Temporal aspects of copper homeostasis and its crosstalk with hormones

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.

Differential expression and co-localization of transcriptional factors during callus transition to differentiation for shoot organogenesis in the water fern Ceratopteris richardii

BACKGROUND AND AIMS

In flowering plants, regeneration can be achieved by a variety of approaches, and different sets of transcriptional factors are involved in these processes. However, regeneration in taxa other than flowering plants remains a mystery. Ceratopteris richardii is a representative fern capable of both direct and indirect organogenesis, and we aimed to investigate the genetics underlying the transition from callus proliferation to differentiation.

METHODS

Morphological and histological analyses were used to determine the type of regeneration involved. RNA sequencing and differential gene expression were used to investigate how the callus switches from proliferation to differentiation. Phylogenetic analysis and RNA in situ hybridization were used to understand whether transcriptional factors are involved in this transition.

KEY RESULTS

The callus formed on nascent leaves and subsequently developed the shoot pro-meristem and shoot meristem, thus completing indirect de novo shoot organogenesis in C. richardii. Genes were differentially expressed during the callus transition from proliferation to differentiation, indicating a role for photosynthesis, stimulus response and transmembrane signalling in this transition and the involvement of almost all cell layers that make up the callus. Transcriptional factors were either downregulated or upregulated, which were generally in many-to-many orthology with genes known to be involved in callus development in flowering plants, suggesting that the genetics of fern callus development are both conserved and divergent. Among them, an STM-like, a PLT-like and an ethylene- and salt-inducible ERF gene3-like gene were expressed simultaneously in the vasculature but not in the other parts of the callus, indicating that the vasculature played a role in the callus transition from proliferation to differentiation.

CONCLUSIONS

Indirect de novo shoot organogenesis could occur in ferns, and the callus transition from proliferation to differentiation required physiological changes, differential expression of transcriptional factors and involvement of the vasculature.

Transcriptional response of Cu-deficient barley (Hordeum vulgare L.) to foliar-applied nano-Cu: Molecular crosstalk between Cu loading into plants and changes in Cu homeostasis genes

For safe and effective nutrient management, the cutting-edge approaches to plant fertilization are continuously developed. The aim of the study was to analyze the transcriptional response of barley suffering from Cu deficiency to foliar application of nanoparticulate Cu (nano-Cu) and its ionic form (CuSO4) at 100 and 1000 mg L-1 for the examination of their supplementing effect. The initial interactions of Cu-compounds with barley leaves were analyzed with spectroscopic (ICP-OES) and microscopic (SEM-EDS) methods. To determine Cu cellular status, the impact of Cu-compounds on the expression of genes involved in regulating Cu homeostasis (PAA1, PAA2, RAN1, COPT5), aquaporins (NIP2.1, PIP1.1, TIP1.1, TIP1.2) and antioxidant defense response (SOD CuZn, SOD Fe, SOD Mn, CAT) after 1 and 7 days of exposure was analyzed. Although Cu accumulation in plant leaves was detected overtime, the Cu content in leaves exposed to nano-Cu for 7 days was 44.5% lower than in CuSO4 at 100 mg L-1. However, nano-Cu aggregates remaining on the leaf surface indicated a potential difference between measured Cu content and the real Cu pool present in the plant. Our study revealed significant changes in the pattern of gene expression overtime depending on Cu-compound type and dose. Despite the initial puzzling patterns of gene expression, after 7 days all Cu transporters showed significant down-regulation under Cu-compounds exposure to prevent Cu excess in plant cells. Conversely, aquaporin gene expression was induced after 7 days, especially by nano-Cu and CuSO4 at 100 mg L-1 due to the stimulatory effect of low Cu doses. Our study revealed that the gradual release of Cu ions from nano-Cu at a lower rate provided a milder molecular response than CuSO4. It might indicate that nano-Cu maintained better metal balance in plants than the conventional compounds, thus may be considered as a long-term supplier of Cu.

Cell cycle-dependent Cu uptake explained the heterogenous responses of Chlamydomonas to Cu exposure

Growing evidence suggested that microorganisms exhibited heterogeneous sensitivity to toxicants, but their underlying mechanisms remain largely unknown. The asynchronous cell cycle progression in natural population implies the connection between cell cycle and heterogeneity. Here, the heterogenous responses of Chlamydomonas reinhardtii upon Cu stress were confirmed with the aid of a fluorometric probe for imaging Cu(I), implying the connection with cell cycle. Our results further indicated that the increase of labile Cu(I) was related to the cell division, leading to the fluctuation of labile Cu(I) with diurnal cycle and cell cycle, respectively. However, lack of Cu mainly influenced the cell division. We demonstrated that G2/M phase was the critical stage requiring high Cu quota during cell division. Specifically, algae at G2/M phase required 10-fold of Cu quota compared with that at G1 phase, which was related to the mitochondrial replication. Eventually, the heterogeneous Cu uptake ability of algae at different cell phases led to the heterogeneous responses to Cu exposure. Overall, Cu could influence the cell cycle through mediating the cell division, and in turn algae at different cell phases exhibited different Cu sensitivities. This study firstly uncovered the underlying mechanisms of heterogeneous Cu sensitivity for phytoplankton, which could help to evaluate the potential ecological risks of Cu.

Deficient copper availability on organoleptic and nutritional quality of tomato fruit

Copper (Cu) is an essential micronutrient for plants because it functions as a redox-active cofactor in vital processes inside the cells. Arable lands are often deficient in micronutrient contents and require the application of enriched fertilisers, whose overuse poses a high risk for human health, the environment and the food safety. Here, we aimed to decipher the effects of Cu deficiency during fruit growth on Cu and other micronutrients contents and on the fruit nutritional value and quality of tomato, the most consumed fruit worldwide, throughout the maturation process. Changes in the contents of important micronutrients for fruit physiology and human health, such as Fe and Mn, occurred in response to Cu deficient growing conditions at different fruit ripening stages, while lower Cu levels were detected in those fruit along the whole maturation process. Cu deficiency delayed changes in lycopene content and fruit colour, but increased acidity, and advanced the rise in antioxidant capacity and vitamin C content during fruit colour change from green to light red in the Moneymaker tomato; although this time lag eventually caught up in the most mature fruit stage. Cu deficiency also increased total phenolic and flavonoid contents only in green fruit.

A YSK-Type Dehydrin from Nicotiana tabacum Enhanced Copper Tolerance in Escherichia coli

Copper is an essential micronutrient for the maintenance of normal cell function but is toxic in excess. Dehydrins are group two late embryogenesis abundant proteins, which facilitate plant survival in harsh environmental conditions. Here, a YSK-type dehydrin, NtDhn17, was cloned from Nicotiana tabacum under copper toxicity and characterized using a heterologous expression system and in vitro or in vivo experiments and exhibited characteristics of intrinsic disorder during in vitro analyses. Heterologous expression of NtDHN17 enhanced the tolerance of E. coli to various metals, osmotic, and oxidative stress. NtDHN17 showed no Cu²⁺-binding properties in vivo or in vitro, indicating that metal ion binding is not universal among dehydrins. In vitro and in vivo experiments suggested that NtDHN17 behaved as a potent anti-aggregation agent providing strong protection to aggregated proteins induced by excess copper ions, an effect dependent on the K-segment but not on the Y- or S-segments. In summary, the protective role of NtDHN17 towards E. coli under conditions of copper toxicity may be related to anti-aggregation ability rather than its acting as an ion scavenger, which might be a valuable target for the genetic improvement of resistance to heavy metal stresses in plants.

