Copper chaperone-dependent and -independent activation of three copper-zinc superoxide dismutase homologs localized in different cellular compartments in Arabidopsis

Increased ascorbic acid synthesis by overexpression of AcGGP3 ameliorates copper toxicity in kiwifruit

The widespread application of copper (Cu) -based fertilizers and pesticides could increase the accumulation of Cu in kiwifruit. According to a global survey, red- and yellow-fleshed kiwifruit contained more elevated amounts of Cu than green-fleshed kiwifruit due to weaker disease resistance and higher use of Cu pesticides. Intriguingly, our research revealed that external and endogenous ascorbic acid (AsA) reduced the phenotypic and physiological injury of Cu toxicity in kiwifruit. Cu stress assays and transcriptional analysis have shown that Cu treatment for 12 h significantly increased the AsA content in kiwifruit leaves and up-regulated key genes involved in AsA biosynthesis, such as GDP-L-galactose phosphorylase3 (GGP3) and GDP-mannose-3',5'-epimerase (GME). Overexpressing GGP3 in transgenic kiwifruit significantly increased the endogenous AsA content of kiwifruit, which was beneficial in mitigating Cu toxicity by decreasing levels of reactive oxygen species, malondialdehyde, and electrolyte leakage, as well as reducing damage to the chloroplast structure and photosystem II. This study presented a novel strategy to ameliorate plant Cu stress by increasing the endogenous antioxidant (AsA) content through transgenesis.

Protein-protein interactions in plant antioxidant defense

The regulation of reactive oxygen species (ROS) levels in plants is ensured by mechanisms preventing their over accumulation, and by diverse antioxidants, including enzymes and nonenzymatic compounds. These are affected by redox conditions, posttranslational modifications, transcriptional and posttranscriptional modifications, Ca²⁺, nitric oxide (NO) and mitogen-activated protein kinase signaling pathways. Recent knowledge about protein-protein interactions (PPIs) of antioxidant enzymes advanced during last decade. The best-known examples are interactions mediated by redox buffering proteins such as thioredoxins and glutaredoxins. This review summarizes interactions of major antioxidant enzymes with regulatory and signaling proteins and their diverse functions. Such interactions are important for stability, degradation and activation of interacting partners. Moreover, PPIs of antioxidant enzymes may connect diverse metabolic processes with ROS scavenging. Proteins like receptor for activated C kinase 1 may ensure coordination of antioxidant enzymes to ensure efficient ROS regulation. Nevertheless, PPIs in antioxidant defense are understudied, and intensive research is required to define their role in complex regulation of ROS scavenging.

Peroxisome-Mediated Reactive Oxygen Species Signals Modulate Programmed Cell Death in Plants

Peroxisomes are a class of simple organelles that play an important role in plant reactive oxygen species (ROS) metabolism. Experimental evidence reveals the involvement of ROS in programmed cell death (PCD) in plants. Plant PCD is crucial for the regulation of plant growth, development and environmental stress resistance. However, it is unclear whether the ROS originated from peroxisomes participated in cellular PCD. Enzymes involved in the peroxisomal ROS metabolic pathways are key mediators to figure out the relationship between peroxisome-derived ROS and PCD. Here, we summarize the peroxisomal ROS generation and scavenging pathways and explain how peroxisome-derived ROS participate in PCD based on recent progress in the functional study of enzymes related to peroxisomal ROS generation or scavenging. We aimed to elucidate the role of the peroxisomal ROS regulatory system in cellular PCD to show its potential in terms of accurate PCD regulation, which contribute to environmental stress resistance.

MSD2-mediated ROS metabolism fine-tunes the timing of floral organ abscission in Arabidopsis

The timely removal of end-of-purpose flowering organs is as essential for reproduction and plant survival as timely flowering. Despite much progress in understanding the molecular mechanisms of floral organ abscission, little is known about how various environmental factors are integrated into developmental programmes that determine the timing of abscission. Here, we investigated whether reactive oxygen species (ROS), mediators of various stress-related signalling pathways, are involved in determining the timing of abscission and, if so, how they are integrated with the developmental pathway in Arabidopsis thaliana. MSD2, encoding a secretory manganese superoxide dismutase, was preferentially expressed in the abscission zone of flowers, and floral organ abscission was accelerated by the accumulation of ROS in msd2 mutants. The expression of the genes encoding the receptor-like kinase HAESA (HAE) and its cognate peptide ligand INFLORESCENCE DEFICIENT IN ABSCISSION (IDA), the key signalling components of abscission, was accelerated in msd2 mutants, suggesting that MSD2 acts upstream of IDA-HAE. Further transcriptome and pharmacological analyses revealed that abscisic acid and nitric oxide facilitate abscission by regulating the expression of IDA and HAE during MSD2-mediated signalling. These results suggest that MSD2-dependent ROS metabolism is an important regulatory point integrating environmental stimuli into the developmental programme leading to abscission.

Glycine-serine-rich effector PstGSRE4 in Puccinia striiformis f. sp. tritici inhibits the activity of copper zinc superoxide dismutase to modulate immunity in wheat

Puccinia striiformis f. sp. tritici (Pst) secretes an array of specific effector proteins to manipulate host immunity and promote pathogen colonization. In a previous study, we functionally characterized a glycine-serine-rich effector PstGSRE1 with a glycine-serine-rich motif (m9). However, the mechanisms of glycine-serine-rich effectors (GSREs) remain obscure. Here we report a new glycine-serine-rich effector, PstGSRE4, which has no m9-like motif but inhibits the enzyme activity of wheat copper zinc superoxide dismutase TaCZSOD2, which acts as a positive regulator of wheat resistance to Pst. By inhibiting the enzyme activity of TaCZSOD2, PstGSRE4 reduces H₂O₂ accumulation and HR areas to facilitate Pst infection. These findings provide new insights into the molecular mechanisms of GSREs of rust fungi in regulating plant immunity.

Pst secretes numerous effectors to modulate host defense systems. However, the mechanisms of these effectors, especially for glycine-rich or serine-rich effectors, remain obscure. In this study, we identified a new glycine-serine-rich effector, PstGSRE4, which exhibits unusual biochemical properties and is highly induced during early stages of infection. Transgenic expression of PstGSRE4-RNAi constructs in wheat significantly reduced virulence of Pst and increased H₂O₂ accumulation in wheat. Overexpression of PstGSRE4 in wheat significantly increased virulence of Pst and reduced H₂O₂ accumulation in wheat. PstGSRE4 was shown to target the ROS-associated regulatory factor TaCZSOD2, which was proved as a positive regulator of wheat immunity in this study. Further study revealed that PstGSRE4 inhibited the enzyme activity of TaCZSOD2 and thus compromises the host immune systems. This work reveals a novel strategy that rust fungi exploit to modulate host defense and facilitate pathogen infection.

