COPT6 is a plasma membrane transporter that functions in copper homeostasis in Arabidopsis and is a novel target of SQUAMOSA promoter-binding protein-like 7

The transcription factor OsSPL9 endows rice with copper deficiency resilience

Copper (Cu) is a crucial micronutrient essential for the growth and development of plants. Rice exhibits remarkable resistance to Cu deficiency, but the underlying molecular mechanisms are not well understood. In this study, we reveal that the plant's ability to withstand Cu deficiency is orchestrated by a transcription factor known as OsSPL9. We have demonstrated that OsSPL9 functions as a central regulator of Cu homeostasis. Disrupting OsSPL9 through knockout significantly reduced the plant's tolerance to Cu deficiency. As a result, the spl9 mutants exhibited reduced Cu accumulation in their shoots when compared with wild-type plants. This reduction was linked to a disruption in the transport of Cu from older leaves to younger ones. Furthermore, we show that OsSPL9 directly bound to GTAC motifs in the promoters of key genes involved in Cu uptake and transport, as well as Cu-miRNAs, and enhanced their transcription under Cu-deficient conditions. Overall, our findings shed light on the molecular basis of rice resilience to Cu deficiency stress and place the transcription factor OsSPL9 as a master regulator of this response.

OsCOPT7 is involved in copper accumulation and transport through xylem

Copper (Cu) is an essential micronutrient for humans, but excessive Cu in rice grains causes health risks. Currently, the mechanisms underlying Cu accumulation in rice are unclear. Here, we identified a novel member of the high-affinity copper transporter (Ctr)-like (COPT) protein family in rice, OsCOPT7, which controls Cu accumulation in rice grains. Mutation in the coding sequence of OsCOPT7 (mutant lc1) leads to inhibition of Cu transport through the xylem, contributing to lower Cu concentrations in the grain of lc1. Knockout or modulation of the expression of OsCOPT7 significantly impacts Cu transportation in the xylem and its accumulation in rice grains. OsCOPT7 localizes at the multi-pass membrane in the cell and the gene is expressed in the exodermis and stele cells, facilitating Cu loading into the xylem. OsCOPT7 expression is upregulated under Cu deficiency and in various organs, implying its contribution to Cu distribution within the rice plant. The variable expression pattern of OsCOPT7 suggests that OsCOPT7 expression responds to Cu stress in rice. Moreover, assays reveal that OsCOPT7 expression level is suppressed by the SQUAMOSA promoter-binding protein-like 9 (OsSPL9) and that OsCOPT7 interacts with Antioxidant Protein1 (OsATX1). This study elucidates the involvement of OsCOPT7 in Cu loading into the xylem, its subsequent distribution within the rice plant, and the potential of this protein in reducing the risk of high Cu concentrations in rice grain grown on Cu-contaminated soil.

Metal Transport Systems in Plants

Plants take up metals, including essential micronutrients [iron (Fe), copper (Cu), zinc (Zn), and manganese (Mn)] and the toxic heavy metal cadmium (Cd), from soil and accumulate these metals in their edible parts, which are direct and indirect intake sources for humans. Multiple transporters belonging to different families are required to transport a metal from the soil to different organs and tissues, but only a few of them have been fully functionally characterized. The transport systems (the transporters required for uptake, translocation, distribution, redistribution, and their regulation) differ with metals and plant species, depending on the physiological roles, requirements of each metal, and anatomies of different organs and tissues. To maintain metal homeostasis in response to spatiotemporal fluctuations of metals in soil, plants have developed sophisticated and tightly regulated mechanisms through the regulation of transporters at the transcriptional and/or posttranscriptional levels. The manipulation of some transporters has succeeded in generating crops rich in essential metals but low in Cd accumulation. A better understanding of metal transport systems will contribute to better and safer crop production.

Rice transcription factor OsWRKY37 positively regulates flowering time and grain fertility under copper deficiency

Efficient uptake, translocation, and distribution of Cu to rice (Oryza sativa) spikelets is crucial for flowering and yield production. However, the regulatory factors involved in this process remain unidentified. In this study, we isolated a WRKY transcription factor gene induced by Cu deficiency, OsWRKY37, and characterized its regulatory role in Cu uptake and transport in rice. OsWRKY37 was highly expressed in rice roots, nodes, leaf vascular bundles, and anthers. Overexpression of OsWRKY37 promoted the uptake and root-to-shoot translocation of Cu in rice under -Cu condition but not under +Cu condition. While mutation of OsWRKY37 significantly decreased Cu concentrations in the stamen, the root-to-shoot translocation and distribution ratio in brown rice affected pollen development, delayed flowering time, decreased fertility, and reduced grain yield under -Cu condition. yeast one-hybrid, transient co-expression and EMSAs, together with in situ RT-PCR and RT-qPCR analysis, showed that OsWRKY37 could directly bind to the upstream promoter region of OsCOPT6 (copper transporter) and OsYSL16 (yellow stripe-like protein) and positively activate their expression levels. Analyses of oscopt6 mutants further validated its important role in Cu uptake in rice. Our study demonstrated that OsWRKY37 acts as a positive regulator involved in the uptake, root-to-shoot translocation, and distribution of Cu through activating the expression of OsCOPT6 and OsYSL16, which is important for pollen development, flowering, fertility, and grain yield in rice under Cu deficient conditions. Our results provide a genetic strategy for improving rice yield under Cu deficient condition.

Molecular Mechanisms of Plant Responses to Copper: From Deficiency to Excess

Copper (Cu) is an essential nutrient for plant growth and development. This metal serves as a constituent element or enzyme cofactor that participates in many biochemical pathways and plays a key role in photosynthesis, respiration, ethylene sensing, and antioxidant systems. The physiological significance of Cu uptake and compartmentalization in plants has been underestimated, despite the importance of Cu in cellular metabolic processes. As a micronutrient, Cu has low cellular requirements in plants. However, its bioavailability may be significantly reduced in alkaline or organic matter-rich soils. Cu deficiency is a severe and widespread nutritional disorder that affects plants. In contrast, excessive levels of available Cu in soil can inhibit plant photosynthesis and induce cellular oxidative stress. This can affect plant productivity and potentially pose serious health risks to humans via bioaccumulation in the food chain. Plants have evolved mechanisms to strictly regulate Cu uptake, transport, and cellular homeostasis during long-term environmental adaptation. This review provides a comprehensive overview of the diverse functions of Cu chelators, chaperones, and transporters involved in Cu homeostasis and their regulatory mechanisms in plant responses to varying Cu availability conditions. Finally, we identified that future research needs to enhance our understanding of the mechanisms regulating Cu deficiency or stress in plants. This will pave the way for improving the Cu utilization efficiency and/or Cu tolerance of crops grown in alkaline or Cu-contaminated soils.

OsCOPT7 is a copper exporter at the tonoplast and endoplasmic reticulum and controls Cu translocation to the shoots and grain of rice

Copper (Cu) is an essential micronutrient for all living organisms but is also highly toxic in excess. Cellular homoeostasis of Cu is maintained by various transporters and metallochaperones. Here, we investigated the biological function of OsCOPT7, a member of the copper transporters (COPT) family, in Cu homoeostasis in rice. OsCOPT7 was mainly expressed in the roots and the expression was upregulated by Cu deficiency. OsCOPT7 was localized at the tonoplast and the endoplasmic reticulum. Knockout of OsCOPT7 increased Cu accumulation in the roots but decreased Cu concentrations in the shoots and grain. The knockout mutants contained higher concentrations of Cu in the roots cell sap but markedly lower concentrations of Cu in the xylem sap than wild-type plants. Seed setting and grain yield were reduced significantly in the knockout mutants grown in a low Cu soil. Knockout mutants were more tolerant to Cu toxicity. Yeast two-hybrid and bimolecular fluorescence complementation assays showed that OsCOPT7 interacts physically with the rice Cu chaperone antioxidant protein 1 (OsATX1). Taken together, our results indicate that OsCOPT7 is a specific Cu transporter functioning to export Cu from the vacuoles and the ER and plays an important role in controlling the root-to-shoot Cu translocation in rice.

