The Arabidopsis heavy metal P-type ATPase HMA5 interacts with metallochaperones and functions in copper detoxification of roots

Bacterial Cu(+)-ATPases: models for molecular structure-function studies

The early discovery of the human Cu(+)-ATPases and their link to Menkes and Wilson's diseases brought attention to the unique role of these transporters in copper homeostasis. The characterization of bacterial Cu(+)-ATPases has significantly furthered our understanding of the structure, selectivity and transport mechanism of these enzymes, as well as their interplay with other elements of Cu(+) distribution networks. This review focuses on the structural-functional insights that have emerged from studies of bacterial Cu(+)-ATPases at the molecular level and how these observations have contributed to drawing up a comprehensive picture of cellular copper homeostasis.

Transition Metal Transport in Plants and Associated Endosymbionts: Arbuscular Mycorrhizal Fungi and Rhizobia

Transition metals such as iron, copper, zinc, or molybdenum are essential nutrients for plants. These elements are involved in almost every biological process, including photosynthesis, tolerance to biotic and abiotic stress, or symbiotic nitrogen fixation. However, plants often grow in soils with limiting metallic oligonutrient bioavailability. Consequently, to ensure the proper metal levels, plants have developed a complex metal uptake and distribution system, that not only involves the plant itself, but also its associated microorganisms. These microorganisms can simply increase metal solubility in soils and making them more accessible to the host plant, as well as induce the plant metal deficiency response, or directly deliver transition elements to cortical cells. Other, instead of providing metals, can act as metal sinks, such as endosymbiotic rhizobia in legume nodules that requires relatively large amounts to carry out nitrogen fixation. In this review, we propose to do an overview of metal transport mechanisms in the plant-microbe system, emphasizing the role of arbuscular mycorrhizal fungi and endosymbiotic rhizobia.

A heavy metal P-type ATPase OsHMA4 prevents copper accumulation in rice grain

Rice is a major source of calories and mineral nutrients for over half the world's human population. However, little is known in rice about the genetic basis of variation in accumulation of copper (Cu), an essential but potentially toxic nutrient. Here we identify OsHMA4 as the likely causal gene of a quantitative trait locus controlling Cu accumulation in rice grain. We provide evidence that OsHMA4 functions to sequester Cu into root vacuoles, limiting Cu accumulation in the grain. The difference in grain Cu accumulation is most likely attributed to a single amino acid substitution that leads to different OsHMA4 transport activity. The allele associated with low grain Cu was found in 67 of the 1,367 rice accessions investigated. Identification of natural allelic variation in OsHMA4 may facilitate the development of rice varieties with grain Cu concentrations tuned to both the concentration of Cu in the soil and dietary needs.

Copper (Cu) is an essential mineral nutrient but high concentrations in rice grain can cause toxicity. Here the authors provide evidence that natural variation in rice grain Cu concentration is caused by altered sequestration of Cu into root vacuoles due to a single amino acid substitution in the OsHMA4 transporter.

Interaction Between ABA Signaling and Copper Homeostasis in Arabidopsis thaliana

ABA is involved in plant responses to non-optimal environmental conditions, including nutrient availability. Since copper (Cu) is a very important micronutrient, unraveling how ABA affects Cu uptake and distribution is relevant to ensure adequate Cu nutrition in plants subjected to stress conditions. Inversely, knowledge about how the plant nutritional status can interfere with ABA biosynthesis and signaling mechanisms is necessary to optimize stress tolerance in horticultural crops. Here the reciprocal influence between ABA and Cu content was addressed by using knockout mutants and overexpressing transgenic plants of high affinity plasma membrane Cu transporters (pmCOPT) with altered Cu uptake. Exogenous ABA inhibited pmCOPT expression and drastically modified COPT2-driven localization in roots. ABA regulated SPL7, the main transcription factor responsive for Cu deficiency responses, and subsequently affected expression of its targets. ABA biosynthesis (aba2) and signaling (hab1-1 abi1-2) mutants differentially responded to ABA according to Cu levels. Alteration of Cu homeostasis in the pmCOPT mutants affected ABA biosynthesis, transport and signaling as genes such as NCED3, WRKY40, HY5 and ABI5 were differentially modulated by Cu status, and also in the pmCOPT and ABA mutants. Altered Cu uptake resulted in modified plant sensitivity to salt-mediated increases in endogenous ABA. The overall results provide evidence for reciprocal cross-talk between Cu status and ABA metabolism and signaling.

Copper Trafficking in Plants and Its Implication on Cell Wall Dynamics

In plants, copper (Cu) acts as essential cofactor of numerous proteins. While the definitive number of these so-called cuproproteins is unknown, they perform central functions in plant cells. As micronutrient, a minimal amount of Cu is needed to ensure cellular functions. However, Cu excess may exert in contrast detrimental effects on plant primary production and even survival. Therefore it is essential for a plant to have a strictly controlled Cu homeostasis, an equilibrium that is both tissue and developmentally influenced. In the current review an overview is presented on the different stages of Cu transport from the soil into the plant and throughout the different plant tissues. Special emphasis is on the Cu-dependent responses mediated by the SPL7 transcription factor, and the crosstalk between this transcriptional regulation and microRNA-mediated suppression of translation of seemingly non-essential cuproproteins. Since Cu is an essential player in electron transport, we also review the recent insights into the molecular mechanisms controlling chloroplastic and mitochondrial Cu transport and homeostasis. We finally highlight the involvement of numerous Cu-proteins and Cu-dependent activities in the properties of one of the major Cu-accumulation sites in plants: the cell wall.

Copper phytoremediation potential of Calandula officinalis L. and the role of antioxidant enzymes in metal tolerance

Cu phytoremediation potential of an ornamental plant, Calandula officinalis, was explored in terms of growth responses, photosynthetic activities and antioxidant enzymes such as SOD, CAT and GPX. The results showed that this plant had high Cu tolerance of up to 400 mg/kg, which is far above the phytotoxic range for non hyperaccumulators. It grew normally in soils at all the doses (150-400 mg/kg) without showing external signs of phytotoxicity. At 150 mg/kg, flowering was augmented; root and shoot biomass, root lengths and leaf soluble protein contents remained same as that of the control. However, chlorophyll and carotenoid pigment contents declined significantly along with significant elevations in lipid peroxidation, at all the doses. Elevations of antioxidant enzymes reflected stress as well as probable mitigation of reactive oxygen species due to Cu stress. Except for the highest conc. (400 mg/kg), leaf accumulation of Cu was higher than root accumulations. The Cu accumulation peaked at 300 mg/kg Cu in soil, with leaf and root accumulations to be respectively, 4675 and 3995 µg/g dry wt., far more than the minimum of 1000 µg/g dry wt. for a Cu hyperaccumulator. The plant root at all the doses tolerated Cu, with the tolerance index ranging from 94-62.7. The soil to plant metal uptake capacity, indicated by extraction coefficient and the root to shoot translocation, indicated by translocation factor, at all the doses of Cu were >1, pointed towards efficient phytoremediation potential.

