Proteomic analysis of Arabidopsis halleri shoots in response to the heavy metals cadmium and zinc and rhizosphere microorganisms

Cadmium tolerance and hyperaccumulation in plants - A proteomic perspective of phytoremediation

Cadmium (Cd) is a major environmental pollutant and poses a risk of transfer into the food chain through contaminated plants. Mechanisms underlying Cd tolerance and hyperaccumulation in plants are not fully understood. Proteomics-based approaches facilitate an in-depth understanding of plant responses to Cd stress at the systemic level by identifying Cd-inducible differentially abundant proteins (DAPs). In this review, we summarize studies related to proteomic changes associated with Cd-tolerance mechanisms in Cd-tolerant crops and Cd-hyperaccumulating plants, especially the similarities and differences across plant species. The enhanced DAPs identified through proteomic studies can be potential targets for developing Cd-hyperaccumulators to remediate Cd-contaminated environments and Cd-tolerant crops with low Cd content in the edible organs. This is of great significance for ensuring the food security of an exponentially growing global population. Finally, we discuss the methodological drawbacks in current proteomic studies and propose that better protocols and advanced techniques should be utilized to further strengthen the reliability and applicability of future Cd-stress-related studies in plants. This review provides insights into the improvement of phytoremediation efficiency and an in-depth study of the molecular mechanisms of Cd enrichment in plants.

Key Pathways and Genes of Arabidopsis thaliana and Arabidopsis halleri Roots under Cadmium Stress Responses: Differences and Similarities

Cadmium (Cd) is among the world's major health concerns, as it renders soils unsuitable and unsafe for food and feed production. Phytoremediation has the potential to remediate Cd-polluted soils, but efforts are still needed to develop a deep understanding of the processes underlying it. In this study, we performed a comprehensive analysis of the root response to Cd stress in A. thaliana, which can phytostabilize Cd, and in A. halleri, which is a Cd hyperaccumulator. Suitable RNA-seq data were analyzed by WGCNA to identify modules of co-expressed genes specifically associated with Cd presence. The results evidenced that the genes of the hyperaccumulator A. halleri mostly associated with the Cd presence are finely regulated (up- and downregulated) and related to a general response to chemical and other stimuli. Additionally, in the case of A. thaliana, which can phytostabilize metals, the genes upregulated during Cd stress are related to a general response to chemical and other stimuli, while downregulated genes are associated with functions which, affecting root growth and development, determine a deep modification of the organ both at the cellular and physiological levels. Furthermore, key genes of the Cd-associated modules were identified and confirmed by differentially expressed gene (DEG) detection and external knowledge. Together, key functions and genes shed light on differences and similarities among the strategies that the plants use to cope with Cd and may be considered as possible targets for future research.

Diversity and activity of soil biota at a post-mining site highly contaminated with Zn and Cd are enhanced by metallicolous compared to non-metallicolous Arabidopsis halleri ecotypes

Hyperaccumulators' ability to take up large quantities of harmful heavy metals from contaminated soils and store them in their foliage makes them promising organisms for bioremediation. Here we demonstrate that some ecotypes of the zinc hyperaccumulator Arabidopsis halleri are more suitable for bioremediation than others, because of their distinct influence on soil biota. In a field experiment, populations originating from metal-polluted and unpolluted soils were transplanted to a highly contaminated metalliferous site in Southern Poland. Effects of plant ecotypes on soil biota were assessed by measurements of feeding activity of soil fauna (bait-lamina test) and catabolic activity and functional diversity of soil bacteria underneath A. halleri plants (Biolog® ECO plates). Chemical soil properties, plant morphological parameters, and zinc concentration in shoots and roots were additionally evaluated. Higher soil fauna feeding activity and higher bacterial community functional diversity were found in soils affected by A. halleri plants originating from metallicolous compared to non-metallicolous ecotypes. Differences in community-level physiological profiles further evidenced changes in microbial communities in response to plant ecotype. These soil characteristics were positively correlated with plant size. No differences in zinc content in shoots and roots, zinc translocation ratio, and plant morphology were observed between metallicolous and non-metallicolous plants. Our results indicate strong associations between A. halleri ecotype and soil microbial community properties. In particular, the improvement of soil biological properties by metallicolous accessions should be further explored to optimize hyperaccumulator-based bioremediation technologies.

Protein Analysis of A. halleri and N. caerulescens Hyperaccumulators When Exposed to Nano and Ionic Forms of Cd and Zn

Hyperaccumulator plant species growing on metal-rich soils can accumulate high quantity of metals and metalloids in aerial tissues, and several proteomic studies on the molecular mechanisms at the basis of metals resistance and hyperaccumulation have been published. Hyperaccumulator are also at the basis of the phytoremediation strategy to remove metals more efficiently from polluted soils or water. Arabidopsis halleri and Noccea caerulescens are both hyperaccumulators of metals and nano-metals. In this study, the change in some proteins in A. halleri and N. caerulescens was assessed after the growth in soil with cadmium and zinc, provided as sulphate salts (CdSO₄ and ZnSO₄) or sulfide quantum dots (CdS QDs and ZnS QDs). The protein extracts obtained from plants after 30 days of growth were analyzed by 2D-gel electrophoresis (2D SDS-PAGE) and identified by MALDI-TOF/TOF mass spectrometry. A bioinformatics analysis was carried out on quantitative protein differences between control and treated plants. In total, 43 proteins resulted in being significatively modulated in A. halleri, while 61 resulted in being modulated in N. caerulescens. Although these two plants are hyperaccumulator of both metals and nano-metals, at protein levels the mechanisms involved do not proceed in the same way, but at the end bring a similar physiological result.

Root and shoot biology of Arabidopsis halleri dissected by WGCNA: an insight into the organ pivotal pathways and genes of an hyperaccumulator

Arabidopsis halleri is a hyperaccumulating pseudo-metallophyte and an emerging model to explore molecular basis of metal tolerance and hyperaccumulation. In this regard, understanding of interacting genes can be a crucial aspect as these interactions regulate several biological functions at molecular level in response to multiple signals. In this current study, we applied a weighted gene co-expression network analysis (WGCNA) on root and shoot RNA-seq data of A. halleri to predict the related scale-free organ specific co-expression networks, for the first time. A total of 19,653 genes of root and 18,081 genes of shoot were grouped into 14 modules and subjected to GO and KEGG enrichment analysis. "Photosynthesis" and "photosynthesis-antenna proteins" were identified as the most enriched and common pathway to both root and shoot. Whereas "glucosinolate biosynthesis," "autophagy," and "SNARE interactions in vesicular transport" were specific to root, and "circadian rhythm" was found to be enriched only in shoot. Later, hub and bottleneck genes were identified in each module by using cytoHubba plugin based on Cytoscape and scoring the relevance of each gene to the topology of the network. The modules with the most significant differential expression pattern across control and treatment (Cd-Zn treatment) were selected and their hub and bottleneck genes were screened to validate their possible involvement in heavy metal stress. Moreover, we combined the analysis of co-expression modules together with protein-protein interactions (PPIs), confirming some genes as potential candidates in plant heavy metal stress and as biomarkers. The results from this analysis shed the light on the pivotal functions to the hyperaccumulative trait of A. halleri, giving perspective to new paths for future research on this species.