The carboxy terminal transmembrane domain of SPL7 mediates interaction with RAN1 at the endoplasmic reticulum to regulate ethylene signalling in Arabidopsis

In Arabidopsis, copper (Cu) transport to the ethylene receptor ETR1 mediated using RAN1, a Cu transporter located at the endoplasmic reticulum (ER), and Cu homeostasis mediated using SPL7, the key Cu-responsive transcription factor, are two deeply conserved vital processes. However, whether and how the two processes interact to regulate plant development remain elusive. We found that its C-terminal transmembrane domain (TMD) anchors SPL7 to the ER, resulting in dual compartmentalisation of the transcription factor. Immunoprecipitation coupled mass spectrometry, yeast-two-hybrid assay, luciferase complementation imaging and subcellular co-localisation analyses indicate that SPL7 interacts with RAN1 at the ER via the TMD. Genetic analysis revealed that the ethylene-induced triple response was significantly compromised in the spl7 mutant, a phenotype rescuable by RAN1 overexpression but not by SPL7 without the TMD. The genetic interaction was corroborated by molecular analysis showing that SPL7 modulates RAN1 abundance in a TMD-dependent manner. Moreover, SPL7 is feedback regulated by ethylene signalling via EIN3, which binds the SPL7 promoter and represses its transcription. These results demonstrate that ER-anchored SPL7 constitutes a cellular mechanism to regulate RAN1 in ethylene signalling and lay the foundation for investigating how Cu homeostasis conditions ethylene sensitivity in the developmental context.

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.

Regulatory role of microRNAs (miRNAs) in the recent development of abiotic stress tolerance of plants

MicroRNAs (miRNAs) are a distinct groups of single-stranded non-coding, tiny regulatory RNAs approximately 20-24 nucleotides in length. miRNAs negatively influence gene expression at the post-transcriptional level and have evolved considerably in the development of abiotic stress tolerance in a number of model plants and economically important crop species. The present review aims to deliver the information on miRNA-mediated regulation of the expression of major genes or Transcription Factors (TFs), as well as genetic and regulatory pathways. Also, the information on adaptive mechanisms involved in plant abiotic stress responses, prediction, and validation of targets, computational tools, and databases available for plant miRNAs, specifically focus on their exploration for engineering abiotic stress tolerance in plants. The regulatory function of miRNAs in plant growth, development, and abiotic stresses consider in this review, which uses high-throughput sequencing (HTS) technologies to generate large-scale libraries of small RNAs (sRNAs) for conventional screening of known and novel abiotic stress-responsive miRNAs adds complexity to regulatory networks in plants. The discoveries of miRNA-mediated tolerance to multiple abiotic stresses, including salinity, drought, cold, heat stress, nutritional deficiency, UV-radiation, oxidative stress, hypoxia, and heavy metal toxicity, are highlighted and discussed in this review.

The role of post-transcriptional modulators of metalloproteins in response to metal deficiencies

Copper and iron proteins have a wide range of functions in living organisms. Metal assembly into metalloproteins is a complex process, where mismetalation is detrimental and energy consuming to cells. Under metal deficiency, metal distribution is expected to reach a metalation ranking, prioritizing essential versus dispensable metalloproteins, while avoiding interference with other metals and protecting metal-sensitive processes. In this review, we propose that post-transcriptional modulators of metalloprotein mRNA (ModMeR) are good candidates in metal prioritization under metal-limited conditions. ModMeR target high quota or redundant metalloproteins and, by adjusting their synthesis, ModMeR act as internal metal distribution valves. Inappropriate metalation of ModMeR targets could compete with metal delivery to essential metalloproteins and interfere with metal-sensitive processes, such as chloroplastic photosynthesis and mitochondrial respiration. Regulation of ModMeR targets could increase or decrease the metal flow through interconnected pathways in cellular metal distribution, helping to achieve adequate differential metal requirements. Here, we describe and compare ModMeR that function in response to copper and iron deficiencies. Specifically, we describe copper-miRNAs from Arabidopsis thaliana and diverse iron ModMeR from yeast, mammals, and bacteria under copper and iron deficiencies, as well as the influence of oxidative stress. Putative functions derived from their role as ModMeR are also discussed.

The Evolution and Functional Roles of miR408 and Its Targets in Plants

MicroRNA408 (miR408) is an ancient and highly conserved miRNA, which is involved in the regulation of plant growth, development and stress response. However, previous research results on the evolution and functional roles of miR408 and its targets are relatively scattered, and there is a lack of a systematic comparison and comprehensive summary of the detailed evolutionary pathways and regulatory mechanisms of miR408 and its targets in plants. Here, we analyzed the evolutionary pathway of miR408 in plants, and summarized the functions of miR408 and its targets in regulating plant growth and development and plant responses to various abiotic and biotic stresses. The evolutionary analysis shows that miR408 is an ancient and highly conserved microRNA, which is widely distributed in different plants. miR408 regulates the growth and development of different plants by down-regulating its targets, encoding blue copper (Cu) proteins, and by transporting Cu to plastocyanin (PC), which affects photosynthesis and ultimately promotes grain yield. In addition, miR408 improves tolerance to stress by down-regulating target genes and enhancing cellular antioxidants, thereby increasing the antioxidant capacity of plants. This review expands and promotes an in-depth understanding of the evolutionary and regulatory roles of miR408 and its targets in plants.

Effect of nano-silicon on the regulation of ascorbate-glutathione contents, antioxidant defense system and growth of copper stressed wheat (Triticum aestivum L.) seedlings

Copper (Cu²⁺) toxicity can inhibit plant growth and development. It has been shown that silicon (Si) can relieve Cu²⁺ stress. However, it is unclear how Si-nanoparticles (SiNPs) relieve Cu²⁺ stress in wheat seedlings. Therefore, the current study was conducted by setting up four treatments: CK, SiNP: (2.5 mM), Cu²⁺: (500 µM), and SiNP+Cu²⁺: (2.5 mM SiNP+500 µM Cu²⁺) to explore whether SiNPs can alleviate Cu²⁺ toxicity in wheat seedlings. The results showed that Cu²⁺ stress hampered root and shoot growth and accumulated high Cu²⁺ concentrations in roots (45.35 mg/kg) and shoots (25.70 mg/kg) of wheat as compared to control treatment. Moreover, Cu²⁺ treatment inhibited photosynthetic traits and chlorophyll contents as well as disturbed the antioxidant defense system by accumulating malondialdehyde (MDA) and hydrogen peroxidase (H₂O₂) contents. However, SiNPs treatment increased root length and shoot height by 15.1% and 22%, respectively, under Cu²⁺ toxicity. Moreover, SiNPs application decreased MDA and H₂O₂ contents by 31.25% and 19.25%, respectively. SiNPs increased non-enzymatic compounds such as ascorbic acid-glutathione (AsA-GSH) and enhanced superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbic peroxidase (APX) activities by 77.5%, 141.7%, 68%, and 80%, respectively. Furthermore, SiNPs decreased Cu²⁺ concentrations in shoots by 26.2%, as compared to Cu²⁺ treatment alone. The results concluded that SiNPs could alleviate Cu²⁺ stress in wheat seedlings. The present investigation may help to increase wheat production in Cu²⁺ contaminated soils.