Metal binding to the N-terminal cytoplasmic domain of the PIB ATPase HMA4 is required for metal transport in Arabidopsis

PIB ATPases are metal cation pumps that transport metals across membranes. These proteins possess N- and C-terminal cytoplasmic extensions that contain Cys- and His-rich high affinity metal binding domains, which may be involved in metal sensing, metal ion selectivity and/or in regulation of the pump activity. The PIB ATPase HMA4 (Heavy Metal ATPase 4) plays a central role in metal homeostasis in Arabidopsis thaliana and has a key function in zinc and cadmium hypertolerance and hyperaccumulation in the extremophile plant species Arabidopsis halleri. Here, we examined the function and structure of the N-terminal cytoplasmic metal-binding domain of HMA4. We mutagenized a conserved CCTSE metal-binding motif in the domain and assessed the impact of the mutations on protein function and localization in planta, on metal-binding properties in vitro and on protein structure by Nuclear Magnetic Resonance spectroscopy. The two Cys residues of the motif are essential for the function, but not for localization, of HMA4 in planta, whereas the Glu residue is important but not essential. These residues also determine zinc coordination and affinity. Zinc binding to the N-terminal domain is thus crucial for HMA4 protein function, whereas it is not required to maintain the protein structure. Altogether, combining in vivo and in vitro approaches in our study provides insights towards the molecular understanding of metal transport and specificity of metal P-type ATPases.

Modulation of copper deficiency responses by diurnal and circadian rhythms in Arabidopsis thaliana

Copper homeostasis under deficiency is regulated by the SQUAMOSA PROMOTER BINDING PROTEIN-LIKE7 (SPL7) transcription factor. The daily oscillating expression of two SPL7-dependent copper deficiency markers, COPPER TRANSPORTER (COPT2) and IRON SUPEROXIDE DISMUTASE (FSD1), has been followed by quantitative PCR and in promoter:LUCIFERASE transgenic plants. Both genes showed circadian and diurnal regulation. Under copper deficiency, their expression decreased drastically in continuous darkness. Accordingly, total copper content was slightly reduced in etiolated seedlings under copper deficiency. The expression of SPL7 and its targets COPT2 and FSD1 was differently regulated in various light signalling mutants. On the other hand, increased copper levels reduced the amplitude of nuclear circadian clock components, such as GIGANTEA (GI). The alteration of copper homeostasis in the COPT1 overexpression line and spl7 mutants also modified the amplitude of a classical clock output, namely the circadian oscillation of cotyledon movements. In the spl7 mutant, the period of the oscillation remained constant. These results suggest a feedback of copper transport on the circadian clock and the integration of rhythmic copper homeostasis into the central oscillator of plants.

Transcriptome Profiling of Louisiana iris Root and Identification of Genes Involved in Lead-Stress Response

Louisiana iris is tolerant to and accumulates the heavy metal lead (Pb). However, there is limited knowledge of the molecular mechanisms behind this feature. We describe the transcriptome of Louisiana iris using Illumina sequencing technology. The root transcriptome of Louisiana iris under control and Pb-stress conditions was sequenced. Overall, 525,498 transcripts representing 313,958 unigenes were assembled using the clean raw reads. Among them, 43,015 unigenes were annotated and their functions classified using the euKaryotic Orthologous Groups (KOG) database. They were divided into 25 molecular families. In the Gene Ontology (GO) database, 50,174 unigenes were categorized into three GO trees (molecular function, cellular component and biological process). After analysis of differentially expressed genes, some Pb-stress-related genes were selected, including biosynthesis genes of chelating compounds, metal transporters, transcription factors and antioxidant-related genes. This study not only lays a foundation for further studies on differential genes under Pb stress, but also facilitates the molecular breeding of Louisiana iris.

Copper-transporting ATPases: The evolutionarily conserved machineries for balancing copper in living systems

Copper ATPases (Cu-ATPases) are ubiquitous transmembrane proteins using energy from ATP to transport copper across different biological membranes of prokaryotic and eukaryotic cells. As they belong to the P-ATPase family, Cu-ATPases contain a characteristic catalytic domain with an evolutionarily conserved aspartate residue phosphorylated by ATP to form a phosphoenzyme intermediate, as well as transmembrane helices containing a cation-binding cysteine-proline-cysteine/histidine/serine (CPx) motif for catalytic activation and cation translocation. In addition, most Cu-ATPases possess the N-terminal Cu-binding CxxC motif required for regulation of enzyme activity. In cells, the Cu-ATPases receive copper from soluble chaperones and maintain intracellular copper homeostasis by efflux of copper from the cell or transport of the metal into the intracellular compartments. In addition, copper pumps play an essential role in cuproprotein biosynthesis by the uptake of copper into the cell or delivery of the metal into the chloroplasts and thylakoid lumen or into the lumen of the secretory pathway, where the metal ion is incorporated into copper-dependent enzymes. In the recent years, significant progress has been made toward understanding the function and regulation of Cu-transporting ATPases in archaea, bacteria, yeast, humans, and plants, providing new insights into the specific physiological roles of these essential proteins in various organisms and revealing some conservative regulatory mechanisms of Cu-ATPase activity. In this review, the structural, biochemical, and functional properties of Cu-ATPases from phylogenetically different organisms are summarized and discussed, with particular attention given to the recent insights into the molecular biology of copper pumps in plants.

The N-terminal degenerated metal-binding domain is involved in the heavy metal transport activity of TaHMA2

We identified key residues of TaHMA2, and the N- and C-terminal regions of the protein have different roles in its transport function when heterologously expressed in yeast. TaHMA2, a P1B-type ATPase from wheat (Triticum aestivum L.), plays an important role in heavy metal homeostasis in plants. A previous study showed that overexpressing TaHMA2 in rice (Oryza sativa L.), Arabidopsis thaliana, or tobacco (Nicotiana tabacum L.) resulted in various responses to heavy metals. Here, we report the heterologous expression of TaHMA2 in the yeast Saccharomyces cerevisiae. TaHMA2 expression increased the yeast's sensitivity to Cd, but not to Zn, Pb or Co, and increased Cd accumulation was concurrently observed. The eGFP-TaHMA2 fusion protein was localized to the plasma membrane and showed a discontinuous pattern. Mutagenesis of the cysteine and glutamate residues in the N-terminal metal-binding domain (N-MBD) impaired the function of TaHMA2. Deletion of most of the C terminus (TaHMA2ΔC, 712-1003) partially abolished the protein's function, whereas deletion of the N terminus (TaHMA2ΔN, 2-699) completely abolished Cd sensitivity. These data suggest that cysteine and glutamate residues are important for the metal-binding/translocation function of TaHMA2. Additional studies are needed to further understand the selectivity of TaHMA2 in planta.