Soil microbial community and abiotic soil properties influence Zn and Cd hyperaccumulation differently in Arabidopsis halleri

Soil contamination with trace metal(loid) elements (TME) is a global concern. This has focused interest on TME-tolerant plants, some of which can hyperaccumulate extraordinary amounts of TME into above-ground tissues, for potential treatment of these soils. However, intra-species variability in TME hyperaccumulation is not yet sufficiently understood to fully harness this potential. Particularly, little is known about the rhizosphere microbial communities associated with hyperaccumulating plants and whether or not they facilitate TME uptake. The aim of this study is to characterize the diversity and structure of Arabidopsis halleri rhizosphere-influenced and background (i.e., non-Arabidopsis) soil microbial communities in four plant populations with contrasting Zn and Cd hyperaccumulation traits, two each from contaminated and uncontaminated sites. Microbial community properties were assessed along with geographic location, climate, abiotic soil properties, and plant parameters to explain variation in Zn and Cd hyperaccumulation. Site type (TME-contaminated vs. uncontaminated) and location explained 44% of bacterial/archaeal and 28% of fungal community variability. A linear discriminant effect size (LEfSe) analysis identified a greater number of taxa defining rhizosphere microbial communities than associated background soils. Further, in TME-contaminated soils, the number of rhizosphere-defining taxa was 6-fold greater than in the background soils. In contrast, the corresponding ratio for uncontaminated sites, was 3 and 1.6 for bacteria/archaea and fungi, respectively. The variables analyzed explained 71% and 76% of the variance in Zn and Cd hyperaccumulation, respectively; however, each hyperaccumulation pattern was associated with different variables. A. halleri rhizosphere fungal richness and diversity associated most strongly with Zn hyperaccumulation, whereas soil Cd and Zn bioavailability had the strongest associations with Cd hyperaccumulation. Our results indicate strong associations between A. halleri TME hyperaccumulation and rhizosphere microbial community properties, a finding that needs to be further explored to optimize phytoremediation technology that is based on hyperaccumulation.

Competition for light induces metal accumulation in a metal hyperaccumulating plant

Plants can respond to competition with a myriad of physiological or morphological changes. Competition has also been shown to affect the foraging decisions of plants belowground. However, a completely unexplored idea is that competition might also affect plants' foraging for specific elements required to inhibit the growth of their competitors. In this study, we examined the effect of simulated competition on root foraging and accumulation of heavy metals in the metal hyperaccumulating perennial plant Arabidopsis halleri, whose metal accumulation has been shown to provide allelopathic ability. A. halleri plants originating from both metalliferous and non-metalliferous soils were grown in a "split-root" setup with one root in a high-metal pot and the other in a low-metal one. The plants were then assigned to either simulated light competition or no-competition (control) treatments, using vertical green or clear plastic filters, respectively. While simulated light competition did not induce greater root allocation into the high-metal pots, it did result in enhanced metal accumulation by A. halleri, particularly in the less metal-tolerant plants, originating from non-metalliferous soils. Interestingly, this accumulation response was particularly enhanced for zinc rather than cadmium. These results provide support to the idea that the accumulation of metals by hyperaccumulating plants can be facultative and change according to their demand following competition.

Evolution of the metal hyperaccumulation and hypertolerance traits

To succeed in life, living organisms have to adapt to the environmental issues to which they are subjected. Some plants, defined as hyperaccumulators, have adapted to metalliferous environments, acquiring the ability to tolerate and accommodate high amounts of toxic metal into their shoot, without showing symptoms of toxicity. The determinants for these traits and their mode of action have long been the subject of research, whose attention lately moved to the evolution of the hypertolerance and hyperaccumulation traits. Genetic evidence indicates that the evolution of both traits includes significant evolutionary events that result in species-wide tolerant and accumulating backgrounds. Different edaphic environments are responsible for subsequent refinement, by local adaptive processes, leading to specific strategies and various degrees of hypertolerance and hyperaccumulation, which characterize metallicolous from non-metallicolous ecotypes belonging to the same genetic unit. In this review, we overview the most updated concepts regarding the evolution of hyperaccumulation and hypertolerance, highlighting also the ecological context concerning the plant populations displaying this fascinating phenomenon.

Excess Copper-Induced Alterations of Protein Profiles and Related Physiological Parameters in Citrus Leaves

This present study examined excess copper (Cu) effects on seedling growth, leaf Cu concentration, gas exchange, and protein profiles identified by a two-dimensional electrophoresis (2-DE) based mass spectrometry (MS) approach after Citrus sinensis and Citrus grandis seedlings were treated for six months with 0.5 (control), 200, 300, or 400 μM CuCl2. Forty-one and 37 differentially abundant protein (DAP) spots were identified in Cu-treated C. grandis and C. sinensis leaves, respectively, including some novel DAPs that were not reported in leaves and/or roots. Most of these DAPs were identified only in C. grandis or C. sinensis leaves. More DAPs increased in abundances than DAPs decreased in abundances were observed in Cu-treated C. grandis leaves, but the opposite was true in Cu-treated C. sinensis leaves. Over 50% of DAPs were associated with photosynthesis, carbohydrate, and energy metabolism. Cu-toxicity-induced reduction in leaf CO2 assimilation might be caused by decreased abundances of proteins related to photosynthetic electron transport chain (PETC) and CO2 assimilation. Cu-effects on PETC were more pronounced in C. sinensis leaves than in C. grandis leaves. DAPs related to antioxidation and detoxification, protein folding and assembly (viz., chaperones and folding catalysts), and signal transduction might be involved in Citrus Cu-toxicity and Cu-tolerance.

Reduction in Hg phytoavailability in soil using Hg-volatilizing bacteria and biochar and the response of the native bacterial community

Biological approaches are considered promising and eco-friendly strategies to remediate Hg contamination in soil. This study investigated the potential of two 'green' additives, Hg-volatilizing bacteria (Pseudomonas sp. DC-B1 and Bacillus sp. DC-B2) and sawdust biochar, and their combination to reduce Hg(II) phytoavailability in soil and the effect of the additives on the soil bacterial community. The results showed that the Hg(II) contents in soils and lettuce shoots and roots were all reduced with these additives, achieving more declines of 12.3-27.4%, 24.8-57.8% and 2.0-48.6%, respectively, within 56 days of incubation compared to the control with no additive. The combination of DC-B2 and 4% biochar performed best in reducing Hg(II) contents in lettuce shoots, achieving a decrease of 57.8% compared with the control. Pyrosequencing analysis showed that the overall bacterial community compositions in the soil samples were similar under different treatments, despite the fact that the relative abundance of dominant genera altered with the additives, suggesting a relatively weak impact of the additives on the soil microbial ecosystem. The low relative abundances of Pseudomonas and Bacillus, close to the background levels, at the end of the experiment indicated a small biological disturbance of the local microbial niche by the exogenous bacteria.