Identification and molecular characterization of the high-affinity copper transporters family in Solanum lycopersicum

Copper (Cu) plays a key role as cofactor in the plant proteins participating in essential cellular processes, such as electron transport and free radical scavenging. Despite high-affinity Cu transporters (COPTs) being key participants in Cu homeostasis maintenance, very little is known about COPTs in tomato (Solanum lycopersicum) even though it is the most consumed fruit worldwide and this crop is susceptible to suboptimal Cu conditions. In this study, a six-member family of COPT (SlCOPT1-6) was identified and characterized. SlCOPTs have a conserved architecture consisting of three transmembrane domains and β-strains. However, the presence of essential methionine residues, a methionine-enriched amino-terminal region, an Mx₃Mx₁₂Gx₃G Cu-binding motif and a cysteine rich carboxy-terminal region, all required for their functionality, is more variable among members. Accordingly, functional complementation assays in yeast indicate that SlCOPT1 and SlCOPT2 are able to transport Cu inside the cell, while SlCOPT3 and SlCOPT5 are only partially functional. In addition, protein interaction network analyses reveal the connection between SlCOPTs and Cu P_IB-type ATPases, other metal transporters, and proteins related to the peroxisome. Gene expression analyses uncover organ-dependency, fruit vasculature tissue specialization and ripening-dependent gene expression profiles, as well as different response to Cu deficiency or toxicity in an organ-dependent manner.

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.

In search for potential biomarkers of copper stress in aquatic plants

Over the last few decades, the use of pesticides and discharge of industrial and domestic wastewater on water surfaces have increased. Especially, Copper (Cu) pollution in aquatic ecosystems could constitute a major health problem, not only for flora and fauna but also for humans. To cope with this challenge, environmental monitoring studies have sought to find Cu-specific biomarkers in terrestrial and aquatic flora and/or fauna. This review discusses the toxic effects caused by Cu on the growth and development of plants, with a special focus on aquatic plants. While copper is considered as an essential metal involved in vital mechanisms for plants, when in excess it becomes toxic and causes alterations on biomarkers: biochemical (oxidative stress, pigment content, phytochelatins, polyamines), physiological (photosynthesis, respiration, osmotic potential), and morphological. In addition, Cu has a detrimental effect on DNA and hormonal balance. An overview of Cu toxicity and detoxification in plants is provided, along with information regarding Cu bioaccumulation and transport. Awareness of the potential use of these reactions as specific biomarkers for copper contamination has indeed become essential.

The Copper-microRNA Pathway Is Integrated with Developmental and Environmental Stress Responses in Arabidopsis thaliana

As an essential nutrient, copper (Cu) scarcity causes a decrease in agricultural production. Cu deficiency responses include the induction of several microRNAs, known as Cu-miRNAs, which are responsible for degrading mRNAs from abundant and dispensable cuproproteins to economize copper when scarce. Cu-miRNAs, such as miR398 and miR408 are conserved, as well as the signal transduction pathway to induce them under Cu deficiency. The Arabidopsis thaliana SQUAMOSA-PROMOTER BINDING PROTEIN-LIKE (SPL) family member SPL7 binds to the cis-regulatory motifs present in the promoter regions of genes expressed under Cu deficiency, including Cu-miRNAs. The expression of several other SPL transcription factor family members is regulated by miR156. This regulatory miR156-SPL module plays a crucial role in developmental phase transitions while integrating internal and external cues. Here, we show that Cu deficiency also affects miR156 expression and that SPL3 overexpressing plants, resistant to miR156 regulation, show a severe decrease in SPL7-mediated Cu deficiency responses. These include the expression of Cu-miRNAs and their targets and is probably due to competition between SPL7 and miR156-regulated SPL3 in binding to cis-regulatory elements in Cu-miRNA promoters. Thus, the conserved SPL7-mediated Cu-miRNA pathway could generally be affected by the miR156-SPL module, thereby underscoring the integration of the Cu-miRNA pathway with developmental and environmental stress responses in Arabidopsis thaliana.

Characterization of the biochemical basis for copper homeostasis and tolerance in Biscutella auriculata L

Biscutella auriculata L. is a plant that belongs to the Brassicaceae family and it has been found growing in a metal-contaminated area of the San Quíntín mine (Ciudad Real, Spain). The purpose of this work was to evaluate the mechanisms that allow this plant to tolerate high concentrations of copper. Seedlings were grown in a semi-hydroponic system for 15 days under 125 μM of Cu (NO₃ )₂ . Exposure to copper resulted in growth inhibition and reduction in the photosynthetic parameters. Copper was mainly accumulated in vascular tissue and vacuoles of the roots and only a minor proportion was transferred to the shoot. Biothiol analysis showed a greater enhancement of reduced glutathione in leaves and increases of phytochelatins (PC2 and PC3) in both leaves and roots. Copper treatment induced oxidative stress, which triggered a response of the enzymatic and non-enzymatic antioxidant mechanisms. The results show that B. auriculata is able to tolerate high metal levels through the activation of specific mechanisms to neutralize the oxidative stress produced and also by metal sequestration through phytochelatins. The preferential accumulation of copper in roots provides clues for further studies on the use of this plant for phytostabilization and environmental recovery purposes in Cu-contaminated areas.

Reactive oxygen species homeostasis and circadian rhythms in plants

Elucidation of the molecular mechanisms by which plants sense and respond to environmental stimuli that influence their growth and yield is a prerequisite for understanding the adaptation of plants to climate change. Plants are sessile organisms and one important factor for their successful acclimation is the temporal coordination of the 24 h daily cycles and the stress response. The crosstalk between second messengers, such as Ca2+, reactive oxygen species (ROS), and hormones is a fundamental aspect in plant adaptation and survival under environmental stresses. In this sense, the circadian clock, in conjunction with Ca2+- and hormone-signalling pathways, appears to act as an important mechanism controlling plant adaptation to stress. The relationship between the circadian clock and ROS-generating and ROS-scavenging mechanisms is still not fully understood, especially at the post-transcriptional level and in stress situations in which ROS levels increase and changes in cell redox state occur. In this review, we summarize the information regarding the relationship between the circadian clock and the ROS homeostasis network. We pay special attention not only to the transcriptional regulation of ROS-generating and ROS-scavenging enzymes, but also to the few studies that have been performed at the biochemical level and those conducted under stress conditions.

