic parameters was observed in selected consortia at lag phase, while a noteworthy decline was observed at exponential and stationary phases in consortia-1 than consortia-2. The results also showed the maximum enhancement in ascorbic acid (2.43 folds), proline (3.34 folds), and cysteine (1.29 folds) in consortia-2, while SOD (1.75 folds), catalase (2.64 folds), and GR (1.19 folds) activity in consortia-1. Therefore, it can be concluded that due to remarkable flexibility and photosynthetic performance, consortia-2 could serve as a potential candidate for sustainable nutrient resource recovery and wastewater treatment, while consortia-1 for bio-fuel production in a natural environment. Thus, formation of algal consortia as the robust biosystem tolerates diverse environmental fluctuations together with wastewater complexity and ultimately can serve appropriate approach for environmental-friendly wastewater treatment and bioenergy production.

The effects of titanium dioxide (TiO2) nanoparticles on physiological, biochemical, and antioxidant properties of Vitex plant (Vitex agnus - Castus L)

Titanium dioxide nanoparticles (TiO2NPs) are widely used in agriculture in order to increase the yield and growth characteristics of plants. This study investigated the effects of TiO2NPs on photosynthetic pigments and several biochemical activities and antioxidant enzymes of the Vitex plant. Different concentrations of nanoparticles (0, 200, 400, 600 and 800 ppm) at five levels were sprayed on Vitex plants on the 30th day of the experiment. TiO2NPs at different concentrations had positive effects on root and shoot dry weight and a negative effect on leaf dry weight. The amount of chlorophyll increased with the concentration of TiO2NPs; however, the amount of chlorophyll b showed a decreasing trend while the total chlorophyll had a constant trend. The highest amount of soluble sugar was obtained in the treatment of 200 ppm nanoparticles. The application of TiO2NPs did not have any effect on the content of proline and soluble proteins of Vitex plant. The effects of foliar TiO2NPs, compared to the control, showed a significant increase in the activity of antioxidant enzymes. In general, TiO2NPs had a favorable effect on dry matter production and some antioxidant and biochemical properties of the Vitex plant.

Mitigation of oxidative stress damage caused by abiotic stress to improve biomass yield of microalgae: A review

Microalgae have been recognized as emerging cell factories due to the high value-added bio-products. However, the balance between algal growth and the accumulation of metabolites is always the main contradiction in algal biomass production. Hence, the security and effectiveness of regulating microalgal growth and metabolism simultaneously have drawn substantial attention. Since the correspondence between microalgal growth and reactive oxygen species (ROS) level has been confirmed, improving its growth under oxidative stress and promoting biomass accumulation under non-oxidative stress by exogenous mitigators is feasible. This paper first introduced ROS generation in microalgae and described the effects of different abiotic stresses on the physiological and biochemical status of microalgae from these aspects associated with growth, cell morphology and structure, and antioxidant system. Secondly, the role of exogenous mitigators with different mechanisms in alleviating abiotic stress was concluded. Finally, the possibility of exogenous antioxidants regulating microalgal growth and improving the accumulation of specific products under non-stress conditions was discussed.

Carbon monoxide/heme oxygenase system in plant: Roles in abiotic stress response and crosstalk with other signals molecules

Carbon monoxide (CO) has been recognized as a crucial gasotransmitter mainly produced by heme oxygenase (HO)-catalyzed heme degradation in plant. Recent studies have shown that CO plays an important role in regulating growth and development of plant, as well as and responding to a variety of abiotic stresses. Meanwhile, many studies have reported on CO working in combination with other signal molecules to mitigate abiotic stress. Here, we presented a comprehensive overview of recent developments in which CO reduces plant damage caused by abiotic stresses. The regulation of antioxidant system, photosynthetic system, ion balance and transport are the main mechanisms of CO-alleviated abiotic stress. We also proposed and discussed the relationship between CO and other signal molecules, including nitric oxide (NO), hydrogen sulfide (H2S), hydrogen gas (H2), abscisic acid (ABA), indole 3-acetic acid (IAA), gibberellin (GA), cytokine (CTK), salicylic acid (SA), jasmonic acid (JA), hydrogen peroxide (H2O2) and calcium ion (Ca2+). Furthermore, the important role of HO genes in alleviating abiotic stress was also discussed. We proposed promising and new research directions for the study of plant CO, which can provide further insights on the role of CO in plant growth and development under abiotic stress.

Lipid ROS- and Iron-Dependent Ferroptotic Cell Death in Unicellular Algae Chlamydomonas reinhardtii

The phenomenon of heat stress leading to ferroptosis-like cell death has recently been observed in bacteria as well as plant cells. Despite recent findings, the evidence of ferroptosis, an iron-dependent cell death remains unknown in microalgae. The present study aimed to investigate if heat shock could induce reactive oxygen species (ROS) and iron-dependent ferroptotic cell death in Chlamydomonas reinhardtii in comparison with RSL3-induced ferroptosis. After RSL3 and heat shock (50 °C) treatments with or without inhibitors, Chlamydomonas cells were evaluated for cell viability and the induction of ferroptotic biomarkers. Both the heat shock and RSL3 treatment were found to trigger ferroptotic cell death, with hallmarks of glutathione-ascorbic acid depletion, GPX5 downregulation, mitochondrial dysfunction, an increase in cytosolic calcium, ROS production, lipid peroxidation, and intracellular iron accumulation via heme oxygenase-1 activation (HO-1). Interestingly, the cells preincubated with ferroptosis inhibitors (ferrostatin-1 and ciclopirox) significantly reduced RSL3- and heat-induced cell death by preventing the accumulation of Fe²⁺ and lipid ROS. These findings reveal that ferroptotic cell death affects the iron homeostasis and lipid peroxidation metabolism of Chlamydomonas, indicating that cell death pathways are evolutionarily conserved among eukaryotes.