Role of nitric oxide in plant responses to heavy metal stress: exogenous application versus endogenous production

Anthropogenic activities, such as industrial processes, mining, and agriculture, lead to an increase in heavy metal concentrations in soil, water, and air. Given their stability in the environment, heavy metals are difficult to eliminate and can constitute a human health risk by entering the food chain through uptake by crop plants. An excess of heavy metals is toxic for plants, which have various mechanisms to prevent their accumulation. However, once metals enter the plant, oxidative damage sometimes occurs, which can lead to plant death. Initial production of nitric oxide (NO), which may play a role in plant perception, signalling, and stress acclimation, has been shown to protect against heavy metals. Very little is known about NO-dependent mechanisms downstream from signalling pathways in plant responses to heavy metal stress. In this review, using bioinformatic techniques, we analyse studies of the involvement of NO in plant responses to heavy metal stress, its possible role as a cytoprotective molecule, and its relationship with reactive oxygen species. Some conclusions are drawn and future research perspectives are outlined to further elucidate the signalling mechanisms underlying the role of NO in plant responses to heavy metal stress.

Isolation and expression analysis of cadmium-induced genes from Cd/Mn hyperaccumulator Phytolacca americana in response to high Cd exposure

Phytolacca americana is recognised as a hyperaccumulator that accumulates cadmium (Cd) and manganese (Mn). Although most studies have provided abundant physiological evidence, little is known about the molecular mechanisms of Cd accumulation in P. americana. In this study, Cd-induced genes were isolated using suppression subtractive hybridisation (SSH) library construction, and gene expression patterns under Cd stress were quantified using real-time quantitative PCR. The functions of PaGST, PaMT and PaFe-SOD were confirmed in transformant yeast. Reactive oxygen species (ROS) formation and cell death in root tips were detected, and SOD and POD activities in leaf tissue were also analysed. There were about 447 expressed sequence tags (ESTs) identified and confirmed. GO analysis showed those genes were mainly involved in metabolism, cell stress and defence, transcription and translation, signal transduction, transport, energy and ion transport, which formed the basis for a molecular understanding of P. americana Cd tolerance mechanisms. Cd also stimulated ROS formation and modified the antioxidant systems. Taken together, our results indicate that ROS formation and Cd-induced gene expression favour P. americana tolerance by activating the defence system and permitting subsequent adaptation to Cd toxicity.

Effect of Cadmium Accumulation on the Performance of Plants and of Herbivores That Cope Differently With Organic Defenses

Some plants are able to accumulate in their shoots metals at levels that are toxic to most other organisms. This ability may serve as a defence against herbivores. Therefore, both metal-based and organic defences may affect herbivores. However, how metal accumulation affects the interaction between herbivores and organic plant defences remains overlooked. To fill this gap, we studied the interactions between tomato (Solanum lycopersicum), a model plant that accumulates cadmium, and two spider-mite species, Tetranychus urticae and Tetranychus evansi that, respectively, induce and suppress organic plant defences, measurable via the activity of trypsin inhibitors. We exposed plants to different concentrations of cadmium and measured its effects on mites and plants. In the plant, despite clear evidence for cadmium accumulation, we did not detect any cadmium effects on traits that reflect the general response of the plant, such as biomass, water content, and carbon/nitrogen ratio. Still, we found effects of cadmium upon the quantity of soluble sugars and on leaf reflectance, where it may indicate structural modifications in the cells. These changes in plant traits affected the performance of spider mites feeding on those plants. Indeed, the oviposition of both spider mite species was higher on plants exposed to low concentrations of cadmium than on control plants, but decreased at concentrations above 0.5 mM. Therefore, herbivores with contrasting responses to organic defences showed a similar hormetic response to metal accumulation by the plants. Additionally, we show that the induction and suppression of plant defences by these spider-mite species was not affected by the amount of cadmium supplied to the plants. Furthermore, the effect of cadmium on the performance of spider mites was not altered by infestation with T. urticae or T. evansi. Together, our results suggest no interaction between cadmium-based and organic plant defences, in our system. This may be useful for plants living in heterogeneous environments, as they may use one or the other defence mechanism, depending on their relative performance in each environment.

Coordinated role of soluble and cell wall bound phenols is a key feature of the metabolic adjustment in a mining woody fleabane (Dittrichia viscosa L.) population under semi-arid conditions

Environmental contamination by hazardous heavy metals/metalloids (metal(loid)s) is growing worldwide. To restrict the migration of toxic contaminants, the establishment of a self-sustainable plant cover is required. Plant growth in multi-polluted soils is a challenging issue not only by metal(loid) toxicities, but also by the co-occurrence of other stressors. Dittrichia viscosa is a pioneer Mediterranean species able to thrive in metal(loid)-enriched tailings in semi-arid areas. The aim of the present work was to examine the metabolic adjustments involved in the acclimation responses of this plant to conditions prevailing in mine-tailings during Mediterranean spring and summer. For this purpose, fully-expanded leaves, and rhizosphere soil of both mining and non-mining populations of D. viscosa grown spontaneously in south-eastern Spain were sampled in two consecutive years. Quantitative analysis of >50 biochemical, physiological and edaphic parameters were performed, including nutrient status, metal(loid) contents, leaf redox components, primary and secondary metabolites, salicylic acid levels, and soil physicochemical properties. Results showed that mining plants exhibited high foliar Zn/Pb co-accumulation capacity, without substantially affecting their photosynthetic metabolism or nutritional status even in the driest summer period. The comparison of the antioxidative/oxidative profile between mining and non-mining D. viscosa populations revealed no major seasonal changes in the content of primary antioxidants (ascorbate and GSH), or in the levels of ROS. Multivariate analysis showed that phenylalanine ammonia-lyase (PAL) and peroxidase (PRX) activities and soluble and cell wall-bound phenols were potential biomarkers for discriminating between both populations. During the dry season, a marked enhancement in the activity of both PAL and soluble PRX resulted in both a drop in the accumulation of soluble phenols and an increase of the strong metal chelator caffeic acid in the cell-wall fraction, supporting the view that the plasticity of phenylpropanoid metabolism provide an effective way to counteract the effects of stress combinations.

Plant glutathione transferase-mediated stress tolerance: functions and biotechnological applications

Plant glutathione transferases (EC 2.5.1.18, GSTs) are an ancient, multimember and diverse enzyme class. Plant GSTs have diverse roles in plant development, endogenous metabolism, stress tolerance, and xenobiotic detoxification. Their study embodies both fundamental aspects and agricultural interest, because of their ability to confer tolerance against biotic and abiotic stresses and to detoxify herbicides. Here we review the biotechnological applications of GSTs towards developing plants that are resistant to biotic and abiotic stresses. We integrate recent discoveries, highlight, and critically discuss the underlying biochemical and molecular pathways involved. We elaborate that the functions of GSTs in abiotic and biotic stress adaptation are potentially a result of both catalytic and non-catalytic functions. These include conjugation of reactive electrophile species with glutathione and the modulation of cellular redox status, biosynthesis, binding, and transport of secondary metabolites and hormones. Their major universal functions under stress underline the potential in developing climate-resilient cultivars through a combination of molecular and conventional breeding programs. We propose that future GST engineering efforts through rational and combinatorial approaches, would lead to the design of improved isoenzymes with purpose-designed catalytic activities and novel functional properties. Concurrent GST-GSH metabolic engineering can incrementally increase the effectiveness of GST biotechnological deployment.