Distinct function of Chlamydomonas CTRA-CTR transporters in Cu assimilation and intracellular mobilization

Successful acclimation to copper (Cu) deficiency involves a fine balance between Cu import and export. In the green alga Chlamydomonas reinhardtii, Cu import is dependent on a transcription factor, Copper Response Regulator 1 (CRR1), responsible for activating genes in Cu-deficient cells. Among CRR1 target genes are two Cu transporters belonging to the CTR/COPT gene family (CTR1 and CTR2) and a related soluble protein (CTR3). The ancestor of these green algal proteins was likely acquired from an ancient chytrid and contained conserved cysteine-rich domains (named the CTR-associated domains, CTRA) that are predicted to be involved in Cu acquisition. We show by reverse genetics that Chlamydomonas CTR1 and CTR2 are canonical Cu importers albeit with distinct affinities, while loss of CTR3 did not result in an observable phenotype under the conditions tested. Mutation of CTR1, but not CTR2, recapitulates the poor growth of crr1 in Cu-deficient medium, consistent with a dominant role for CTR1 in high-affinity Cu(I) uptake. On the other hand, the overaccumulation of Cu(I) (20 times the quota) in zinc (Zn) deficiency depends on CRR1 and both CTR1 and CTR2. CRR1-dependent activation of CTR gene expression needed for Cu over-accumulation can be bypassed by the provision of excess Cu in the growth medium. Over-accumulated Cu is sequestered into the acidocalcisome but can become remobilized by restoring Zn nutrition. This mobilization is also CRR1-dependent, and requires activation of CTR2 expression, again distinguishing CTR2 from CTR1 and consistent with the lower substrate affinity of CTR2.

ONE SENTENCE SUMMARY

Regulation of Cu uptake and sequestration by members of the CTR family of proteins in Chlamydomonas.

Distinct function of Chlamydomonas CTRA-CTR transporters in Cu assimilation and intracellular mobilization

Successful acclimation to copper (Cu) deficiency involves a fine balance between Cu import and export. In the unicellular green alga Chlamydomonas reinhardtii , Cu import is dependent on C opper R esponse R egulator 1 (CRR1), the master regulator of Cu homeostasis. Among CRR1 target genes are two Cu transporters belonging to the CTR/COPT gene family ( CTR1 and CTR2 ) and a related soluble cysteine-rich protein (CTR3). The ancestor of these green algal proteins was likely acquired from an ancient chytrid and contained conserved cysteine-rich domains (named the CTR-associated domains, CTRA) that are predicted to be involved in Cu acquisition. We show by reverse genetics that Chlamydomonas CTR1 and CTR2 are canonical Cu importers albeit with distinct affinities, while loss of CTR3 did not result in an observable phenotype under the conditions tested. Mutation of CTR1 , but not CTR2 , recapitulate the poor growth of crr1 in Cu-deficient medium, consistent with a dominant role for CTR1 in high affinity Cu(I) uptake. Notably, the over-accumulation of Cu(I) in Zinc (Zn)-deficiency (20 times the quota) depends on CRR1 and both CTR1 and CTR2. CRR1-dependent activation of CTR gene expression needed for Cu over-accumulation can be bypassed by the provision of excess Cu in the growth medium. Over-accumulated Cu is sequestered into the acidocalcisome but can become remobilized by restoring Zn nutrition. This mobilization is also CRR1-dependent, and requires activation of CTR2 expression, again distinguishing CTR2 from CTR1 and is consistent with the lower substrate affinity of CTR2.

Beyond NPK: Mineral Nutrient-Mediated Modulation in Orchestrating Flowering Time

Flowering time in plants is a complex process regulated by environmental conditions such as photoperiod and temperature, as well as nutrient conditions. While the impact of major nutrients like nitrogen, phosphorus, and potassium on flowering time has been well recognized, the significance of micronutrient imbalances and their deficiencies should not be neglected because they affect the floral transition from the vegetative stage to the reproductive stage. The secondary major nutrients such as calcium, magnesium, and sulfur participate in various aspects of flowering. Micronutrients such as boron, zinc, iron, and copper play crucial roles in enzymatic reactions and hormone biosynthesis, affecting flower development and reproduction as well. The current review comprehensively explores the interplay between microelements and flowering time, and summarizes the underlying mechanism in plants. Consequently, a better understanding of the interplay between microelements and flowering time will provide clues to reveal the roles of microelements in regulating flowering time and to improve crop reproduction in plant industries.

Zn deficiency disrupts Cu and S homeostasis in Chlamydomonas resulting in over accumulation of Cu and Cysteine

Growth of Chlamydomonas reinhardtii in zinc (Zn) limited medium leads to disruption of copper (Cu) homeostasis, resulting in up to 40-fold Cu over-accumulation relative to its typical Cu quota. We show that Chlamydomonas controls its Cu quota by balancing Cu import and export, which is disrupted in a Zn deficient cell, thus establishing a mechanistic connection between Cu and Zn homeostasis. Transcriptomics, proteomics and elemental profiling revealed that Zn-limited Chlamydomonas cells up-regulate a subset of genes encoding "first responder" proteins involved in sulfur (S) assimilation and consequently accumulate more intracellular S, which is incorporated into L-cysteine, γ-glutamylcysteine, and homocysteine. Most prominently, in the absence of Zn, free L-cysteine is increased ∼80-fold, corresponding to ∼2.8 × 109 molecules/cell. Interestingly, classic S-containing metal binding ligands like glutathione and phytochelatins do not increase. X-ray fluorescence microscopy showed foci of S accumulation in Zn-limited cells that co-localize with Cu, phosphorus and calcium, consistent with Cu-thiol complexes in the acidocalcisome, the site of Cu(I) accumulation. Notably, cells that have been previously starved for Cu do not accumulate S or Cys, causally connecting cysteine synthesis with Cu accumulation. We suggest that cysteine is an in vivo Cu(I) ligand, perhaps ancestral, that buffers cytosolic Cu.

Loss of OPT3 function decreases phloem copper levels and impairs crosstalk between copper and iron homeostasis and shoot-to-root signaling in Arabidopsis thaliana

Copper (Cu) and iron (Fe) are essential micronutrients that are toxic when accumulating in excess in cells. Thus, their uptake by roots is tightly regulated. While plants sense and respond to local Cu availability, the systemic regulation of Cu uptake has not been documented in contrast to local and systemic control of Fe uptake. Fe abundance in the phloem has been suggested to act systemically, regulating the expression of Fe uptake genes in roots. Consistently, shoot-to-root Fe signaling is disrupted in Arabidopsis thaliana mutants lacking the phloem companion cell-localized Fe transporter, OLIGOPEPTIDE TRANSPORTER 3 (AtOPT3). We report that AtOPT3 also transports Cu in heterologous systems and contributes to its delivery from sources to sinks in planta. The opt3 mutant contained less Cu in the phloem, was sensitive to Cu deficiency and mounted a transcriptional Cu deficiency response in roots and young leaves. Feeding the opt3 mutant and Cu- or Fe-deficient wild-type seedlings with Cu or Fe via the phloem in leaves downregulated the expression of both Cu- and Fe-deficiency marker genes in roots. These data suggest the existence of shoot-to-root Cu signaling, highlight the complexity of Cu/Fe interactions, and the role of AtOPT3 in fine-tuning root transcriptional responses to the plant Cu and Fe needs.