Carbon monoxide promotes stomatal initiation by regulating the expression of two EPF genes in Arabidopsis cotyledons

The gaseous molecule carbon monoxide (CO) can freely pass through the cell membrane and participate in signal transduction in the cell to regulate physiological activities in plants. Here, we report that CO has a positive regulatory role in stomatal development. Exogenous CO donor CORM-2 [Tricarbonyldichlororuthenium (II) dimer] treatment resulted in an increase of stomatal index (SI) on the abaxial epidermis of cotyledons in wild-type, which can be reversed by the addition of the CO biosynthesis inhibitor ZnPPIX [Protoporphyrin IX zinc (II)]. Consistent with this result, mutation of the CO biosynthesis gene HY1 resulted in a decrease of SI in hy1-100 plants, while overexpression of HY1 led to an increase of SI. Further investigation revealed that CO acts upstream of SPCH and YDA in the stomatal development pathway, since the loss of function mutants spch-1 and yda-2 were insensitive to CORM-2. The expression of EPF2 was inhibited by CORM-2 treatment in wild type and is lower in hy1 than in wild-type plants. In contrast, the expression of STOMAGEN was promoted by CORM-2 treatment and is higher in HY1-overexpression lines. Loss of function mutants of both epf2 and stomagen are insensitive to CORM-2 treatment. These results indicated that CO positively regulates stomatal initiation and distribution by modulating the expression of EPF2 and STOMAGEN.

Putative Protein Discovery from Microalgal Genomes as a Synthetic Biology Protein Library for Heavy Metal Bio-Removal

Simple Summary

Nowadays, heavy metal polluted wastewater is one of the global challenges that leads to an insufficient supply of clean water. Taking advantage of what nature has to offer, several organisms, including microalgae, can natively bioremediate these heavy metals. However, the effectiveness of such processes does not meet expectations, especially with the increasing amount of pollution in today’s world. Therefore, with the goal of creating effective strains, synthetic biology via bioengineering is widely used as a strategy to enhance the heavy metal bio-removing capability, either by directly engineering the native ability of organisms or by transferring the ability to a more suitable host. In order to do so, a list of genes or proteins involved in the processes is crucial for stepwise engineering. Yet, a large amount of information remains to be discovered. In this work, a comprehensive library of putative proteins that are involved in heavy metal bio-removal from microalgae was constructed. Moreover, with the development of machine learning, the 3D structures of these proteins are also predicted, using machine learning-based methods, to aid the use of synthetic biology further.

Abstract

Synthetic biology is a principle that aims to create new biological systems with particular functions or to redesign the existing ones through bioengineering. Therefore, this principle is often utilized as a tool to put the knowledge learned to practical use in actual fields. However, there is still a great deal of information remaining to be found, and this limits the possible utilization of synthetic biology, particularly on the topic that is the focus of the present work—heavy metal bio-removal. In this work, we aim to construct a comprehensive library of putative proteins that might support heavy metal bio-removal. Hypothetical proteins were discovered from Chlorella and Scenedesmus genomes and extensively annotated. The protein structures of these putative proteins were also modeled through Alphafold2. Although a portion of this workflow has previously been demonstrated to annotate hypothetical proteins from whole genome sequences, the adaptation of such steps is yet to be done for library construction purposes. We also demonstrated further downstream steps that allow a more accurate function prediction of the hypothetical proteins by subjecting the models generated to structure-based annotation. In conclusion, a total of 72 newly discovered putative proteins were annotated with ready-to-use predicted structures available for further investigation.

Heavy metal-induced stress in eukaryotic algae-mechanisms of heavy metal toxicity and tolerance with particular emphasis on oxidative stress in exposed cells and the role of antioxidant response

Heavy metals is a collective term describing metals and metalloids with a density higher than 5 g/cm3. Some of them are essential micronutrients; others do not play a positive role in living organisms. Increased anthropogenic emissions of heavy metal ions pose a serious threat to water and land ecosystems. The mechanism of heavy metal toxicity predominantly depends on (1) their high affinity to thiol groups, (2) spatial similarity to biochemical functional groups, (3) competition with essential metal cations, (4) and induction of oxidative stress. The antioxidant response is therefore crucial for providing tolerance to heavy metal-induced stress. This review aims to summarize the knowledge of heavy metal toxicity, oxidative stress and antioxidant response in eukaryotic algae. Types of ROS, their formation sites in photosynthetic cells, and the damage they cause to the cellular components are described at the beginning. Furthermore, heavy metals are characterized in more detail, including their chemical properties, roles they play in living cells, sources of contamination, biochemical mechanisms of toxicity, and stress symptoms. The following subchapters contain the description of low-molecular-weight antioxidants and ROS-detoxifying enzymes, their properties, cellular localization, and the occurrence in algae belonging to different clades, as well as the summary of the results of the experiments concerning antioxidant response in heavy metal-treated eukaryotic algae. Other mechanisms providing tolerance to metal ions are briefly outlined at the end.

Activation of the ROS/HO-1/NQO1 signaling pathway contributes to the copper-induced oxidative stress and autophagy in duck renal tubular epithelial cells

The aim of this study was to investigate the crosstalk between oxidative stress and autophagy through the ROS/HO-1/NQO1 pathway caused by copper (Cu). Duck renal tubular epithelial cells were treated in Cu sulfate (CuSO₄) (0, 100 and 200 μM) for 12 h, and in the combination of CuSO₄ (200 μM) and reactive oxygen species (ROS) scavenger (butyl hydroxyanisole, BHA, 100 μM), or HO-1 inhibitor (zinc protoporphyrin, ZnPP, 10 μM) for 12 h. Results revealed that Cu could significantly elevate the levels of intracellular ROS, superoxide dismutase, hydrogen peroxide, malondialdehyde, glutathione, simultaneously reduce catalase and glutathione peroxidase levels, and upregulate HO-1, SOD-1, CAT, NQO1, GCLM mRNA levels and HO-1, SOD-1 protein levels. Additionally, Cu could observably increase the number of autophagosomes, acidic vesicle organelles (AVOs) and LC3 puncta; upregulate mRNA levels of mTOR, Beclin-1, ATG7, ATG5, ATG3, LC3II and protein levels of Beclin-1, LC3II/LC3I, downregulate LC3I mRNA level. Both treatments with BHA and ZnPP could significantly alleviate the changes of antioxidant indexes levels and ROS accumulation, reduce the increase of the number of autophagosomes, AVOs and LC3 puncta, and mitigate the above changed oxidative stress and autophagy related mRNA and protein levels induced by Cu. In summary, our findings indicated that excessive Cu could induce oxidative stress and autophagy by activating the ROS/HO-1/NQO1 pathway, and inhibition of HO-1 might attenuate Cu-induced oxidative stress and autophagy in duck renal tubular epithelial cells.