An Arabidopsis thaliana copper-sensitive mutant suggests a role of phytosulfokine in ethylene production

To increase our understanding of the adaptation for copper (Cu) deficiency, Arabidopsis mutants with apparent alterations under Cu deficiency were identified. In this report, a novel mutant, tpst-2, was found to be more sensitive than wild-type (Col-0) plants to Cu deficiency during root elongation. The positional cloning of tpst-2 revealed that this gene encodes a tyrosylprotein sulfotransferase (TPST). Moreover, the ethylene production of tpst-2 mutant was higher than that of Col-0 under Cu deficiency, and adding the ethylene response inhibitor AgNO3 partially rescued defects in root elongation. Interestingly, peptide hormone phytosulfokine (PSK) treatment also repressed the ethylene production of tpst-2 mutant plants. Our results revealed that TPST suppressed ethylene production through the action of PSK.

Functional and Biochemical Characterization of Cucumber Genes Encoding Two Copper ATPases CsHMA5.1 and CsHMA5.2

Plant copper P1B-type ATPases appear to be crucial for maintaining copper homeostasis within plant cells, but until now they have been studied mostly in model plant systems. Here, we present the molecular and biochemical characterization of two cucumber copper ATPases, CsHMA5.1 and CsHMA5.2, indicating a different function for HMA5-like proteins in different plants. When expressed in yeast, CsHMA5.1 and CsHMA5.2 localize to the vacuolar membrane and are activated by monovalent copper or silver ions and cysteine, showing different affinities to Cu(+) (Km ∼1 or 0.5 μM, respectively) and similar affinity to Ag(+) (Km ∼2.5 μM). Both proteins restore the growth of yeast mutants sensitive to copper excess and silver through intracellular copper sequestration, indicating that they contribute to copper and silver detoxification. Immunoblotting with specific antibodies revealed the presence of CsHMA5.1 and CsHMA5.2 in the tonoplast of cucumber cells. Interestingly, the root-specific CsHMA5.1 was not affected by copper stress, whereas the widely expressed CsHMA5.2 was up-regulated or down-regulated in roots upon copper excess or deficiency, respectively. The copper-induced increase in tonoplast CsHMA5.2 is consistent with the increased activity of ATP-dependent copper transport into tonoplast vesicles isolated from roots of plants grown under copper excess. These data identify CsHMA5.1 and CsHMA5.2 as high affinity Cu(+) transporters and suggest that CsHMA5.2 is responsible for the increased sequestration of copper in vacuoles of cucumber root cells under copper excess.

Molecular and biochemical properties of two P1B2-ATPases, CsHMA3 and CsHMA4, from cucumber

P1B-ATPases (heavy metal ATPases, HMAs) constitute a multigenic subfamily of P-ATPases involved in the transport of monovalent and divalent heavy metals in plant cells. Here, we present the organization of genes encoding the HMA family in the cucumber genome and report the function and biochemical properties of two cucumber proteins homologous to the HMA2-4-like plant HMAs. Eight genes encoding putative P1B -ATPases were identified in the cucumber genome. Among them, CsHMA3 was predominantly expressed in roots and up-regulated by Pb, Zn and Cd excess, whereas the CsHMA4 transcript was most abundant in roots and flowers of cucumber plants, and elevated under Pb and Zn excess. Expression of CsHMA3 in Saccharomyces cerevisiae enhanced yeast tolerance to Cd and Pb, whereas CsHMA4 conferred increased resistance of yeast cells to Cd and Zn. Immunostaining with specific antibodies raised against cucumber proteins revealed tonoplast localization of CsHMA3 and plasma membrane localization of CsHMA4 in cucumber root cells. Kinetic studies of CsHMA3 and CsHMA4 in yeast membranes indicated differing heavy metal cation affinities of these two proteins. Altogether, the results suggest an important role of CsHMA3 and CsHMA4 in Cd and Pb detoxification and Zn homeostasis in cucumber cells.

Effects of copper-oxide nanoparticles, dissolved copper and ultraviolet radiation on copper bioaccumulation, photosynthesis and oxidative stress in the aquatic macrophyte Elodea nuttallii

In this study, the uptake and sub-toxic effects of CuO nanoparticles (CuO-NPs), dissolved Cu(II) alone or in combination with UV radiation on the aquatic macrophyte Elodea nuttallii were studied. Emphasis was on Cu accumulation, growth, photosynthesis and the oxidative stress related enzymes peroxidase (POD) and superoxide dismutase (SOD). The results showed stronger Cu accumulation in plants exposed to 10 mg L(-1) CuO-NPs, corresponding to 1.4-2 mg L(-1) dissolved Cu(II), than to 256 μg L(-1) Cu(II). However, the ratio between the accumulated Cu and dissolved Cu in CuO treatments was lower than in Cu(II) treatments. Additional UV exposure increased accumulation in both treatments, with the effect being stronger for Cu accumulation from CuO-NPs than for dissolved Cu(II). Photosynthetic capacity was strongly reduced by UV treatment, whereas remained unaffected by Cu(II) or CuO-NP treatments. Similarly, the increase of SOD activity was more pronounced in the UV treatments. On the other hand, POD activity enhancement was strongest in the plants exposed to CuO-NPs for 24 h. Expression of the copper transporter COPT1 as revealed by RT-qPCR was inhibited by Cu(II) and CuO-NP treatment, limiting the uptake of excess Cu into the cells. Overall, the combined exposure of E. nuttallii to UV radiation with CuO-NPs or Cu(II) has a higher impact than exposure to CuO-NPs or Cu(II) alone. The results imply that heavy pollution of natural water with CuO-NPs or dissolved Cu might have stronger effects in combination with natural UV irradiation on organisms in situ.

Cation transporters/channels in plants: Tools for nutrient biofortification

Cation transporters/channels are key players in a wide range of physiological functions in plants, including cell signaling, osmoregulation, plant nutrition and metal tolerance. The recent identification of genes encoding some of these transport systems has allowed new studies toward further understanding of their integrated roles in plant. This review summarizes recent discoveries regarding the function and regulation of the multiple systems involved in cation transport in plant cells. The role of membrane transport in the uptake, distribution and accumulation of cations in plant tissues, cell types and subcellular compartments is described. We also discuss how the knowledge of inter- and intra-species variation in cation uptake, transport and accumulation as well as the molecular mechanisms responsible for these processes can be used to increase nutrient phytoavailability and nutrients accumulation in the edible tissues of plants. The main trends for future research in the field of biofortification are proposed.