Functional components of the bacterial CzcCBA efflux system reduce cadmium uptake and accumulation in transgenic tobacco plants

Cadmium (Cd) is a toxic trace element released into the environment by industrial and agricultural practices, threatening the health of plants and contaminating the food/feed chain. Biotechnology can be used to develop plant varieties with a higher capacity for Cd accumulation (for use in phytoremediation programs) or a lower capacity for Cd accumulation (to reduce Cd levels in food and feed). Here we generated transgenic tobacco plants expressing components of the Pseudomonas putida CzcCBA efflux system. Plants were transformed with combinations of the CzcC, CzcB and CzcA genes, and the impact on Cd mobilization was analysed. Plants expressing PpCzcC showed no differences in Cd accumulation, whereas those expressing PpCzcB or PpCzcA accumulated less Cd in the shoots, but more Cd in the roots. Plants expressing both PpCzcB and PpCzcA accumulated less Cd in the shoots and roots compared to controls, whereas plants expressing all three genes showed a significant reduction in Cd levels only in shoots. These results show that components of the CzcCBA system can be expressed in plants and may be useful for developing plants with a reduced capacity to accumulate Cd in the shoots, potentially reducing the toxicity of food/feed crops cultivated in Cd-contaminated soils.

Toxicity of heavy metals and metal-containing nanoparticles on plants

Plants are under the continual threat of changing climatic conditions that are associated with various types of abiotic stresses. In particular, heavy metal contamination is a major environmental concern that restricts plant growth. Plants absorb heavy metals along with essential elements from the soil and have evolved different strategies to cope with the accumulation of heavy metals. The use of proteomic techniques is an effective approach to investigate and identify the biological mechanisms and pathways affected by heavy metals and metal-containing nanoparticles. The present review focuses on recent advances and summarizes the results from proteomic studies aimed at understanding the response mechanisms of plants under heavy metal and metal-containing nanoparticle stress. Transport of heavy metal ions is regulated through the cell wall and plasma membrane and then sequestered in the vacuole. In addition, the role of different metal chelators involved in the detoxification and sequestration of heavy metals is critically reviewed, and changes in protein profiles of plants exposed to metal-containing nanoparticles are discussed in detail. Finally, strategies for gaining new insights into plant tolerance mechanisms to heavy metal and metal-containing nanoparticle stress are presented. This article is part of a Special Issue entitled: Plant Proteomics--a bridge between fundamental processes and crop production, edited by Dr. Hans-Peter Mock.

Functional and Integrative Analysis of the Proteomic Profile of Radish Root under Pb Exposure

Lead (Pb) is one of the most abundant heavy metal (HM) pollutants, which can penetrate the plant through the root and then enter the food chain causing potential health risks for human beings. Radish is an important root vegetable crop worldwide. To investigate the mechanism underlying plant response to Pb stress in radish, the protein profile changes of radish roots respectively upon Pb(NO₃)₂ at 500 mg L^-1(Pb500) and 1000 mg L^-1(Pb1000), were comprehensively analyzed using iTRAQ (Isobaric Tag for Relative and Absolute Quantification). A total of 3898 protein species were successfully detected and 2141 were quantified. Among them, a subset of 721 protein species were differentially accumulated upon at least one Pb treatment, and 135 ones showed significantly abundance changes under both two Pb-stressed conditions. Many critical protein species related to protein translation, processing, and degradation, reactive oxygen species (ROS) scavenging, photosynthesis, and respiration and carbon metabolism were successfully identified. Gene Ontology (GO) and pathway enrichment analysis of the 135 differential abundance protein species (DAPS) revealed that the overrepresented GO terms included "cell wall," "apoplast," "response to metal ion," "vacuole," and "peroxidase activity," and the critical enriched pathways were involved in "citric acid (TCA) cycle and respiratory electron transport," "pyruvate metabolism," "phenylalanine metabolism," "phenylpropanoid biosynthesis," and "carbon metabolism." Furthermore, the integrative analysis of transcriptomic, miRNA, degradome, metabolomics and proteomic data provided a strengthened understanding of radish response to Pb stress at multiple levels. Under Pb stress, many key enzymes (i.e., ATP citrate lyase, Isocitrate dehydrogenase, fumarate hydratase and malate dehydrogenase) involved in the glycolysis and TCA cycle were severely affected, which ultimately cause alteration of some metabolites including glucose, citrate and malate. Meanwhile, a series of other defense responses including ascorbate (ASA)-glutathione (GSH) cycle for ROS scavenging and Pb-defense protein species (glutaredoxin, aldose 1-epimerase malate dehydrogenase and thioredoxin), were triggered to cope with Pb-induced injuries. These results would be helpful for further dissecting molecular mechanism underlying plant response to HM stresses, and facilitate effective management of HM contamination in vegetable crops by genetic manipulation.

Inoculation with endophytic Bacillus megaterium 1Y31 increases Mn accumulation and induces the growth and energy metabolism-related differentially-expressed proteome in Mn hyperaccumulator hybrid pennisetum

In this study, a hydroponic culture experiment was conducted in a greenhouse to investigate the molecular and microbial mechanisms involved in the endophytic Bacillus megaterium 1Y31-enhanced Mn tolerance and accumulation in Mn hyperaccumulator hybrid pennisetum. Strain 1Y31 significantly increased the dry weights (ranging from 28% to 94%) and total Mn uptake (ranging from 23% to 112%) of hybrid pennisetum treated with 0, 2, and 10mM Mn compared to the control. Total 98 leaf differentially expressed proteins were identified between the live and dead bacterial inoculated hybrid pennisetum. The major leaf differentially expressed proteins were involved in energy generation, photosynthesis, response to stimulus, metabolisms, and unknown function. Furthermore, most of the energy generation and photosynthesis-related proteins were up-regulated, whereas most of the response to stimulus and metabolism-related proteins were down-regulated under Mn stress. Notably, the proportion of indole-3-acetic acid (IAA)-producing endophytic bacteria was significantly higher in the bacterial inoculated plants under Mn stress. The results suggested that strain 1Y31 increased the growth and Mn uptake of hybrid pennisetum through increasing the efficiency of photosynthesis and energy metabolism as well as the proportion of plant growth-promoting endophytic bacteria in the plants.

Effect of elevated CO2 concentration on photosynthetic characteristics of hyperaccumulator Sedum alfredii under cadmium stress

The combined effects of elevated CO2 and cadmium (Cd) on photosynthetic rate, chlorophyll fluorescence and Cd accumulation in hyperaccumulator Sedum alfredii Hance were investigated to predict plant growth under Cd stress with rising atmospheric CO2 concentration. Both pot and hydroponic experiments were conducted and the plants were grown under ambient (350 µL L(-1)) or elevated (800 µL L(-1)) CO2 . Elevated CO2 significantly (P < 0.05) increased Pn (105%-149%), Pnmax (38.8%-63.0%) and AQY (20.0%-34.8%) of S. alfredii in all the Cd treatments, but reduced chlorophyll concentration, dark respiration and photorespiration. After 10 days growth in medium with 50 µM Cd under elevated CO2 , PSII activities were significantly enhanced (P < 0.05) with Pm, Fv/Fm, Φ(II) and qP increased by 66.1%, 7.5%, 19.5% and 16.4%, respectively, as compared with ambient-grown plants. Total Cd uptake in shoot of S. alfredii grown under elevated CO2 was increased by 44.1%-48.5%, which was positively correlated with the increase in Pn. These results indicate that elevated CO2 promoted the growth of S. alfredii due to increased photosynthetic carbon uptake rate and photosynthetic light-use efficiency, and showed great potential to improve the phytoextraction of Cd by S. alfredii.