Arabidopsis COPT1 copper transporter uses a single histidine to regulate transport activity and protein stability

Copper acquisition and subsequent delivery to target proteins are essential for many biological processes. However, the cellular levels of this trace element must be controlled because of its potential toxicity. The COPT1 protein rich in potential metal-binding amino acids functions in high affinity copper uptake at the plasma membrane of Arabidopsis cells. The functional role of these putative metal-binding residues is largely unknown. Through truncations and site-directed mutagenesis, we identified His43, a single residue within the extracellular N-terminal domain as absolutely critical for copper uptake of COPT1. Substitution of this residue with leucine, methionine or cysteine almost inactivated transport function of COPT1, implying that His43 fails to serves as a copper ligand in the regulation of COPT1 activity. Deletion of all extracellular N-terminal metal-binding residues completely blocked copper-stimulated degradation but did not alter the subcellular distribution and multimerization of COPT1. Although mutation of His43 to alanine and serine retained the transporter activity in yeast cells, the mutant protein was unstable and degraded in the proteasome in Arabidopsis cells. Our results demonstrate a pivotal role for the extracellular residue His43 in high affinity copper transport activity, and suggest common molecular mechanisms for regulating both metal transport and protein stability of COPT1.

Cysteine: an ancestral Cu binding ligand in green algae?

Growth of Chlamydomonas reinhardtii in zinc (Zn) limited medium leads to disruption of copper (Cu) homeostasis, resulting in up to 40-fold Cu over-accumulation relative to its typical Cu quota. We show that Chlamydomonas controls its Cu quota by balancing Cu import and export, which is disrupted in a Zn deficient cell, thus establishing a mechanistic connection between Cu and Zn homeostasis. Transcriptomics, proteomics and elemental profiling revealed that Zn-limited Chlamydomonas cells up-regulate a subset of genes encoding "first responder" proteins involved in sulfur (S) assimilation and consequently accumulate more intracellular S, which is incorporated into L-cysteine, γ-glutamylcysteine and homocysteine. Most prominently, in the absence of Zn, free L-cysteine is increased ~80-fold, corresponding to ~ 2.8 × 10 ⁹ molecules/cell. Interestingly, classic S-containing metal binding ligands like glutathione and phytochelatins do not increase. X-ray fluorescence microscopy showed foci of S accumulation in Zn-limited cells that co-localize with Cu, phosphorus and calcium, consistent with Cu-thiol complexes in the acidocalcisome, the site of Cu(I) accumulation. Notably, cells that have been previously starved for Cu do not accumulate S or Cys, causally connecting cysteine synthesis with Cu accumulation. We suggest that cysteine is an in vivo Cu(I) ligand, perhaps ancestral, that buffers cytosolic Cu.

Principles to recover copper-conducting CTR proteins for the purpose of structural and functional studies

Transition metals such as copper and zinc are essential elements required for the survival of most organisms, from bacteria to humans. Yet, elevated levels of these elements are highly toxic. The Copper TRansporter protein family (CTRs) represents the only identified copper uptake proteins in eukaryotes and hence serves as key components for the maintenance of appropriate levels of the metal. Moreover, CTRs have been proposed to serve as an entry point into cells of certain cancer drugs and to constitute attractive drug-targets for novel antifungals. Nevertheless, the structure, function, and regulation of the CTRs remain elusive, limiting valuable information also for applied sciences. To this end, here we report procedures to isolate a range of CTR members using Saccharomyces cerevisiae as a production host, focusing on three homologs, human CTR1, human CTR2, and Candida albicans CTR. Using forms C-terminally-linked to a protease cleavage sequence, Green Fluorescent Protein (GFP), and a His-tag, assessment of the localization, quantification and purification was facilitated. Cellular accumulation of the proteins was investigated via live-cell imaging. Detergents compatible with acceptable solubilization yields were identified and fluorescence-detection size-exclusion-chromatography (F-SEC) revealed preferred membrane extraction conditions for the targets. For purification purposes, the solubilized CTR members were subjected to affinity chromatography and SEC, reaching near homogeneity. The quality and quantity of the CTRs studied will permit downstream efforts to uncover imperative biophysical aspects of these proteins, paving the way for subsequent drug-discovery studies.

Molecular mechanisms underlying the toxicity and detoxification of trace metals and metalloids in plants

Plants take up a wide range of trace metals/metalloids (hereinafter referred to as trace metals) from the soil, some of which are essential but become toxic at high concentrations (e.g., Cu, Zn, Ni, Co), while others are non-essential and toxic even at relatively low concentrations (e.g., As, Cd, Cr, Pb, and Hg). Soil contamination of trace metals is an increasing problem worldwide due to intensifying human activities. Trace metal contamination can cause toxicity and growth inhibition in plants, as well as accumulation in the edible parts to levels that threatens food safety and human health. Understanding the mechanisms of trace metal toxicity and how plants respond to trace metal stress is important for improving plant growth and food safety in contaminated soils. The accumulation of excess trace metals in plants can cause oxidative stress, genotoxicity, programmed cell death, and disturbance in multiple physiological processes. Plants have evolved various strategies to detoxify trace metals through cell-wall binding, complexation, vacuolar sequestration, efflux, and translocation. Multiple signal transduction pathways and regulatory responses are involved in plants challenged with trace metal stresses. In this review, we discuss the recent progress in understanding the molecular mechanisms involved in trace metal toxicity, detoxification, and regulation, as well as strategies to enhance plant resistance to trace metal stresses and reduce toxic metal accumulation in food crops.

The role of metal transporters in phytoremediation: A closer look at Arabidopsis

Pollution of the environment by heavy metals (HMs) has recently become a global issue, affecting the health of all living organisms. Continuous human activities (industrialization and urbanization) are the major causes of HM release into the environment. Over the years, two methods (physical and chemical) have been widely used to reduce HMs in polluted environment. However, these two methods are inefficient and very expensive to reduce the HMs released into the atmosphere. Alternatively, researchers are trying to remove the HMs by employing hyper-accumulator plants. This method, referred to phytoremediation, is highly efficient, cost-effective, and eco-friendly. Phytoremediation can be divided into five types: phytostabilization, phytodegradation, rhizofiltration, phytoextraction, and phytovolatilization, all of which contribute to HMs removal from the polluted environment. Brassicaceae family members (particularly Arabidopsis thaliana) can accumulate more HMs from the contaminated environment than those of other plants. This comprehensive review focuses on how HMs pollute the environment and discusses the phytoremediation measures required to reduce the impact of HMs on the environment. We discuss the role of metal transporters in phytoremediation with a focus on Arabidopsis. Then draw insights into the role of genome editing tools in enhancing phytoremediation efficiency. This review is expected to initiate further research to improve phytoremediation by biotechnological approaches to conserve the environment from pollution.

Physiological and Molecular Mechanisms of Plant Responses to Copper Stress

Copper (Cu) is an essential micronutrient for humans, animals, and plants, and it participates in various morphological, physiological, and biochemical processes. Cu is a cofactor for a variety of enzymes, and it plays an important role in photosynthesis, respiration, the antioxidant system, and signal transduction. Many studies have demonstrated the adverse effects of excess Cu on crop germination, growth, photosynthesis, and antioxidant activity. This review summarizes the biological functions of Cu, the toxicity of excess Cu to plant growth and development, the roles of Cu transport proteins and chaperone proteins, and the transport process of Cu in plants, as well as the mechanisms of detoxification and tolerance of Cu in plants. Future research directions are proposed, which provide guidelines for related research.

The TaWRKY22–TaCOPT3D Pathway Governs Cadmium Uptake in Wheat

Cadmium (Cd) is a heavy metal nonessential for plants; this toxic metal accumulation in crops has significant adverse effects on human health. The crosstalk between copper (Cu) and Cd has been reported; however, the molecular mechanisms remain unknown. The present study investigated the function of wheat Cu transporter 3D (TaCOPT3D) in Cd tolerance. The TaCOPT3D transcripts significantly accumulated in wheat roots under Cd stress. Furthermore, TaCOPT3D-overexpressing lines were compared to wildtype (WT) plants to test the role of TaCOPT3D in Cd stress response. Under 20 mM Cd treatment, TaCOPT3D-overexpressing lines exhibited more biomass and lower root, shoot, and grain Cd accumulation than the WT plants. In addition, overexpression of TaCOPT3D decreased the reactive oxygen species (ROS) levels and increased the active antioxidant enzymes under Cd conditions. Moreover, the transcription factor (TF) TaWRKY22, which targeted the TaCOPT3D promoter, was identified in the regulatory pathway of TaCOPT3D under Cd stress. Taken together, these results show that TaCOPT3D plays an important role in regulating plant adaptation to cadmium stress through bound by TaWRKY22. These findings suggest that TaCOPT3D is a potential candidate for decreasing Cd accumulation in wheat through genetic engineering.