Evidence for a role of nitric oxide in iron homeostasis in plants

Nitric oxide (NO), once regarded as a poisonous air pollutant, is now understood as a regulatory molecule essential for several biological functions in plants. In this review, we summarize NO generation in different plant organs and cellular compartments, and also discuss the role of NO in iron (Fe) homeostasis, particularly in Fe-deficient plants. Fe is one of the most limiting essential nutrient elements for plants. Plants often exhibit Fe deficiency symptoms despite sufficient tissue Fe concentrations. NO appears to not only up-regulate Fe uptake mechanisms but also makes Fe more bioavailable for metabolic functions. NO forms complexes with Fe, which can then be delivered into target cells/tissues. NO generated in plants can alleviate oxidative stress by regulating antioxidant defense processes, probably by improving functional Fe status and by inducing post-translational modifications in the enzymes/proteins involved in antioxidant defense responses. It is hypothesized that NO acts in cooperation with transcription factors such as bHLHs, FIT, and IRO to regulate the expression of enzymes and proteins essential for Fe homeostasis. However, further investigations are needed to disentangle the interaction of NO with intracellular target molecules that leads to enhanced internal Fe availability in plants.

Ferrous iron-induced increases in capitate glandular trichome density and upregulation of CbHO-1 contributes to increases in blinin content in Conyza blinii

Ferrous iron can promote the development of glandular trichomes and increase the content of blinin, which depends on CbHO-1 expression. Conyza blinii (C. blinii) is a unique Chinese herbal medicine that grows in Sichuan Province, China. Because the habitat of C. blinii is an iron ore mining area with abundant iron content, this species can be used as one of the best materials to study the mechanism of plant tolerance to iron. In this study, C. blinii was treated with ferrous-EDTA solutions at different concentrations, and it was found that the tolerance value of C. blinii to iron was 200 μM. Under this concentration, the plant height, root length, biomass, and iron content of C. blinii increased to the maximum values, and the effect was dependent on the upregulated expression of CbHO-1. At the same time, under ferrous iron, the photosynthetic capacity and capitate glandular trichome density of C. blinii also significantly increased, providing precursors and sites for the synthesis of blinin, thus significantly increasing the content of blinin. These processes were also dependent on the high expression of CbHO-1. Correlation analysis showed that there were strong positive correlations between iron content, capitate glandular trichome density, CbHO-1 gene expression, and blinin content. This study explored the effects of ferrous iron on the physiology and biochemistry of C. blinii, greatly improving our understanding of the mechanism of iron tolerance in C. blinii.

Trifluralin and Atrazine Sensitivity to Selected Cereal and Legume Crops

Soil-applied herbicides can persist in sufficient concentrations to affect the growth of crops in rotations. The sensitivity of wheat, barley, oat, lucerne and lentil to trifluralin and atrazine residues were investigated with three glasshouse experiments in 2018 and 2019. Each bioassay crop species was tested against different concentrations of trifluralin and atrazine in sandy soil using a full factorial design. Shoot and root parameters of the tested crop species were fitted in logistic equations against herbicide concentrations to calculate effective doses for 50% growth inhibition (ED50). Results revealed that both shoot and root parameters of all the test crop species were significantly affected by trifluralin and atrazine. Trifluralin delayed crop emergence at the lower concentrations examined, while higher concentrations prevented emergence entirely. Low concentrations of atrazine did not affect emergence but significantly reduced plant height, soil–plant analyses development (SPAD) index, shoot dry weight, root length, root dry weight and number of nodules of all the crop species. At high concentration, atrazine resulted in plant death. Legumes were found to be more sensitive than cereals when exposed to both trifluralin and atrazine treatments, with lucerne being the most sensitive to both herbicides, ED50 ranging from 0.01 to 0.07 mg/kg soil for trifluralin; and from 0.004 to 0.01 mg/kg for atrazine. Barley was the most tolerant species observed in terms of the two herbicides tested. Lucerne can be used to develop a simple but reliable bioassay technique to estimate herbicide residues in the soil so that a sound crop rotation strategy can be implemented.

The improved methods of heavy metals removal by biosorbents: A review

For decades, a vast array of innovative biosorbents have been found out and used in the removal of heavy metals, including bacteria, algae and fungi, etc. Although extensive biological species have been tried as a biosorbent for heavy metals removal, for removal efficiency or economy efficiency limited, it has failed to make a substantial breakthrough in practical application. Thus, many improved methods based on biosorbents emerged. In this review, based on the literature and our research results, we highlight three types of novel methods for biosorbents removal of heavy metals: chemical modification of biosorbents; biomass and chemical materials combination; multiple biomass complex systems. We mainly focus on their configuration, biosorption performance, their creation method, regeneration/reuse, their application and development in the future. Through the comparative analysis of various methods, we think that intracellular autogenous nanomaterials may open up another window in biosorption of heavy metals area. At the same time, the combination of various treatment methods will be the development tendency of heavy metal pollution treatment in the future.

OsNHAD is a chloroplast membrane-located transporter required for resistance to salt stress in rice (Oryza sativa)

Salt stress is one of the major environmental factors limiting crop productivity. Although physiological and molecular characterization of salt stress response in plants has been the focus for many years, research on transporters for sodium ion (Na⁺) uptake, translocation and accumulation in plants, particularly in food crops like rice is limited. In this study, we functionally identified an uncharacterized sodium ion transporter named OsNHAD which encodes a putative Na⁺ ⁄ H⁺ antiporter in rice. Homology search shows its close relation to the Arabidopsis Na⁺/H⁺ antiporter AtNHD1 with 72.74% identity of amino acids. OsNHAD transcripts mainly express in leaves and are induced by Na⁺ stress. Confocal laser scanning microscopy analysis of OsNHAD::GFP fusion in tobacco leaves shows that OsNHAD resides in the chloroplast envelop. Knock-down of OsNHAD by RNA interference led to increased rice sensitivity to Na⁺, manifested by stunted plant growth, enhanced cellular damage, reduced PSII activity and changed chloroplast morphology. Mutation of OsNHAD also resulted in accumulation of more Na⁺ in chloroplasts and in shoots as well, suggesting that OsNHAD is involved in mediating efflux and detoxification of Na⁺ but does not affect K⁺ accumulation in plant cells. Complementation test reveals that OsNHAD was able to functionally restore the Arabidopsis mutant atnhd1-1 growth phenotype. These results suggest that OsNHAD possibly mediates homeostasis of sodium ions in the subcellular compartments and tissues of the plants when challenged to salt stress.