The PIF1-miR408-PLANTACYANIN repression cascade regulates light-dependent seed germination

Light-dependent seed germination is a vital process for many seed plants. A decisive event in light-induced germination is degradation of the central repressor PHYTOCHROME INTERACTING FACTOR 1 (PIF1). The balance between gibberellic acid (GA) and abscisic acid (ABA) helps to control germination. However, the cellular mechanisms linking PIF1 turnover to hormonal balancing remain elusive. Here, employing far-red light-induced Arabidopsis thaliana seed germination as the experimental system, we identified PLANTACYANIN (PCY) as an inhibitor of germination. It is a blue copper protein associated with the vacuole that is both highly expressed in mature seeds and rapidly silenced during germination. Molecular analyses showed that PIF1 binds to the miR408 promoter and represses miR408 accumulation. This in turn posttranscriptionally modulates PCY abundance, forming the PIF1-miR408-PCY repression cascade for translating PIF1 turnover to PCY turnover during early germination. Genetic analysis, RNA-sequencing, and hormone quantification revealed that PCY is necessary and sufficient to maintain the PIF1-mediated seed transcriptome and the low-GA-high-ABA state. Furthermore, we found that PCY domain organization and regulation by miR408 are conserved features in seed plants. These results revealed a cellular mechanism whereby PIF1-relayed external light signals are converted through PCY turnover to internal hormonal profiles for controlling seed germination.

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.

Comparative effects of ascobin and glutathione on copper homeostasis and oxidative stress metabolism in mitigation of copper toxicity in rice

Copper (Cu) pollution of agricultural land is a major threat to crop production. Exogenous chemical treatment is an easily accessible and rapid approach to remediate metal toxicity, including Cu toxicity in plants. We compared the effects of ascobin (ASC; ascorbic acid:citric acid at 2:1) and glutathione (GSH) in mitigation of Cu toxicity in rice. Plants subjected to Cu stress displayed growth inhibition and biomass reduction, which were connected to reduced levels of chlorophylls, RWC, total phenolic compounds, carotenoids and Mg²⁺ . Increased accumulation of ROS and malondialdehyde indicated oxidative stress in Cu-stressed plants. However, application of ASC or GSH minimized the inhibitory effects of Cu stress on rice plants by restricting Cu²⁺ uptake and improving mineral balance, chlorophyll content and RWC. Both ASC and GSH pretreatments reduced levels of ROS and malondialdehyde and improved activities of antioxidant enzymes, suggesting their roles in alleviating oxidative damage. A comparison on the effects of ASC and GSH under Cu stress revealed that ASC was more effective in restricting Cu²⁺ accumulation (69.5% by ASC and 57.1% by GSH), Ca²⁺ and Mg²⁺ homeostasis, protection of photosynthetic pigments and activation of antioxidant defence mechanisms [catalase (110.4%), ascorbate peroxidase (76.5%) and guaiacol peroxidase (39.0%) by ASC, and catalase (58.9%) and ascorbate peroxidase (59.9%) by GSH] in rice than GSH, eventually resulting in better protection of ASC-pretreated plants against Cu stress. In conclusion, although ASC and GSH differed in induction of stress protective mechanisms, both were effective in improving rice performance in response to Cu phytotoxicity.

Main Molecular Pathways Associated with Copper Tolerance Response in Imperata cylindrica by de novo Transcriptome Assembly

The metallophyte Imperata cylindrica inhabits copper (Cu) polluted soils in large areas from Central Chile. Here, we subjected clonal vegetative plantlets to 300 mg Cu kg-1 of substrate for 21 days to identify the main molecular pathways involved in the response to Cu stress. Transcriptomic analyses were performed for shoots and roots, with and without Cu supply. RNA-Seq and de novo transcriptome assembly were performed to identify the gene response associated with molecular mechanisms of Cu tolerance in I. cylindrica. De novo transcriptome revealed a total of 200,521 transcripts (1777 bp) comprising ~91% complete ultra-conserved genes in the eukaryote and Plantae database. The differentially expressed genes (DEGs) in roots were 7386, with 3558 of them being up-regulated and the other 3828 down-regulated. The transcriptome response in shoots was significantly less, showing only 13 up-regulated and 23 down-regulated genes. Interestingly, DEGs mainly related with actin and cytoskeleton formation, and to a minor degree, some DEGs associated with metal transporters and superoxide dismutase activity in root tissues were found. These transcriptomic results suggest that cytoskeleton could be acting as a mechanism of Cu-binding in the root, resulting in a high Cu tolerance response in this metallophyte, which deserve to be analyzed ultra-structurally. Our study contributes to reinforcing the potential of I. cylindrica as a candidate plant species to be used as a phytoremediation agent in Cu-contaminated environments.

Understanding the role of miRNAs for improvement of tea quality and stress tolerance

MicroRNAs (miRNAs) are an emerging class of small non-coding RNAs that exhibit important role in regulation of gene expression, mostly through the mechanism of cleavage and/or inhibition of translation of target mRNAs during or after transcription. Although much has been unravelled about the role of miRNAs in diverse biological processes like maintenance of functional integrity of genes and genome, growth and development, metabolism, and adaptive responses towards biotic and abiotic stresses in plants, not much is known on their specific roles in majority of cash crops - an area of investigation with potentially significant and gainful economic implications. Tea (Camellia sinensis) is globally the second most consumed beverage after water and its cultivation has major agro-economic and social ramifications. In recent years, global tea production has been greatly challenged by many biotic and abiotic stress factors and a deeper understanding of molecular processes regulating stress adaptation in this largely under investigated crop stands to significantly facilitate potential crop improvement strategies towards durable stress tolerance. This review endeavours to highlight recent advances in our understanding of the role of miRNAs in regulating stress tolerance traits in tea plant with additional focus on their role in determining tea quality attributes.

MtCOPT2 is a Cu+ transporter specifically expressed in Medicago truncatula mycorrhizal roots

Arbuscular mycorrhizal fungi are critical participants in plant nutrition in natural ecosystems and in sustainable agriculture. A large proportion of the phosphorus, nitrogen, sulfur, and transition metal elements that the host plant requires are obtained from the soil by the fungal mycelium and released at the arbuscules in exchange for photosynthates. While many of the plant transporters responsible for obtaining macronutrients at the periarbuscular space have been characterized, the identities of those mediating transition metal uptake remain unknown. In this work, MtCOPT2 has been identified as the only member of the copper transporter family COPT in the model legume Medicago truncatula to be specifically expressed in mycorrhizal roots. Fusing a C-terminal GFP tag to MtCOPT2 expressed under its own promoter showed a distribution pattern that corresponds with arbuscule distribution in the roots. When expressed in tobacco leaves, MtCOPT2-GFP co-localizes with a plasma membrane marker. MtCOPT2 is intimately related to the rhizobial nodule-specific MtCOPT1, which is suggestive of a shared evolutionary lineage that links transition metal nutrition in the two main root endosymbioses in legumes.

SYNERGISTIC ON AUXIN AND CYTOKININ 1 positively regulates growth and attenuates soil pathogen resistance

Plants as non-mobile organisms constantly integrate varying environmental signals to flexibly adapt their growth and development. Local fluctuations in water and nutrient availability, sudden changes in temperature or other abiotic and biotic stresses can trigger changes in the growth of plant organs. Multiple mutually interconnected hormonal signaling cascades act as essential endogenous translators of these exogenous signals in the adaptive responses of plants. Although the molecular backbones of hormone transduction pathways have been identified, the mechanisms underlying their interactions are largely unknown. Here, using genome wide transcriptome profiling we identify an auxin and cytokinin cross-talk component; SYNERGISTIC ON AUXIN AND CYTOKININ 1 (SYAC1), whose expression in roots is strictly dependent on both of these hormonal pathways. We show that SYAC1 is a regulator of secretory pathway, whose enhanced activity interferes with deposition of cell wall components and can fine-tune organ growth and sensitivity to soil pathogens.