The key micronutrient copper orchestrates broad-spectrum virus resistance in rice

Copper is a critical regulator of plant growth and development. However, the mechanisms by which copper responds to virus invasion are unclear. We previously showed that SPL9-mediated transcriptional activation of miR528 adds a previously unidentified regulatory layer to the established ARGONAUTE (AGO18)-miR528-L-ascorbate oxidase (AO) antiviral defense. Here, we report that rice promotes copper accumulation in shoots by inducing copper transporter genes, including HMA5 and COPT, to counteract viral infection. Copper suppresses the transcriptional activation of miR528 by inhibiting the protein level of SPL9, thus alleviating miR528-mediated cleavage of AO transcripts to strengthen the antiviral response. Loss-of-function mutations in HMA5, COPT1, and COPT5 caused a significant reduction in copper accumulation and plant viral resistance because of the increased SPL9-mediated miR528 transcription. Gain in viral susceptibility was mitigated when SPL9 was mutated in the hma5 mutant background. Our study elucidates the molecular mechanisms and regulatory networks of copper homeostasis and the SPL9-miR528-AO antiviral pathway.

CITF1 Functions Downstream of SPL7 to Specifically Regulate Cu Uptake in Arabidopsis

Copper (Cu) is one of the most indispensable micronutrients, and proper Cu homeostasis is required for plants to maintain essential cellular functions. Plants activate the Cu uptake system during Cu limitation. Although SPL7 (SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 7) and CITF1 (Cu-DEFICIENCY INDUCED TRANSCRIPTION FACTOR 1) are two transcription factors in Cu homeostasis, it remains unclear how SPL7 and CITF1 control the Cu uptake system. Here, we reveal that overexpression of CITF1 causes the enhanced tolerance to Cu deficiency and the elevated expression of Cu uptake genes COPT2, FRO4 and FRO5. Electrophoretic mobility shift assays (EMSA) and transient expression assays indicate that SPL7 directly binds to and activates the promoter of CITF1. The overexpression of CITF1 partially rescues the sensitivity of spl7-1 to Cu deficiency. Transcriptome data suggest that SPL7 and CITF1 coregulate the Cu-homeostasis-signaling network, and CITF1 has its own independent functions. Moreover, both SPL7 and CITF1 can directly bind to and activate the promoters of three Cu uptake genes COPT2, FRO4 and FRO5. This work shows the functions of CITF1 in the Cu-homeostasis-signaling network, providing insights into the complicated molecular mechanism underlying Cu homeostasis.

Latent Benefits and Toxicity Risks Transmission Chain of High Dietary Copper along the Livestock-Environment-Plant-Human Health Axis and Microbial Homeostasis: A Review

The extensive use of high-concentration copper (Cu) in feed additives, fertilizers, pesticides, and nanoparticles (NPs) inevitably causes significant pollution in the ecological environment. This type of chain pollution begins with animal husbandry: first, Cu accumulation in animals poisons them; second, high Cu enters the soil and water sources with the feces and urine to cause toxicity, which may further lead to crop and plant pollution; third, this process ultimately endangers human health through consumption of livestock products, aquatic foods, plants, and even drinking water. High Cu potentially alters the antibiotic resistance of soil and water sources and further aggravates human disease risks. Thus, it is necessary to formulate reasonable Cu emission regulations because the benefits of Cu for livestock and plants cannot be ignored. The present review evaluates the potential hazards and benefits of high Cu in livestock, the environment, the plant industry, and human health. We also discuss aspects related to bacterial and fungal resistance and homeostasis and perspectives on the application of Cu-NPs and microbial high-Cu removal technology to reduce the spread of toxicity risks to humans.

The Ubiquitin Proteasome System and Nutrient Stress Response

Plants utilize different molecular mechanisms, including the Ubiquitin Proteasome System (UPS) that facilitates changes to the proteome, to mitigate the impact of abiotic stresses on growth and development. The UPS encompasses the ubiquitination of selected substrates followed by the proteasomal degradation of the modified proteins. Ubiquitin ligases, or E3s, are central to the UPS as they govern specificity and facilitate the attachment of one or more ubiquitin molecules to the substrate protein. From recent studies, the UPS has emerged as an important regulator of the uptake and translocation of essential macronutrients and micronutrients. In this review, we discuss select E3s that are involved in regulating nutrient uptake and responses to stress conditions, including limited or excess levels of nitrogen, phosphorus, iron, and copper.

Insights to improve the plant nutrient transport by CRISPR/Cas system

We need to improve food production to feed the ever growing world population especially in a changing climate. Nutrient deficiency in soils is one of the primary bottlenecks affecting the crop production both in developed and developing countries. Farmers are forced to apply synthetic fertilizers to improve the crop production to meet the demand. Understanding the mechanism of nutrient transport is helpful to improve the nutrient-use efficiency of crops and promote the sustainable agriculture. Many transporters involved in the acquisition, export and redistribution of nutrients in plants are characterized. In these studies, heterologous systems like yeast and Xenopus were most frequently used to study the transport function of plant nutrient transporters. CRIPSR/Cas system introduced recently has taken central stage for efficient genome editing in diverse organisms including plants. In this review, we discuss the key nutrient transporters involved in the acquisition and redistribution of nutrients from soil. We draw insights on the possible application CRISPR/Cas system for improving the nutrient transport in plants by engineering key residues of nutrient transporters, transcriptional regulation of nutrient transport signals, engineering motifs in promoters and transcription factors. CRISPR-based engineering of plant nutrient transport not only helps to study the process in native plants with conserved regulatory system but also aid to develop non-transgenic crops with better nutrient use-efficiency. This will reduce the application of synthetic fertilizers and promote the sustainable agriculture strengthening the food and nutrient security.

Two ubiquitin-associated ER proteins interact with COPT copper transporters and modulate their accumulation

The endoplasmic reticulum (ER) contains an elaborate protein quality control network that promotes protein folding and prevents accumulation of misfolded proteins. Evolutionarily conserved UBIQUITIN-ASSOCIATED DOMAIN-CONTAINING PROTEIN 2 (UBAC2) is involved in ER-associated protein degradation in metazoans. We have previously reported that two close UBAC2 homologs from Arabidopsis (Arabidopsis thaliana) not only participate in selective autophagy of ER components but also interact with plant-specific PATHOGEN-ASSOCIATED MOLECULAR PATTERN (PAMP)-INDUCED COILED COIL (PICC) protein to increase the accumulation of POWDERY MILDEW-RESISTANT 4 callose synthase. Here, we report that UBAC2s also interacted with COPPER (Cu) TRANSPORTER 1 (COPT1) and plasma membrane-targeted members of the Cu transporter family. The ubac2 mutants were significantly reduced in both the accumulation of COPT proteins and Cu content, and also displayed increased sensitivity to a Cu chelator. Therefore, UBAC2s positively regulate the accumulation of COPT transporters, thereby increasing Cu uptake by plant cells. Unlike with POWDERY MILDEW RESISTANCE 4, however, the positive role of UBAC2s in the accumulation of COPT1 is not dependent on PICC or the UBA domain of UBAC2s. When COPT1 was overexpressed under the CaMV 35S promoter, the increased accumulation of COPT1 was strongly UBAC2-dependent, particularly when a signal peptide was added to the N-terminus of COPT1. Further analysis using inhibitors of protein synthesis and degradation strongly suggested that UBAC2s stabilize newly synthesized COPT proteins against degradation by the proteasome system. These results indicate that plant UBAC2s are multifunctional proteins that regulate the degradation and accumulation of specific ER-synthesized proteins.

