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People are constantly moving to and within Kampala, Uganda. When choosing a place to settle, they have to find a balance between several housing preferences and constraints imposed by their socio-e...

Cadmium (Cd) is well known to inhibit vegetative plant growth. However, knowledge regarding its influence on the cell cycle is scarce and mainly limited to cell cultures and root tissue. Therefore, the main aim of this study was to investigate the effects of Cd exposure on cell division and endoreduplication in Arabidopsis thaliana leaves. In order to do so, we first investigated whether different leaves of the same rosette harvested at one time point (i.e. leaf position) could be used as an alternative for leaf age when investigating the cell cycle. To this end, wild-type A. thaliana plants were grown hydroponically with or without the addition of 5 μM CdSO4. Leaf growth and development, cell division, endoreduplication and the expression of cell cycle-related genes were investigated in separate leaves. The results show that different leaf positions constitute a developmental series that can be used to deduce the development of a single leaf over time. Furthermore, our data indicate that the concentration of nuclei in leaf extracts measured via flow cytometry can be used as a proxy to determine the effects of stress factors on the extent of cell division in A. thaliana leaves, reducing the need to perform time consuming microscopic analyses. Finally, we show that Cd exposure significantly reduces cell number, cell size and nuclear DNA content, implying an inhibition of both cell division and endoreduplication. These effects accumulate over time and contribute to the Cd-induced disturbance of leaf growth and development. At the molecular and cellular level, Cd increases hydrogen peroxide levels and induces the expression of marker genes for oxidative stress and DNA damage and genes encoding CDK inhibitors of the SIAMESE-RELATED family, suggesting that the Cd-induced inhibition of cell cycle progression is intertwined with oxidative stress and subsequent DNA damage.

Made available in DSpace on 2019-10-04T12:30:57Z (GMT). No. of bitstreams: 0 Previous issue date: 2018-11-01 Brazilian Federal Government Agency Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Research Foundation Flanders - FWO Exposure to cadmium (Cd) can cause oxidative stress and injuries to the photosynthetic apparatus of plants. Otherwise, plant metabolism impaired by Cd toxicity can be aided by a proper sulfur (S) nutrition, since S is a component of antioxidants and photosynthetic systems. In this sense, our aim in this study was to evaluate the effect of S supply (0.1, 1.9 and 3.7 mmol L-1) on the antioxidant and photosynthetic systems of Massai grass exposed to Cd (0.0, 0.1 and 0.5 mmol L-1) as a prerequisite for studies of Cd phytoextraction. Plants supplied with 3.7 mmol L-1 S showed the lowest growth and the highest Cd concentrations when exposed to 0.5 mmol L-1 Cd. However, Massai grass grown without S deprivation showed the highest GSH + GSSG concentrations in all tissues and the lowest lipid peroxidation in stems and sheaths and roots. In general, the activities of SOD, CAT, APX, GPOX and GR did not increase in Massai grass exposed to Cd, even with proper S supply. Severe Cd exposure halted the photosynthetic system of Massai grass, although well-nourished plants with S exhibited greatest photosynthetic rate probably due to the action of GSH in the control of the oxidative stress. Univ Sao Paulo, Ctr Nucl Energy Agr, BR-13416000 Piracicaba, Brazil Hasselt Univ, Ctr Environm Sci, B-3590 Diepenbeek, Belgium Univ Fed Acre, BR-69920900 Rio Branco, Brazil Univ Sao Paulo, Luiz de Queiroz Coll Agr, BR-13418900 Piracicaba, Brazil Sao Paulo State Univ, BR-17602496 Tupa, Brazil Sao Paulo State Univ, BR-17602496 Tupa, Brazil CAPES: 1332394 FAPESP: 2014/16731-7 FAPESP: 2014/18735-0 FAPESP: 2015/21562-2 FAPESP: 2017/12367-7