Defective copper transport in the copt5 mutant affects cadmium tolerance

Cadmium toxicity interferes with essential metal homeostasis, which is a problem for both plant nutrition and the consumption of healthy food by humans. Copper uptake is performed by the members of the Arabidopsis high affinity copper transporter (COPT) family. One of the members, COPT5, is involved in copper recycling from the vacuole toward the cytosolic compartment. We show herein that copt5 mutants are more sensitive to cadmium stress than wild-type plants, as indicated by reduced growth. Exacerbated cadmium toxicity in copt5 mutants is due specifically to altered copper traffic through the COPT5 transporter. Three different processes which have been shown to affect cadmium tolerance are altered in copt5 mutants. First, ethylene biosynthesis diminishes under copper deficiency and, in the presence of cadmium, ethylene production diminishes further. Copper deficiency responses are also attenuated under cadmium treatment. Remarkably, while copt5 roots present higher oxidative stress toxicity symptoms than controls, aerial copt5 parts display lower oxidative stress, as seen by reduced cadmium delivery to shoots. Taken together, these results demonstrate that copper transport plays a key role in cadmium resistance, and suggest that oxidative stress triggers an NADPH oxidase-mediated signaling pathway, which contributes to cadmium translocation and basal plant resistance. The slightly lower cadmium levels that reach aerial parts in the copt5 mutants, irrespective of the copper content in the media, suggest a new biotechnological approach to minimize toxic cadmium entry into food chains.

Copper Delivery to Chloroplast Proteins and its Regulation

Copper is required for photosynthesis in chloroplasts of plants because it is a cofactor of plastocyanin, an essential electron carrier in the thylakoid lumen. Other chloroplast copper proteins are copper/zinc superoxide dismutase and polyphenol oxidase, but these proteins seem to be dispensable under conditions of low copper supply when transcripts for these proteins undergo microRNA-mediated down regulation. Two ATP-driven copper transporters function in tandem to deliver copper to chloroplast compartments. This review seeks to summarize the mechanisms of copper delivery to chloroplast proteins and its regulation. We also delineate some of the unanswered questions that still remain in this field.

Temporal aspects of copper homeostasis and its crosstalk with hormones

To cope with the dual nature of copper as being essential and toxic for cells, plants temporarily adapt the expression of copper homeostasis components to assure its delivery to cuproproteins while avoiding the interference of potential oxidative damage derived from both copper uptake and photosynthetic reactions during light hours. The circadian clock participates in the temporal organization of coordination of plant nutrition adapting metabolic responses to the daily oscillations. This timely control improves plant fitness and reproduction and holds biotechnological potential to drive increased crop yields. Hormonal pathways, including those of abscisic acid, gibberellins, ethylene, auxins, and jasmonates are also under direct clock and light control, both in mono and dicotyledons. In this review, we focus on copper transport in Arabidopsis thaliana and Oryza sativa and the presumable role of hormones in metal homeostasis matching nutrient availability to growth requirements and preventing metal toxicity. The presence of putative hormone-dependent regulatory elements in the promoters of copper transporters genes suggests hormonal regulation to match special copper requirements during plant development. Spatial and temporal processes that can be affected by hormones include the regulation of copper uptake into roots, intracellular trafficking and compartmentalization, and long-distance transport to developing vegetative and reproductive tissues. In turn, hormone biosynthesis and signaling are also influenced by copper availability, which suggests reciprocal regulation subjected to temporal control by the central oscillator of the circadian clock. This transcriptional regulatory network, coordinates environmental and hormonal signaling with developmental pathways to allow enhanced micronutrient acquisition efficiency.

Impact of heavy metal stresses on the growth and auxin homeostasis of Arabidopsis seedlings

The phytohormone auxin is an essential mediator in many aspects of plant development. Its dynamic and differential distribution within the plant is regulated by a variety of environmental cues including heavy metal stimuli. In the present study, we first evaluated the toxic effects of seven heavy metals including Pb(2+), Cd(2+), Hg(2+), Ni(2+). Zn(2+), Co(2+) and Cu(2+) in their excess on the model plant, Arabidopsis thaliana. Various morphological defects including loss in fresh weight and leaf area, decrease of the primary root length and stimulation of the lateral root density occurred to a different extent among seven heavy metals. Next, using an indicative DR5:GUS reporter line of Arabidopsis, the auxin accumulation and distribution within plant seedlings were found to be dramatically and differentially affected by these heavy metals. We further analyzed the transcriptional changes of 27 selected auxin homeostasis-related genes by qRT-PCR technique and found that upon various heavy metals, the expressions of the candidate genes were distinctly altered in shoots and roots. Our data indicated that when confronted with excessive heavy metals, plants could dynamically and differentially regulate the transcription of auxin-related genes to adjust the location and effective accumulation of auxin within the plant for better adaptation and survival under the adverse environment.

Identification and characterization of a novel copper transporter gene family TaCT1 in common wheat

Copper is an essential micronutrient for plant growth and development, and copper transporter plays a pivotal role for keeping copper homeostasis. However, little is known about copper transporters in wheat. Here, we report a novel copper transporter gene family, TaCT1, in common wheat. Three TaCT1 homoeologous genes were isolated and assigned to group 5 chromosomes. Each of the TaCT1 genes (TaCT1-5A, -5B or -5D) possesses 12 transmembrane domains. TaCT1 genes exhibited higher transcript levels in leaf than in root, culm and spikelet. Excess copper down-regulated the transcript levels of TaCT1 and copper deficiency-induced TaCT1 expression. Subcellular experiments localized the TaCT1 to the Golgi apparatus. Yeast expression experiments and virus-induced gene silencing analysis indicated that the TaCT1 functioned in copper transport. Site-directed mutagenesis demonstrated that three amino acid residues, Met(35), Met(38) and Cys(365), are required for TaCT1 function. Phylogenetic and functional analyses suggested that homologous genes shared high similarity with TaCT1 may exist exclusively in monocot plants. Our work reveals a novel wheat gene family encoding major facilitator superfamily (MFS)-type copper transporters, and provides evidence for their functional involvement in promoting copper uptake and keeping copper homeostasis in common wheat.

Metallothionein deficiency impacts copper accumulation and redistribution in leaves and seeds of Arabidopsis

Most angiosperm genomes contain several genes encoding metallothionein (MT) proteins that can bind metals including copper (Cu) and zinc (Zn). Metallothionein genes are highly expressed under various conditions but there is limited information about their function. We have studied Arabidopsis mutants that are deficient in multiple MTs to learn about the functions of MTs in plants. T-DNA insertions were identified in four of the five Arabidopsis MT genes expressed in vegetative tissues. These were crossed to produce plants deficient in four MTs (mt1a/mt2a/mt2b/mt3). The concentration of Cu was lower in seeds but higher in old leaves of the quad-MT mutant compared to wild-type plants. Experiments with stable isotopes showed that Cu in seeds came from two sources: directly from roots and via remobilization from other organs. Mobilization of Cu out of senescing leaves was disrupted in MT-deficient plants. Tolerance to Cu, Zn and paraquat was unaffected by MT deficiency but these plants were slightly more sensitive to cadmium (Cd). The quad-MT mutant showed no change in resistance to a number of microbial pathogens, or in the progression of leaf senescence. Although these MTs are not required to complete the plant's life cycle, MTs are important for Cu homeostasis and distribution in Arabidopsis.