MSD2, an apoplastic Mn-SOD, contributes to root skotomorphogenic growth by modulating ROS distribution in Arabidopsis

Reactive oxygen species (ROS) play essential roles as a second messenger in various physiological processes in plants. Due to their oxidative nature, ROS can also be harmful. Thus, the generation and homeostasis of ROS are tightly controlled by multiple enzymes. Membrane-localized NADPH oxidases are well known to generate ROS during developmental and stress responses, but the metabolic pathways of the superoxide (O2^-) generated by them in the apoplast are poorly understood, and the identity of the apoplastic superoxide dismutase (SOD) is unknown in Arabidopsis. Here, we show that a putative manganese SOD, MSD2 is secreted and possesses a SOD activity that can be inhibited by nitration at tyrosine 68. The expression of MSD2 in roots is light condition-dependent, suggesting that MSD2 may act on ROS metabolism in roots during the light-to-dark transition. Root architecture is governed by ROS distribution that exhibits opposite gradient of H₂O₂ and O2^-, which is indeed altered in etiolated msd2 mutants and accompanied by changes in the onset of differentiation. These results provide a missing link in our understanding of ROS metabolism and suggest that MSD2 plays a role in root skotomorphogenesis by regulating ROS distribution, thereby playing a pivotal role in plant growth and development.

Plasma membrane-localized protein BcHIPP16 promotes the uptake of copper and cadmium in planta

Cadmium (Cd) is one of the toxic heavy metals in soil, which not only suppresses crop production but also threatens human health. In this study, we aim to clarify the biological function of Cd-related gene BcHIPP16, so as to provide potential genetic solutions to decrease the Cd levels of pak choi. Tissue expression analysis showed that BcHIPP16 expressed in almost all the plant bodies. The transcriptional level of BcHIPP16 in roots was higher than that in shoots, which was significantly induced by copper (Cu) deficiency and Cd exposure conditions. Subcellular localization revealed that BcHIPP16 localized in plasma membrane. Expressing BcHIPP16 in yeast cells improved the sensitivity to Cu and Cd and improved their accumulation in yeast. Furthermore, the Cu and Cd content of Arabidopsis seedlings were increased and complemented, respectively when expressing BcHIPP16 in wild type (WT) and hip16 mutants. Non-invasive Micro-test Technology (NMT) was used to measure the real-time Cd²⁺ influx from the root surface of BcHIPP16 transgenic Arabidopsis lines, and the result demonstrated that BcHIPP16 promoted Cd²⁺ influx into Arabidopsis root cells. Taken together, our study showed that BcHIPP16 contributed to absorbing nutrient metal Cu and heavy metal Cd in planta.

Plant Copper Metalloenzymes As Prospects for New Metabolism Involving Aromatic Compounds

Copper is an important transition metal cofactor in plant metabolism, which enables diverse biocatalysis in aerobic environments. Multiple classes of plant metalloenzymes evolved and underwent genetic expansions during the evolution of terrestrial plants and, to date, several representatives of these copper enzyme classes have characterized mechanisms. In this review, we give an updated overview of chemistry, structure, mechanism, function and phylogenetic distribution of plant copper metalloenzymes with an emphasis on biosynthesis of aromatic compounds such as phenylpropanoids (lignin, lignan, flavonoids) and cyclic peptides with macrocyclizations via aromatic amino acids. We also review a recent addition to plant copper enzymology in a copper-dependent peptide cyclase called the BURP domain. Given growing plant genetic resources, a large pool of copper biocatalysts remains to be characterized from plants as plant genomes contain on average more than 70 copper enzyme genes. A major challenge in characterization of copper biocatalysts from plant genomes is the identification of endogenous substrates and catalyzed reactions. We highlight some recent and future trends in filling these knowledge gaps in plant metabolism and the potential for genomic discovery of copper-based enzymology from plants.

Adaptive Responses of Citrus grandis Leaves to Copper Toxicity Revealed by RNA-Seq and Physiology

Copper (Cu)-toxic effects on Citrus grandis growth and Cu uptake, as well as gene expression and physiological parameters in leaves were investigated. Using RNA-Seq, 715 upregulated and 573 downregulated genes were identified in leaves of C. grandis seedlings exposed to Cu-toxicity (LCGSEC). Cu-toxicity altered the expression of 52 genes related to cell wall metabolism, thus impairing cell wall metabolism and lowering leaf growth. Cu-toxicity downregulated the expression of photosynthetic electron transport-related genes, thus reducing CO₂ assimilation. Some genes involved in thermal energy dissipation, photorespiration, reactive oxygen species scavenging and cell redox homeostasis and some antioxidants (reduced glutathione, phytochelatins, metallothioneins, l-tryptophan and total phenolics) were upregulated in LCGSEC, but they could not protect LCGSEC from oxidative damage. Several adaptive responses might occur in LCGSEC. LCGSEC displayed both enhanced capacities to maintain homeostasis of Cu via reducing Cu uptake by leaves and preventing release of vacuolar Cu into the cytoplasm, and to improve internal detoxification of Cu by accumulating Cu chelators (lignin, reduced glutathione, phytochelatins, metallothioneins, l-tryptophan and total phenolics). The capacities to maintain both energy homeostasis and Ca homeostasis might be upregulated in LCGSEC. Cu-toxicity increased abscisates (auxins) level, thus stimulating stomatal closure and lowering water loss (enhancing water use efficiency and photosynthesis).

Reactive oxygen species and organellar signaling

The evolution of photosynthesis and its associated metabolic pathways has been crucial to the successful establishment of plants, but has also challenged plant cells in the form of production of reactive oxygen species (ROS). Intriguingly, multiple forms of ROS are generated in virtually every plant cell compartment through diverse pathways. As a result, a sophisticated network of ROS detoxification and signaling that is simultaneously tailored to individual organelles and safeguards the entire cell is necessary. Here we take an organelle-centric view on the principal sources and sinks of ROS across the plant cell and provide insights into the ROS-induced organelle to nucleus retrograde signaling pathways needed for operational readjustments during environmental stresses.

Ectopic expression of GmHP08 enhances resistance of transgenic Arabidopsis toward drought stress

Ectopic expression of Glycine max two-component system member GmHP08 in Arabidopsis enhanced drought tolerance of transgenic plants, possibly via ABA-dependent pathways. Phosphorelay by two-component system (TCS) is a signal transduction mechanism which has been evolutionarily conserved in both prokaryotic and eukaryotic organisms. Previous studies have provided lines of evidence on the involvement of TCS genes in plant perception and responses to environmental stimuli. In this research, drought-associated functions of GmHP08, a TCS member from soybean (Glycine max L.), were investigated via its ectopic expression in Arabidopsis system. Results from the drought survival assay showed that GmHP08-transgenic plants exhibited higher survival rates compared with their wild-type (WT) counterparts, indicating better drought resistance of the former group. Analyses revealed that the transgenic plants outperformed the WT in various regards, i.e. capability of water retention, prevention of hydrogen peroxide accumulation and enhancement of antioxidant enzymatic activities under water-deficit conditions. Additionally, the expression of stress-marker genes, especially antioxidant enzyme-encoding genes, in the transgenic plants were found greater than that of the WT plants. In contrary, the expression of SAG13 gene, one of the senescence-associated genes, and of several abscisic acid (ABA)-related genes was repressed. Data from this study also revealed that the ectopic expression lines at germination and early seedling development stages were hypersensitive to exogenous ABA treatment. Taken together, our results demonstrated that GmHP08 could play an important role in mediating plant response to drought, possibly via an ABA-dependent manner.