Cadmium (Cd) is a non-essential, toxic heavy metal that poses serious threats to both ecosystems and human health. Plants employ various cellular and molecular mechanisms to minimise the impact of Cd toxicity and cell walls function as a defensive barrier during Cd exposure. In this study, we adopted a quantitative gel-based proteomic approach (two-dimensional difference gel electrophoresis) to investigate changes in the abundance of cell wall and soluble proteins in stems of Medicago sativa L. upon long-term exposure to Cd (10mg Cdkg(-1) soil as CdSO4). Obtained protein data were complemented with targeted gene expression analyses. Plants were affected by Cd exposure at an early growth stage but seemed to recover at a more mature stage as no difference in biomass was observed. The accumulation of Cd was highest in roots followed by stems and leaves. Quantitative proteomics revealed a changed abundance for 179 cell wall proteins and 30 proteins in the soluble fraction upon long-term Cd exposure. These proteins are involved in cell wall remodelling, defence response, carbohydrate metabolism and promotion of the lignification process. The data indicate that Cd exposure alters the cell wall proteome and underline the role of cell wall proteins in defence against Cd stress. The identified proteins are linked to alterations in cell wall structure and lignification process in stems of M.sativa, underpinning the function of the cell wall as an effective barrier against Cd stress. This study is funded as part of the bilateral project CadWALL (FNR/FWO project INTER/FWO/12/14). JFH, JR, AC, KS and AG conceived the study and designed the experiments. AG prepared the samples and performed most of the experiments and data analyses. JR, GG, KS and EK contributed to analysis of the data. AG and KS drafted the manuscript, which was critically revised and approved by all co-authors. We especially thank Sebastien Planchon and Laurent Solinhac for technical assistance, Johanna Ziebel for performing all ICP-MS analyses and Bruno Printz for support. We thank the group of Ann Cuypers at the University of Hasselt for great support and assistance in performing the gene expression analysis.

Cadmium (Cd) exposure causes an oxidative challenge and inhibits cell cycle progression, ultimately impacting plant growth. Stress-induced effects on the cell cycle are often a consequence of activation of the DNA damage response (DDR). The main aim of this study was to investigate the role of the transcription factor SUPPRESSOR OF GAMMA RESPONSE 1 (SOG1) and three downstream cyclin-dependent kinase inhibitors of the SIAMESE-RELATED (SMR) family in the Cd-induced DDR and oxidative challenge in leaves of Arabidopsis thaliana. Effects of Cd on plant growth, cell cycle regulation and the expression of DDR genes were highly similar between the wildtype and smr4/5/7 mutant. In contrast, sog1-7 mutant leaves displayed a much lower Cd sensitivity within the experimental time-frame and significantly less pronounced upregulations of DDR-related genes, indicating the involvement of SOG1 in the Cd-induced DDR. Cadmium-induced responses related to the oxidative challenge were disturbed in the sog1-7 mutant, as indicated by delayed Cd-induced increases of hydrogen peroxide and glutathione concentrations and lower upregulations of oxidative stress-related genes. In conclusion, our results attribute a novel role to SOG1 in regulating the oxidative stress response and connect oxidative stress to the DDR in Cd-exposed plants.

Background The heavy metal cadmium (Cd) accumulates in the environment due to anthropogenic influences. It is unessential and harmful to all life forms. The plant cell wall forms a physical barrier against environmental stress and changes in the cell wall structure have been observed upon Cd exposure. In the current study, changes in the cell wall composition and structure of Medicago sativa stems were investigated after long-term exposure to Cd. Liquid chromatography coupled to mass spectrometry (LC-MS) for quantitative protein analysis was complemented with targeted gene expression analysis and combined with analyses of the cell wall composition. Results Several proteins determining for the cell wall structure changed in abundance. Structural changes mainly appeared in the composition of pectic polysaccharides and data indicate an increased presence of xylogalacturonan in response to Cd. Although a higher abundance and enzymatic activity of pectin methylesterase was detected, the total pectin methylation was not affected. Conclusions An increased abundance of xylogalacturonan might hinder Cd binding in the cell wall due to the methylation of its galacturonic acid backbone. Probably, the exclusion of Cd from the cell wall and apoplast limits the entry of the heavy metal into the symplast and is an important factor during tolerance acquisition. Electronic supplementary material The online version of this article (10.1186/s12870-019-1859-y) contains supplementary material, which is available to authorized users.

The mitochondrial alternative oxidase (AOX) is suggested to modulate metabolic homeostasis and is activated at transcript and protein level in cadmium (Cd)-exposed Arabidopsis thaliana plants. In this study, the importance of AOX in the metabolic response to Cd stress was investigated. Primary metabolites were determined in roots and leaves of wild-type plants and aox1a knockout mutants exposed to sublethal Cd concentrations for 24 and 72 h. Additional parameters (gene expression, enzyme capacity) were included to support the results at the metabolic level. Whereas sugar and organic acid levels initially dropped in roots, the opposite was observed in leaves of both genotypes exposed to Cd. Regarding genotype-related differences in the leaves, our results strengthen the previously proposed link between AOX1a and ethylene. Furthermore, respiratory flux dependent on amino acid degradation could be related to AOX1a function in leaves of Cd-exposed plants. In roots, a relation between AOX1a and antioxidative defence – in particular glutathione (GSH) – is demonstrated during Cd exposure. In conclusion, A. thaliana plants show a tissue-specific response to sublethal Cd exposure, with a differential role for AOX1a in roots and leaves.