Genotypes of the aquatic plant Myriophyllum spicatum with different growth strategies show contrasting sensitivities to copper contamination

Genotypic variability has been considered for years as a key attribute in species adaptation to new environments. It has been extensively studied in a context of chemical resistance, but remains poorly studied in response to chemical exposure in a context of global change. As aquatic ecosystems are particularly affected by environmental changes, we aimed to study how genotypic variability could inflect the sensitivity of aquatic plants to chemicals. Seven genotypes of Myriophyllum spicatum were exposed to three copper concentrations at 0, 0.15 and 0.5 mg/L. The sensitivity of the different genotypes was assessed through several endpoints such as relative growth rate (RGR) and morphological traits, as well as physiological markers, such as plant biomacromolecular composition. Our results showed that genotypes exhibited significant differences in their life-history traits in absence of chemical contamination. Some trait syndromes were observed, and three growth strategies were identified: (1) biomass production and main shoot elongation, (2) dry matter storage with denser whorls to promote resource conservation and (3) lateral shoot production. An up to eightfold difference in sensitivity for growth-related endpoints was observed among genotypes. Differences in sensitivity were partly attributed to morphological life-history traits. Our results confirm that genotypic variability can significantly affect M. spicatum sensitivity to Cu, and may influence the outcomes of laboratory testing based on the study of one single genotype. We recommend including genotypic variation as an assessment factor in ecological risk assessment and to study this source of variability more in depth as a possible driver of ecosystem resilience.

Genome-wide and SNP network analyses reveal genetic control of spikelet sterility and yield-related traits in wheat

Revealing the genetic factors underlying yield and agronomic traits in wheat are an imperative need for covering the global food demand. Yield boosting requires a deep understanding of the genetic basis of grain yield-related traits (e.g., spikelet fertility and sterility). Here, we have detected much natural variation among ancient hexaploid wheat accessions in twenty-two agronomic traits collected over eight years of field experiments. A genome-wide association study (GWAS) using 15 K single nucleotide polymorphisms (SNPs) was applied to detect the genetic basis of studied traits. Subsequently, the GWAS output was reinforced via other statistical and bioinformatics analyses to detect putative candidate genes. Applying the genome-wide SNP-phenotype network defined the most decisive SNPs underlying the traits. Six pivotal SNPs, co-located physically within the genes encoding enzymes, hormone response, metal ion transport, and response to oxidative stress have been identified. Of these, metal ion transport and Gibberellin 2-oxidases (GA2oxs) genes showed strong involvement in controlling the spikelet sterility, which had not been reported previously in wheat. SNP-gene haplotype analysis confirmed that these SNPs influence spikelet sterility, especially the SNP co-located on the exon of the GA2ox gene. Interestingly, these genes were highly expressed in the grain and spike, demonstrating their pivotal role in controlling the trait. The integrative analysis strategy applied in this study, including GWAS, SNP-phenotype network, SNP-gene haplotype, expression analysis, and genome-wide prediction (GP), empower the identification of functional SNPs and causal genes. GP outputs obtained in this study are encouraging for the implementation of the traits to accelerate yield improvement by making an early prediction of complex yield-related traits in wheat. Our findings demonstrate the usefulness of the ancient wheat material as a valuable resource for yield-boosting. This is the first comprehensive genome-wide analysis for spikelet sterility in wheat, and the results provide insights into yield improvement.

Deregulated High Affinity Copper Transport Alters Iron Homeostasis in Arabidopsis

The present work describes the effects on iron homeostasis when copper transport was deregulated in Arabidopsis thaliana by overexpressing high affinity copper transporters COPT1 and COPT3 (COPT^OE ). A genome-wide analysis conducted on COPT1^OE plants, highlighted that iron homeostasis gene expression was affected under both copper deficiency and excess. Among the altered genes were those encoding the iron uptake machinery and their transcriptional regulators. Subsequently, COPT^OE seedlings contained less iron and were more sensitive than controls to iron deficiency. The deregulation of copper (I) uptake hindered the transcriptional activation of the subgroup Ib of basic helix-loop-helix (bHLH-Ib) factors under copper deficiency. Oppositely, copper excess inhibited the expression of the master regulator FIT but activated bHLH-Ib expression in COPT^OE plants, in both cases leading to the lack of an adequate iron uptake response. As copper increased in the media, iron (III) was accumulated in roots, and the ratio iron (III)/iron (II) was increased in COPT^OE plants. Thus, iron (III) overloading in COPT^OE roots inhibited local iron deficiency responses, aimed to metal uptake from soil, leading to a general lower iron content in the COPT^OE seedlings. These results emphasized the importance of appropriate spatiotemporal copper uptake for iron homeostasis under non-optimal copper supply. The understanding of the role of copper uptake in iron metabolism could be applied for increasing crops resistance to iron deficiency.

Effects of copper on oxidative stress and autophagy in hypothalamus of broilers

The purpose of this research was to discuss the effects of copper (Cu)-induced toxicity on oxidative stress and autophagy in hypothalamus of broilers. In this study, 240 one-day-old broilers were randomly divided into 4 groups and the contents of dietary Cu in 4 groups were 11 mg/kg (control group), 110 mg/kg (group I), 220 mg/kg (group II), and 330 mg/kg (group III). The experiment lasted for 49 days and the hypothalamus tissues were collected for histological observation and detection of Cu content. Additionally, the indicators related to oxidative stress in hypothalamus were determined. Moreover, the mRNA expression levels of autophagy-related genes and the protein expression levels of Beclin1, LC3-II/LC3-I, and p62 in hypothalamus were measured. Results showed that the treated groups were observed vacuolar degeneration in hypothalamus compared to control group, and the Cu content in hypothalamus was increased with the increase of dietary Cu. Furthermore, the activities of SOD, CAT, T-AOC were increased in group I and group II and then decreased in group III, and the content of MDA and the mRNA levels of Nrf2, HO-1, SOD-1, CAT, GCLC, GCLM, and GST in treated groups were elevated compared to control group. Moreover, the mRNA expression levels of Beclin1, Atg5, LC3-I, LC3-II and the protein expression levels of Beclin1 and LC3-II/LC3-I up-regulated significantly with the increasing levels of Cu. However, the mRNA expression levels of p62 and mTOR and the protein expression level of p62 down-regulated remarkably. Taken together, our present study evidenced that excessive intake of Cu could induce oxidative stress and autophagy in hypothalamus of broilers.