The copper economy response is partially conserved in rice (Oryza sativa L.)

Copper (Cu) is an essential element for plants, especially in photosynthesis, as it is required for plastocyanin function in electron transfer reactions at thylakoid membranes. In Arabidopsis thaliana, Cu deficiency leads to the Cu economy response, in which plants prioritize Cu usage by plastocyanin in detriment of non-essential cupric proteins. In rice (Oryza sativa), however, this response has not been characterized. Rice OsHMA5 is a Cu xylem-loading transporter involved in Cu translocation from roots to shoots, as suggested by the analysis of oshma5 mutant plants. Aiming to understand how rice plants respond to Cu deficiency and how decreased Cu translocation to shoots can affect this response, we characterized the physiological and molecular responses of WT and oshma5 plants under control and Cu deficiency treatments. We found evidence that shoots of oshma5 plants are more prone to Cu deficiency compared to shoots of WT plants, as demonstrated by decreased chlorophyll and Cu concentrations, and electron transport rate. Gene expression analysis revealed that Cu high-affinity transporters OsCOPT1 and OsCOPT5, along with a set of miRNAs and three Cu/Zn superoxide dismutases are responsive to Cu deficiency in both WT and oshma5 plants, suggesting their involvement in the Cu economy response. However, Fe superoxide dismutase was not up-regulated in rice, indicating a difference compared to the A. thaliana Cu economy model. Therefore, we provide evidence for a partially conserved Cu economy response in rice, in comparison to A. thaliana.

Mitigation of Copper Stress in Maize by Inoculation with Paenibacillus polymyxa and Bacillus circulans

Copper (Cu) is a micronutrient that assumes a principal role in plant growth and development. However, its excess concentration in soil is imperiling crop productivity. Inoculation with different bacterial strains in cereals could modify growth traits, photosynthetic effectiveness, and generation of strong antioxidant defense systems to make them more tolerant of Cu stress. Therefore, a pot study was designed to test plant growth-promoting rhizobacteria (PGPR) including Paenibacillus polymyxa and Bacillus circulans to Cu exposed maize (Zea mays L.) plants. Increasing Cu (100 to 500 µM of CuSO₄) concentration decreased growth traits, photosynthetic pigments, soluble sugars, phosphorous (P) and potassium (K) contents, and the activity of catalase (CAT) but increased proline and malondialdehyde (MDA) content, the activity of peroxidase (POD) and Cu ions at root and shoot level. Moreover, the bacterial treatment also modulated the antioxidant capability in stress-free plants. Nevertheless, inoculation with P. polymyxa and B. circulans alleviated Cu-induced growth, photosynthetic pigments and mineral nutrient (P and K) on one hand and regulating the pools of osmolytes and antioxidant enzymes, whilst simultaneously reducing MDA and Cu root and shoot contents. These improved activities of antioxidant enzymes and the regulation of osmolytes content elicited by the blend of bacterial inoculation would have retained the ability of maize plants to confer resilience to Cu stress. This study further affirms that the application of two specific bacterial strains to maize plants proved very effective to ameliorate the Cu toxicity.

Effects of ozone treatment on SOD activity and genes in postharvest cantaloupe

Ozone has been shown to play a positive role in the storage and preservation of agricultural products. However, there is little research on the cantaloupe preservation mechanism of ozone treatment (OT), especially the effect on superoxide dismutase (SOD) and the mechanism of scavenging superoxide anion In this study, xizhoumi 25 was used as a typical cantaloupe material to detect content, hydrogen peroxide (H₂O₂) and SOD enzyme activity in the pericarp and pulp, respectively, and transcriptomics and qRT-RCR were used for cantaloupe SOD family gene expression. The results showed that the rate of and H₂O₂ content were inhibited and SOD activity was higher in the treatment group compared with the control (CK) group in the pericarp and pulp; SOD was more active in the pericarp and was higher than that in the pulp. The transcription level of Cu/Zn-SOD, identified as the most abundant component of the cantaloupe SOD gene family, was promoted in the OT group, especially the key gene Cu/Zn-SOD-1. The expression level of the Fe-SOD gene was promoted in the pericarp but regulated in the pulp, while the expression of the Mn-SOD gene was down-regulated in the OT group in both pericarp and pulp. In addition, the results of qRT-PCR were consistent with the transcriptome results. Correlation analysis showed that OT not only enhanced the positive correlation between and H₂O₂ in the whole cantaloupe and the negative correlation between and SOD activity in the pericarp but also altered the correlation between SOD genes and The mechanism of regulation in postharvest cantaloupe treated with ozone may be through stimulating the SOD activity and altering the expression of related genes in the pericarp and pulp.

Ozone has been shown to play a positive role in the storage and preservation of agricultural products.

Heterologous Expression of a Soybean Gene RR34 Conferred Improved Drought Resistance of Transgenic Arabidopsis

Two-component systems (TCSs) have been identified as participants in mediating plant response to water deficit. Nevertheless, insights of their contribution to plant drought responses and associated regulatory mechanisms remain limited. Herein, a soybean response regulator (RR) gene RR34, which is the potential drought-responsive downstream member of a TCS, was ectopically expressed in the model plant Arabidopsis for the analysis of its biological roles in drought stress response. Results from the survival test revealed outstanding recovery ratios of 52%-53% in the examined transgenic lines compared with 28% of the wild-type plants. Additionally, remarkedly lower water loss rates in detached leaves as well as enhanced antioxidant enzyme activities of catalase and superoxide dismutase were observed in the transgenic group. Further transcriptional analysis of a subset of drought-responsive genes demonstrated higher expression in GmRR34-transgenic plants upon exposure to drought, including abscisic acid (ABA)-related genes NCED3, OST1, ABI5, and RAB18. These ectopic expression lines also displayed hypersensitivity to ABA treatment at germination and post-germination stages. Collectively, these findings indicated the ABA-associated mode of action of GmRR34 in conferring better plant performance under the adverse drought conditions.

Overexpression of an AP2/ERF family gene, BpERF13, in birch enhances cold tolerance through upregulating CBF genes and mitigating reactive oxygen species

The AP2/ERF (APETALA2/ethylene-responsive factor) family of transcription factors (TF) is involved in regulating biotic and abiotic stress responses in plants. To explore the role of AP2/ERFs in cold tolerance in woody plants, BpERF13 was cloned and characterized in Betula platyphylla (white birch), a species primarily found in Asia in temperate and boreal climates. Based on phylogenetic analysis, BpERF13 is a member of the IXb subfamily of ERFs. Using qRT-PCR, we found that BpERF13 was differentially expressed in different tissues, and its expression could be induced by cold treatment (4 °C). BpERF13 protein, fused with GFP, was exclusively localized to nuclei. To further assess the role of BpERF13 in cold tolerance, BpERF13 overexpression (OE) transgenic lines were generated in B. platyphylla and used for cold stress treatment and biochemical/physiological studies. BpERF13 overexpression lines had significantly increased tolerance to subfreezing treatment and reduced reactive oxygen species. Using a TF-centered yeast one-hybrid (Y1H) experimental system, we showed that BpERF13 could bind to LTRECOREATCOR15 and MYBCORE cis-elements to activate a reporter gene. ChIP-seq and ChIP-PCR experiments further demonstrated that BpERF13 bound to these cis-elements when present in the 5' proximal regions of superoxide dismutase (SOD), peroxidase (POD), and C-repeat-binding factor (CBF) genes. qRT-PCR was employed to examine the expression levels of these genes in response to cold stress; SOD, POD, and CBF genes were significantly upregulated in BpERF13 transgenic lines compared to wild-type plants in response to cold stress. These results indicate that the transcription factor BpERF13 regulates physiological processes underlying cold tolerance in woody plants.