The natural accession Columbia (Col-0) is considered as the reference genome of the model plant Arabidopsis thaliana. Nonetheless, Col-0 plants are more sensitive to excess copper (Cu) and cadmium (Cd) than other widely used accessions such as Wassilewskija (Ws) plants. In the current study, this accession-specific metal sensitivity is further explored by comparing the responses in leaves of Col-0 and Ws plants exposed to excess Cu and Cd. Our results suggest that different life strategies favored by both accessions under physiological conditions affect their response to metal exposure. While Col-0 plants mainly invest in metal detoxification, Ws plants center on nutrient homeostasis. In particular, the higher expression of genes related to Cu homeostasis genes in non-exposed conditions indicates that Ws plants possess a constitutively efficient metal homeostasis. On the other hand, oxidative stress-related MAPK signaling appears to be boosted in leaves of Col-0 plants exposed to excess Cu. Furthermore, the upregulation of the glutathione (GSH) biosynthesis GSH2 gene and the increased GSH concentration after Cd exposure suggest the activation of detoxification mechanisms, such as phytochelatin production, to counteract the more severe Cd-induced oxidative stress in leaves of Col-0 plants. Exposure to Cd also led to a more pronounced ethylene signaling response in leaves of Col-0 as compared to Ws plants, which could be related to Cd-induced GSH metabolism. In conclusion, accession-specific life strategies clearly affect the way in which leaves of A. thaliana plants cope with excess Cu and Cd. This work was supported by BOF funding from Hasselt University through a PhD grant for Rafaela Amaral dos Reis and the Research Foundation Flanders (FWO) by a postdoctoral grant for Els Keunen. Additional funding came from FWO projects [G0D3414; G0B6716].

Sugar, amino and organic acid, glutathione (GSH) and phytochelatin (PC) content in plant tissues can be altered by S nutrition, and these metabolites have the potential to lower a plant’s susceptibility to cadmium (Cd) toxicity. In the current study, our aim was to analyze the effect of S nutrition on the metabolic profile and the synthesis of GSH and PCs in Panicum maximum cv. Massai (Massai grass) used for Cd phytoextraction, since this forage grass shows fast growth, high biomass production, and adaptation to soil and climatic adversities. We evaluated in a greenhouse combinations of three S (0.1, 1.9 and 3.7 mmol L−1) and two Cd concentrations (0.1 and 0.5 mmol L−1) in nutrient solution, within a single growth period. The tissues of plants exposed to Cd showed distinct responses related to the primary metabolism. Tryptophan, lysine, and histidine were more accumulated in all tissues when Massai grass was exposed to Cd and was grown with 1.9 and/or 3.7 mmol L−1 S, which indicates that these amino acids are probably involved in Cd accumulation and detoxification in this plant. Among the sugars and sugar derivatives detected, galactinol appears to be the most active in decrease Cd-induced oxidative stress. Although there was no effect of Cd/S combinations on the expression of the genes encoding GSH1 and PCS2, the levels of GSH and PCs were strongly increased by Cd, mainly in roots and samples comprising both stem and sheath material of Massai grass and independently of different metabolic changes occurring in these tissues. Synthesis of the majority of metabolites evaluated in this study was mostly induced when Cd-exposed Massai grass was supplied with 1.9 mmol L−1 S, but more studies in field conditions are necessary to define the optimal S concentration for the plants grown in soil.

Cadmium (Cd) induces the generation of reactive oxygen species (ROS) and stimulates ethylene biosynthesis. The phytohormone ethylene is a regulator of many developmental and physiological plant processes as well as stress responses. Previous research indicated various links between ethylene signalling and oxidative stress. Our results support a correlation between the Cd-induced oxidative challenge and ethylene signalling in Arabidopsis thaliana leaves. The effects of 24 or 72 h exposure to 5 μM Cd on plant growth and several oxidative stress-related parameters were compared between wild-type (WT) and ethylene insensitive mutants (etr1-1, ein2-1, ein3-1). Cadmium-induced responses observed in WT plants were mainly affected in etr1-1 and ein2-1 mutants, of which the growth was less inhibited by Cd exposure as compared to WT and ein3-1 mutants. Both etr1-1 and ein2-1 showed a delayed response in the glutathione (GSH) metabolism, including GSH levels and transcript levels of GSH synthesising and recycling enzymes. Furthermore, the expression of different oxidative stress marker genes was significantly lower in Cd-exposed ein2-1 mutants, evidencing that ethylene signalling is involved in early responses to Cd stress. A model for the cross-talk between ethylene signalling and oxidative stress is proposed.