Transcriptomic and metabolomic analysis of copper stress acclimation in Ectocarpus siliculosus highlights signaling and tolerance mechanisms in brown algae

BACKGROUND

Brown algae are sessile macro-organisms of great ecological relevance in coastal ecosystems. They evolved independently from land plants and other multicellular lineages, and therefore hold several original ontogenic and metabolic features. Most brown algae grow along the coastal zone where they face frequent environmental changes, including exposure to toxic levels of heavy metals such as copper (Cu).

RESULTS

We carried out large-scale transcriptomic and metabolomic analyses to decipher the short-term acclimation of the brown algal model E. siliculosus to Cu stress, and compared these data to results known for other abiotic stressors. This comparison demonstrates that Cu induces oxidative stress in E. siliculosus as illustrated by the transcriptomic overlap between Cu and H2O2 treatments. The common response to Cu and H2O2 consisted in the activation of the oxylipin and the repression of inositol signaling pathways, together with the regulation of genes coding for several transcription-associated proteins. Concomitantly, Cu stress specifically activated a set of genes coding for orthologs of ABC transporters, a P1B-type ATPase, ROS detoxification systems such as a vanadium-dependent bromoperoxidase, and induced an increase of free fatty acid contents. Finally we observed, as a common abiotic stress mechanism, the activation of autophagic processes on one hand and the repression of genes involved in nitrogen assimilation on the other hand.

CONCLUSIONS

Comparisons with data from green plants indicate that some processes involved in Cu and oxidative stress response are conserved across these two distant lineages. At the same time the high number of yet uncharacterized brown alga-specific genes induced in response to copper stress underlines the potential to discover new components and molecular interactions unique to these organisms. Of particular interest for future research is the potential cross-talk between reactive oxygen species (ROS)-, myo-inositol-, and oxylipin signaling.

Investigating the toxicity, uptake, nanoparticle formation and genetic response of plants to gold

We have studied the physiological and genetic responses of Arabidopsis thaliana L. (Arabidopsis) to gold. The root lengths of Arabidopsis seedlings grown on nutrient agar plates containing 100 mg/L gold were reduced by 75%. Oxidized gold was subsequently found in roots and shoots of these plants, but gold nanoparticles (reduced gold) were only observed in the root tissues. We used a microarray-based study to monitor the expression of candidate genes involved in metal uptake and transport in Arabidopsis upon gold exposure. There was up-regulation of genes involved in plant stress response such as glutathione transferases, cytochromes P450, glucosyl transferases and peroxidases. In parallel, our data show the significant down-regulation of a discreet number of genes encoding proteins involved in the transport of copper, cadmium, iron and nickel ions, along with aquaporins, which bind to gold. We used Medicago sativa L. (alfalfa) to study nanoparticle uptake from hydroponic culture using ionic gold as a non-nanoparticle control and concluded that nanoparticles between 5 and 100 nm in diameter are not directly accumulated by plants. Gold nanoparticles were only observed in plants exposed to ionic gold in solution. Together, we believe our results imply that gold is taken up by the plant predominantly as an ionic form, and that plants respond to gold exposure by up-regulating genes for plant stress and down-regulating specific metal transporters to reduce gold uptake.

HMA1 and PAA1, two chloroplast-envelope PIB-ATPases, play distinct roles in chloroplast copper homeostasis

Copper is an essential micronutrient but it is also potentially toxic as copper ions can catalyse the production of free radicals, which result in various types of cell damage. Therefore, copper homeostasis in plant and animal cells must be tightly controlled. In the chloroplast, copper import is mediated by a chloroplast-envelope PIB-type ATPase, HMA6/PAA1. Copper may also be imported by HMA1, another chloroplast-envelope PIB-ATPase. To get more insights into the specific functional roles of HMA1 and PAA1 in copper homeostasis, this study analysed the phenotypes of plants affected in the expression of both HMA1 and PAA1 ATPases, as well as of plants overexpressing HMA1 in a paa1 mutant background. The results presented here provide new evidence associating HMA1 with copper homeostasis in the chloroplast. These data suggest that HMA1 and PAA1 behave as distinct pathways for copper import and targeting to the chloroplast. Finally, this work also provides evidence for an alternative route for copper import into the chloroplast mediated by an as-yet unidentified transporter that is neither HMA1 nor PAA1.

Comparison of global responses to mild deficiency and excess copper levels in Arabidopsis seedlings

Copper is an essential micronutrient in higher plants, but it is toxic in excess. The fine adjustments required to fit copper nutritional demands for optimal growth are illustrated by the diverse, severe symptoms resulting from copper deficiency and excess. Here, a differential transcriptomic analysis was done between Arabidopsis thaliana plants suffering from mild copper deficiency and those with a slight copper excess. The effects on the genes encoding cuproproteins or copper homeostasis factors were included in a CuAt database, which was organised to collect additional information and connections to other databases. The categories overrepresented under copper deficiency and copper excess conditions are discussed. Different members of the categories overrepresented under copper deficiency conditions were both dependent and independent of the general copper deficiency transcriptional regulator SPL7. The putative regulatory elements in the promoter of the copper deficiency overrepresented genes, particularly of the iron superoxide dismutase gene FSD1, were also analysed. A 65 base pair promoter fragment, with at least three GTAC sequences, was found to be not only characteristic of them all, but was responsible for most of the FSD1 copper-dependent regulations. Moreover, a new molecular marker for the slight excess copper nutritional status is proposed. Taken together, these data further contribute to characterise copper nutritional responses in higher plants.

An overview of heavy metal challenge in plants: from roots to shoots

Heavy metals are often present naturally in soils, but many human activities (e.g. mining, agriculture, sewage processing, the metal industry and automobiles) increase their prevalence in the environment resulting in concentrations that are toxic to animals and plants. Excess heavy metals affect plant physiology by inducing stress symptoms, but many plants have adapted to avoid the damaging effects of metal toxicity, using strategies such as metal chelation, transport and compartmentalization. Understanding the molecular basis of heavy metal tolerance in plants will facilitate the development of new strategies to create metal-tolerant crops, biofortified foods and plants suitable for the phytoremediation of contaminated sites.