Hydrogen peroxide modulate photosynthesis and antioxidant systems in tomato (Solanum lycopersicum L.) plants under copper stress

Plant growth and development could be modulated by minute concentrations of hydrogen peroxide (H₂O₂) which serves as a signaling molecule for various processes. The present work was conducted with an aim that H₂O₂ could also modify root morphology, morphology and movement of stomata, photosynthetic responses, activity of carbonic anhydrase, and antioxidant systems in tomato (Solanum lycopersicum L.) plants under copper stress (Cu; 10 or 100 mg kg^-1 soil). Roots of 20 d old plants were dipped in 0.1 or 0.5 mM of H₂O₂ solution for 4 h and then transplanted to the soil filled in earthen pots. High Cu stress (100 mg kg^-1 soil) altered root morphology, reduced chlorophyll content and photosynthetic capacity and also affected movement of stomata and generation of antioxidant species at 40 d after transplantation. Further, root dipping treatment of H₂O₂ to plants under stress and stress-free conditions enhanced accumulation of proline and activity of catalase, peroxidase, and superoxide dismutase, whereas production of superoxide radical (O₂^•¯) and H₂O₂ were decreased. Overall, H₂O₂ treatment improved growth, photosynthesis, metabolic state of the plants which provided tolerance and helped the plants to cope well under Cu stress.

Biotechnological Potential of LSD1, EDS1, and PAD4 in the Improvement of Crops and Industrial Plants

Lesion Simulating Disease 1 (LSD1), Enhanced Disease Susceptibility (EDS1) and Phytoalexin Deficient 4 (PAD4) were discovered a quarter century ago as regulators of programmed cell death and biotic stress responses in Arabidopsis thaliana. Recent studies have demonstrated that these proteins are also required for acclimation responses to various abiotic stresses, such as high light, UV radiation, drought and cold, and that their function is mediated through secondary messengers, such as salicylic acid (SA), reactive oxygen species (ROS), ethylene (ET) and other signaling molecules. Furthermore, LSD1, EDS1 and PAD4 were recently shown to be involved in the modification of cell walls, and the regulation of seed yield, biomass production and water use efficiency. The function of these proteins was not only demonstrated in model plants, such as Arabidopsis thaliana or Nicotiana benthamiana, but also in the woody plant Populus tremula x tremuloides. In addition, orthologs of LSD1, EDS1, and PAD4 were found in other plant species, including different crop species. In this review, we focus on specific LSD1, EDS1 and PAD4 features that make them potentially important for agricultural and industrial use.

LSD1-, EDS1- and PAD4-dependent conditional correlation among salicylic acid, hydrogen peroxide, water use efficiency and seed yield in Arabidopsis thaliana

In Arabidopsis thaliana, LESION SIMULATING DISEASE 1 (LSD1), ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1) and PHYTOALEXIN DEFICIENT 4 (PAD4) proteins are regulators of cell death (CD) in response to abiotic and biotic stresses. Hormones, such as salicylic acid (SA), and reactive oxygen species, such as hydrogen peroxide (H₂ O₂ ), are key signaling molecules involved in plant CD. The proposed mathematical models presented in this study suggest that LSD1, EDS1 and PAD4 together with SA and H₂ O₂ are involved in the control of plant water use efficiency (WUE), vegetative growth and generative development. The analysis of Arabidopsis wild-type and single mutants lsd1, eds1, and pad4, as well as double mutants eds1/lsd1 and pad4/lsd1, demonstrated the strong conditional correlation between SA/H₂ O₂ and WUE that is dependent on LSD1, EDS1 and PAD4 proteins. Moreover, we found a strong correlation between the SA/H₂ O₂ homeostasis of 4-week-old Arabidopsis leaves and a total seed yield of 9-week-old plants. Altogether, our results prove that SA and H₂ O₂ are conditionally regulated by LSD1/EDS/PAD4 to govern WUE, biomass accumulation and seed yield. Conditional correlation and the proposed models presented in this study can be used as the starting points in the creation of a plant breeding algorithm that would allow to estimate the seed yield at the initial stage of plant growth, based on WUE, SA and H₂ O₂ content.

Reactive Oxygen Species and the Redox-Regulatory Network in Cold Stress Acclimation

Cold temperatures restrict plant growth, geographical extension of plant species, and agricultural practices. This review deals with cold stress above freezing temperatures often defined as chilling stress. It focuses on the redox regulatory network of the cell under cold temperature conditions. Reactive oxygen species (ROS) function as the final electron sink in this network which consists of redox input elements, transmitters, targets, and sensors. Following an introduction to the critical network components which include nicotinamide adenine dinucleotide phosphate (NADPH)-dependent thioredoxin reductases, thioredoxins, and peroxiredoxins, typical laboratory experiments for cold stress investigations will be described. Short term transcriptome and metabolome analyses allow for dissecting the early responses of network components and complement the vast data sets dealing with changes in the antioxidant system and ROS. This review gives examples of how such information may be integrated to advance our knowledge on the response and function of the redox regulatory network in cold stress acclimation. It will be exemplarily shown that targeting the redox network might be beneficial and supportive to improve cold stress acclimation and plant yield in cold climate.

Genome-wide analysis of superoxide dismutase genes in Larix kaempferi

Superoxide dismutase is a key enzyme that scavenges superoxide anion and plays vital roles in plant antioxidant system. This study identified six SOD genes from the deciduous conifer Larix kaempferi, which is widely distributed across the cooler regions of the northern hemisphere. These six SOD genes were classified into three types: Cu/Zn-SOD (LkSOD1, 2, 3 and 4), Fe-SOD (LkSOD5) and Mn-SODs (LkSOD6). Three Cu/Zn-SOD proteins (LkSOD1, 3 and 4) were cytosolic-localized, while the other three proteins (LkSOD2, 5 and 6) were chloroplast-localized. Larix SOD proteins displayed catalytic activities toward superoxide anion, and retained >55% of its maximum enzymatic activity between 10 °C and 40 °C. Over expressions of three Larix SOD genes (LkSOD2, 4 and 6) in Arabidopsis thaliana, respectively, showed increased germination rates and root lengths during salt stress. LkSOD5 gene could rescue pale green and dwarf phenotype of Arabidopsis atfsd2-2 mutant. Taken together, this study provided comprehensive insight into the gymnosperm SOD gene family.