Cadmium (Cd) pollution renders many soils across the world unsuited or unsafe for food- or feed-orientated agriculture. The main mechanism of Cd phytotoxicity is the induction of oxidative stress, amongst others through the depletion of glutathione. Oxidative stress can damage lipids, proteins, and nucleic acids, leading to growth inhibition or even cell death. The plant cell has a variety of tools to defend itself against Cd stress. First and foremost, cell walls might prevent Cd from entering and damaging the protoplast. Both the primary and secondary cell wall have an array of defensive mechanisms that can be adapted to cope with Cd. Pectin, which contains most of the negative charges within the primary cell wall, can sequester Cd very effectively. In the secondary cell wall, lignification can serve to immobilize Cd and create a tougher barrier for entry. Changes in cell wall composition are, however, dependent on nutrients and conversely might affect their uptake. Additionally, the role of ascorbate (AsA) as most important apoplastic antioxidant is of considerable interest, due to the fact that oxidative stress is a major mechanism underlying Cd toxicity, and that AsA biosynthesis shares several links with cell wall construction. In this review, modifications of the plant cell wall in response to Cd exposure are discussed. Focus lies on pectin in the primary cell wall, lignification in the secondary cell wall and the importance of AsA in the apoplast. Regarding lignification, we attempt to answer the question whether increased lignification is merely a consequence of Cd toxicity, or rather an elicited defense response. We propose a model for lignification as defense response, with a central role for hydrogen peroxide as substrate and signaling molecule.

Environmental contamination with metals and organic compounds poses a serious threat to human health. Investigating plant responses to these contaminants at the molecular and cellular level is crucial to optimize phytoremediation strategies to clean up contaminated soils. Two key players in plant stress responses are the sulphur-containing amino acids cysteine and methionine. Cysteine is an important constituent of the metal-chelating metallothioneins and is also the precursor for glutathione and subsequent phytochelatin synthesis. During stress conditions, glutathione is involved in (1) metal chelation, (2) xenobiotic detoxification and (3) antioxidative defence. The activated form of methionine, S-adenosylmethionine, is involved in the synthesis of ethylene and polyamines, both playing important roles in signal transduction. This review provides an overview of sulphur uptake and assimilation and its conversion into basic metabolites essential for detoxification and signal transduction during metal and organic contaminant exposure in plants. Furthermore, the cross talk between these pathways and their relation to the contaminant-induced oxidative challenge are discussed.

Reverse transcription-quantitative PCR (RT-qPCR) has been widely adopted to measure differences in mRNA levels; however, biological and technical variation strongly affects the accuracy of the reported differences. RT-qPCR specialists have warned that, unless researchers minimize this variability, they may report inaccurate differences and draw incorrect biological conclusions. The Minimum Information for Publication of Quantitative Real-Time PCR Experiments (MIQE) guidelines describe procedures for conducting and reporting RT-qPCR experiments. The MIQE guidelines enable others to judge the reliability of reported results; however, a recent literature survey found low adherence to these guidelines. Additionally, even experiments that use appropriate procedures remain subject to individual variation that statistical methods cannot correct. For example, since ideal reference genes do not exist, the widely used method of normalizing RT-qPCR data to reference genes generates background noise that affects the accuracy of measured changes in mRNA levels. However, current RT-qPCR data reporting styles ignore this source of variation. In this commentary, we direct researchers to appropriate procedures, outline a method to present the remaining uncertainty in data accuracy, and propose an intuitive way to select reference genes to minimize uncertainty. Reporting the uncertainty in data accuracy also serves for quality assessment, enabling researchers and peer reviewers to confidently evaluate the reliability of gene expression data.