Transcriptomic Analysis Reveals the Roles of Detoxification Systems in Response to Mercury in Chromera velia

Heavy metal pollution is an increasing global concern. Among heavy metals, mercury (Hg) is especially dangerous because of its massive release into the environment and high toxicity, especially for aquatic organisms. The molecular response mechanisms of algae to Hg exposure are mostly unknown. Here, we combine physiological, biochemical, and transcriptomic analysis to provide, for the first time, a comprehensive view on the pathways activated in Chromera velia in response to toxic levels of Hg. Production of hydrogen peroxide and superoxide anion, two reactive oxygen species (ROS), showed opposite patterns in response to Hg²⁺ while reactive nitrogen species (RNS) levels did not change. A deep RNA sequencing analysis generated a total of 307,738,790 high-quality reads assembled in 122,874 transcripts, representing 89,853 unigenes successfully annotated in databases. Detailed analysis of the differently expressed genes corroborates the biochemical results observed in ROS production and suggests novel putative molecular mechanisms in the algal response to Hg²⁺. Moreover, we indicated that important transcription factor (TF) families associated with stress responses differentially expressed in C. velia cultures under Hg stress. Our study presents the first in-depth transcriptomic analysis of C. velia, focusing on the expression of genes involved in different detoxification defense systems in response to heavy metal stress.

Characterisation of Pb-induced changes and prediction of Pb exposure in microalgae using infrared spectroscopy

Macromolecular changes in Chlorella sp. FleB1 and Scenedesmus acutus YaA6 exposed to Pb were characterised by Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy. Spectra were collected in the mid-infrared spectral region and a chemometric approach was used to analyse the spectra. Principal Component Analysis showed that under Pb treatment, carbohydrates and lipids increased while proteins and phosphorylated molecules decreased in both the isolates. These conclusions were corroborated by conventional measurements of photosynthesis and neutral lipids. In the two strains, Pb inhibited photosynthesis, which directly affects the synthesis of polysaccharides and other macromolecules. Neutral lipids as measured by Nile Red were increased in lead-treated samples. Modelling of spectral data against Pb exposure levels allowed Pb-exposed samples to be readily discriminated from unexposed controls and the level of Pb exposure could be predicted with good accuracy in independent validation testing. This study demonstrates the utility of the spectroscopic approach as a rapid, inexpensive, non-destructive method to understand changes in cellular composition induced by Pb and allowing detection and quantification of Pb levels in metal-contaminated cells in a "snapshot".

Genome-wide association analysis identifies loci governing mercury accumulation in maize

Owing to the rapid development of urbanisation and industrialisation, heavy metal pollution has become a widespread environmental problem. Maize planted on mercury (Hg)-polluted soil can absorb and accumulate Hg in its edible parts, posing a potential threat to human health. To understand the genetic mechanism of Hg accumulation in maize, we performed a genome-wide association study using a mixed linear model on an association population consisting of 230 maize inbred lines with abundant genetic variation. The order of relative Hg concentrations in different maize tissues was as follows: leaves > bracts > stems > axes > kernels. Combined two locations, a total of 37 significant single-nucleotide polymorphisms (SNPs) associated with kernels, 12 with axes, 13 with stems, 27 with bracts and 23 with leaves were detected with p < 0.0001. Each significant SNP was calculated and the SNPs significant associated with kernels, axes, stems, bracts and leaves explained 6.96%–10.56%, 7.19%–15.87%, 7.11%–10.19%, 7.16%–8.71% and 6.91%–9.17% of the phenotypic variation, respectively. Among the significant SNPs, nine co-localised with previously detected quantitative trait loci. This study will aid in the selection of Hg-accumulation inbred lines that satisfy the needs for pollution-safe cultivars and maintaining maize production.

Physiological characterization of Chlamydomonas reinhardtii acclimated to chronic stress induced by Ag, Cd, Cr, Cu and Hg ions

Acclimation to heavy metal-induced stress is a complex phenomenon. Among the mechanisms of heavy metal toxicity, an important one is the ability to induce oxidative stress, so that the antioxidant response is crucial for providing tolerance to heavy metal ions. The effect of chronic stress induced by ions of five heavy metals, Ag, Cu, Cr (redox-active metals) Cd, Hg (nonredox-active metals) on the green microalga Chlamydomonas reinhardtii was examined at two levels - the biochemical (content of photosynthetic pigments and prenyllipid antioxidants, lipid peroxidation) and the physiological (growth rate, photosynthesis and respiration rates, induction of nonphotochemical quenching of chlorophyll fluorescence). The expression of the genes which encode the enzymes participating in the detoxification of reactive oxygen species (APX1, CAT1, FSD1, MSD1) was measured. The other gene measured was one required for plastoquinone and α-tocopherol biosynthesis (VTE3). The application of heavy metal ions partly inhibited growth and biosynthesis of chlorophyll. The growth inhibition was accompanied by enhanced lipid peroxidation. An increase in the content of prenyllipid antioxidants was observed in cultures exposed to Cr2O7(2-), Cd(2+) (α- and γ-tocopherol and plastoquinone) and Cu(2+) (only tocopherols). The induction of nonphotochemical quenching was enhanced in cultures exposed to Cu(2+), Cr2O7(2-) and Cd(2+), as compared to the control. Chronic heavy metal-induced stress led to changes in gene expression dependent on the type and concentration of heavy metal ions. The up-regulation of antioxidant enzymes was usually accompanied by the up-regulation of the VTE3 gene.

Biological responses of wheat (Triticum aestivum) plants to the herbicide simetryne in soils

The rotation of rice and wheat is widely used and highly endorsed, and simetryne (s-triazine herbicide) is one of the principal herbicides widely used in this rotation for weed and grass control. However, little is known regarding the mechanism of the ecological and physiological effects of simetryne on wheat crops. In this study, we performed a comprehensive investigation of crop response to simetryne to elucidate the accumulation and phytotoxicity of the herbicide in wheat crops. Wheat plants exposed to 0.8 to 8.0mgkg(-1) simetryne for 7 d exhibited suppressed growth and decreased chlorophyll content. With simetryne concentration in the soil varied from 0.8mgkg(-1) to 8.0mgkg(-1), simetryne was progressively accumulated by the wheat plants. The accumulation of simetryne in the wheat plants not only induced the over production of ROS and injured the membrane lipids but also stimulated the production of antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX), glutathione reductase (GR) and glutathione S-transferase (GST). A test of enzymatic activity and gene expression illustrated that the wheat plants were wise enough to motivate the antioxidant enzymes through both molecular and physiological mechanisms to alleviate the simetryne-induced stress. This study offers an illuminating insight into the effective adaptive response of the wheat plants to the simetryne stress.