Biochemical and functional characterization of OsCSD3, a novel CuZn superoxide dismutase from rice

Superoxide dismutases (SODs, EC 1.15.1.1) belong to an important group of antioxidant metalloenzymes. Multiple SODs exist for scavenging of reactive oxygen species (ROS) in different cellular compartments to maintain an intricate ROS balance. The present study deals with molecular and biochemical characterization of CuZn SOD encoded by LOC_Os03g11960 (referred to as OsCSD3), which is the least studied among the four rice isozymes. The OsCSD3 showed higher similarity to peroxisomal SODs in plants. The OsCSD3 transcript was up-regulated in response to salinity, drought, and oxidative stress. Full-length cDNA encoding OsCSD3 was cloned and expressed in Escherichia coli and analyzed for spectral characteristics. UV (ultraviolet)–visible spectroscopic analysis showed evidences of d–d transitions, while circular dichroism analysis indicated high β-sheet content in the protein. The OsCSD3 existed as homodimer (∼36 kDa) with both Cu2+ and Zn2+ metal cofactors and was substantially active over a wide pH range (7.0–10.8), with optimum pH of 9.0. The enzyme was sensitive to diethyldithiocarbamate but insensitive to sodium azide, which are the characteristics features of CuZn SODs. The enzyme also exhibited bicarbonate-dependent peroxidase activity. Unlike several other known CuZn SODs, OsCSD3 showed higher tolerance to hydrogen peroxide and thermal inactivation. Heterologous overexpression of OsCSD3 enhanced tolerance of E. coli sod double-knockout (ΔsodA ΔsodB) mutant and wild-type strain against methyl viologen-induced oxidative stress, indicating the in vivo function of this enzyme. The results show that the locus LOC_Os03g11960 of rice encodes a functional CuZn SOD with biochemical characteristics similar to the peroxisomal isozymes.

Origin and roles of a novel copper-zinc superoxide dismutase (CuZnSOD) gene from the harmful dinoflagellate Prorocentrum minimum

Superoxide dismutase (SOD) acts as the first line of defence against reactive oxygen species (ROS) within cells. In this study, we characterized a novel SOD gene (PmCuZnSOD) from the dinoflagellate Prorocentrum minimum, and examined its structural features, putative origin and gene expression. The SOD cDNA is 895 bp in length, containing dinoflagellate splice-leader (dinoSL) sequence, 714-bp ORF (237 aa), and poly (A) tail. In addition, PmCuZnSOD is coded on the dinoflagellate nuclear genome without introns and in a non-tandem repeat manner; however, the encoded protein is probably localized in chloroplasts. Phylogenetic analysis indicated that it might be acquired from cyanobacteria via horizontal gene transfer (HGT) and then the gene possibly relocated from the chloroplast to the nuclear genome. Excess copper dramatically increased the PmCuZnSOD transcripts and SOD activity in cells, caused by ROS generation and decrease of photosynthetic efficiency in the treated cells. These suggest that CuZnSOD may function to defend against oxidative stress for the survival of the dinoflagellate.

The superoxide dismutase genes might be required for appropriate development of the ovule after fertilization in Xanthoceras sorbifolium

Superoxide dismutase genes were expressed differentially along with developmental stages of fertilized ovules in Xanthoceras sorbifolium, and the XsMSD gene silencing resulted in the arrest of fertilized ovule development. A very small percentage of mature fruits (ca. 5%) are produced relative to the number of bisexual flowers in Xanthoceras sorbifolium because seeds and fruits are aborted at early stages of development after pollination. Reactive oxygen species (ROS) in plants are implicated in an extensive range of biological processes, such as programmed cell death and senescence. Superoxide dismutase (SOD) activity might be required to regulate ROS homeostasis in the fertilized ovules of X. sorbifolium. The present study identified five SOD genes and one SOD copper chaperone gene in the tree. Their transcripts were differentially expressed along different stages of fertilized ovule development. These genes showed maximum expression in the ovules at 3 days after pollination (DAP), a time point in which free nuclear endosperm and nucleus tissues rapidly develop. The XsCSD1, XsFSD1 and XsMSD contained seven, eight, and five introns, respectively. Analysis of the 5'-flanking region of XsFSD1 and XsMSD revealed many cis-acting regulatory elements. Evaluation of XsMSD gene function based on virus-induced gene silencing (VIGS) indicated that the gene was closely related to early development of the fertilized ovules and fruits. This study suggested that SOD genes might be closely associated with the fate of ovule development (aborted or viable) after fertilization in X. sorbifolium.

A comprehensive proteomic analysis of elaioplasts from citrus fruits reveals insights into elaioplast biogenesis and function

Elaioplasts of citrus peel are colorless plastids which accumulate significant amounts of terpenes. However, other functions of elaioplasts have not been fully characterized to date. Here, a LC-MS/MS shotgun technology was applied to identify the proteins from elaioplasts that were highly purified from young fruit peel of kumquat. A total of 655 putative plastid proteins were identified from elaioplasts according to sequence homology in silico and manual curation. Based on functional classification via Mapman, ~50% of the identified proteins fall into six categories, including protein metabolism, transport, and lipid metabolism. Of note, elaioplasts contained ATP synthase and ADP, ATP carrier proteins at high abundance, indicating important roles for ATP generation and transport in elaioplast biogenesis. Additionally, a comparison of proteins between citrus chromoplast and elaioplast proteomes suggest a high level of functional conservation. However, some distinctive protein profiles were also observed in both types of plastids notably for isoprene biosynthesis in elaioplasts, and carotenoid metabolism in chromoplasts. In conclusion, this comprehensive proteomic study provides new insights into the major metabolic pathways and unique characteristics of elaioplasts and chromoplasts in citrus fruit.

Different roles of glutathione in copper and zinc chelation in Brassica napus roots

We investigated the specific features of copper and zinc excess action on the roots of canola (Brassica napus L.) plants. Copper rapidly accumulated in canola root cells and reached saturation during several hours of treatment, whereas the root zinc content increased relatively slowly. Excessive copper and zinc entry inside the cell resulted in significant cell damage, as evidenced by alterations in plasmalemma permeability and decreases in cellular enzymatic activity. Zinc excess specifically damaged root hair cells, which correlated with a pronounced elevation of their labile zinc level. In vitro, we showed that reduced glutathione (GSH) readily reacted with copper ions to form complexes with blocked sulfhydryl groups. In contrast, zinc ions were ineffective as glutathione blockers, and glutathione molecules did not lose their specific chemical activity in the presence of Zn²⁺ ions. The effect of copper and zinc excess on the glutathione pool in canola root cells was analysed by a combination of biochemical determination of total and oxidized glutathione contents and fluorescent staining of free reduced glutathione with monochlorobimane dye. Excess copper led to dose-dependent diminution of free reduced glutathione contents in the root cells, which could not be explained by the loss of total cellular glutathione or its oxidation. In contrast, we observed little effect of much higher intracellular zinc concentrations on the free reduced glutathione content. We concluded that GSH plays an important role in copper excess, but not zinc excess chelation, in canola root cells.