Cadmium (Cd) is a toxic metal with a long half-life in biological systems. This half-life is partly as a result of metallothioneins (MTs), metal-binding proteins with a high affinity for Cd. The high retention properties of the kidneys reside in proximal tubular cells that possess transport mechanisms for Cd-MT uptake, ultimately leading to more Cd accumulation. Researchers have studied MT-metal interactions using various techniques including quantitative real-time PCR (qPCR), an efficient tool for quantifying gene expression. Often a poor choice of reference genes, which is represented by their instability and condition dependency, leads to inefficient normalization of gene expression data and misinterpretations. This study demonstrates the importance of an efficient normalization strategy in toxicological research. A selection of stable reference genes was proposed in order to acquire reliable and reproducible gene quantification under metal stress using MT expression as an example. Moreover, in vitro and in vivo setups were compared to identify the influence of toxicological compounds in function of the experimental design. This study shows that glyceraldehyde-3-phosphate dehydrogenase (Gapdh), tyrosine monooxygenase/tryptophan5-monooxygenase activation-protein, zeta polypeptide (Ywhaz) and beta-actin (Actb) are the most stable reference genes in a kidney proximal tubular cell line exposed to moderate and high Cd concentrations, applied as CdCl2 . A slightly different sequence in reference gene stability was found in renal cells isolated from rats in vivo exposed to Cd. It was further shown that three reference genes are required for efficient normalization in this experimental setup. This study demonstrates the importance of an efficient normalization strategy in toxicological research.

This study aims to uncover the spatiotemporal involvement of glutathione (GSH) in two major mechanisms of cadmium (Cd)-induced detoxification (i.e. chelation and antioxidative defence). A kinetic study was conducted on hydroponically grown Arabidopsis thaliana (L. Heyhn) to gain insight into the early events after exposure to Cd. Cadmium detoxification was investigated at different levels, including gene transcripts, enzyme activities and metabolite content. Data indicate a time-dependent response both within roots and between plant organs. Early on in roots, GSH was preferentially allocated to phytochelatin (PC) synthesis destined for Cd chelation. This led to decreased GSH levels, without alternative pathways activated to complement GSH's antioxidative functions. After one day however, multiple antioxidative pathways increased including superoxide dismutase (SOD), ascorbate (AsA) and catalase (CAT) to ensure efficient neutralization of Cd-induced reactive oxygen species (ROS). As a consequence of Cd retention and detoxification in roots, a delayed response occurred in leaves. Together with high leaf thiol contents and possibly signalling responses from the roots, the leaves were protected, allowing them sufficient time to activate their defence mechanisms. © 2014 Elsevier Masson SAS.

Plant growth and development can be highly restricted by environmental stressors such as cadmium (Cd) pollution. The mitochondrial non-phosphorylating alternative respiratory pathway, mediated by alternative oxidase (AOX), alternative NAD(P)H dehydrogenases (NDs) and uncoupling protein (UCP), was suggested to be crucial in the acclimation of plants to fluctuating environmental conditions. Therefore, we examined the effects of environmentally realistic Cd exposure (5 and 10 μM) on the alternative respiratory chain in Arabidopsis thaliana using a kinetic exposure setup. We demonstrated that during exposure to Cd, Arabidopsis seedlings show a mitochondrial oxidative challenge to which they acutely respond by increasing the transcript level of several AOX , ND and UCP isoforms in both roots and leaves. In addition, AOX protein levels increased during acute Cd exposure (2 and 24 h). Based on our data, we suggest the formation of a condensed non-phosphorylating electron transport chain (ETC) functioning through cytosolic NDs and AOX, with co-regulation of ND and AOX expression during Cd stress. Therefore, both enzymes might cooperate in the potential acclimation of Arabidopsis seedlings to environmentally realistic Cd exposure by modulating the extent of mitochondrial ROS production.

Plants suffering from abiotic stress are commonly facing an enhanced accumulation of reactive oxygen species (ROS) with damaging as well as signalling effects at organellar and cellular levels. The outcome of an environmental challenge highly depends on the delicate balance between ROS production and scavenging by both enzymatic and metabolic antioxidants. However, this traditional classification is in need of renewal and reform, as it is becoming increasingly clear that soluble sugars such as disaccharides, raffinose family oligosaccharides and fructans--next to their associated metabolic enzymes--are strongly related to stress-induced ROS accumulation in plants. Therefore, this review aims at extending the current concept of antioxidants functioning during abiotic stress, with special focus on the emanate role of sugars as true ROS scavengers. Examples are given based on their cellular location, as different organelles seem to exploit distinct mechanisms. Moreover, the vacuole comes into the picture as important player in the ROS signalling network of plants. Elucidating the interplay between the mechanisms controlling ROS signalling during abiotic stress will facilitate the development of strategies to enhance crop tolerance to stressful environmental conditions.