Intraspecific variability of cadmium tolerance and accumulation, and cadmium-induced cell wall modifications in the metal hyperaccumulator Arabidopsis halleri

Certain molecular mechanisms of Cd tolerance and accumulation have been identified in the model species Arabidopsis halleri, while intraspecific variability of these traits and the mechanisms of shoot detoxification were little addressed. The Cd tolerance and accumulation of metallicolous and non-metallicolous A. halleri populations from different genetic units were tested in controlled conditions. In addition, changes in shoot cell wall composition were investigated using Fourier transform infrared spectroscopy. Indeed, recent works on A. halleri suggest Cd sequestration both inside cells and in the cell wall/apoplast. All A. halleri populations tested were hypertolerant to Cd, and the metallicolous populations were on average the most tolerant. Accumulation was highly variable between and within populations, and populations that were non-accumulators of Cd were identified. The effect of Cd on the cell wall composition was quite similar in the sensitive species A. lyrata and in A. halleri individuals; the pectin/polysaccharide content of cell walls seems to increase after Cd treatment. Nevertheless, the changes induced by Cd were more pronounced in the less tolerant individuals, leading to a correlation between the level of tolerance and the extent of modifications. This work demonstrated that Cd tolerance and accumulation are highly variable traits in A. halleri, suggesting adaptation at the local scale and involvement of various molecular mechanisms. While in non-metallicolous populations drastic modifications of the cell wall occur due to higher Cd toxicity and/or Cd immobilization in this compartment, the increased tolerance of metallicolous populations probably involves other mechanisms such as vacuolar sequestration.

Wounding of Arabidopsis halleri leaves enhances cadmium accumulation that acts as a defense against herbivory

Approximately 0.2 % of all angiosperms are classified as metal hyperaccumulators based on their extraordinarily high leaf metal contents, for example >1 % zinc, >0.1 % nickel or >0.01 % cadmium (Cd) in dry biomass. So far, metal hyperaccumulation has been considered to be a taxon-wide, constitutively expressed trait, the extent of which depends solely on available metal concentrations in the soil. Here we show that in the facultative metallophyte Arabidopsis halleri, both insect herbivory and mechanical wounding of leaves trigger an increase specifically in leaf Cd accumulation. Moreover, the Cd concentrations accumulated in leaves can serve as an elemental defense against herbivory by larvae of the Brassicaceae specialist small white (Pieris rapae), thus allowing the plant to take advantage of this non-essential trace element and toxin. Metal homeostasis genes are overrepresented in the systemic transcriptional response of roots to the wounding of leaves in A. halleri, supporting that leaf Cd accumulation is preceded by systemic signaling events. A similar, but quantitatively less pronounced transcriptional response was observed in A. thaliana, suggesting that the systemically regulated modulation of metal homeostasis in response to leaf wounding also occurs in non-hyperaccumulator plants. This is the first report of an environmental stimulus influencing metal hyperaccumulation.

Selenium hyperaccumulators harbor a diverse endophytic bacterial community characterized by high selenium resistance and plant growth promoting properties

Selenium (Se)-rich plants may be used to provide dietary Se to humans and livestock, and also to clean up Se-polluted soils or waters. This study focused on endophytic bacteria of plants that hyperaccumulate selenium (Se) to 0.5-1% of dry weight. Terminal restriction fragment length polymorphism (T-RFLP) analysis was used to compare the diversity of endophytic bacteria of hyperaccumulators Stanleya pinnata (Brassicaceae) and Astragalus bisulcatus (Fabaceae) with those from related non-accumulators Physaria bellii (Brassicaceae) and Medicago sativa (Fabaceae) collected on the same, seleniferous site. Hyperaccumulators and non-accumulators showed equal T-RF diversity. Parsimony analysis showed that T-RFs from individuals of the same species were more similar to each other than to those from other species, regardless of plant Se content or spatial proximity. Cultivable endophytes from hyperaccumulators S. pinnata and A. bisulcatus were further identified and characterized. The 66 bacterial morphotypes were shown by MS MALDI-TOF Biotyper analysis and 16S rRNA gene sequencing to include strains of Bacillus, Pseudomonas, Pantoea, Staphylococcus, Paenibacillus, Advenella, Arthrobacter, and Variovorax. Most isolates were highly resistant to selenate and selenite (up to 200 mM) and all could reduce selenite to red elemental Se, reduce nitrite and produce siderophores. Seven isolates were selected for plant inoculation and found to have plant growth promoting properties, both in pure culture and when co-cultivated with crop species Brassica juncea (Brassicaceae) or M. sativa. There were no effects on plant Se accumulation. We conclude that Se hyperaccumulators harbor an endophytic bacterial community in their natural seleniferous habitat that is equally diverse to that of comparable non-accumulators. The hyperaccumulator endophytes are characterized by high Se resistance, capacity to produce elemental Se and plant growth promoting properties.

Rhizosphere microbial community composition affects cadmium and zinc uptake by the metal-hyperaccumulating plant Arabidopsis halleri

The remediation of metal-contaminated soils by phytoextraction depends on plant growth and plant metal accessibility. Soil microorganisms can affect the accumulation of metals by plants either by directly or indirectly stimulating plant growth and activity or by (im)mobilizing and/or complexing metals. Understanding the intricate interplay of metal-accumulating plants with their rhizosphere microbiome is an important step toward the application and optimization of phytoremediation. We compared the effects of a "native" and a strongly disturbed (gamma-irradiated) soil microbial communities on cadmium and zinc accumulation by the plant Arabidopsis halleri in soil microcosm experiments. A. halleri accumulated 100% more cadmium and 15% more zinc when grown on the untreated than on the gamma-irradiated soil. Gamma irradiation affected neither plant growth nor the 1 M HCl-extractable metal content of the soil. However, it strongly altered the soil microbial community composition and overall cell numbers. Pyrosequencing of 16S rRNA gene amplicons of DNA extracted from rhizosphere samples of A. halleri identified microbial taxa (Lysobacter, Streptomyces, Agromyces, Nitrospira, "Candidatus Chloracidobacterium") of higher relative sequence abundance in the rhizospheres of A. halleri plants grown on untreated than on gamma-irradiated soil, leading to hypotheses on their potential effect on plant metal uptake. However, further experimental evidence is required, and wherefore we discuss different mechanisms of interaction of A. halleri with its rhizosphere microbiome that might have directly or indirectly affected plant metal accumulation. Deciphering the complex interactions between A. halleri and individual microbial taxa will help to further develop soil metal phytoextraction as an efficient and sustainable remediation strategy.