Carbon Monoxide as a Signaling Molecule in Plants

Carbon monoxide (CO), a gaseous molecule, has emerged as a signaling molecule in plants, due to its ability to trigger a series of physiological reactions. This article provides a brief update on the synthesis of CO, its physiological functions in plant growth and development, as well as its roles in abiotic stress tolerance such as drought, salt, ultraviolet radiation, and heavy metal stress. CO has positive effects on seed germination, root development, and stomatal closure. Also, CO can enhance plant abiotic stress resistance commonly through the enhancement of antioxidant defense system. Moreover, CO shows cross talk with other signaling molecules including NO, phytohormones (IAA, ABA, and GA) and other gas signaling molecules (H2S, H2, CH4).

CrGNAT gene regulates excess copper accumulation and tolerance in Chlamydomonas reinhardtii

Excess copper (Cu) in environment affects the growth and metabolism of plants and green algae. However, the molecular mechanism for regulating plant tolerance to excess Cu is not fully understood. Here, we report a gene CrGNAT enconding an acetyltransferase in Chlamydomonas reinhardtii and identified its role in regulating tolerance to Cu toxicity. Expression of CrGNAT was significantly induced by 75-400μM Cu. The top induction occurred at 100μM. Transgenic algae overexpressing CrGNAT (35S::CrGNAT) in C. reinhardtii showed high tolerance to excess Cu, with improved cell population, chlorophyll accumulation and photosynthesis efficiency, but with low degree of oxidation with regard to reduced hydrogen peroxide, lipid peroxides and non-protein thiol compounds. In contrast, CrGNAT knock-down lines with antisense led to sensitivity to Cu stress. 35S::CrGNAT algae accumulated more Cu and other metals (Zn, Fe, Cu, Mn and Mg) than wild-type, whereas the CrGNAT down-regulated algae (35S::AntiCrGNAT) had moderate levels of Cu and Mn, but no effects on Zn, Fe and Mg accumulation as compared to wild-type. The elevated metal absorption in CrGNAT overexpression algae implies that the metals can be removed from water media. Quantitative RT-PCR analysis revealed that expression of two genes encoding N-lysine histone methyltransferases was repressed in 35S::CrGNAT algae, suggesting that CrGNAT-regulated algal tolerance to Cu toxicity is likely associated with histone methylation and chromatin remodeling. The present work provided an example a basis to develop techniques for environmental restoration of metal-contaminated aquatic ecosystems.

Indole acetic acid is responsible for protection against oxidative stress caused by drought in soybean plants: the role of heme oxygenase induction

Objectives This study was focused on the role of indole acetic acid (IAA) in the defense against oxidative stress damage caused by drought in soybean plants and to elucidate whether heme oxygenase-1 (HO-1) and nitric oxide (NO) are involved in this mechanism. IAA is an auxin that participates in many plant processes including oxidative stress defense, but to the best of our knowledge no information is yet available about its possible action in drought stress. Methods To this end, soybean plants were treated with 8% polyethylene glycol (PEG) or 100 µM IAA. To evaluate the behavior of IAA, plants were pretreated with this compound previous to PEG addition. Lipid peroxidation levels (thiobarbituric acid reactive substances (TBARS)), glutathione (GSH) and ascorbate (AS) contents, catalase (CAT), superoxide dismutase (SOD), and guaiacol peroxidase (POD) activities were determined to evaluate oxidative damage. Results Drought treatment (8% PEG) caused a significant increase in TBARS levels as well as a marked decrease in the non-enzymatic (GSH and AS) and enzymatic (CAT, SOD, and POD) antioxidant defense systems. Pre-treatment with IAA prevented the alterations of stress parameters caused by drought, while treatment with IAA alone did not produce changes in TBARS levels, or GSH and AS contents. Moreover, the activities of the classical enzymes involved in the enzymatic defense system (SOD, CAT, and POD) remained similar to control values. Furthermore, this hormone could enhance HO-1 activity (75% with respect to controls), and this increase was positively correlated with protein content as well as gene expression. The direct participation of HO-1 as an antioxidant enzyme was established by performing experiments in the presence of Zn-protoporphyrin IX, a well-known irreversible inhibitor of this enzyme. It was also demonstrated that HO-1 is modulated by NO, as shown by experiments performed in the presence of an NO donor (sodium nitroprusside), an NO scavenger (2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide), or an NO synthesis inhibitor (N-nitro-l-arginine methyl ester, NAME). Discussion It is concluded that IAA is responsible, at least in part, for the protection against oxidative stress caused by drought in soybean plants through the modulation of NO levels which, in turn, enhances HO-1 synthesis and activity.

Quantitative trait loci for mercury accumulation in maize (Zea mays L.) identified using a RIL population

To investigate the genetic mechanism of mercury accumulation in maize (Zea mays L.), a population of 194 recombinant inbred lines derived from an elite hybrid Yuyu 22, was used to identify quantitative trait loci (QTLs) for mercury accumulation at two locations. The results showed that the average Hg concentration in the different tissues of maize followed the order: leaves > bracts > stems > axis > kernels. Twenty-three QTLs for mercury accumulation in five tissues were detected on chromosomes 1, 4, 7, 8, 9 and 10, which explained 6.44% to 26.60% of the phenotype variance. The QTLs included five QTLs for Hg concentration in kernels, three QTLs for Hg concentration in the axis, six QTLs for Hg concentration in stems, four QTLs for Hg concentration in bracts and five QTLs for Hg concentration in leaves. Interestingly, three QTLs, qKHC9a, qKHC9b, and qBHC9 were in linkage with two QTLs for drought tolerance. In addition, qLHC1 was in linkage with two QTLs for arsenic accumulation. The study demonstrated the concentration of Hg in Hg-contaminated paddy soil could be reduced, and maize production maintained simultaneously by selecting and breeding maize Hg pollution-safe cultivars (PSCs).