Plants exposed to excess metals are challenged by an increased generation of reactive oxygen species (ROS) such as superoxide ([Formula: see text]), hydrogen peroxide (H2O2) and the hydroxyl radical ((•)OH). The mechanisms underlying this oxidative challenge are often dependent on metal-specific properties and might play a role in stress perception, signaling and acclimation. Although ROS were initially considered as toxic compounds causing damage to various cellular structures, their role as signaling molecules became a topic of intense research over the last decade. Hydrogen peroxide in particular is important in signaling because of its relatively low toxicity, long lifespan and its ability to cross cellular membranes. The delicate balance between its production and scavenging by a plethora of enzymatic and metabolic antioxidants is crucial in the onset of diverse signaling cascades that finally lead to plant acclimation to metal stress. In this review, our current knowledge on the dual role of ROS in metal-exposed plants is presented. Evidence for a relationship between H2O2 and plant metal tolerance is provided. Furthermore, emphasis is put on recent advances in understanding cellular damage and downstream signaling responses as a result of metal-induced H2O2 production. Finally, special attention is paid to the interaction between H2O2 and other signaling components such as transcription factors, mitogen-activated protein kinases, phytohormones and regulating systems (e.g. microRNAs). These responses potentially underlie metal-induced senescence in plants. Elucidating the signaling network activated during metal stress is a pivotal step to make progress in applied technologies like phytoremediation of polluted soils.

Metals, like cadmium (Cd) and copper (Cu), have the ability to induce the production of reactive oxygen species (ROS) at the cellular level. It is widely known that these ROS can cause irreversible damage to cellular components, like DNA, proteins and lipids. On the other hand, ROS can also act as signaling molecules and in this way they play an essential role in many normal physiological processes, but also in defense responses against stress. ROS signaling in plants uses mitogen-activated protein kinase (MAPK) pathways leading to the transcriptional control of target genes involved in the scavenging or production of ROS. Here, oxidative signaling induced by exposure to excess Cd or Cu was investigated in relation to anti-oxidative defense responses to these metals. Three-week-old Arabidopsis thaliana plants were exposed to environmentally realistic concentrations of Cu and Cd and immediate responses were measured at the level of hydrogen peroxide (H2O2) content, lipid peroxidation and transcript levels of genes involved in ROS homeostasis and signaling. Our findings show immediate (after 2 h exposure) effects in the roots following Cu exposure, whereas effects in the leaves were generally more delayed. Effects of Cd exposure in leaves and roots were observed only after 24 h exposure. On one hand, exposure of roots to Cu leads via activation of NADPH oxidases and Fenton reactions to H2O2 production that can induce MAPK and oxylipin signaling to control the cellular redox status. On the other hand, conversion of H2O2 to the more damaging hydroxyl radical by Fenton and Haber–Weiss reactions can initiate lipid peroxidation leading to membrane damage. In roots exposed to elevated Cd concentrations only oxidative signaling was initiated, possibly via NADPH oxidase-mediated ROS production. In leaves, time-dependent activation of MAPK and oxylipin signaling was seen after exposure to both metals, Cu or Cd, independent of changes in H2O2 content.

Plant responses to the toxic effects of soil contaminants, such as excess metals or organic substances, have been studied mainly at physiological, biochemical and molecular levels, but the influence on root system architecture has received little attention. Nevertheless, the precise position, morphology and extent of roots can influence contaminant uptake. Here, data are discussed that aim to increase the molecular and ecological understanding of the influence of contaminants on root system architecture. Furthermore, the potential of plant-associated bacteria to influence root growth by their growth-promoting and stress-relieving capacities is explored.Root growth parameters of Arabidopsis thaliana seedlings grown in vertical agar plates are quantified. Mutants are used in a reverse genetics approach to identify molecular components underlying quantitative changes in root architecture after exposure to excess cadmium, copper or zinc. Plant-associated bacteria are isolated from contaminated environments, genotypically and phenotypically characterized, and used to test plant root growth improvement in the presence of contaminants.The molecular determinants of primary root growth inhibition and effects on lateral root density by cadmium were identified. A vertical split-root system revealed local effects of cadmium and copper on root development. However, systemic effects of zinc exposure on root growth reduced both the avoidance of contaminated areas and colonization of non-contaminated areas. The potential for growth promotion and contaminant degradation of plant-associated bacteria was demonstrated by improved root growth of inoculated plants exposed to 2,4-di-nitro-toluene (DNT) or cadmium.Knowledge concerning the specific influence of different contaminants on root system architecture and the molecular mechanisms by which this is achieved can be combined with the exploitation of plant-associated bacteria to influence root development and increase plant stress tolerance, which should lead to more optimal root systems for application in phytoremediation or safer biomass production.