Molecular diversity of arbuscular mycorrhizal fungi at a large-scale antimony mining area in southern China

Arbuscular mycorrhizal fungi (AMF) have great potential for assisting heavy metal hyperaccumulators in the remediation of contaminated soils. However, little information is available about the community composition of AMF under natural conditions in soils contaminated by antimony (Sb). The objective of this study was to investigate the characteristics of AMF molecular diversity, and to explore the effects of Sb content and soil properties on the AMF community structure in an Sb mining area. Four Sb mine spoils and one adjacent reference area were selected from around the Xikuangshan mine in southern China. The association of AMF molecular diversity and community composition with the rhizosphere soils of the dominant plant species was studied by Polymerase Chain Reaction-Denaturing Gradient Gel Electrophoresis (PCR-DGGE). Results from all five studied sites showed that the diversity of AMF decreased with increasing Sb concentration. Principal component analysis (PCA) indicated that the AMF community structure was markedly different among these groups. Further redundancy analysis (RDA) showed that Sb contamination was the dominating factor influencing the AMF community structure in the Sb mine area. However, the multivariate analysis showed that, apart from the soil Sb content, extractable nitrogen content and organic matter content also attributed to AMF sequence distribution type. Some AMF sequences were only found in the highly contaminated area and these might be ideal candidates for improving phytoremediation efficiency in Sb mining regions. Gene sequencing analysis revealed that most species were affiliated with Glomus, suggesting that Glomus was the dominant AMF genus in the studied Sb mining area.

Excess copper induced proteomic changes in the marine brown algae Sargassum fusiforme

Copper (Cu) is an essential micronutrient for algal growth and development; however, it is also generally considered to be one of the most toxic metals when present at higher levels. Seaweeds are often exposed to low concentrations of metals, including Cu, for long time periods. In cases of ocean outfall, they may even be abruptly exposed to high levels of metals. The physiological processes that are active under Cu stress are largely unknown. In this study, the brown macroalga Sargassum fusiforme was cultured in fresh seawater at final Cu concentrations of 0, 4, 8, 24 and 47 μM. The Cu(2+) concentration and chlorophyll autofluorescence were measured to establish the toxic effects of Cu on this economically important seaweed. The accumulation of Cu by S. fusiforme was also dependent upon the external Cu concentration. Algal growth displayed a general decline with increasing media Cu concentrations, indicating that S. fusiforme was able to tolerate Cu stress at low concentrations, while it was negatively impacted at high concentrations. The term "acute stress" was employed to indicate exposure to high Cu concentrations for 1 day in this study. On the other hand, "chronic stress" was defined as exposure to lower sub-lethal Cu concentrations for 7 days. Proteins were extracted from control and Cu-treated S. fusiforme samples and separated by two-dimensional gel electrophoresis. Distinct patterns of protein expression in the acute and chronic stress conditions were observed. Proteins related to energy metabolism and photosynthesis were reduced significantly, whereas those related to carbohydrate metabolism, protein destination, RNA degradation and signaling regulation were induced in S. fusiforme in response to acute copper stress. Energy metabolism-related proteins were significantly induced by chronic Cu stress. Proteins from other functional groups, such as those related to membranes and transport, were present in minor quantities. These results suggest that S. fusiforme is sensitive to excess Cu, regardless of the presence of acute or chronic stress. We discuss the possible function of these identified proteins, taking into consideration the information available from other plant models.

Culturable endophytic bacteria enhance Ni translocation in the hyperaccumulator Noccaea caerulescens

In this work, both culture-dependent and independent approaches were used to identify and isolate endophytic bacteria from roots of the Ni hyperaccumulator Noccaea caerulescens. A total of 17 isolates were cultured from root samples, selected for tolerance to 6mM Ni and grouped by restriction analysis of 16S rDNA. Bacterial species cultivated from roots belonged to seven genera, Microbacterium, Arthrobacter, Agreia, Bacillus, Sthenotrophomonas, Kocuria and Variovorax. The culture-independent approach confirmed the presence of Microbacterium and Arthrobacter while only other five clones corresponding to different amplified ribosomal DNA restriction patterns were detected. Five selected highly Ni-resistant bacteria showing also plant growth promoting activities, were inoculated into seeds of N. caerulescens, and in vivo microscopic analysis showed rapid root colonisation. Inoculated plants showed increased shoot biomass, root length and root-to-shoot Ni translocation. Root colonisation was also evident, but not effective, in the non-hyperaccumulating Thlaspi perfoliatum. Seed inoculation with selected Ni-resistant endophytic bacteria may represent a powerful tool in phytotechnologies, although transferring it to biomass species still requires further studies and screening.

Roles of rhizobial symbionts in selenium hyperaccumulation in Astragalus (Fabaceae)

PREMISE OF THE STUDY

Are there dimensions of symbiotic root interactions that are overlooked because plant mineral nutrition is the foundation and, perhaps too often, the sole explanation through which we view these relationships? In this paper we investigate how the root nodule symbiosis in selenium (Se) hyperaccumulator and nonaccumulator Astragalus species influences plant selenium (Se) accumulation.

METHODS

In greenhouse studies, Se was added to nodulated and nonnodulated hyperaccumulator and nonaccumulator Astragalus plants, followed by investigation of nitrogen (N)-Se relationships. Selenium speciation was also investigated, using x-ray microprobe analysis and liquid chromatography-mass spectrometry (LC-MS).

KEY RESULTS

Nodulation enhanced biomass production and Se to S ratio in both hyperaccumulator and nonaccumulator plants. The hyperaccumulator contained more Se when nodulated, while the nonaccumulator contained less S when nodulated. Shoot [Se] was positively correlated with shoot N in Se-hyperaccumulator species, but not in nonhyperaccumulator species. The x-ray microprobe analysis showed that hyperaccumulators contain significantly higher amounts of organic Se than nonhyperaccumulators. LC-MS of A. bisulcatus leaves revealed that nodulated plants contained more γ-glutamyl-methylselenocysteine (γ-Glu-MeSeCys) than nonnodulated plants, while MeSeCys levels were similar.

CONCLUSIONS

Root nodule mutualism positively affects Se hyperaccumulation in Astragalus. The microbial N supply particularly appears to contribute glutamate for the formation of γ-Glu-MeSeCys. Our results provide insight into the significance of symbiotic interactions in plant adaptation to edaphic conditions. Specifically, our findings illustrate that the importance of these relationships are not limited to alleviating macronutrient deficiencies.

Differential cadmium and zinc distribution in relation to their physiological impact in the leaves of the accumulating Zygophyllum fabago L

Cadmium and zinc share many similar physiochemical properties, but their compartmentation, complexation and impact on other mineral element distribution in plant tissues may drastically differ. In this study, we address the impact of 10 μm Cd or 50 μm Zn treatments on ion distribution in leaves of a metallicolous population of the non-hyperaccumulating species Zygophyllum fabago at tissue and cell level, and the consequences on the plant response through a combined physiological, proteomic and metabolite approach. Micro-proton-induced X-ray emission and laser ablation inductively coupled mass spectrometry analyses indicated hot spots of Cd concentrations in the vicinity of vascular bundles in response to Cd treatment, essentially bound to S-containing compounds as revealed by extended X-ray absorption fine structure and non-protein thiol compounds analyses. A preferential accumulation of Zn occurred in vascular bundle and spongy mesophyll in response to Zn treatment, and was mainly bound to O/N-ligands. Leaf proteomics and physiological status evidenced a protection of photosynthetically active tissues and the maintenance of cell turgor through specific distribution and complexation of toxic ions, reallocation of some essential elements, synthesis of proteins involved in photosynthetic apparatus or C-metabolism, and metabolite synthesis with some specificities regarding the considered heavy metal treatment.