TpIRT1 from Polish wheat (Triticum polonicum L.) enhances the accumulation of Fe, Mn, Co, and Cd in Arabidopsis

Uptake and internal transport of micronutrients are essential for plant growth, development, and yield. In this regard, Iron Regulated Transporters (IRTs) from the Zinc Regulated Transporter (ZRT)/IRT-related protein (ZIP) family play an important role in transition metal uptake. Most studies have been focused on IRT1-like proteins in diploid species. Information on IRT1-like proteins in polyploids is limited. Here, we studied the function of TpIRT1A and TpIRT1B homoeologs in a tetraploid crop, Polish wheat (Triticum polonicum L.). Our results highlighted the importance of TpIRT1 in mediating the uptake and translocation of Fe, Mn, Co, and Cd with direct implications for wheat yield potential. Both TpIRT1A and TpIRT1B were located at the plasma membrane and internal vesicle-like organelle in protoplasts of Arabidopsis thaliana L. and increased Cd and Co sensitivity in yeast. The over-expression of TpIRT1B in A. thaliana increased Fe, Mn, Co, and Cd concentration in its tissues and improved plant growth under Fe, Mn, and Co deficiencies, while increased the sensitivity to Cd compared to wild type. Functional analysis of IRT1 homoeologs from tetraploid and diploid ancestral wheat species in yeast disclosed four distinct amino acid residues in TdiIRT1B (T. dicoccum L. (Schrank)) and TtuIRT1B (T. turgidum L.). Together, our results increase the knowledge of IRT1 function in a globally important crop, wheat.

Adaptation to coastal soils through pleiotropic boosting of ion and stress hormone concentrations in wild Arabidopsis thaliana

Local adaptation in coastal areas is driven chiefly by tolerance to salinity stress. To survive high salinity, plants have evolved mechanisms to specifically tolerate sodium. However, the pathways that mediate adaptive changes in these conditions reach well beyond Na⁺ . Here we perform a high-resolution genetic, ionomic, and functional study of the natural variation in Molybdenum transporter 1 (MOT1) associated with coastal Arabidopsis thaliana accessions. We quantify the fitness benefits of a specific deletion-harbouring allele (MOT1^DEL ) present in coastal habitats that is associated with lower transcript expression and molybdenum accumulation. Analysis of the leaf ionome revealed that MOT1^DEL  plants accumulate more copper (Cu) and less sodium (Na⁺ ) than plants with the noncoastal MOT1 allele, revealing a complex interdependence in homeostasis of these three elements. Our results indicate that under salinity stress, reduced MOT1 function limits leaf Na⁺  accumulation through abscisic acid (ABA) signalling. Enhanced ABA biosynthesis requires Cu. This demand is met in Cu deficient coastal soils through MOT1^DEL  increasing the expression of SPL7 and the copper transport protein COPT6. MOT1^DEL  is able to deliver a pleiotropic suite of phenotypes that enhance salinity tolerance in coastal soils deficient in Cu. This is achieved by inducing ABA biosynthesis and promoting reduced uptake or better compartmentalization of Na⁺ , leading to coastal adaptation.

Multi-omics analyses on the response mechanisms of 'Shine Muscat' grapevine to low degree of excess copper stress (Low-ECS)

Copper stress is one of the most severe heavy metal stresses in plants. Grapevine has a relatively higher copper tolerance than other fruit crops. However, there are no reports regarding the tolerance mechanisms of the 'Shine Muscat' ('SM') grape to a low degree of excess copper stress (Low-ECS). Based on the physiological indicators and multi-omics (transcriptome, proteome, metabolome, and microRNAome) data, 8 h (h) after copper treatment was the most severe stress time point. Nonetheless, copper stress was alleviated 64 h after treatment. Cu ion transportation, photosynthesis pathway, antioxidant system, hormone metabolism, and autophagy were the primary response systems in 'SM' grapevine under Low-ECS. Numerous genes and proteins, such as HMA5, ABC transporters, PMM, GME, DHAR, MDHAR, ARGs, and ARPs, played essential roles in the 'SM' grapevine's response to Low-ECS. This work was carried out to gain insights into the multi-omics responses of 'SM' grapevine to Low-ECS. This study provides genetic and agronomic information that will guide better vinery management and breeding copper-resistant grape cultivars.

YSL3-mediated copper distribution is required for fertility, seed size and protein accumulation in Brachypodium

Addressing the looming global food security crisis requires the development of high-yielding crops. In agricultural soils, deficiency in the micronutrient copper significantly decreases grain yield in wheat (Triticum aestivum), a globally important crop. In cereals, grain yield is determined by inflorescence architecture, flower fertility, grain size, and weight. Whether copper is involved in these processes, and how it is delivered to the reproductive organs is not well understood. We show that copper deficiency alters not only the grain set but also flower development in both wheat and its recognized model, Brachypodium distachyon. We then show that the Brachypodium yellow stripe-like 3 (YSL3) transporter localizes to the phloem, transports copper in frog (Xenopus laevis) oocytes, and facilitates copper delivery to reproductive organs and grains. Failure to deliver copper, but not iron, zinc, or manganese to these structures in the ysl3 CRISPR-Cas9 mutant results in delayed flowering, altered inflorescence architecture, reduced floret fertility, grain size, weight, and protein accumulation. These defects are rescued by copper supplementation and are complemented by YSL3 cDNA. This knowledge will help to devise sustainable approaches for improving grain yield in regions where soil quality is a major obstacle for crop production. Copper distribution by a phloem-localized transporter is essential for the transition to flowering, inflorescence architecture, floret fertility, size, weight, and protein accumulation in seeds.

Copper: uptake, toxicity and tolerance in plants and management of Cu-contaminated soil

Copper (Cu) is an essential mineral nutrient for the proper growth and development of plants; it is involved in myriad morphological, physiological, and biochemical processes. Copper acts as a cofactor in various enzymes and performs essential roles in photosynthesis, respiration and the electron transport chain, and is a structural component of defense genes. Excess Cu, however, imparts negative effects on plant growth and productivity. Many studies have summarized the adverse effects of excess Cu on germination, growth, photosynthesis, and antioxidant response in agricultural crops. Its inhibitory influence on mineral nutrition, chlorophyll biosynthesis, and antioxidant enzyme activity has been verified. The current review focuses on the availability and uptake of Cu by plants. The toxic effects of excess Cu on seed germination, plant growth and development, photosynthesis, and antioxidant response in plants are discussed. Plant tolerance mechanisms against Cu stress, and management of Cu-contaminated soils are presented.

Molecular characterization and expression analysis of the SQUAMOSA PROMOTER BINDING PROTEIN-LIKE gene family in Paeonia suffruticosa

A total of 16 PsSPL genes were identified in tree peony. PsSPLs potentially regulated flowering time, lateral bud and seed development, and the juvenile-to-adult phase transition. SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factors are important for plant growth and development. Here, we report the identification of 16 full-length PsSPLs in tree peony (Peaonia suffruticosa Andr.) and 9 PsSPLs that have miR156 target sites. Phylogenetic analysis of the relationship of SPLs in P. suffruticosa and Arabidopsis suggested that they can be classified into six groups, and PsSPLs were highly correlated with Arabidopsis SPLs counterparts in the same group. Cis-element of promoter region analysis suggested that PsSPL genes play roles in physiological processes and developmental events. Expression analysis indicated that most PsSPL genes exhibited high expression levels in the tissues and organs examined here. The increasing expression levels of PsSPL1, PsSPL2, PsSPL8, PsSPL9, PsSPL12, and PsSPL16, and decreasing expression levels of PsSPL1A and PsSPL1B in buds over time suggested that they were probably regulated by the juvenile-to-adult phase transition. In addition, the expression profiles of PsSPL genes in different developmental buds and seeds suggested that PsSPL2, PsSPL3, PsSPL9, PsSPL10, PsSPL13, and PsSPL13A were important genes for regulating the flowering time of the tree peony; PsSPL2 and PsSPL8 might play a role in suppressing lateral bud development, and PsSPL2, PsSPL13, and PsSPL14 positively controlled grain size and number, and pod branching. These results provide a foundation for future functional analysis of PsSPL genes in tree peony growth and development.