Accumulation and toxicological response of atrazine in rice crops

Atrazine is one of the most widely used herbicides for controlling weeds and grasses. Due to its intensive use, it has become a serious contaminant in soil and water. To evaluate impact of atrazine on graminaceous crops, experiments focusing on atrazine accumulation and toxic response in rice (Oryza sativa) were carried out. Treatment with atrazine at 0.05-0.8 mg L(-1) for 6 d reduced elongation of shoot and root. Compared with a mock treatment, the elongation of shoot with atrazine was 67.1 percent of the control, whereas that of root was 79.5 percent, indicating that the shoot was more affected than the root. Atrazine was readily absorbed by rice from media. Although the quantitative absorption of atrazine was positively correlated with the external supply of the herbicide, translocation of atrazine from roots to the above-ground was reduced from 39.88±6.26 (at 0.05 mg L(-1)) to 9.25±0.27 (0.8 mg L(-1)). While accumulation of atrazine in rice plants led to toxic responses such as over-generation of hydrogen peroxide and superoxide anions, it triggered the plant defense system against the herbicide-induced oxidative stress. This was best presented by the enhanced activities of several antioxidant enzymes (e.g. superoxide dismutase, catalase and peroxidase) and expression of genes responsible for the tolerance to atrazine toxicity.

HISN3 mediates adaptive response of Chlamydomonas reinhardtii to excess nickel

Investigation of genes for heavy metal [e.g. nickel (Ni) and zinc (Zn)] absorption and detoxification in green algae is of great importance because some of the metals have become one of the major contaminants in the aquatic ecosystem. In plants, overload of heavy metals modifies many aspects of biological processes. However, the mechanisms by which heavy metals exert detrimental effects are not fully understood. The present study identified a biological role for HISN3 (the gene coding for phosphoribosylformimino-5-aminoimidazole carboxamide ribonucleotide isomerase) in regulating the response of Chlamydomonas reinhardtii, a unicellular green alga, to Ni toxicity. In higher plants, HISN3 encodes an enzyme catalyzing the fourth step in the histidine biosynthesis pathway, but its functional importance is yet to be identified. Transgenic algae overexpressing HISN3 in C. reinhardtii showed high tolerance to excess Ni, with a 48.3-57.4% increase in cell population and moderate histidine accumulation compared with the wild type. HISN3 overexpression improved accumulation of Chl and photosynthesis efficiency, but suppressed Ni-induced generation of reactive oxygen species and lipid peroxides. Interestingly, more Ni and other metals [Zn, iron (Fe), copper (Cu), manganese (Mn) and magnesium (Mg)] were accumulated in HISN3-overexpressing cells than in the wild type. In contrast, RNA interference of HISN3 depressed Ni accumulation but caused cellular sensitivity to Ni. The elevated metal absorption in the HISN3-overexpressing algae implies that the metals can be removed from water media. Thus, our work presents an example for algae genetically designed to improve tolerance to metal toxicity and environmental restoration of metal-contaminated aquatic ecosystems.

Bioaccumulation and degradation of atrazine in several Chinese ryegrass genotypes

Soil pollution with herbicides is a global problem. Before phytoremediation technology is developed for the plant-based clean-up of polluted soils, investigation of potential plants that can be used to accumulate and degrade herbicides is a critical step. In this study, three selected genotypes of ryegrass were comprehensively analyzed with regard to the atrazine accumulation, degradation and toxicological response. Under the conditions of soil with 0.8 mg kg(-1) atrazine, the maximum value for atrazine accumulation was 2.70 mg kg(-1) in shoots and 0.58 mg kg(-1) in roots. The residue of atrazine in soil with ryegrass cultivation was much lower than that in soil without ryegrass cultivation. Also, the content of atrazine residues in the rhizosphere was significantly lower than that in the non-rhizosphere soil. Activities of several enzymes (urease, invertase, polyphenol oxidase, acid phosphatase and alkaline phosphatase) in soil were assayed. These enzymes were depressed by atrazine but activated by ryegrass cultivation, even in the presence of atrazine. Finally, comparative studies have been conducted on the ryegrass genotypes in response to atrazine. They showed different capacities of degradation and bioaccumulation of atrazine. One of the grass cultivars Changjiang II (CJ) had better growth and higher levels of chlorophyll, but displayed less oxidative injury than two others, Abode (AB) and Jiewei (JW), under atrazine exposure.

Comparison of the toxicity of silver nanoparticles and silver ions on the growth of terrestrial plant model Arabidopsis thaliana

Silver nanoparticles (AgNPs) are one of the most widely used nanomaterials, but the mechanism of AgNP toxicity in terrestrial plants is still unclear. We compared the toxic effects of AgNPs and Ag+ on Arabidopsis thaliana at the physiological, ultrastructural and molecular levels. AgNPs did not affect seed germination; however, they showed stronger inhibitory effect on root elongation than Ag+. The results of transmission electron microscopy and metal content analysis showed that AgNPs could be accumulated in leaves. These absorbed AgNPs disrupted the thylakoid membrane structure and decreased chlorophyll content, which can inhibit plant growth. By comparison, a small amount of Ag+ was absorbed by seedlings, and it did not pronouncedly affect chloroplast structure and other metal ion absorption as AgNPs did. Compared with Ag+, AgNPs could alter the transcription of antioxidant and aquaporin genes, indicating that AgNPs changed the balance between the oxidant and antioxidant systems, and also affected the homeostasis of water and other small molecules within the plant body. All the data from physiological, ultrastructural and molecular levels suggest that AgNPs were more toxic than Ag+.

miR395 is involved in detoxification of cadmium in Brassica napus

The toxic metal cadmium (Cd) constitutes one of the major inorganic contaminants in environments. microRNAs (miRNAs) are a class of endogenous non-coding small RNAs. miR395 is conserved and regulates sulfate assimilation and distribution in higher plants, but whether it is involved in detoxification of Cd in plants has not been described. In this study, transgenic rapeseed (Brassica napus) over-expressing miR395 was identified under Cd stress. miR395-over-expressing plants showed a lower degree of Cd-induced oxidative stress than wild type. By contrast, chlorophyll, glutathione and non-protein thiols contents were higher in the transformants than wild type. Determination of growth response showed that 35S::MIR395 plants accumulated higher levels of biomass and sulfur than wild type under Cd exposure. miR395 transgenic plants had higher levels of Cd in plants, particularly at the high supply of Cd in the medium, but they tended to repress Cd translocation from roots to shoots. Simultaneously, expression of metal-tolerance genes such as BnPCS1, BnHO1 and Sultr1;1 was up-regulated under Cd stress, and the expression of the genes was more pronounced in 35S::MIR395 plants than in wild type. These results suggest that miR395 would be involved in detoxification of Cd in B. napus.