Genome-Wide Identification and Transcriptional Expression Analysis of Cucumber Superoxide Dismutase (SOD) Family in Response to Various Abiotic Stresses

Superoxide dismutase (SOD) proteins are widely present in the plant kingdom and play important roles in different biological processes. However, little is known about the SOD genes in cucumber. In this study, night SOD genes were identified from cucumber (Cucumis sativus) using bioinformatics-based methods, including 5 Cu/ZnSODs, 3 FeSODs, and 1 MnSOD. Gene structure and motif analysis indicated that most of the SOD genes have relatively conserved exon/intron arrangement and motif composition. Phylogenetic analyses with SODs from cucumber and several other species revealed that these SOD proteins can be traced back to two ancestral SODs before the divergence of monocot and dicot plants. Many cis-elements related to stress responses and plant hormones were found in the promoter sequence of each CsSOD gene. Gene expression analysis revealed that most of the CsSOD genes are expressed in almost all the tested tissues. qRT-PCR analysis of 8 selected CsSOD genes showed that these genes could respond to heat, cold, osmotic, and salt stresses. Our results provide a basis for further functional research on SOD gene family in cucumber and facilitate their potential applications in the genetic improvement of cucumber.

The Heat Stress Factor HSFA6b Connects ABA Signaling and ABA-Mediated Heat Responses

Heat stress response (HSR) is a conserved mechanism developed to increase the expression of heat shock proteins (HSPs) via a heat shock factor (HSF)-dependent mechanism. Signaling by the stress phytohormone abscisic acid (ABA) is involved in acquired thermotolerance as well. Analysis of Arabidopsis (Arabidopsis thaliana) microarray databases revealed that the expression of HSFA6b, a class A HSF, extensively increased with salinity, osmotic, and cold stresses, but not heat. Here, we show that HSFA6b plays a pivotal role in the response to ABA and in thermotolerance. Salt-inducible HSFA6b expression was down-regulated in ABA-insensitive and -deficient mutants; however, exogenous ABA application restored expression in ABA-deficient, but not -insensitive plants. Thus, ABA signaling is required for proper HSFA6b expression. A transcriptional activation assay of protoplasts revealed that ABA treatment and coexpression of an ABA signaling master effector, ABA-RESPONSIVE ELEMENT-BINDING PROTEIN1, could activate the HSFA6b promoter. In addition, HSFA6b directly bound to the promoter of DEHYDRATION-RESPONSIVE ELEMENT-BINDING PROTEIN2A and enhanced its expression. Analysis of ABA responses in seed germination, cotyledon greening, and root growth as well as salt and drought tolerance in HSFA6b-null, overexpression, and dominant negative mutants revealed that HSFA6b is a positive regulator participating in ABA-mediated salt and drought resistance. Thermoprotection tests showed that HSFA6b was required for thermotolerance acquisition. Our study reveals a network in which HSFA6b operates as a downstream regulator of the ABA-mediated stress response and is required for heat stress resistance. This new ABA-signaling pathway is integrated into the complex HSR network in planta.

Identification of Two Conserved Residues Involved in Copper Release from Chloroplast PIB-1-ATPases

Copper is an essential transition metal for living organisms. In the plant model Arabidopsis thaliana, half of the copper content is localized in the chloroplast, and as a cofactor of plastocyanin, copper is essential for photosynthesis. Within the chloroplast, copper delivery to plastocyanin involves two transporters of the PIB-1-ATPases subfamily: HMA6 at the chloroplast envelope and HMA8 in the thylakoid membranes. Both proteins are high affinity copper transporters but share distinct enzymatic properties. In the present work, the comparison of 140 sequences of PIB-1-ATPases revealed a conserved region unusually rich in histidine and cysteine residues in the TMA-L1 region of eukaryotic chloroplast copper ATPases. To evaluate the role of these residues, we mutated them in HMA6 and HMA8. Mutants of interest were selected from phenotypic tests in yeast and produced in Lactococcus lactis for further biochemical characterizations using phosphorylation assays from ATP and Pi Combining functional and structural data, we highlight the importance of the cysteine and the first histidine of the CX3HX2H motif in the process of copper release from HMA6 and HMA8 and propose a copper pathway through the membrane domain of these transporters. Finally, our work suggests a more general role of the histidine residue in the transport of copper by PIB-1-ATPases.

miR408 is involved in abiotic stress responses in Arabidopsis

MicroRNAs (miRNAs) are small RNAs that regulate the expression of target genes post-transcriptionally; they are known to play major roles in development and responses to abiotic stress. miR408 is a highly conserved miRNA in plants that responds to the availability of copper and targets genes encoding copper-containing proteins. It was recently recognized to be an important component of the HY5-SPL7 gene network that mediates a coordinated response to light and copper, illustrating its central role in the response of plants to the environment. Expression of miR408 is significantly affected by a variety of developmental and ‏environmental conditions; however, its biological function is ‏unknown. Involvement of miR408 in the abiotic stress response was investigated in Arabidopsis. Expression of miR408, as well as its target genes, was investigated in response to salinity, cold, oxidative stress, drought and osmotic stress. Analyses of transgenic plants with modulated miR408 expression revealed that higher miR408 expression leads to improved tolerance to salinity, cold and oxidative stress, but enhanced sensitivity to drought and osmotic stress. Cellular antioxidant capacity was enhanced in plants with elevated miR408 expression, as manifested by reduced levels of reactive oxygen species and induced expression of genes associated with antioxidative functions, including Cu/Zn superoxide dismutases (CSD1 and CSD2) and glutathione-S-transferase (GST-U25), as well as auxiliary genes: the copper chaperone CCS1 and the redox stress-associated gene SAP12. Overall, the results demonstrate significant involvement of miR408 in abiotic stress responses, emphasizing the central function of miR408 in plant survival.

Mitogen-activated protein kinase kinase 5 (MKK5)-mediated signalling cascade regulates expression of iron superoxide dismutase gene in Arabidopsis under salinity stress

Superoxide dismutases (SODs) are involved in plant adaptive responses to biotic and abiotic stresses but the upstream signalling process that modulates their expression is not clear. Expression of two iron SODs, FSD2 and FSD3, was significantly increased in Arabidopsis in response to NaCl treatment but blocked in transgenic MKK5-RNAi plant, mkk5. Using an assay system for transient expression in protoplasts, it was found that mitogen-activated protein kinase kinase 5 (MKK5) was also activated in response to salt stress. Overexpression of MKK5 in wild-type plants enhanced their tolerance to salt treatments, while mkk5 mutant exhibited hypersensitivity to salt stress in germination on salt-containing media. Moreover, another kinase, MPK6, was also involved in the MKK5-mediated iron superoxide dismutase (FSD) signalling pathway in salt stress. The kinase activity of MPK6 was totally turned off in mkk5, whereas the activity of MPK3 was only partially blocked. MKK5 interacted with the MEKK1 protein that was also involved in the salt-induced FSD signalling pathway. These data suggest that salt-induced FSD2 and FSD3 expressions are influenced by MEKK1 via MKK5-MPK6-coupled signalling. This MAP kinase cascade (MEKK1, MKK5, and MPK6) mediates the salt-induced expression of iron superoxide dismutases.