Arabidopsis COPPER TRANSPORTER 1 undergoes degradation in a proteasome-dependent manner

The essential nutrient copper is toxic in excess. Therefore, plants must tightly control copper uptake and distribution. Arabidopsis thaliana high-affinity copper transporters (COPTs) mediate copper uptake, partitioning, and redistribution. Here we show that COPT1 localizes to the plasma membrane and endoplasmic reticulum in stably transgenic plants expressing a COPT1-green fluorescent protein (GFP) fusion protein, and the fusion protein is rapidly degraded upon plant exposure to excess copper. MG132 treatment largely abolished copper-induced degradation of COPT1, implying a link between the proteasome and COPT1 activity in modulating copper uptake. Co-immunoprecipitation analyses revealed that COPT1 cannot be ubiquitinated in the presence of excess copper and MG132. Through site-directed mutagenesis, we identified Lys159 in the C-terminal cytoplasmic tail of COPT1 as critical for copper acquisition, but not for copper-mediated down-regulation of COPT1, in plants. Furthermore, pharmacological analysis showed that treatment with a vesicle trafficking inhibitor or a V-ATPase inhibitor does not alter the subcellular dynamics of COPT1-GFP, consistent with the absence of a connection between the endosomal recycling/vacuolar system and COPT1 degradation. Together, our data suggest that proteasomal degradation rather than vacuolar proteolysis is important for the regulation of copper transport to maintain copper homeostasis in plants.

Mechanisms and Role of Nitric Oxide in Phytotoxicity-Mitigation of Copper

Phytotoxicity of metals significantly contributes to the major loss in agricultural productivity. Among all the metals, copper (Cu) is one of essential metals, where it exhibits toxicity only at its supra-optimal level. Elevated Cu levels affect plants developmental processes from initiation of seed germination to the senescence, photosynthetic functions, growth and productivity. The use of plant growth regulators/phytohormones and other signaling molecules is one of major approaches for reversing Cu-toxicity in plants. Nitric oxide (NO) is a versatile and bioactive gaseous signaling molecule, involved in major physiological and molecular processes in plants. NO modulates responses of plants grown under optimal conditions or to multiple stress factors including elevated Cu levels. The available literature in this context is centered mainly on the role of NO in combating Cu stress with partial discussion on underlying mechanisms. Considering the recent reports, this paper: (a) overviews Cu uptake and transport; (b) highlights the major aspects of Cu-toxicity on germination, photosynthesis, growth, phenotypic changes and nutrient-use-efficiency; (c) updates on NO as a major signaling molecule; and (d) critically appraises the Cu-significance and mechanisms underlying NO-mediated alleviation of Cu-phytotoxicity. The outcome of the discussion may provide important clues for future research on NO-mediated mitigation of Cu-phytotoxicity.

Whole-transcriptome RNA sequencing reveals the global molecular responses and ceRNA regulatory network of mRNAs, lncRNAs, miRNAs and circRNAs in response to copper toxicity in Ziyang Xiangcheng (Citrus junos Sieb. Ex Tanaka)

BACKGROUND

Copper (Cu) toxicity has become a potential threat for citrus production, but little is known about related mechanisms. This study aims to uncover the global landscape of mRNAs, long non-coding RNAs (lncRNAs), circular RNAs (circRNAs) and microRNAs (miRNAs) in response to Cu toxicity so as to construct a regulatory network of competing endogenous RNAs (ceRNAs) and to provide valuable knowledge pertinent to Cu response in citrus.

RESULTS

Tolerance of four commonly used rootstocks to Cu toxicity was evaluated, and 'Ziyang Xiangcheng' (Citrus junos) was found to be the most tolerant genotype. Then the roots and leaves sampled from 'Ziyang Xiangcheng' with or without Cu treatment were used for whole-transcriptome sequencing. In total, 5734 and 222 mRNAs, 164 and 5 lncRNAs, 45 and 17 circRNAs, and 147 and 130 miRNAs were identified to be differentially expressed (DE) in Cu-treated roots and leaves, respectively, in comparison with the control. Gene ontology enrichment analysis showed that most of the DEmRNAs and targets of DElncRNAs and DEmiRNAs were annotated to the categories of 'oxidation-reduction', 'phosphorylation', 'membrane', and 'ion binding'. The ceRNA network was then constructed with the predicted pairs of DEmRNAs-DEmiRNAs and DElncRNAs-DEmiRNAs, which further revealed regulatory roles of these DERNAs in Cu toxicity.

CONCLUSIONS

A large number of mRNAs, lncRNAs, circRNAs, and miRNAs in 'Ziyang Xiangcheng' were altered in response to Cu toxicity, which may play crucial roles in mitigation of Cu toxicity through the ceRNA regulatory network in this Cu-tolerant rootstock.

Copper accumulation, subcellular partitioning and physiological and molecular responses in relation to different copper tolerance in apple rootstocks

To unravel the physiological and molecular regulation mechanisms underlying the variation in copper (Cu)accumulation, translocation and tolerance among five apple rootstocks, seedlings were exposed to either basal or excess Cu. Excess Cu suppressed plant biomass and root architecture, which was less pronounced in Malus prunifolia Borkh., indicating its relatively higher Cu tolerance. Among the five apple rootstocks, M. prunifolia exhibited the highest Cu concentration and bio-concentration factor in roots but the lowest translocation factor, indicating its greater ability to immobilize Cu and restrict translocation to the aerial parts. Higher Cu concentration in cell wall fraction but lower Cu proportion in membrane-containing and organelle-rich fractions were found in M. prunifolia. Compared with the other four apple rootstocks under excess Cu conditions, M. prunifolia had a lower increment of hydrogen peroxide in roots and leaves and malondialdehyde in roots, but higher concentrations of carbohydrates and enhanced antioxidants. Transcript levels of genes involved in Cu uptake, transport and detoxification revealed species-specific differences that are probably related to alterations in Cu tolerance. M. prunifolia had relatively higher gene transcript levels including copper transporters 2 (COPT2), COPT6 and zinc/iron-regulated transporter-related protein 2 (ZIP2), which probably took part in Cu uptake, and C-type ATP-binding cassette transporter 2 (ABCC2), copper chaperone for Cu/Zn superoxide dismutase (CCS), Cu/Zn superoxide dismutase 1 (CSD1) and metallothionein 2 (MT2) probably implicated in Cu detoxification, and relatively lower mRNA levels of yellow stripe-like transporter 3 (YSL3) and heavy metal ATPase 5 (HMA5) involved in transport of Cu to aerial parts. These results suggest that M. prunifolia is more tolerant to excess Cu than the other four apple rootstocks under the current experimental conditions, which is probably attributed to more Cu retention in roots, subcellular partitioning, well-coordinated antioxidant defense mechanisms and transcriptional expression of genes involved in Cu uptake, translocation and detoxification.

Characterization of the Copper Transporters from Lotus spp. and Their Involvement under Flooding Conditions

Forage legumes are an important livestock nutritional resource, which includes essential metals, such as copper. Particularly, the high prevalence of hypocuprosis causes important economic losses to Argentinian cattle agrosystems. Copper deficiency in cattle is partially due to its low content in forage produced by natural grassland, and is exacerbated by flooding conditions. Previous results indicated that incorporation of Lotus spp. into natural grassland increases forage nutritional quality, including higher copper levels. However, the biological processes and molecular mechanisms involved in copper uptake by Lotus spp. remain poorly understood. Here, we identify four genes that encode putative members of the Lotus copper transporter family, denoted COPT in higher plants. A heterologous functional complementation assay of the Saccharomyces cerevisiae ctr1∆ctr3∆ strain, which lacks the corresponding yeast copper transporters, with the putative Lotus COPT proteins shows a partial rescue of the yeast phenotypes in restrictive media. Under partial submergence conditions, the copper content of L. japonicus plants decreases and the expression of two Lotus COPT genes is induced. These results strongly suggest that the Lotus COPT proteins identified in this work function in copper uptake. In addition, the fact that environmental conditions affect the expression of certain COPT genes supports their involvement in adaptive mechanisms and envisages putative biotechnological strategies to improve cattle copper nutrition.