Joint ecotoxicology of cadmium and metsulfuron-methyl in wheat (Triticum aestivum)

Herbicide is indispensable for crop production. However, substantial usage of herbicide has led to its increasing accumulation in soils and crops. In addition, cadmium has become one of the widely occurring contaminants in soils due to its significant release into environment via anthropogenic activities. In this study, ecotoxicological investigations were made by exposing the food crop wheat to joint contaminations of Cd and metsulfuron-methyl, a sulfonylurea herbicide. We analyzed growth and physiological and molecular responses in wheat exposed to 0.5 mg kg(-1) Cd and 0.02 mg kg(-1) metsulfuron-methyl (MSM). Soils contaminated with Cd and MSM complex caused significantly detrimental effect on wheat growth and physiological process. Combinative treatments with Cd and MSM damage more severely the plant cells as compared with Cd or MSM treatment alone. Compared with the growth parameter, the biochemical and molecular responses of wheat appeared more pronounced to Cd and MSM complex. Furthermore, compared with control, wheat plants exposed to Cd + MSM generated more O(2-.)and H2O2, both of which were shown to be the cause of enhanced activity of several antioxidant enzymes. Native polyacrylamide gel eletrophoresis and molecular response analyses were performed to validate the results indicated above. Our results indicated that joint contamination with Cd and MSM was more toxic to wheat than a single contamination. These sensitive biological parameters can be used as biomarkers monitoring the ecotoxicological process in plants.

Gasotransmitters are emerging as new guard cell signaling molecules and regulators of leaf gas exchange

Specialized guard cells modulate plant gas exchange through the regulation of stomatal aperture. The size of the stomatal pore is a direct function of the volume of the guard cells. The transport of solutes across channels in plasma membrane is a crucial process in the maintenance of guard cell water status. The fine tuned regulation of that transport requires an integrated convergence of multiple endogenous and exogenous signals perceived at both the cellular and the whole plant level. Gasotransmitters are novel signaling molecules with key functions in guard cell physiology. Three gasotransmitters, nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H(2)S) are involved in guard cell regulatory processes. These molecules are endogenously produced by plant cells and are part of the guard cells responses to drought stress conditions through ABA-dependent pathways. In this review, we summarize the current knowledge of gasotransmitters as versatile molecules interacting with different components of guard cell signaling network and propose them as players in new paradigms to study ABA-independent guard cell responses to water deficit.

Phytotoxicity, bioaccumulation and degradation of isoproturon in green algae

Isoproturon (IPU) is a pesticide used for protection of land crops from weed or pathogen attack. Recent survey shows that IPU has been detected as a contaminant in aquatic systems and may have negative impact on aquatic organisms. To understand the phytotoxicity and potential accumulation and degradation of IPU in algae, a comprehensive study was performed with the green alga Chlamydomonas reinhardtii. Algae exposed to 5-50 μg L(-1) IPU for 3d displayed progressive inhibition of cell growth and reduced chlorophyll fluorescence. Time-course experiments with 25 μg L(-1) IPU for 6d showed similar growth responses. The 72 h EC50 value for IPU was 43.25 μg L(-1), NOEC was 5 μg L(-1) and LOEC was 15 μg L(-1). Treatment with IPU induced oxidative stress. This was validated by a group of antioxidant enzymes, whose activities were promoted by IPU exposure. The up-regulation of several genes coding for the enzymes confirmed the observation. IPU was shown to be readily accumulated by C. reinhardtii. However, the alga showed a weak ability to degrade IPU accumulated in its cells, which was best presented at the lower concentration (5 μg L(-1)) of IPU in the medium. The imbalance of accumulation and degradation of IPU may be the cause that resulted in the detrimental growth and cellular damage.

Overexpression of a protein disulfide isomerase-like protein from Methanothermobacter thermoautotrophicum enhances mercury tolerance in transgenic rice

MTH1745, from thermophilic archaea Methanothermobacter thermoautotrophicum, is a protein disulfide isomerase-like protein (PDIL) with a chaperone function and disulfide isomerase activity. Mercuric cations have a high affinity for sulfhydryl groups and consequently inhibit plant growth. Disulfide compounds (e.g., copper-zinc superoxide dismutase, Cu/Zn SOD) and sulfhydryl compounds (e.g., glutathione, phytochelatins, and metallothioneins) play important roles in mercury (Hg) response. To study the relationship between Hg detoxification and PDILs, we overexpressed MTH1745 in Oryza sativa L. cv. Nipponbare by Agrobacterium-mediated transformation. The transgenic rice seedlings displayed Hg tolerance with obvious phenotypes and more effective photosynthesis compared to wild-type plants. Furthermore, lower levels of superoxide anion radicals, hydrogen peroxide, and malondialdehyde were observed in leaves or roots of transgenic plants. Antioxidant enzyme activities of superoxide dismutase and peroxidase were notably higher in transgenic seedlings under different concentrations of mercuric chloride. Moreover, increased content of non-protein thiols, reduced glutathione (GSH), and GSH/GSSG (GSSG, oxidized glutathione) ratio were also observed in the detoxification of Hg. These results indicated that heterologous expression of a PDIL from extremophiles in rice could protect the synthesis, increase stability of proteins, and enhance Hg tolerance in rice.

BjHO-1 is involved in the detoxification of heavy metal in India mustard (Brassica juncea)

Heme oxygenase-1 (HO-1) is a stress-responsive gene coding for an enzyme catalyzing the catabolism of heme to yield biliverdin IXα, carbon monoxide (CO) and iron. However, its biological role in regulating metal homeostasis, particularly the tolerance to toxic heavy metals is poorly understood. In this study, a novel gene encoding a Brassica juncea heme oxygenase-1 (designated as BjHO-1) was cloned and functionally identified. Spatial expression of BjHO-1 showed that it was differentially expressed in cotyledon, hypocotyl, leaf and root. BjHO-1 was found to be induced significantly by heavy metal Hg. To understand whether BjHO-1 is able to regulate plant tolerance to Hg, we constructed transgenic B. juncea plants overexpressing HO-1, and showed that 35S::BjHO1 plants confer the plant resistance to Hg toxicity by improving plant dry mass, reducing Hg accumulation, and attenuating Hg-induced oxidative stress. We further cloned a 1,099 bp promoter sequence upstream of BjHO-1 using genome walking approach. Multiple stress-responsive elements were detected in the BjHO-1 promoter regions. The promoter can be activated by Zn, Cd, Hg and Pb exposure. Our results indicate that up-regulation of BjHO-1 is beneficial for limiting the uptake or accumulation of heavy metals into plants. This work also provides a new example for molecular breeding designed for plants that do not accumulate or minimizing accumulation of toxic trace metals growing on heavy metal-contaminated soils.