Exposure to Copper Oxide Nanoparticles and Arsenic Causes Intergenerational Effects on Rice (Oryza sativa japonica Koshihikari) Seed Germination and Seedling Growth

Offspring generation (F1) rice (Oryza sativa japonica Koshihikari) seed germination and seedling growth tests were conducted for 18 d to investigate intergenerational effects of arsenic (As) and copper oxide nanoparticles (nCuO), with seeds harvested from a life cycle study exposed to As (0 and 10 mg/kg) and nCuO (0, 0.1, 1.0, 10, 50, and 100 mg/L). Seed germination and seedling growth of F1 plants were influenced by treatments experienced by parent generation (F0) plants (p < 0.05). Seeds produced from plants in F0 treatment with nCuO 50 mg/L had the lowest germination percentage and shortest seedling shoot length and root length in F1 control (F1C) and As at 10 mg/kg (F1As) alone treatments (p < 0.05). The shoot length and root length were decreased, whereas the number of root branches was increased in F1As treatment compared with F1C (p < 0.001). Interaction of As and nCuO also caused differential seed germination and seedling growth at various nCuO concentrations in quasi-F0 treatment (seeds receiving the same exposure as F0 plants; p < 0.05). Copper and As uptake in F1C seedlings were not affected by seeds' F0 exposure; this indicated that the transgenerational effects on rice seedling growth were not dependent on total Cu or As uptake in seedlings. The enhanced effects on seedlings from quasi-F0 treatment were influenced by additional exposure to nCuO and As that also interacted to affect Cu and As uptake in seedlings. Environ Toxicol Chem 2019;38:1978-1987. © 2019 SETAC.

The Nutrient Response Transcriptional Regulome of Arabidopsis

Plants respond actively to changes in their environment. Variations in nutrient availability elicit substantial transcriptional reprogramming, and we aimed to systematically describe these adjustments and identify the regulators responsible. Using gene coexpression analysis based on 13 different nutrient availability anomalies, we defined and analyzed nutrient stress response signatures. We identified known regulators and could predict functions in nutrient responses for transcriptional regulators previously associated with other processes, thus linking development and environmental interaction. Three of the identified transcriptional regulators, PIF4, HY5, and NF-Y, known from their role in light signaling, targeted a substantial part of the network and may participate in remodeling the global Arabidopsis transcriptome in response to variations of nutrient availability. We present gene coexpression and transcriptional regulation networks, which can be used as tools to further explore regulatory events and dependencies even by users with basic informatics skills.

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Gene coexpression analysis is a powerful tool for elucidating nutrient stress

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Nutrient stress elicits unique signatures of modular transcriptional response

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Master transcriptional regulators coordinate plant growth and nutrient utilization

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Analysis suggests PIF4, HY5, and the NF-Y to be master regulators

Distribution and Speciation of Copper and Arsenic in Rice Plants ( Oryza sativa japonica 'Koshihikari') Treated with Copper Oxide Nanoparticles and Arsenic during a Life Cycle

A 6 × 2 factorial study was conducted to investigate the effects of copper oxide nanoparticles (nCuO, 0-100 mg/L), arsenic (As, 0-10 mg/kg), and their interaction on uptake, distribution, and speciation of Cu and As in rice plants ( Oryza sativa japonica 'Koshihikari'). Arsenic (in As-addition treatments) and Cu in seedling roots (SRs) were 1.45 and 1.58 times those in soil, respectively. Arsenic and Cu concentrations further increased in mature plant roots (MRs), which were 2.06 and 2.35 times those in soil, respectively. Arsenic and Cu concentrations in seedling shoots (SSs) were 79% and 54% lower than those in SRs, respectively. The mature stems, however, contained only 3% and 44% of As and Cu in SSs. Copper in flag leaves did not vary much compared to that in stems, whereas As was 14.5 times that in stems. Species transformations of Cu and As were observed in rice including reductions of Cu(II) to Cu(I) and As(V) to As(III). Arsenic in dehusked grains was negatively correlated with Cu and was lowered by nCuO below the WHO (World Health Organization) maximum safe concentration for white rice (200 ng/g). This may alleviate As adverse effects on humans from rice consumption.

Copper transporter COPT5 participates in the crosstalk between vacuolar copper and iron pools mobilisation

Copper (Cu) deficiency affects iron (Fe) homeostasis in several plant processes, including the increased Fe requirements due to cuproprotein substitutions for the corresponding Fe counterpart. Loss-of-function mutants from Arabidopsis thaliana high affinity copper transporter COPT5 and Fe transporters NATURAL RESISTANCE-ASSOCIATED MACROPHAGE PROTEIN 3/4 (NRAMP3 and NRAMP4) were used to study the interaction between metals internal pools. A physiological characterisation showed that the copt5 mutant is sensitive to Fe deficiency, and that nramp3nramp4 mutant growth was severely affected under limiting Cu. By a transcriptomic analysis, we observed that NRAMP4 expression was highly induced in the copt5 mutant under Cu deficiency, while COPT5 was overexpressed in the nramp3nramp4 mutant. As a result, an enhanced mobilisation of the vacuolar Cu or Fe pools, when the other metal export through the tonoplast is impaired in the mutants, has been postulated. However, metals coming from internal pools are not used to accomplish the increased requirements that derive from metalloprotein substitution under metal deficiencies. Instead, the metal concentrations present in aerial parts of the copt5 and nramp3nramp4 mutants conversely show compensated levels of these two metals. Together, our data uncover an interconnection between Cu and Fe vacuolar pools, whose aim is to fulfil interorgan metal translocation.

Characterization of biomass sorghum for copper phytoremediation: photosynthetic response and possibility as a bioenergy feedstock from contaminated land

In order to decrease the concentration of toxic metals in contaminated lands, phytoextraction can be suitable considering the use of plant species with high potential for biomass production, such as biomass sorghum (Sorghum bicolor L.). We assessed a biomass sorghum (BRS716) potential as a copper phytoextractor as well as the physiological stability under this stressful condition. A completely randomized experimental design was used for a greenhouse experiment in which sorghum plants were submitted to a range of Cu²⁺ concentrations: 2.3, 10.9, 19.6, 30.5, 37.6 and 55.6 mg dm^-3. The plant growth was not affected by increasing Cu²⁺ concentrations, suggesting that this species is tolerant to copper. There was a decrease in photosynthetic rate according to the increase in Cu²⁺ concentration, but not at a level that could disturb plant metabolism and eventual death. The values obtained for transfer index ranged from 0.62 to 0.11 which evidenced the restriction of Cu²⁺ transport to the aerial parts. The more Cu²⁺ available in soil, the smaller the amount of Cu²⁺ transported to aerial parts of sorghum. So, our results show that biomass sorghum has potential to be used for Cu²⁺ phytoextraction in concentration of up to 20 mg dm^-3. Also, in heavily Cu²⁺ polluted sites, it can be used to produce biomass for bioenergy purpose, promoting a low rate of Cu²⁺ extraction.