Towards the discovery of novel genetic component involved in stress resistance in Arabidopsis thaliana

The exposure of plants to high concentrations of trace metallic elements such as copper involves a remodeling of the root system, characterized by a primary root growth inhibition and an increase in the lateral root density. These characteristics constitute easy and suitable markers for screening mutants altered in their response to copper excess. A forward genetic approach was undertaken in order to discover novel genetic factors involved in the response to copper excess. A Cu(2+) -sensitive mutant named copper modified resistance1 (cmr1) was isolated and a causative mutation in the CMR1 gene was identified by using positional cloning and next-generation sequencing. CMR1 encodes a plant-specific protein of unknown function. The analysis of the cmr1 mutant indicates that the CMR1 protein is required for optimal growth under normal conditions and has an essential role in the stress response. Impairment of the CMR1 activity alters root growth through aberrant activity of the root meristem, and modifies potassium concentration and hormonal balance (ethylene production and auxin accumulation). Our data support a putative role for CMR1 in cell division regulation and meristem maintenance. Research on the role of CMR1 will contribute to the understanding of the plasticity of plants in response to changing environments.

Silicon mitigates heavy metal stress by regulating P-type heavy metal ATPases, Oryza sativa low silicon genes, and endogenous phytohormones

BACKGROUND

Silicon (Si) application has been known to enhance the tolerance of plants against abiotic stresses. However, the protective mechanism of Si under heavy metals contamination is poorly understood. The aim of this study was to assess the role of Si in counteracting toxicity due to cadmium (Cd) and copper (Cu) in rice plants (Oryza sativa).

RESULTS

Si significantly improved the growth and biomass of rice plants and reduced the toxic effects of Cd/Cu after different stress periods. Si treatment ameliorated root function and structure compared with non-treated rice plants, which suffered severe root damage. In the presence of Si, the Cd/Cu concentration was significantly lower in rice plants, and there was also a reduction in lipid peroxidation and fatty acid desaturation in plant tissues. The reduced uptake of metals in the roots modulated the signaling of phytohormones involved in responses to stress and host defense, such as abscisic acid, jasmonic acid, and salicylic acid. Furthermore, the low concentration of metals significantly down regulated the mRNA expression of enzymes encoding heavy metal transporters (OsHMA2 and OsHMA3) in Si-metal-treated rice plants. Genes responsible for Si transport (OsLSi1 and OsLSi2), showed a significant up-regulation of mRNA expression with Si treatment in rice plants.

CONCLUSION

The present study supports the active role of Si in the regulation of stresses from heavy metal exposure through changes in root morphology.

Origin and evolution of metal P-type ATPases in Plantae (Archaeplastida)

Metal ATPases are a subfamily of P-type ATPases involved in the transport of metal cations across biological membranes. They all share an architecture featuring eight transmembrane domains in pairs of two and are found in prokaryotes as well as in a variety of Eukaryotes. In Arabidopsis thaliana, eight metal P-type ATPases have been described, four being specific to copper transport and four displaying a broader metal specificity, including zinc, cadmium, and possibly copper and calcium. So far, few efforts have been devoted to elucidating the origin and evolution of these proteins in Eukaryotes. In this work, we use large-scale phylogenetics to show that metal P-type ATPases form a homogenous group among P-type ATPases and that their specialization into either monovalent (Cu) or divalent (Zn, Cd…) metal transport stems from a gene duplication that took place early in the evolution of Life. Then, we demonstrate that the four subgroups of plant metal ATPases all have a different evolutionary origin and a specific taxonomic distribution, only one tracing back to the cyanobacterial progenitor of the chloroplast. Finally, we examine the subsequent evolution of these proteins in green plants and conclude that the genes thoroughly characterized in model organisms are often the result of lineage-specific gene duplications, which calls for caution when attempting to infer function from sequence similarity alone in non-model organisms.

Whole shoot mineral partitioning and accumulation in pea (Pisum sativum)

Several grain legumes are staple food crops that are important sources of minerals for humans; unfortunately, our knowledge is incomplete with respect to the mechanisms of translocation of these minerals to the vegetative tissues and loading into seeds. Understanding the mechanism and partitioning of minerals in pea could help in developing cultivars with high mineral density. A mineral partitioning study was conducted in pea to assess whole-plant growth and mineral content and the potential source-sink remobilization of different minerals, especially during seed development. Shoot and root mineral content increased for all the minerals, although tissue-specific partitioning differed between the minerals. Net remobilization was observed for P, S, Cu, and Fe from both the vegetative tissues and pod wall, but the amounts remobilized were much below the total accumulation in the seeds. Within the mature pod, more minerals were partitioned to the seed fraction (>75%) at maturity than to the pod wall for all the minerals except Ca, where only 21% was partitioned to the seed fraction. Although there was evidence for net remobilization of some minerals from different tissues into seeds, continued uptake and translocation of minerals to source tissues during seed fill is as important, if not more important, than remobilization of previously stored minerals.

Comparative study of root growth and sucrose-cleaving enzymes in metallicolous and non-metallicolous populations of Rumex dentatus under copper stress

Sucrose metabolism in roots of metallophytes is very important for root growth and maintenance of heavy metal tolerance. However, rare researches have been carried out on this topic so far. We tested here a hypothesis that roots of copper-tolerant plants should manifest higher activities of sucrose-cleaving enzymes than non-tolerant plants for maintaining root growth under Cu stress. Plants of two contrasting populations of metallophyte Rumex dentatus, one from an ancient Cu mine (MP) and the other from a non-mine site (NMP), were treated with Cu in controlled experiments. Cu treatment resulted in a higher root biomass and root/shoot biomass ratio in MP compared to NMP. More complicated root system architecture was showed in MP under Cu stress. Activities and transcript levels of acid invertase as well as contents of sucrose and reducing sugar in MP were elevated under Cu treatment, while activities of neutral/alkaline invertase and sucrose synthase showed no significant differences between two populations. The results indicate important roles of acid invertase in governing root growth under Cu stress.