Toxicology of isoproturon to the food crop wheat as affected by salicylic acid

PURPOSE

Isoproturon, a herbicide belonging to the phenylurea family, is widely used to kill weeds in soils. Recent study indicated that isoproturon has become a contaminant in ecosystems due to its intensive use, thus bringing environmental risks to crop production safety. Salicylic acid (SA) is one of the components in plant defense signaling pathways and regulates diverse physiological responses to biotic and environmental stresses. The purpose of the study is to help to understand how SA mediates the biological process in wheat under isoproturon stress.

METHODS

Wheat seeds (Triticum aestivum, cv. Yangmai 13) were surface-sterilized and placed on moist filter paper for germination. After 24 h, the germinating seeds were placed on a plastic pot (1 L) containing 1,120 g soil mixed with isoproturon at 4 mg kg(-1) soil. After 4 days, wheat leaves were sprayed with 5 mg L(-1) SA. The SA treatment was undertaken once a day and lasted for 6 days, when the third true leaf was well developed. For control seedlings, only water was sprayed. Seedlings were grown under a light intensity of 300 µmol m(-2) s(-1) with a light/dark cycle of 12/12 h at 25°C, and watered to keep 70% relative water content in soils.

RESULTS AND DISCUSSION

We investigated the role of SA in alleviating isoproturon-induced toxicity in the food crop wheat (T. aestivum). Plants exposed to 4 mg kg(-1) isoproturon showed growth stunt and oxidative damage, but concomitant treatment with 5 mg L(-1) SA was able to attenuate the toxic effect. Isoproturon in soils was readily accumulated by wheat, but such accumulation can be blocked significantly by SA application. Treatment with SA decreased the abundance of O(2) (.-) and H(2)O(2), as well as activities of antioxidant enzymes, and increased activities of catalase in isoproturon-exposed plants. The enzyme activities were confirmed by the native polyacrylamide gel electrophoresis. Further, an RT-PCR-based assay was performed to show that several transcripts coding antioxidant enzymes were increased with isoproturon but decreased by SA.

CONCLUSION

The present results indicate that exogenous SA is able to improve the wheat tolerance to isoproturon toxicity.

Heme oxygenase is involved in nitric oxide- and auxin-induced lateral root formation in rice

Lateral root (LR) development performs the essential tasks of providing water, nutrients, and physical support to plants. Therefore, understanding the regulation of LR development is of agronomic importance. In this study, we examined the effect of nitric oxide (NO), auxin, and hemin (Hm) on LR formation in rice. Treatment with Hm [a highly effective heme oxygenase (HO) inducer], sodium nitroprusside (SNP, an NO donor), or indole-3-butyric acid (IBA, a naturally occurring auxin) induced LR formation and HO activity. LR formation and HO activity induced by SNP and IBA but not Hm was reduced by the specific NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide. As well, Hm, SNP, and IBA could induce OsHO1 mRNA expression. Zn protoporphyrin IX (the specific inhibitor of HO) and hemoglobin (the carbon monoxide/NO scavenger) reduced LR number and HO activity induced by Hm, SNP, and IBA. Our data suggest that HO is required for Hm-, auxin-, and NO-induced LR formation in rice.

Mercury toxicity, molecular response and tolerance in higher plants

Mercury (Hg) contamination in soils has become a great concern as a result of its natural release and anthropogenic activities. This review presents broad aspects of our recent understanding of mercury contamination and toxicology in plants including source of Hg contamination, toxicology, tolerant regulation in plants, and minimization strategy. We first introduced the sources of mercury contamination in soils. Mercury exists in different forms, but ionic mercury (Hg(2+)) is the predominant form in soils and readily absorbed by plants. The second issue to be discussed is the uptake, transport, and localization of Hg(2+) in plants. Mercury accumulated in plants evokes severe phytotoxicity and impairs numerous metabolic processes including nutrient uptake, water status, and photosynthesis. The mechanisms of mercury-induced toxicology, molecular response and gene networks for regulating plant tolerance will be reviewed. In the case of Hg recent much progress has been made in profiling of transcriptome and more importantly, uncovering a group of small RNAs that potentially mediates plant tolerance to Hg. Several newly discovered signaling molecules such as nitric oxide and carbon monoxide have now been described as regulators of plant tolerance to Hg. A recently emerged strategy, namely selection and breeding of plant cultivars to minimize Hg (or other metals) accumulation will be discussed in the last part of the review.

Bioaccumulation and catabolism of prometryne in green algae

Investigation on organic xenobiotics bioaccumulation/biodegradation in green algae is of great importance from environmental point of view because widespread distribution of these compounds in agricultural areas has become one of the major problems in aquatic ecosystem. Also, new technology needs to be developed for environmental detection and re-usage of the compounds as bioresources. Prometryne as a herbicide is widely used for killing annual grasses in China and other developing countries. However, overuse of the pesticide results in high risks to contamination to aquatic environments. In this study, we focused on analysis of bioaccumulation and degradation of prometryne in Chlamydomonas reinhardtii, a green alga, along with its adaptive response to prometryne toxicity. C. reinhardtii treated with prometryne at 2.5-12.5 μg L(-1) for 4 d or 7.5 μg L(-1) for 1-6 d accumulated a large quantity of prometryne, with more than 2 mg kg(-1) fresh weight in cells exposed to 10 μg L(-1) prometryne. Moreover, it showed a great ability to degrade simultaneously the cell-accumulated prometryne. Such uptake and catabolism of prometryne led to the rapid removal of prometryne from media. Physiological and molecular analysis revealed that toxicology was associated with accumulation of prometryne in the cells. The biological processes of degradation can be interpreted as an internal tolerance mechanism. These results suggest that the green alga is useful in bioremediation of prometryne-contaminated aquatic ecosystems.