Differential inhibition of Arabidopsis superoxide dismutases by peroxynitrite-mediated tyrosine nitration

Despite the importance of superoxide dismutases (SODs) in the plant antioxidant defence system little is known about their regulation by post-translational modifications. Here, we investigated the in vitro effects of nitric oxide derivatives on the seven SOD isoforms of Arabidopsis thaliana. S-nitrosoglutathione, which causes S-nitrosylation of cysteine residues, did not influence SOD activities. By contrast, peroxynitrite inhibited the mitochondrial manganese SOD1 (MSD1), peroxisomal copper/zinc SOD3 (CSD3), and chloroplastic iron SOD3 (FSD3), but no other SODs. MSD1 was inhibited by up to 90% but CSD3 and FSD3 only by a maximum of 30%. Down-regulation of these SOD isoforms correlated with tyrosine (Tyr) nitration and both could be prevented by the peroxynitrite scavenger urate. Site-directed mutagenesis revealed that-amongst the 10 Tyr residues present in MSD1-Tyr63 was the main target responsible for nitration and inactivation of the enzyme. Tyr63 is located nearby the active centre at a distance of only 5.26 Å indicating that nitration could affect accessibility of the substrate binding pocket. The corresponding Tyr34 of human manganese SOD is also nitrated, suggesting that this might be an evolutionarily conserved mechanism for regulation of manganese SODs.

Copper Delivery to Chloroplast Proteins and its Regulation

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

Redox regulated peroxisome homeostasis

Peroxisomes are ubiquitous organelles present in nearly all eukaryotic cells. Conserved functions of peroxisomes encompass beta-oxidation of fatty acids and scavenging of reactive oxygen species generated from diverse peroxisomal metabolic pathways. Peroxisome content, number, and size can change quickly in response to environmental and/or developmental cues. To achieve efficient peroxisome homeostasis, peroxisome biogenesis and degradation must be orchestrated. We review the current knowledge on redox regulated peroxisome biogenesis and degradation with an emphasis on yeasts and plants.

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Conserved functions of peroxisomes include β-oxidation of fatty acids and scavenging of ROS.

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Peroxisome homeostasis is achieved by coordinating biogenesis and degradation.

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Repression of peroxisome biogenesis under oxidative stress.

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Superfluous and oxidative damaged peroxisomes are degraded by pexophagy.

Differential response of Arabidopsis leaves and roots to cadmium: glutathione-related chelating capacity vs antioxidant capacity

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

Chlorovirus PBCV-1 encodes an active copper-zinc superoxide dismutase

Superoxide dismutases (SODs) are metalloproteins that protect organisms from toxic reactive oxygen species by catalyzing the conversion of superoxide anion to hydrogen peroxide and molecular oxygen. Chlorovirus PBCV-1 encodes a 187-amino-acid protein that resembles a Cu-Zn SOD with all of the conserved amino acid residues for binding copper and zinc (named cvSOD). cvSOD has an internal Met that results in a 165-amino-acid protein (named tcvSOD). Both cvSOD and tcvSOD recombinant proteins inhibited nitroblue tetrazolium reduction of superoxide anion generated in a xanthine-xanthine oxidase system in solution. tcvSOD was chosen for further characterization because it was easier to produce. Recombinant tcvSOD also inhibited a riboflavin photochemical reduction system in a polyacrylamide gel assay, which was blocked by the Cu-Zn SOD inhibitor cyanide but not by azide, which inhibits Fe and Mn SODs. A k(cat)/K(m) value for cvSOD was determined by stop-flow spectrophotometry as 1.28 × 10(8) M(-1) s(-1), suggesting that cvSOD-catalyzed O2 (-) dismutation was not a diffusion controlled encounter. The cvsod gene was expressed as a late gene, and cvSOD activity was detected in purified virions. Superoxide accumulated rapidly during virus infection, and circumstantial evidence indicates that cvSOD aids its decomposition to benefit virus replication. Cu-Zn SOD homologs have been described to occur in 3 other families of large DNA viruses, poxviruses, baculoviruses, and mimiviruses, which group as a clade. Interestingly, cvSOD does not group in the same clade as the other virus SODs but instead groups in an expanded clade that includes Cu-Zn SODs from many cellular organisms.

IMPORTANCE

Virus infection often leads to an increase in toxic reactive oxygen species in the host, which can be detrimental to virus replication. Viruses have developed various ways to overcome this barrier. As reported in this article, the chloroviruses often encode and package a functional Cu-Zn superoxide dismutase in the virion that presumably lowers the concentration of reactive oxygen induced early during virus infection.

Function and Regulation of the Plant COPT Family of High-Affinity Copper Transport Proteins

Copper (Cu) is an essential micronutrient for all eukaryotes because it participates as a redox active cofactor in multiple biological processes, including mitochondrial respiration, photosynthesis, oxidative stress protection, and iron (Fe) transport. In eukaryotic cells, Cu transport toward the cytoplasm is mediated by the conserved CTR/COPT family of high-affinity Cu transport proteins. This outlook paper reviews the contribution of our research group to the characterization of the function played by the Arabidopsis thaliana COPT1–6 family of proteins in plant Cu homeostasis. Our studies indicate that the different tissue specificity, Cu-regulated expression, and subcellular localization dictate COPT-specialized contribution to plant Cu transport and distribution. By characterizing lack-of-function Arabidopsis mutant lines, we conclude that COPT1 mediates root Cu acquisition, COPT6 facilitates shoot Cu distribution, and COPT5 mobilizes Cu from storage organelles. Furthermore, our work with copt2 mutant and COPT-overexpressing plants has also uncovered Cu connections with Fe homeostasis and the circadian clock, respectively. Future studies on the interaction between COPT transporters and other components of the Cu homeostasis network will improve our knowledge of plant Cu acquisition, distribution, regulation, and utilization by Cu-proteins.

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

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

Regulation of the chloroplastic copper chaperone (CCS) and cuprozinc superoxide dismutase (CSD2) by alternative splicing and copper excess in Glycine max

Metal homeostasis is an important aspect of plant physiology, and the copper transport into the chloroplast and its fate after delivery is of special relevance for plants. In this work, the regulation of the chloroplastic copper chaperone for the cuprozinc superoxide dismutase (GmCCS) and its target, the cuprozinc superoxide dismutase (GmCSD2), was investigated in photosynthetic cell suspensions and entire plants from Glycine max (L.) Merr. Both genes were expressed in cell suspensions and in all plant tissues analysed, and their RNAs matured by alternative splicing with intron retention (IntronR). This mechanism generated a spliced and three non-spliced mRNAs in the case of GmCCS but only a spliced and a non-spliced mRNAs in GmCSD2. Copper excess strongly upregulated the expression of both fully spliced mRNAs but mostly unaffected the non-spliced forms. In entire plants, some tissue specificity was also observed depending on copper content status. At the protein level, the GmCCS was mostly unaffected but the GmCSD2 was strongly induced under copper excess in all subcellular fractions analysed, suggesting a post-transcriptional regulation for the former. This different protein regulation of the chaperone and its target may indicate some additional function for the CSD2 protein. In addition to its well-known superoxide dismutase (SOD) activity, it may also function as a metal sink in copper excess availability to avoid metal cell damage. Furthermore, the GmCCS seems to be present in the stroma only but the GmCSD2 was present in both stroma and thylakoids despite the general idea that the SOD enzymes are typically soluble stroma proteins. The presence of the SOD enzyme on the surface of the thylakoid membranes is reasonable considering that the superoxide radical (O2-) is preferentially formed at the acceptor side of the PSI.