Expression of HMA4 cDNAs of the zinc hyperaccumulator Noccaea caerulescens from endogenous NcHMA4 promoters does not complement the zinc-deficiency phenotype of the Arabidopsis thaliana hma2hma4 double mutant

Noccaea caerulescens (Nc) exhibits a very high constitutive expression of the heavy metal transporting ATPase, HMA4, as compared to the non-hyperaccumulator Arabidopsis thaliana (At), due to copy number expansion and altered cis-regulation. We screened a BAC library for HMA4 and found that HMA4 is triplicated in the genome of a N. caerulescens accession from a former Zn mine near La Calamine (LC), Belgium. We amplified multiple HMA4 promoter sequences from three calamine N. caerulescens accessions, and expressed AtHMA4 and different NcHMA4 cDNAs under At and Nc HMA4 promoters in the A. thaliana (Col) hma2hma4 double mutant. Transgenic lines expressing HMA4 under the At promoter were always fully complemented for root-to-shoot Zn translocation and developed normally at a 2-μM Zn supply, whereas the lines expressing HMA4 under Nc promoters usually showed only slightly enhanced root to shoot Zn translocation rates in comparison with the double mutant, probably owing to ectopic expression in the roots, respectively. When expression of the Zn deficiency responsive marker gene ZIP4 was tested, the transgenic lines expressing AtHMA4 under an NcHMA4-1-LC promoter showed on average a 7-fold higher expression in the leaves, in comparison with the double hma2hma4 mutant, showing that this construct aggravated, rather than alleviated the severity of foliar Zn deficiency in the mutant, possible owing to expression in the leaf mesophyll.

Arabidopsis copper transport protein COPT2 participates in the cross talk between iron deficiency responses and low-phosphate signaling

Copper and iron are essential micronutrients for most living organisms because they participate as cofactors in biological processes, including respiration, photosynthesis, and oxidative stress protection. In many eukaryotic organisms, including yeast (Saccharomyces cerevisiae) and mammals, copper and iron homeostases are highly interconnected; yet, such interdependence is not well established in higher plants. Here, we propose that COPT2, a high-affinity copper transport protein, functions under copper and iron deficiencies in Arabidopsis (Arabidopsis thaliana). COPT2 is a plasma membrane protein that functions in copper acquisition and distribution. Characterization of the COPT2 expression pattern indicates a synergic response to copper and iron limitation in roots. We characterized a knockout of COPT2, copt2-1, that leads to increased resistance to simultaneous copper and iron deficiencies, measured as reduced leaf chlorosis and improved maintenance of the photosynthetic apparatus. We propose that COPT2 could play a dual role under iron deficiency. First, COPT2 participates in the attenuation of copper deficiency responses driven by iron limitation, possibly to minimize further iron consumption. Second, global expression analyses of copt2-1 versus wild-type Arabidopsis plants indicate that low-phosphate responses increase in the mutant. These results open up new biotechnological approaches to fight iron deficiency in crops.

The plant vascular system: evolution, development and functions

The emergence of the tracheophyte-based vascular system of land plants had major impacts on the evolution of terrestrial biology, in general, through its role in facilitating the development of plants with increased stature, photosynthetic output, and ability to colonize a greatly expanded range of environmental habitats. Recently, considerable progress has been made in terms of our understanding of the developmental and physiological programs involved in the formation and function of the plant vascular system. In this review, we first examine the evolutionary events that gave rise to the tracheophytes, followed by analysis of the genetic and hormonal networks that cooperate to orchestrate vascular development in the gymnosperms and angiosperms. The two essential functions performed by the vascular system, namely the delivery of resources (water, essential mineral nutrients, sugars and amino acids) to the various plant organs and provision of mechanical support are next discussed. Here, we focus on critical questions relating to structural and physiological properties controlling the delivery of material through the xylem and phloem. Recent discoveries into the role of the vascular system as an effective long-distance communication system are next assessed in terms of the coordination of developmental, physiological and defense-related processes, at the whole-plant level. A concerted effort has been made to integrate all these new findings into a comprehensive picture of the state-of-the-art in the area of plant vascular biology. Finally, areas important for future research are highlighted in terms of their likely contribution both to basic knowledge and applications to primary industry.

Barley HvHMA1 is a heavy metal pump involved in mobilizing organellar Zn and Cu and plays a role in metal loading into grains

Heavy metal transporters belonging to the P(1B)-ATPase subfamily of P-type ATPases are key players in cellular heavy metal homeostasis. Heavy metal transporters belonging to the P(1B)-ATPase subfamily of P-type ATPases are key players in cellular heavy metal homeostasis. In this study we investigated the properties of HvHMA1, which is a barley orthologue of Arabidopsis thaliana AtHMA1 localized to the chloroplast envelope. HvHMA1 was localized to the periphery of chloroplast of leaves and in intracellular compartments of grain aleurone cells. HvHMA1 expression was significantly higher in grains compared to leaves. In leaves, HvHMA1 expression was moderately induced by Zn deficiency, but reduced by toxic levels of Zn, Cu and Cd. Isolated barley chloroplasts exported Zn and Cu when supplied with Mg-ATP and this transport was inhibited by the AtHMA1 inhibitor thapsigargin. Down-regulation of HvHMA1 by RNA interference did not have an effect on foliar Zn and Cu contents but resulted in a significant increase in grain Zn and Cu content. Heterologous expression of HvHMA1 in heavy metal-sensitive yeast strains increased their sensitivity to Zn, but also to Cu, Co, Cd, Ca, Mn, and Fe. Based on these results, we suggest that HvHMA1 is a broad-specificity exporter of metals from chloroplasts and serve as a scavenging mechanism for mobilizing plastid Zn and Cu when cells become deficient in these elements. In grains, HvHMA1 might be involved in mobilizing Zn and Cu from the aleurone cells during grain filling and germination.

Engineering copper hyperaccumulation in plants by expressing a prokaryotic copC gene

In this work, engineering Cu-hyperaccumulation in plants was approached. First, the copC gene from Pseudomonas sp. Az13, encoding a periplasmic Cu-binding protein, was expressed in Arabidopsis thaliana driven by the CaMV35S promoter (transgenic lines 35S-copC). 35S-copC lines showed up to 5-fold increased Cu accumulation in roots (up to 2000 μg Cu. g(-1)) and shoots (up to 400 μg Cu. g(-1)), compared to untransformed plants, over the limits established for Cu-hyperaccumulators. 35S lines showed enhanced Cu sensitivity. Second, copC was engineered under the control of the cab1 (chlorophyll a/b binding protein 1) promoter, in order to drive copC expression to the shoots (transgenic lines cab1-copC). cab1-copC lines showed increased Cu translocation factors (twice that of wild-type plants) and also displayed enhanced Cu sensitivity. Finally, subcellular targeting the CopC protein to plant vacuoles was addressed by expressing a modified copC gene containing specific vacuole sorting determinants (transgenic lines 35S-copC-V). Unexpectedly, increased Cu-accumulation was not achieved-neither in roots nor in shoots-when compared to 35S-copC lines. Conversely, 35S-copC-V lines did display greatly enhanced Cu-hypersensitivity. Our results demonstrate the feasibility of obtaining Cu-hyperaccumulators by engineering a prokaryotic Cu-binding protein, but they highlight the difficulty of altering the exquisite Cu homeostasis in plants.