Problems inherent to a meta-analysis of proteomics data: a case study on the plants' response to Cd in different cultivation conditions

This meta-analysis focuses on plant-proteome responses to cadmium (Cd) stress. Initially, some general topics related to a proteomics meta-analysis are discussed: (1) obstacles encountered during data analysis, (2) a consensus in proteomic research, (3) validation and good reporting practices for protein identification and (4) guidelines for statistical analysis of differentially abundant proteins. In a second part, the Cd responses in leaves and roots obtained from a proteomics meta-analysis are discussed in (1) a time comparison (short versus long term exposure), and (2) a culture comparison (hydroponics versus soil cultivation). Data of the meta-analysis confirmed the existence of an initial alarm phase upon Cd exposure. Whereas no metabolic equilibrium is established in hydroponically exposed plants, an equilibrium seems to be manifested in roots of plants grown in Cd-contaminated soil after long term exposure. In leaves, the carbohydrate metabolism is primarily affected independent of the exposure time and the cultivation method. In addition, a metabolic shift from CO2-fixation towards respiration is manifested, independent of the cultivation system. Finally, some ideas for the improvement of proteomics setups and for comparisons between studies are discussed.

BIOLOGICAL SIGNIFICANCE

This meta-analysis focuses on the plant responses to Cd stress in leaves and roots at the proteome level. This meta-analysis points out the encountered obstacles when performing a proteomics meta-analysis related to inherent technologies, but also related to experimental setups. Furthermore, the question is addressed whether an extrapolation of results obtained in hydroponic cultivation towards soil-grown plants is possible.

Cd²⁺-induced alteration of the global proteome of human skin fibroblast cells

Cadmium (Cd²⁺) is a toxic heavy metal and a well-known human carcinogen. The toxic effects of Cd²⁺ on biological systems are diverse and thought to be exerted through a complex array of mechanisms. Despite the large number of studies aimed to elucidate the toxic mechanisms of action of Cd²⁺, few have been targeted toward investigating the ability of Cd²⁺ to disrupt multiple cellular pathways simultaneously and the overall cellular responses toward Cd²⁺ exposure. In this study, we employed a quantitative proteomic method, relying on stable isotope labeling by amino acids in cell culture (SILAC) and LC–MS/MS, to assess the Cd²⁺-induced simultaneous alterations of multiple cellular pathways in cultured human skin fibroblast cells. By using this approach, we were able to quantify 2931 proteins, and 400 of them displayed significantly changed expression following Cd²⁺ exposure. Our results unveiled that Cd²⁺ treatment led to the marked upregulation of several antioxidant enzymes (e.g., metallothionein-1G, superoxide dismutase, pyridoxal kinase, etc.), enzymes associated with glutathione biosynthesis and homeostasis (e.g., glutathione S-transferases, glutathione synthetase, glutathione peroxidase, etc.), and proteins involved in cellular energy metabolism (e.g., glycolysis, pentose phosphate pathway, and the citric acid cycle). Additionally, we found that Cd²⁺ treatment resulted in the elevated expression of two isoforms of dimethylarginine dimethylaminohydrolase (DDAH I and II), enzymes known to play a key role in regulating nitric oxide biosynthesis. Consistent with these findings, we observed elevated formation of nitric oxide in human skin (GM00637) and lung (IMR-90) fibroblast cells following Cd²⁺ exposure. The upregulation of DDAH I and II suggests a role of nitric oxide synthesis in Cd²⁺-induced toxicity in human cells.

The bacterial rhizobiome of hyperaccumulators: future perspectives based on omics analysis and advanced microscopy

Hyperaccumulators are plants that can extract heavy metal ions from the soil and translocate those ions to the shoots, where they are sequestered and detoxified. Hyperaccumulation depends not only on the availability of mobilized metal ions in the soil, but also on the enhanced activity of metal transporters and metal chelators which may be provided by the plant or its associated microbes. The rhizobiome is captured by plant root exudates from the complex microbial community in the soil, and may colonize the root surface or infiltrate the root cortex. This community can increase the root surface area by inducing hairy root proliferation. It may also increase the solubility of metals in the rhizosphere and promote the uptake of soluble metals by the plant. The bacterial rhizobiome, a subset of specialized microorganisms that colonize the plant rhizosphere and endosphere, makes an important contribution to the hyperaccumulator phenotype. In this review, we discuss classic and more recent tools that are used to study the interactions between hyperaccumulators and the bacterial rhizobiome, and consider future perspectives based on the use of omics analysis and microscopy to study plant metabolism in the context of metal accumulation. Recent data suggest that metal-resistant bacteria isolated from the hyperaccumulator rhizosphere and endosphere could be useful in applications such as phytoextraction and phytoremediation, although more research is required to determine whether such properties can be transferred successfully to non-accumulator species.

Cadmium–zinc exchange and their binary relationship in the structure of Zn-related proteins: a mini review

Here we give an overview of ongoing work on discovering the structural mechanisms of Cd–Zn exchange and the potentially diverse roles of Cd at Zn functional sites in proteins.

Managing heavy metal toxicity stress in plants: biological and biotechnological tools

The maintenance of ion homeostasis in plant cells is a fundamental physiological requirement for sustainable plant growth, development and production. Plants exposed to high concentrations of heavy metals must respond in order to avoid the deleterious effects of heavy metal toxicity at the structural, physiological and molecular levels. Plant strategies for coping with heavy metal toxicity are genotype-specific and, at least to some extent, modulated by environmental conditions. There is considerable interest in the mechanisms underpinning plant metal tolerance, a complex process that enables plants to survive metal ion stress and adapt to maintain growth and development without exhibiting symptoms of toxicity. This review briefly summarizes some recent cell biological, molecular and proteomic findings concerning the responses of plant roots to heavy metal ions in the rhizosphere, metal ion-induced reactions at the cell wall-plasma membrane interface, and various aspects of heavy metal ion uptake and transport in plants via membrane transporters. The molecular and genetic approaches that are discussed are analyzed in the context of their potential practical applications in biotechnological approaches for engineering increased heavy metal tolerance in crops and other useful plants.