Salicylic acid determines differential senescence produced by two Turnip mosaic virus strains involving reactive oxygen species and early transcriptomic changes

Losses produced by virus diseases depend mostly on symptom severity. Turnip mosaic virus (TuMV) is one of the most damaging and widespread potyvirus infecting members of the family Brassicaceae, including Arabidopsis thaliana. We used JPN1 and UK1 TuMV strains to characterize viral infections regarding symptom development, senescence progression, antioxidant response, reactive oxygen species (ROS) accumulation, and transcriptional profiling. Both isolates, despite accumulating similar viral titers, induced different symptomatology and strong differences in oxidative status. Early differences in several senescence-associated genes linked to the ORE1 and ORS1 regulatory networks as well as persistent divergence in key ROS production and scavenging systems of the plant were detected. However, at a later stage, both strains induced nutrient competition, indicating that senescence rates are influenced by different mechanisms upon viral infections. Analyses of ORE1 and ORS1 levels in infected Brassica juncea plants showed a similar pattern, suggesting a conserved differential response to both strains in Brassicaceae spp. Transcriptional analysis of the ORE1 and ORS1 regulons showed similarities between salicylic acid (SA) response and the early induction triggered by UK1, the most severe strain. By means of SA-defective NahG transgenic plants, we found that differential senescence progression and ROS accumulation between strains rely on an intact SA pathway.

Species-specific activation of Cu/Zn SOD by its CCS copper chaperone in the pathogenic yeast Candida albicans

Candida albicans is a pathogenic yeast of important public health relevance. Virulence of C. albicans requires a copper and zinc containing superoxide dismutase (SOD1), but the biology of C. albicans SOD1 is poorly understood. To this end, C. albicans SOD1 activation was examined in baker's yeast (Saccharomyces cerevisiae), a eukaryotic expression system that has proven fruitful for the study of SOD1 enzymes from invertebrates, plants, and mammals. In spite of the 80% similarity between S. cerevisiae and C. albicans SOD1 molecules, C. albicans SOD1 is not active in S. cerevisiae. The SOD1 appears incapable of productive interactions with the copper chaperone for SOD1 (CCS1) of S. cerevisiae. C. albicans SOD1 contains a proline at position 144 predicted to dictate dependence on CCS1. By mutation of this proline, C. albicans SOD1 gained activity in S. cerevisiae, and this activity was independent of CCS1. We identified a putative CCS1 gene in C. albicans and created heterozygous and homozygous gene deletions at this locus. Loss of CCS1 resulted in loss of SOD1 activity, consistent with its role as a copper chaperone. C. albicans CCS1 also restored activity to C. albicans SOD1 expressed in S. cerevisiae. C. albicans CCS1 is well adapted for activating its partner SOD1 from C. albicans, but not SOD1 from S. cerevisiae. In spite of the high degree of homology between the SOD1 and CCS1 molecules in these two fungal species, there exists a species-specific barrier in CCS-SOD interactions which may reflect the vastly different lifestyles of the pathogenic versus the noninfectious yeast.

Transcriptome profiling reveals differential transcript abundance in response to chilling stress in Populus simonii

We report global gene expression patterns of poplar in response to chilling stress. A total of 1,085 significantly differentially expressed genes, involved in photosynthesis, signal transduction, and regulation of transcription, were identified. To understand the gene network underlying the response to chilling stress in the poplar, Populus simonii, we determined the genome transcript expression profile using an Affymetrix GeneChip with 56,000 genes. Our results revealed 11,626 cold-responsive genes, with 5,267 upregulated and 6,359 downregulated. In terms of biological processes, gene ontology (GO) analysis indicated that cold-induced genes were enriched in response to temperature stimulus, reactive oxygen species, and hormone stimulus. GO terms including cellular nitrogen compound metabolic processes, photosynthesis, and generation of precursor metabolites and energy were enriched in the cold-repressed genes. The functional annotation of differentially expressed genes revealed genes involved in photosynthesis, calcium/calmodulin-mediated signal transduction, abscisic acid (ABA) homeostasis and transport, and antioxidant defense systems. Gene expression analysis showed that the majority of genes involved in photosynthesis were repressed, but the THF1 gene was induced, suggesting that it may play an important role in the production of vesicles for leaf development under low-temperature conditions. Several genes involved in calcium/calmodulin-mediated signal transduction, ABA homeostasis and transport, and antioxidant defense systems were significantly induced under chilling stress, suggesting that they may act as positive regulators in the enhanced low-temperature tolerance of poplar. Several transcription factors had divergent expression patterns, suggesting they have variable functional responses to abiotic stress. This profile of global gene expression patterns during chilling stress will be valuable for future studies on the molecular mechanisms of chilling tolerance in woody plants.

Evaluation of higher plant virus resistance genes in the green alga, Chlorella variabilis NC64A, during the early phase of infection with Paramecium bursaria chlorella virus-1

With growing industrial interest in algae plus their critical roles in aquatic systems, the need to understand the effects of algal pathogens is increasing. We examined a model algal host-virus system, Chlorella variabilis NC64A and virus, PBCV-1. C. variabilis encodes 375 homologs to genes involved in RNA silencing and in response to virus infection in higher plants. Illumina RNA-Seq data showed that 325 of these homologs were expressed in healthy and early PBCV-1 infected (≤60min) cells. For each of the RNA silencing genes to which homologs were found, mRNA transcripts were detected in healthy and infected cells. C. variabilis, like higher plants, may employ certain RNA silencing pathways to defend itself against virus infection. To our knowledge this is the first examination of RNA silencing genes in algae beyond core proteins, and the first analysis of their transcription during virus infection.

Heavy metal-associated isoprenylated plant protein (HIPP): characterization of a family of proteins exclusive to plants

Metallochaperones are key proteins for the safe transport of metallic ions inside the cell. HIPPs (heavy metal-associated isoprenylated plant proteins) are metallochaperones that contain a metal binding domain (HMA) and a C-terminal isoprenylation motif. In this study, we provide evidence that proteins of this family are found only in vascular plants and may be separated into five distinct clusters. HIPPs may be involved in (a) heavy metal homeostasis and detoxification mechanisms, especially those involved in cadmium tolerance, (b) transcriptional responses to cold and drought, and (c) plant-pathogen interactions. In particular, our results show that the rice (Oryza sativa) HIPP OsHIPP41 gene is highly expressed in response to cold and drought stresses, and its product is localized in the cytosol and the nucleus. The results suggest that HIPPs play an important role in the development of vascular plants and in plant responses to environmental changes.