YSL16 is a phloem-localized transporter of the copper-nicotianamine complex that is responsible for copper distribution in rice

Cu is an essential element for plant growth, but the molecular mechanisms of its distribution and redistribution within the plants are unknown. Here, we report that Yellow stripe-like16 (YSL16) is involved in Cu distribution and redistribution in rice (Oryza sativa). Rice YSL16 was expressed in the roots, leaves, and unelongated nodes at the vegetative growth stage and highly expressed in the upper nodes at the reproductive stage. YSL16 was expressed at the phloem of nodes and vascular tissues of leaves. Knockout of this gene resulted in a higher Cu concentration in the older leaves but a lower concentration in the younger leaves at the vegetative stage. At the reproductive stage, a higher Cu concentration was found in the flag leaf and husk, but less Cu was present in the brown rice, resulting in a significant reduction in fertility in the knockout line. Isotope labeling experiments with (65)Cu showed that the mutant lost the ability to transport Cu-nicotianamine from older to younger leaves and from the flag leaf to the panicle. Rice YSL16 transported the Cu-nicotianamine complex in yeast. Taken together, our results indicate that Os-YSL16 is a Cu-nicotianamine transporter that is required for delivering Cu to the developing young tissues and seeds through phloem transport.

Interaction of brassinosteroids and polyamines enhances copper stress tolerance in raphanus sativus

Brassinosteroids (BRs) and polyamines (PAs) regulate various responses to abiotic stress, but their involvement in the regulation of copper (Cu) homeostasis in plants exposed to toxic levels of Cu is poorly understood. This study provides an analysis of the effects of exogenously applied BRs and PAs on radish (Raphanus sativus) plants exposed to toxic concentrations of Cu. The interaction of 24-epibrassinolide (EBR, an active BR) and spermidine (Spd, an active PA) on gene expression and the physiology of radish plants resulted in enhanced tolerance to Cu stress. Results indicated that the combined application of EBR and Spd modulated the expression of genes encoding PA enzymes and genes that impact the metabolism of indole-3-acetic acid (IAA) and abscisic acid (ABA) resulting in enhanced Cu stress tolerance. Altered expression of genes implicated in Cu homeostasis appeared to be the main effect of EBR and Spd leading to Cu stress alleviation in radish. Ion leakage, in vivo imaging of H(2)O(2), comet assay, and improved tolerance of Cu-sensitive yeast strains provided further evidence for the ability of EBR and Spd to improve Cu tolerance significantly. The study indicates that co-application of EBR and Spd is an effective approach for Cu detoxification and the maintenance of Cu homeostasis in plants. Therefore, the use of these compounds in agricultural production systems should be explored.

The CopRS two-component system is responsible for resistance to copper in the cyanobacterium Synechocystis sp. PCC 6803

Photosynthetic organisms need copper for cytochrome oxidase and for plastocyanin in the fundamental processes of respiration and photosynthesis. However, excess of free copper is detrimental inside the cells and therefore organisms have developed homeostatic mechanisms to tightly regulate its acquisition, sequestration, and efflux. Herein we show that the CopRS two-component system (also known as Hik31-Rre34) is essential for copper resistance in Synechocystis sp. PCC 6803. It regulates expression of a putative heavy-metal efflux-resistance nodulation and division type copper efflux system (encoded by copBAC) as well as its own expression (in the copMRS operon) in response to the presence of copper in the media. Mutants in this two-component system or the efflux system render cells more sensitive to the presence of copper in the media and accumulate more intracellular copper than the wild type. Furthermore, CopS periplasmic domain is able to bind copper, suggesting that CopS could be able to detect copper directly. Both operons (copMRS and copBAC) are also induced by the photosynthetic inhibitor 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone but this induction requires the presence of copper in the media. The reduced response of two mutant strains to copper, one lacking plastocyanin and a second one impaired in copper transport to the thylakoid, due to the absence of the P(I)-type ATPases PacS and CtaA, suggests that CopS can detect intracellular copper. In addition, a tagged version of CopS with a triple HA epitope localizes to both the plasma and the thylakoid membranes, suggesting that CopS could be involved in copper detection in both the periplasm and the thylakoid lumen.

Copper chaperone antioxidant protein1 is essential for copper homeostasis

Copper (Cu) is essential for plant growth but toxic in excess. Specific molecular mechanisms maintain Cu homeostasis to facilitate its use and avoid the toxicity. Cu chaperones, proteins containing a Cu-binding domain(s), are thought to assist Cu intracellular homeostasis by their Cu-chelating ability. In Arabidopsis (Arabidopsis thaliana), two Cu chaperones, Antioxidant Protein1 (ATX1) and ATX1-Like Copper Chaperone (CCH), share high sequence homology. Previously, their Cu-binding capabilities were demonstrated and interacting molecules were identified. To understand the physiological functions of these two chaperones, we characterized the phenotype of atx1 and cch mutants and the cchatx1 double mutant in Arabidopsis. The shoot and root growth of atx1 and cchatx1 but not cch was specifically hypersensitive to excess Cu but not excess iron, zinc, or cadmium. The activities of antioxidant enzymes in atx1 and cchatx1 were markedly regulated in response to excess Cu, which confirms the phenotype of Cu hypersensitivity. Interestingly, atx1 and cchatx1 were sensitive to Cu deficiency. Overexpression of ATX1 not only enhanced Cu tolerance and accumulation in excess Cu conditions but also tolerance to Cu deficiency. In addition, the Cu-binding motif MXCXXC of ATX1 was required for these physiological functions. ATX1 was previously proposed to be involved in Cu homeostasis by its Cu-binding activity and interaction with the Cu transporter Heavy metal-transporting P-type ATPase5. In this study, we demonstrate that ATX1 plays an essential role in Cu homeostasis in conferring tolerance to excess Cu and Cu deficiency. The possible mechanism is discussed.

Mutations in rice (Oryza sativa) heavy metal ATPase 2 (OsHMA2) restrict the translocation of zinc and cadmium

Widespread soil contamination with heavy metals has fostered the need for plant breeders to develop new crops that do not accumulate heavy metals. Metal-transporting transmembrane proteins that transport heavy metals across the plant plasma membrane are key targets for developing these new crops. Oryza sativa heavy metal ATPase 3 (OsHMA3) is known to be a useful gene for limiting cadmium (Cd) accumulation in rice. OsHMA2 is a close homolog of OsHMA3, but the function of OsHMA2 is unknown. To gain insight into the function of OsHMA2, we analyzed three Tos17 insertion mutants. The translocation ratios of zinc (Zn) and Cd were clearly lower in all mutants than in the wild type, suggesting that OsHMA2 is a major transporter of Zn and Cd from roots to shoots. By comparing each allele in the OsHMA2 protein structure and measuring the Cd translocation ratio, we identified the C-terminal region as essential for Cd translocation into shoots. Two alleles were identified as good material for breeding rice that does not contain Cd in the grain but does contain some Zn, and that grows normally.