Phytoextraction has a promising potential as an environmentally friendly clean-up method for soils contaminated with toxic metals. To improve the development of efficient phytoextraction strategies, better knowledge regarding metal uptake, translocation and detoxification in planta is a prerequisite. This review highlights our current understanding on these mechanisms, and their impact on plant growth and health. Special attention is paid to the central role of glutathione (GSH) in this process. Because of the high affinity of metals to thiols and as a precursor for phytochelatins (PCs), GSH is an essential metal chelator. Being an important antioxidant, a direct link between metal detoxification and the oxidative challenge in plants growing on contaminated soils is observed, where GSH could be a key player. In addition, as redox couple, oxidized and reduced GSH transmits specific information, in this way tuning cellular signalling pathways under environmental stress conditions. Possible improvements of phytoextraction could be achieved by using transgenic plants or plant-associated microorganisms. Joined efforts should be made to cope with the challenges faced with phytoextraction in order to successfully implement this technique in the field.

The phytohormone ethylene is known to mediate a diverse array of signaling processes during abiotic stress in plants. Whereas many reports have demonstrated enhanced ethylene production in metal-exposed plants, the underlying molecular mechanisms are only recently investigated. Increasing evidence supports a role for ethylene in the regulation of plant metal stress responses. Moreover, crosstalk appears to exist between ethylene and the cellular redox balance, nutrients and other phytohormones. This review highlights our current understanding of the key role ethylene plays during responses to metal exposure. Moreover, particular attention is paid to the integration of ethylene within the broad network of plant responses to metal stress.

Lipoxygenases (LOXes, EC 1.13.11.12) are involved in growth, development and responses to stress. Earlier results suggested a role in stress generation, signalling and/or responses when Arabidopsis thaliana is exposed to cadmium (Cd), and expression of the cytosolic LOX1 was highly upregulated in the roots after Cd exposure. To investigate the involvement of LOX1 in early metal stress responses, three-week-old wild-type and lox1-1 mutant A. thaliana plants were acutely (24 h) exposed to realistic Cd concentrations (5 and 10 mu M) and several oxidative stress and signalling related parameters were studied at transcriptional and biochemical levels. Transcription of several genes encoding ROS producing and scavenging enzymes failed to be induced up to wild-type levels after Cd exposure. Expression of 9-LOX enzymes was inhibited in lox1-1 mutant roots due to lack of functional LOX1 and downregulated LOX5 expression, and the lox1-1 mutation also interfered with the expression of genes involved in jasmonate biosynthesis. LOX1 and RBOHD may be involved in stress signalling from roots to shoots, as the induction of APX2 expression, which is dependent on RBOHD activity, was disrupted in lox1-1 while RBOHD failed to be upregulated. A different pattern of H2O2 production and ascorbate and glutathione levels in lox1-1 mutants after Cd exposure may have indirectly influenced gene expression patterns. Although indirect effects of the lox1-1 mutation on gene expression complicate the determination of exact sensing - signalling - response pathways, the results presented here outline a more refined LOX1 functioning in Cd-induced stress responses that could be used in studies determining the exact involvement of LOX1 in these pathways. (c) 2012 Elsevier Masson SAS. All rights reserved.

Plants suffering from abiotic stress are commonly facing an enhanced accumulation of reactive oxygen species (ROS) with damaging as well as signalling effects at organellar and cellular levels. The outcome of an environmental challenge highly depends on the delicate balance between ROS production and scavenging by both enzymatic and metabolic antioxidants. However, this traditional classification is in need of renewal and reform, as it is becoming increasingly clear that soluble sugars such as disaccharides, raffinose family oligosaccharides and fructans--next to their associated metabolic enzymes--are strongly related to stress-induced ROS accumulation in plants. Therefore, this review aims at extending the current concept of antioxidants functioning during abiotic stress, with special focus on the emanate role of sugars as true ROS scavengers. Examples are given based on their cellular location, as different organelles seem to exploit distinct mechanisms. Moreover, the vacuole comes into the picture as important player in the ROS signalling network of plants. Elucidating the interplay between the mechanisms controlling ROS signalling during abiotic stress will facilitate the development of strategies to enhance crop tolerance to stressful environmental conditions.