Role of Azospirillum brasilense in triggering different Fe chelate reductase enzymes in cucumber plants subjected to both nutrient deficiency and toxicity

Azospirillum brasilense was reported to up-regulate iron (Fe) uptake mechanisms, such as Fe reduction and rhizosphere acidification, in both Fe sufficient and deficient cucumber plants (Cucumis sativus L.). Strategy I plants take up both Fe and copper (Cu) after their reduction mediated by the ferric-chelate reductase oxidase (FRO) enzyme. Interestingly, in cucumber genome only one FRO gene is reported. Thus, in the present study we applied a bioinformatics approach to identify the member of cucumber FRO gene family and allowed the identification of at least three CsFRO genes, one of which was the already identified, i.e. CsFRO1. The expression patterns of the newly identified transcripts were investigated in hydroponically grown cucumber plants treated with different Fe and Cu nutritional regimes. Gene expression was then correlated with morphological (i.e. root architecture) and physiological (Fe(III) reducing activity) parameters to shed light on: i) the CsFRO homologue responsible of the increased reduction activity in Fe-sufficient plants inoculated with A. brasilense cucumber plants, and ii) the possible effect of A. brasilense in ameliorating the symptoms of Cu toxicity in cucumber plants. The data obtained showed that all the CsFRO genes were expressed in the root tissues of cucumber plants and responded to Cu starvation, combined Cu/Fe deficiency and Cu toxicity. Only CsFRO3 was modulated by the A. brasilense in Fe-sufficient plants suggesting for the first time a different specificity of action of the three isoenzymes depending not only on the nutritional regime (either deficiency or toxicity) but also on the presence of the PGPR. Furthermore, results suggest that the PGPR could even ameliorate the stress symptoms caused by both the double (i.e. Cu and Fe) and Cu deficiency as well as Cu toxicity modulating, on one hand, the growth of the root system and, on the other hand, the root nutrient uptake.

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.

Copper in the suprachiasmatic circadian clock: A possible link between multiple circadian oscillators

The mammalian circadian clock in the suprachiasmatic nucleus (SCN) is very robust, able to coordinate our daily physiological and behavioral rhythms with exquisite accuracy. Simultaneously, the SCN clock is highly sensitive to environmental timing cues such as the solar cycle. This duality of resiliency and sensitivity may be sustained in part by a complex intertwining of three cellular oscillators: transcription/translation, metabolic/redox, and membrane excitability. We suggest here that one of the links connecting these oscillators may be forged from copper (Cu). Cellular Cu levels are highly regulated in the brain and peripherally, and Cu affects cellular metabolism, redox state, cell signaling, and transcription. We have shown that both Cu chelation and application induce nighttime phase shifts of the SCN clock in vitro and that these treatments affect glutamate, N-methyl-D-aspartate receptor, and associated signaling processes differently. More recently we found that Cu induces mitogen-activated protein kinase-dependent phase shifts, while the mechanisms by which Cu removal induces phase shifts remain unclear. Lastly, we have found that two Cu transporters are expressed in the SCN, and that one of these transporters (ATP7A) exhibits a day/night rhythm. Our results suggest that Cu homeostasis is tightly regulated in the SCN, and that changes in Cu levels may serve as a time cue for the circadian clock. We discuss these findings in light of the existing literature and current models of multiple coupled circadian oscillators in the SCN.

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.

Expression of the Intracellular COPT3-Mediated Cu Transport Is Temporally Regulated by the TCP16 Transcription Factor

Copper is an essential element in plants. When scarce, copper is acquired from extracellular environment or remobilized from intracellular sites, through members of the high affinity copper transporters family COPT located at the plasma membrane and internal membrane, respectively. Here, we show that COPT3 is an intracellular copper transporter, located at a compartment of the secretory pathway, that is mainly expressed in pollen grains and vascular bundles. Contrary to the COPT1 plasma membrane member, the expression of the internal COPT3 membrane transporter was higher at 12 h than at 0 h of a neutral photoperiod day under copper deficiency. The screening of a library of conditionally overexpressed transcription factors implicated members of the TCP family in the COPT3 differential temporal expression pattern. Particularly, in vitro, TCP16 was found to bind to the COPT3 promoter and down-regulated its expression. Accordingly, TCP16 was mainly expressed at 0 h under copper deficiency and induced at 12 h by copper excess. Moreover, TCP16 overexpression resulted in increased sensitivity to copper deficiency, whereas the tcp16 mutant was sensitive to copper excess. Both copper content and the expression of particular copper status markers were altered in plants with modified levels of TCP16. Consistent with TCP16 affecting pollen development, the lack of COPT3 function led to altered pollen morphology. Furthermore, analysis of copt3 and COPT3 overexpressing plants revealed that COPT3 function exerted a negative effect on TCP16 expression. Taken together, these results suggest a differential daily regulation of copper uptake depending on the external and internal copper pools, in which TCP16 inhibits copper remobilization at dawn through repression of intracellular transporters.

Morphological and functional responses of a metal-tolerant sunflower mutant line to a copper-contaminated soil series

The potential use of a metal-tolerant sunflower mutant line for biomonitoring Cu phytoavailability, Cu-induced soil phytotoxicity, and Cu phytoextraction was assessed on a Cu-contaminated soil series (13-1020 mg Cu kg^-1) obtained by fading a sandy topsoil from a wood preservation site with a similar uncontaminated soil. Morphological and functional plant responses as well as shoot, leaf, and root ionomes were measured after a 1-month pot experiment. Hypocotyl length, shoot and root dry weight (DW) yields, and leaf area gradually decreased as soil Cu exposure rose. Their dose-response curves (DRC) plotted against indicators of Cu exposure were generally well fitted by sigmoidal curves. The half-maximal effective concentration (EC₅₀) of morphological parameters ranged between 203 and 333 mg Cu kg^-1 soil, corresponding to 290-430 μg Cu L^-1 in the soil pore water, and 20 ± 5 mg Cu kg^-1 DW in the shoots. The EC₁₀ for shoot Cu concentration (13-15 mg Cu kg^-1 DW) coincided to 166 mg Cu kg^-1 soil. Total chlorophyll content and total antioxidant capacity (TAC) were early biomarkers (EC₁₀: 23 and 51 mg Cu kg^-1 soil). Their DRC displayed a biphasic response. Photosynthetic pigment contents, e.g., carotenoids, correlated with TAC. Ionome was changed in Cu-stressed roots, shoots, and leaves. Shoot Cu removal peaked roughly at 280 μg Cu L^-1 in the soil pore water.

Dietary Ingestion of Calories and Micronutrients Modulates the DNA Methylation Profile of Leukocytes from Older Individuals

OBJECTIVE

Several lines of evidence from the last decade support the connection between nutrition and epigenetic mechanisms. In the present study we evaluated the impact of the daily dietary intake of calories and the micronutrients vitamin A, D, B1, B2, B5, C, E, copper, calcium, phosphorus, iron, iodine, selenium, manganese, potassium and sodium on the global DNA methylation profile of blood cells from older individuals.

RESEARCH METHODS AND PROCEDURES

The study enrolled 126 physically independent elderly of both sexes (60 men and 66 women). For the molecular analysis, DNA samples were extracted from leukocytes and global DNA methylation was evaluated using a high throughput Elisa-based method. Correlations between global DNA methylation and the daily intake of calorie or micronutrients were evaluated using Prism5 GraphPad Software.