Mapping the leaf proteome of Miscanthus sinensis and its application to the identification of heat-responsive proteins

Miscanthus sinensis is a promising bioenergy crop; however, its genome is poorly represented in sequence databases. As an initial step in the comprehensive analysis of the M. sinensis proteome, we report a reference 2-DE protein map of the leaf. A total of 316 protein spots were excised from the gels, digested with trypsin and subjected to matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) or MALDI-TOF/TOF MS. Two hundred and thirty-two protein spots were identified, which are involved in a variety of cellular functions through distinct metabolic pathways. Functional annotation of the proteins revealed a nearly complete C3 and C4 cycle, starch and sugar synthesis pathway, glycolysis pathway, a significant portion of the pentose phosphate pathway, and many enzymes involved in secondary metabolism such as flavonoid/isoflavonoid, kaurene, chalcone, sesquiterpene and lignin biosynthesis. Other proteins belong to primary metabolism, transcription, protein synthesis, protein destination/storage, disease/defense, cell growth/division, transportation and signal transduction. To test the applicability of the constructed map, we studied the effect of heat stress on M. sinensis leaf proteome. Twenty-five protein spots were upregulated, five were newly induced and twenty-five spots were downregulated by heat treatment. The differentially accumulated proteins were involved in photosynthesis, energy metabolism, gene transcription, protein kinases and phosphatases, signal transduction, protein synthesis and heat shock responses. C4-specific pyruvate orthophosphate dikinase, Rubisco large subunit, Rubisco activase and some associated proteins were upregulated during heat stress and tend to restore upon recovery. Identification of these proteins provides some important clues regarding the way M. sinensis copes with hot climate. This work represents the first extensive proteomic description of M. sinensis and provides a reference map and heat-responsive candidates for future molecular and physiological studies of this bioenergy crop.

Recent advances in the analysis of metal hyperaccumulation and hypertolerance in plants using proteomics

Hyperaccumulator/hypertolerant plant species have evolved strategies allowing them to grow in metal-contaminated soils, where they accumulate high concentrations of heavy metals in their shoots without signs of toxicity. The mechanisms that allow enhanced metal uptake, root-to-shoot translocation and detoxification in these species are not fully understood. Complementary approaches such as transcriptomic-based DNA microarrays and proteomics have recently been used to gain insight into the molecular pathways evolved by metal hyperaccumulator/hypertolerant species. Proteomics has the advantage of focusing on the translated portion of the genome and it allows to analyze complex networks of proteins. This review discusses the recent analysis of metal hyperaccumulator/hypertolerant plant species using proteomics. Changes in photosynthetic proteins, sulfur, and glutathione metabolism, transport, biotic and xenobiotic defenses as well as the differential regulation of proteins involved in signaling and secondary metabolism are discussed in relation to metal hyperaccumulation. We also consider the potential contribution of several proteins to the hyperaccumulation phenotype.

The proteomics of heavy metal hyperaccumulation by plants

Hyperaccumulators are distinguished from non-hyperaccumulators on the basis of their capacity to extract heavy metal ions from the soil, their more efficient root-to-shoot translocation of these ions and their greater ability to detoxify and sequester heavy metals in the shoot. The understanding of the mechanisms underlying metal ion accumulation has progressed beyond the relevant biochemistry and physiology to encompass the genetic and molecular regulatory systems which differentiate hyperaccumulators from non-hyperaccumulators. This paper reviews the literature surrounding the application of proteomics technology to plant metal hyperaccumulation, in particular involving the elements As, Cd, Cu, Ni, Pb and Zn. The hyperaccumulation process across a number of unrelated plant species appears to be associated with proteins involved in energy metabolism, the oxidative stress response and abiotic and biotic stress. The relevance of transducers of the metal stress response to the phenomenon of hyperaccumulation is summarized. Proteomic data complement the more voluminous genomic and transcriptomic data sets in providing a more nuanced picture of the process, and should therefore help in the identification of the major genetic determinants of the hyperaccumulation phenomenon.

Early Zn2+-induced effects on membrane potential account for primary heavy metal susceptibility in tolerant and sensitive Arabidopsis species

BACKGROUND AND AIMS

Uptake of heavy metals by plant root cells depends on electro-physiological parameters of the plasma membrane. In this study, responses of the plasma membrane in root cells were analysed where early reactions to the metal ion-induced stress are localized. Three different Arabidopsis species with diverse strategies of their adaptation to heavy metals were compared: sensitive Arabidopsis thaliana and tolerant A. halleri and A. arenosa.

METHODS

Plants of A. thaliana Col-0 ecotype and plants of A. arenosa and A. halleri originating from natural metallicolous populations were exposed to high concentrations of Zn(2+). Plants were tested for root growth rate, cellular tolerance, plant morphology and cell death in the root apex. In addition, the membrane potential (E(M)) of mature cortical root cells and changes in the pH of the liquid culture media were measured.

KEY RESULTS

Primary roots of A. halleri and A. arenosa plants grew significantly better at increased Zn(2+) concentrations than A. thaliana plants. Elevated Zn(2+) concentrations in the culture medium induced rapid changes in E(M). The reaction was species-specific and concentration-dependent. Arabidopsis halleri revealed the highest insensitivity of the plasma membrane and the highest survival rate under prolonged treatment with extra-high concentrations. Plants were able to effectively adjust the pH in the control, but much less at Zn(2+)-induced lower pH.

CONCLUSIONS

The results indicate a similar mode of early reaction to Zn(2+), but with different extent in tolerant and sensitive species of Arabidopsis. The sensitivity of A. thaliana and a high tolerance of A. halleri and A. arenosa were demonstrated. Plasma membrane depolarization was lowest in the hyperaccumulator A. halleri and highest in A. thaliana. This indicates that rapid membrane voltage changes are an excellent tool to monitor the effects of heavy metals.

Proteomic identification of differentially expressed proteins in Gossypium thurberi inoculated with cotton Verticillium dahliae

Thurber's cotton (Gossypium thurberi) is the wild relative of cultivated cotton. It is highly resistant to cotton Verticillium wilt, a disease that significantly affects cotton yield and quality. To reveal the mechanism of disease resistance in G. thurberi and to clone resistance-related genes, we used two-dimensional electrophoresis (2-DE) and tandem time-of-flight mass spectrometry (MALDI-TOF-MS) to identify differentially expressed proteins in Thurber's cotton after inoculation with Verticillium dahliae. A total of 57 different protein spots were upregulated, including 52 known proteins representing 11% of the total protein spots. These proteins are involved in resistance to stress and disease, transcriptional regulation, signal transduction, protein processing and degradation, photosynthesis, production capacity, basic metabolism, and other processes. In addition, five disease resistance proteins showed intense upregulation, indicating that resistance genes (R genes) may play a critical role in resistance to Verticillium wilt in Thurber's cotton. Our results suggest that disease and stress resistance are the combined effects of multiple co-expressed genes. This provides a basis for further, detailed investigation into the mechanisms underlying Verticillium wilt resistance of G. thurberi and for cloning essential genes into cotton cultivars to produce Verticillium wilt resistant plants.

The adaptive response of lichens to mercury exposure involves changes in the photosynthetic machinery

Lichens are an excellent model to study the bioaccumulation of heavy metals but limited information is available on the molecular mechanisms occurring during bioaccumulation. We investigated the changes of the lichen proteome during exposure to constant concentrations of mercury. We found that most of changes involves proteins of the photosynthetic pathway, such as the chloroplastic photosystem I reaction center subunit II, the oxygen-evolving protein and the chloroplastic ATP synthase β-subunit. This suggests that photosynthesis is a target of the toxic effects of mercury. These findings are also supported by changes in the content of photosynthetic pigments (chlorophyll a and b, and β-carotene). Alterations to the photosynthetic machinery also reflect on the structure of thylakoid membranes of algal cells. Response of lichens to mercury also involves stress-related proteins (such as Hsp70) but not cytoskeletal proteins. Results suggest that lichens adapt to mercury exposure by changing the metabolic production of energy.