Contamination of soils with the potentially toxic elements cadmium (Cd) and copper (Cu) affects plant growth and crop production, and bioaccumulation in the food chain poses a threat to human health. Toxic levels of Cd or Cu both impose an oxidative challenge on plants, even though these trace elements have a different chemical (non-redox active versus redox-active) and biological (non-essential versus essential element) behaviour. Through (in)direct mechanisms, Cd and Cu cause an increased production of reactive oxygen species (ROS) as well as interference with redox-regulated compounds in different cellular compartments. This chapter highlights general and/or specific mechanisms of interference with the cellular redox homeostasis by Cd and Cu, which may be part of the sensing mechanism to these stresses. Furthermore, it emphasises the metal-induced oxidative challenge and its involvement in either cellular damage and/or downstream signalling responses.

A general status of oxidative stress in plants caused by exposure to elevated metal concentrations in the environment coincides with a constraint on mitochondrial electron transport, which enhances ROS accumulation at the mitochondrial level. As mitochondria are suggested to be involved in redox signaling under environmental stress conditions, mitochondrial ROS can initiate a signaling cascade mediating the overall stress response, i.e., damage versus adaptation. This review highlights our current understanding of metal-induced responses in plants, with focus on the production and detoxification of mitochondrial ROS. In addition, the potential involvement of retrograde signaling in these processes will be discussed.

Plants exposed to cadmium (Cd) show morphological and physiological disorders. To increase our knowledge regarding Cd-induced signalling, most often the effects of acute exposure are investigated. However, this does not allow in-depth analysis of morphological effects. Therefore, we chronically exposed Arabidopsis thaliana plants to environmentally realistic Cd concentrations (5 or 10 μM) and, using a described phenotypic framework methodology, we determined the impact of Cd on the plant’s ability to complete its life cycle and produce germinative seeds. Visible Cd-induced morphological changes were observed within a short exposure period, with chlorotic and anthocyanous leaf colouring occurring dose-dependently. Although rosette growth was severely reduced in Cd-exposed plants, all plants were able to emerge inflorescences and produce siliques containing germinative seeds, thus confirming the non-lethality of the used Cd concentrations. Although the growth inhibition of Cd-exposed plants was dependent on the dose, both concentrations had similar effects on inflorescence height and silique counts. In conclusion, vegetative growth of plants chronically exposed to Cd is inhibited in a concentration-dependent manner. However, the effect on plant regeneration is clearly stress-determined but independent on the Cd concentration applied. In Arabidopsis thaliana, vegetative and reproductive growth are differentially influenced by Cd.

The cellular redox state is an important determinant of metal phytotoxicity. In this study we investigated the influence of cadmium (Cd) and copper (Cu) stress on the cellular redox balance in relation to oxidative signalling and damage in Arabidopsis thaliana. Both metals were easily taken up by the roots, but the translocation to the aboveground parts was restricted to Cd stress. In the roots, Cu directly induced an oxidative burst, whereas enzymatic ROS (reactive oxygen species) production via NADPH oxidases seems important in oxidative stress caused by Cd. Furthermore, in the roots, the glutathione metabolism plays a crucial role in controlling the gene regulation of the antioxidative defence mechanism under Cd stress. Metal-specific alterations were also noticed with regard to the microRNA regulation of CuZnSOD gene expression in both roots and leaves. The appearance of lipid peroxidation is dual: it can be an indication of oxidative damage as well as an indication of oxidative signalling as lipoxygenases are induced after metal exposure and are initial enzymes in oxylipin biosynthesis. In conclusion, the metal-induced cellular redox imbalance is strongly dependent on the chemical properties of the metal and the plant organ considered. The stress intensity determines its involvement in downstream responses in relation to oxidative damage or signalling.

At the cellular level, cadmium (Cd) induces both damaging and repair processes in which the cellular redox status plays a crucial role. Being not redox-active, Cd is unable to generate reactive oxygen species (ROS) directly, but Cd-induced oxidative stress is a common phenomenon observed in multiple studies. The current review gives an overview on Cd-induced ROS production and anti-oxidative defense in organisms under different Cd regimes. Moreover, the Cd-induced oxidative challenge is discussed with a focus on damage and signaling as downstream responses. Gathering these data, it was clear that oxidative stress related responses are affected during Cd stress, but the apparent discrepancies observed in between the different studies points towards the necessity to increase our knowledge on the spatial and temporal ROS signature under Cd stress. This information is essential in order to reveal the exact role of Cd-induced oxidative stress in the modulation of downstream responses under a diverse array of conditions.