RESULTS

A statistically significant correlation was observed between global DNA methylation and the daily caloric value (p=0.019, r=-0.21), and the intake of vitamin A (p=0.03, r=-0.18), Vitamin E (p=0.027, r=-0.20) and copper (p=0.04, r=-0.18). No correlation was observed between global DNA methylation and the daily intake of vitamin D, B1, B2, B5, C, calcium, phosphorus, iron, iodine, selenium, manganese and potassium (p>0.05).

CONCLUSION

Our data demonstrate that the daily intake of calories or the micronutrients vitamin A, vitamin E and copper can potentially modulate the global DNA methylation profile of leukocytes in older adults and corroborate the notion of nutritional influences on epigenetic mechanisms.

Copper and ectopic expression of the Arabidopsis transport protein COPT1 alter iron homeostasis in rice (Oryza sativa L.)

Copper deficiency and excess differentially affect iron homeostasis in rice and overexpression of the Arabidopsis high-affinity copper transporter COPT1 slightly increases endogenous iron concentration in rice grains. Higher plants have developed sophisticated mechanisms to efficiently acquire and use micronutrients such as copper and iron. However, the molecular mechanisms underlying the interaction between both metals remain poorly understood. In the present work, we study the effects produced on iron homeostasis by a wide range of copper concentrations in the growth media and by altered copper transport in Oryza sativa plants. Gene expression profiles in rice seedlings grown under copper excess show an altered expression of genes involved in iron homeostasis compared to standard control conditions. Thus, ferritin OsFER2 and ferredoxin OsFd1 mRNAs are down-regulated whereas the transcriptional iron regulator OsIRO2 and the nicotianamine synthase OsNAS2 mRNAs rise under copper excess. As expected, the expression of OsCOPT1, which encodes a high-affinity copper transport protein, as well as other copper-deficiency markers are down-regulated by copper. Furthermore, we show that Arabidopsis COPT1 overexpression (C1 ^OE ) in rice causes root shortening in high copper conditions and under iron deficiency. C1 ^OE rice plants modify the expression of the putative iron-sensing factors OsHRZ1 and OsHRZ2 and enhance the expression of OsIRO2 under copper excess, which suggests a role of copper transport in iron signaling. Importantly, the C1 ^OE rice plants grown on soil contain higher endogenous iron concentration than wild-type plants in both brown and white grains. Collectively, these results highlight the effects of rice copper status on iron homeostasis, which should be considered to obtain crops with optimized nutrient concentrations in edible parts.

The Plant Circadian Clock: From a Simple Timekeeper to a Complex Developmental Manager

The plant circadian clock allows organisms to anticipate the predictable changes in the environment by adjusting their developmental and physiological traits. In the last few years, it was determined that responses known to be regulated by the oscillator are also able to modulate clock performance. These feedback loops and their multilayer communications create a complex web, and confer on the clock network a role that exceeds the measurement of time. In this article, we discuss the current knowledge of the wiring of the clock, including the interplay with metabolism, hormone, and stress pathways in the model species Arabidopsis thaliana We outline the importance of this system in crop agricultural traits, highlighting the identification of natural alleles that alter the pace of the timekeeper. We report evidence supporting the understanding of the circadian clock as a master regulator of plant life, and we hypothesize on its relevant role in the adaptability to the environment and the impact on the fitness of most organisms.

The arbuscular mycorrhizal fungus Rhizophagus irregularis differentially regulates the copper response of two maize cultivars differing in copper tolerance

Arbuscular mycorrhiza can increase plant tolerance to heavy metals. The effects of arbuscular mycorrhiza on plant metal tolerance vary depending on the fungal and plant species involved. Here, we report the effect of the arbuscular mycorrhizal fungus Rhizophagus irregularis on the physiological and biochemical responses to Cu of two maize genotypes differing in Cu tolerance, the Cu-sensitive cv. Orense and the Cu-tolerant cv. Oropesa. Development of the symbiosis confers an increased Cu tolerance to cv. Orense. Root and shoot Cu concentrations were lower in mycorrhizal than in non-mycorrhizal plants of both cultivars. Shoot lipid peroxidation increased with soil Cu content only in non-mycorrhizal plants of the Cu-sensitive cultivar. Root lipid peroxidation increased with soil Cu content, except in mycorrhizal plants grown at 250mg Cu kg^-1soil. In shoots of mycorrhizal plants of both cultivars, superoxide dismutase, ascorbate peroxidase, catalase and glutathione reductase activities were not affected by soil Cu content. In Cu-supplemented soils, total phytochelatin content increased in shoots of mycorrhizal cv. Orense but decreased in cv. Oropesa. Overall, these data suggest that the increased Cu tolerance of mycorrhizal plants of cv. Orense could be due to an increased induction of shoot phytochelatin biosynthesis by the symbiosis in this cultivar.

Small RNA-mediated responses to low- and high-temperature stresses in cotton

MicroRNAs (miRNAs) are one class of endogenous non-coding RNAs modulating the expression of target genes involved in plant development and stress tolerance, by degrading mRNA or repressing translation. In this study, small RNA and mRNA degradome sequencing were used to identify low- and high-temperature stress-responsive miRNAs and their targets in cotton (Gossypium hirsutum). Cotton seedlings were treated under different temperature conditions (4, 12, 25, 35, and 42 °C) and then the effects were investigated. In total, 319 known miRNAs and 800 novel miRNAs were identified, and 168 miRNAs were differentially expressed between different treatments. The targets of these miRNAs were further analysed by degradome sequencing. Based on studies from Gene Ontology and Kyoto Encyclopedia of Genes and Genomes, the majority of the miRNAs are from genes that are likely involved in response to hormone stimulus, oxidation-reduction reaction, photosynthesis, plant-pathogen interaction and plant hormone signal transduction pathways. This study provides new insight into the molecular mechanisms of plant response to extreme temperature stresses, and especially the roles of miRNAs under extreme temperatures.

Phytohormone profile in Lactuca sativa and Brassica oleracea plants grown under Zn deficiency

Phytohormones, structurally diverse compounds, are involved in multiple processes within plants, such as controlling plant growth and stress response. Zn is an essential micronutrient for plants and its deficiency causes large economic losses in crops. Therefore, the purpose of this study was to analyse the role of phytohormones in the Zn-deficiency response of two economically important species, i.e. Lactuca sativa and Brassica oleracea. For this, these two species were grown hydroponically with different Zn-application rates: 10 μM Zn as control and 0.1 μM Zn as deficiency treatment and phytohormone concentration was determined by U-HPLC-MS. Zn deficiency resulted in a substantial loss of biomass in L. sativa plants that was correlated with a decline in growth-promoting hormones such as indole-3-acetic acid (IAA), cytokinins (CKs), and gibberellins (GAs). However these hormones increased or stabilized their concentrations in B. oleracea and could help to maintain the biomass in this species. A lower concentration of stress-signaling hormones such as ethylene precursor aminocyclopropane-1-carboxylic acid (ACC), abscisic acid (ABA), salicylic acid (SA) and jasmonic acid (JA) and also CKs might be involved in Zn uptake in L. sativa while a rise in GA4, isopentenyl adenine (iP), and ACC and a fall in JA and SA might contribute to a better Zn-utilization efficiency (ZnUtE), as observed in B. oleracea plants.

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.

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.