The R2R3-MYB Transcription Factor MYB49 Regulates Cadmium Accumulation

Abscisic acid (ABA) reduces accumulation of potentially toxic cadmium (Cd) in plants. How the ABA signal is transmitted to modulate Cd uptake remains largely unclear. Here, we report that the basic region/Leu zipper transcription factor ABSCISIC ACID-INSENSITIVE5 (ABI5), a central ABA signaling molecule, is involved in ABA-repressed Cd accumulation in plants by physically interacting with a previously uncharacterized R2R3-type MYB transcription factor, MYB49. Overexpression of the Cd-induced MYB49 gene in Arabidopsis (Arabidopsis thaliana) resulted in a significant increase in Cd accumulation, whereas myb49 knockout plants and plants expressing chimeric repressors of MYB49:ERF-associated amphiphilic repression motif repression domain (SRDX49) exhibited reduced accumulation of Cd. Further investigations revealed that MYB49 positively regulates the expression of the basic helix-loop-helix transcription factors bHLH38 and bHLH101 by directly binding to their promoters, leading to activation of IRON-REGULATED TRANSPORTER1, which encodes a metal transporter involved in Cd uptake. MYB49 also binds to the promoter regions of the heavy metal-associated isoprenylated plant proteins (HIPP22) and HIPP44, resulting in up-regulation of their expression and subsequent Cd accumulation. On the other hand, as a feedback mechanism to control Cd uptake and accumulation in plant cells, Cd-induced ABA up-regulates the expression of ABI5, whose protein product interacts with MYB49 and prevents its binding to the promoters of downstream genes, thereby reducing Cd accumulation. Our results provide new insights into the molecular feedback mechanisms underlying ABA signaling-controlled Cd uptake and accumulation in plants.

Synergism and antagonisms between nutrients induced by copper toxicity in grapevine rootstocks: Monocropping vs. intercropping

The long-term use of Cu-containing fungicides contaminates vineyards soils, which can induce Cu toxicity and nutrient imbalances in several plant species. The aim of this work was to evaluate the effect of Cu toxicity on two grapevine rootstocks, Fercal and 196.17, and to elucidate if intercropping with oat can alleviate grapevine Cu toxicity. Plants were hydroponically-cultivated and treated with different Cu concentrations. At harvest the biomass accumulation, the SPAD index and the symplastic and apoplastic root and leaves ionome were measured to evaluate possible synergistic and/or antagonistic effects on other micro- and macronutrients. The root exudation analysis was correlated with genes expression (VvPEZ-like), whereas PCA analysis performed on the grapevine and oat ionome revealed that both mono- and intercropped 196.17 rootstock display a positive effect on Zn and Mn in the root tissues at high Cu concentrations. An increase of Zn and Mn in roots was also reported for the intercropped Fercal rootstock at high Cu concentrations while an antagonistic relation was reported for root Zn concentration in the monocropped Fercal rootstock. Our results showed that grapevine and oat compete for nutrient uptake and that this phenomenon can possibly alleviate grapevine Cu toxicity. However, Fercal rootstock is able to take advantage from oat, while 196.17 is disadvantaged by the intercropping system. Even though intercropping system seems to be a valuable tool to counteract grapevine Cu toxicity, the application of this agricultural practice has shown to be species dependent and should be evaluated for each rootstock.

Copper uptake mechanism of Arabidopsis thaliana high-affinity COPT transporters

Copper (Cu) is an essential plant micronutrient. Under scarcity, Cu²⁺ is reduced to Cu⁺ and taken up through specific high-affinity transporters (COPTs). In Arabidopsis, the COPT family consists of six members, either located at the plasma membrane (COPT1, COPT2, and COPT6) or in internal membranes (COPT3 and COPT5). Cu uptake by COPT proteins has been mainly assessed through complementation studies in corresponding yeast mutants, but the mechanism of this transport has not been elucidated. To test whether Cu is incorporated by an electrogenic mechanism, electrophysiological changes induced by Cu addition were studied in Arabidopsis thaliana. Mutant (T-DNA insertion mutants, copt2-1 and copt5-2) and overexpressing lines (COPT1^OE and COPT5^OE) with altered expression of COPT transporters were compared to wild-type plants. No significant changes of the membrane potential (E_m) were detected, regardless of genotype or Cu concentration supplied. In contrast, membrane depolarization was detected in response to iron supply in both wild-type and in mutant or transgenic plants. Similar results were obtained for trans-plant potentials (TPP). GFP fusions of the plasma membrane COPT2 and the internal COPT5 transporters were expressed in Xenopus laevis oocytes to potentiate Cu uptake signals, and the cRNA-injected oocytes were tested for electrical currents upon Cu addition using two-electrode voltage clamp. Results with oocytes confirmed those obtained in plants. Cu accumulation in injected oocytes was measured by ICP-OES, and a significant increase in Cu content with respect to controls occurred in oocytes expressing COPT2:GFP. The possible mechanisms driving this transport are discussed in this manuscript.

The Yellow Stripe-Like (YSL) Gene Functions in Internal Copper Transport in Peanut

Copper (Cu) is involved in fundamental biological processes for plant growth and development. However, Cu excess is harmful to plants. Thus, Cu in plant tissues must be tightly regulated. In this study, we found that the peanut Yellow Stripe-Like family gene AhYSL3.1 is involved in Cu transport. Among five AhYSL genes, AhYSL3.1 and AhYSL3.2 were upregulated by Cu deficiency in peanut roots and expressed mainly in young leaves. A yeast complementation assay suggested that the plasma membrane-localized AhYSL3.1 was a Cu-nicotianamine complex transporter. High expression of AhYSL3.1 in tobacco and rice plants with excess Cu resulted in a low concentration of Cu in young leaves. These transgenic plants were resistant to excess Cu. The above results suggest that AhYSL3.1 is responsible for the internal transport of Cu in peanut.

LeSPL-CNR negatively regulates Cd acquisition through repressing nitrate reductase-mediated nitric oxide production in tomato

An SPL-type transcription factor, LeSPL-CNR, is negatively involved in NO production by modulating SlNR expression and nitrate reductase activity, which contributes to Cd tolerance. Cadmium (Cd) is a highly toxic pollutant. Identifying factors affecting Cd accumulation in plants is a prerequisite for minimizing dietary uptake of Cd from crops grown with contaminated soil. Here, we report the involvement of a SQUAMOSA PROMOTER-BINDING PROTEIN-LIKE (SPL) transcription factor LeSPL-CNR in Cd tolerance in tomato (Solanum lycopersicum). In comparison with the wild-type Ailsa Craig (AC) plants, the Colourless non-ripening (Cnr) epimutant displayed increased Cd accumulation and enhanced sensitivity to Cd, which was in well accordance with the repression of LeSPL-CNR expression. Cd stress-induced NO production was inhibited by nitrate reductase (NR) inhibitor, but not NO synthase-like enzyme inhibitor. Expression of LeSPL-CNR was negatively correlated with SlNR expression and the NR activity. We also demonstrated that LeSPL-CNR inhibited the SlNR promoter activity in vivo and bound to SlNR promoter sequence that does not contain a known SBP-binding motif. In addition, expression of an IRON-REGULATED TRANSPORTER1, SlIRT1, was more abundant in Cnr roots than AC roots under Cd stress. LeSPL-CNR may thus provide a molecular mechanism linking Cd stress response to regulation of NR-dependent NO production, which then contributes to Cd uptake via SlIRT1 expression in tomato.

OsYSL16 is Required for Preferential Cu Distribution to Floral Organs in Rice

Deficiency of copper (Cu) causes low fertility in many plant species, but the molecular mechanisms underlying distribution of Cu to the floral organs are poorly understood. Here, we found that a member of yellow-stripe like (YSL) family, YSL16 encoding the Cu-nicotianamine (Cu-NA) transporter, was highly expressed in the rachilla, with less expression in the palea and lemma of rice (Oryza sativa). β-Glucuronidase (GUS) staining of transgenic rice carrying the OsYSL16 promoter-GUS showed that OsYSL16 was mainly expressed in vascular bundles of the rachilla as well as the palea and lemma. Knockout of OsYSL16 resulted in decreased Cu distribution to the stamens, but increased distribution to the palea and lemma. A short-term (24 h) 65Cu labeling experiment confirmed increased Cu concentration of palea and lemma in the mutant. Furthermore, we found that redistribution of Cu from the palea and lemma was impaired in the osysl16 mutant after exposure to Cu-free solution. The osysl16 mutant showed low pollen germination, but this was rescued by addition of Cu in the medium. Our results indicate that OsYSL16 expressed in the vascular bundles of the rachilla is important for preferential distribution of Cu to the stamens, while OsYSL16 in vascular bundles of the palea and lemma is involved in Cu redistribution under Cu-limited conditions in rice.

OsATX1 Interacts with Heavy Metal P1B-Type ATPases and Affects Copper Transport and Distribution

Copper (Cu) is an essential micronutrient for plant growth. However, the molecular mechanisms underlying Cu trafficking and distribution to different organs in rice (Oryza sativa) are poorly understood. Here, we report the function and role of Antioxidant Protein1 (OsATX1), a Cu chaperone in rice. Knocking out OsATX1 resulted in increased Cu concentrations in roots, whereas OsATX1 overexpression reduced root Cu concentrations but increased Cu accumulation in the shoots. At the reproductive stage, the concentrations of Cu in developing tissues, including panicles, upper nodes and internodes, younger leaf blades, and leaf sheaths of the main tiller, were increased significantly in OsATX1-overexpressing plants and decreased in osatx1 mutants compared with the wild type. The osatx1 mutants also showed a higher Cu concentration in older leaves. Yeast two-hybrid and bimolecular fluorescence complementation assays showed that OsATX1 interacts with the rice heavy metal P1B-ATPases HMA4, HMA5, HMA6, and HMA9. These results suggest that OsATX1 may function to deliver Cu to heavy metal P1B-ATPases for Cu trafficking and distribution in order to maintain Cu homeostasis in different rice tissues. In addition, heterologous expression of OsATX1 in the yeast (Saccharomyces cerevisiae) cadmium-sensitive mutant Δycf1 increased the tolerance to Cu and cadmium by decreasing their respective concentrations in the transformed yeast cells. Taken together, our results indicate that OsATX1 plays an important role in facilitating root-to-shoot Cu translocation and the redistribution of Cu from old leaves to developing tissues and seeds in rice.

Arabidopsis Pollen Fertility Requires the Transcription Factors CITF1 and SPL7 That Regulate Copper Delivery to Anthers and Jasmonic Acid Synthesis

A deficiency of the micronutrient copper (Cu) leads to infertility and grain/seed yield reduction in plants. How Cu affects fertility, which reproductive structures require Cu, and which transcriptional networks coordinate Cu delivery to reproductive organs is poorly understood. Using RNA-seq analysis, we showed that the expression of a gene encoding a novel transcription factor, CITF1 (Cu-DEFICIENCY INDUCED TRANSCRIPTION FACTOR1), was strongly upregulated in Arabidopsis thaliana flowers subjected to Cu deficiency. We demonstrated that CITF1 regulates Cu uptake into roots and delivery to flowers and is required for normal plant growth under Cu deficiency. CITF1 acts together with a master regulator of copper homeostasis, SPL7 (SQUAMOSA PROMOTER BINDING PROTEIN LIKE7), and the function of both is required for Cu delivery to anthers and pollen fertility. We also found that Cu deficiency upregulates the expression of jasmonic acid (JA) biosynthetic genes in flowers and increases endogenous JA accumulation in leaves. These effects are controlled in part by CITF1 and SPL7. Finally, we show that JA regulates CITF1 expression and that the JA biosynthetic mutant lacking the CITF1- and SPL7-regulated genes, LOX3 and LOX4, is sensitive to Cu deficiency. Together, our data show that CITF1 and SPL7 regulate Cu uptake and delivery to anthers, thereby influencing fertility, and highlight the relationship between Cu homeostasis, CITF1, SPL7, and the JA metabolic pathway.

COPT2, a plasma membrane located copper transporter, is involved in the uptake of Au in Arabidopsis

The mechanism of gold nanoparticle formation and genes involved in such processes, especially Au transport in plants are not understood. Previous reports pointed to the probable role of COPT2 in Au transport based on the transcript accumulation of COPT2 under Au exposure. Here, we provide evidence revealing the additional role of COPT2 for Au mobilization in yeast and Arabidopsis. The COPT2 transcripts significantly accumulated in the root of Arabidopsis under Au exposure. The expression of COPT2 restores Cu uptake ability in ctr1Δctr3Δ mutants and leads to Au sensitivity in yeast, which is comparable to Cu in growth kinetics experiments. The metal measurement data showed that the Au level was increased in COPT2, expressing yeast cells compared to vector transformed control. The copt2 mutant of Arabidopsis displayed a similar growth pattern to that of Col-0 under Au treatment. However, a notable phenotypic difference was noticed in three-week-old plants treated with and without Au. Consistent with yeast, Au uptake was reduced in the copt2 mutant of Arabidopsis. Together, these results clearly reveal the Au uptake capability of COPT2 in yeast and Arabidopsis. This is the first report showing the potential role of any transporter towards uptake and accumulation of Au in plants.

The copper microRNAs

1030 I. 1030 II. 1030 III. 1031 IV. 1031 V. 1032 VI. 1033 VII. 1034 VIII. 1034 1034 References 1034 SUMMARY: Copper (Cu) microRNAs are upregulated by Cu deficiency and mediate the post-transcriptional downregulation of transcripts that encode Cu proteins, suggesting a role directly related to Cu. However, expression and phenotypic analyses of copper microRNA mutants and over-expressors have suggested roles mainly in tolerance to abiotic stresses. To reconcile available data, a model is proposed which emphasizes the mobile nature of copper microRNA molecules in the regulation of Cu homeostasis. It is proposed that the Cu-microRNA regulatory circuits are further co-opted by plants to regulate both beneficial and pathogenic interactions with microbes. Further exploration of Cu-microRNA functions that account for the cell-to-cell mobility should give novel insight into plant microbe interactions and the integration of micronutrition and development.

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.

CsNIP2;1 is a Plasma Membrane Transporter from Cucumis sativus that Facilitates Urea Uptake When Expressed in Saccharomyces cerevisiae and Arabidopsis thaliana

Urea is an important source of nitrogen (N) for the growth and development of plants. It occurs naturally in soils, is the major N source in agricultural fertilizers and is an important N metabolite in plants. Therefore, the identification and characterization of urea transporters in higher plants is important for the fundamental understanding of urea-based N nutrition in plants and for designing novel strategies for improving the N-use efficiency of urea based-fertilizers. Progress in this area, however, is hampered due to scarce knowledge of plant urea transporters. From what is known, urea uptake from the soil into plant roots is mediated by two types of transporters: the major intrinsic proteins (MIPs) and the DUR3 orthologs, mediating low- and high-affinity urea transport, respectively. Here we characterized a MIP family member from Cucumis sativus, CsNIP2;1, with regard to its contribution to urea transport. We show that CsNIP2;1 is a plasma membrane transporter that mediates pH-dependent urea uptake when expressed in yeast. We also found that ectopic expression of CsNIP2;1 improves growth of wild-type Arabidopsis thaliana and rescues growth and development of the atdur3-3 mutant on medium with urea as the sole N source. In addition, CsNIP2;1 is transcriptionally up-regulated by N deficiency, urea and NO3 (-). These data and results from the analyses of the pattern of CsNIP2;1 expression in A. thaliana and cucumber suggest that CsNIP2;1 might be involved in multiple steps of urea-based N nutrition, including urea uptake and internal transport during N remobilization throughout seed germination and N delivery to developing tissues.