Sulphate assimilation under Cd2+ stress in Physcomitrella patens--combined transcript, enzyme and metabolite profiling

Metal toxicity in Bryum coronatum Schwaegrichen: impact on chlorophyll content, lamina cell structure, and metal accumulation

This research examined the impact of heavy metals, including Cd, Pb, and Zn, on chlorophyll content and lamina cell structure in Bryum coronatum. After exposure to varying metal concentrations (0.015, 0.065, 0.250, 1, and 4 mg/L), chlorophyll content, chloroplast numbers, lamina cell change, and metal accumulation were investigated. Chlorophyll content was assessed using spectrophotometry, whereas chloroplast numbers and lamina cell changes were examined under a light microscope. Metal accumulation was quantified through ICP-MS. The findings revealed that Cd notably reduced chlorophyll a content, while Pb and Zn showed minimal influence. Cd and Pb exposure decreased the number of chloroplasts in lamina cells, with no impact from Zn. The moss's capacity to absorb metals increased with higher exposure levels, indicating its potential as a biomonitor for heavy metal pollution. Cell mortality occurred in response to Cd and Pb, primarily in the median and apical lamina regions, while Zn had no effect. This study sheds light on heavy metal toxicity in B. coronatum, underscoring its significance for environmental monitoring. Further research on the mechanisms and consequences of heavy metal toxicity in bryophytes is essential for a comprehensive understanding of this critical issue.

Non-Protein Thiol Compounds and Antioxidant Responses Involved in Bryophyte Heavy-Metal Tolerance

Heavy-metal pollution represents a problem which has been widely discussed in recent years. The biological effects of heavy metals have been studied in both animals and plants, ranging from oxidative stress to genotoxicity. Plants, above all metal-tolerant species, have evolved a wide spectrum of strategies to counteract exposure to toxic metal concentrations. Among these strategies, the chelation and vacuolar sequestration of heavy metals are, after cell-wall immobilization, the first line of defence that prevent heavy metals from interacting with cell components. Furthermore, bryophytes activate a series of antioxidant non-enzymatic and enzymatic responses to counteract the effects of heavy metal in the cellular compartments. In this review, the role of non-protein thiol compounds and antioxidant molecules in bryophytes will be discussed.

Responses to Cadmium in Early-Diverging Streptophytes (Charophytes and Bryophytes): Current Views and Potential Applications

Several transition metals are essential for plant growth and development, as they are involved in various fundamental metabolic functions. By contrast, cadmium (Cd) is a metal that can prove extremely toxic for plants and other organisms in a dose-dependent manner. Charophytes and bryophytes are early-diverging streptophytes widely employed for biomonitoring purposes, as they are able to cope with high concentrations of toxic metal(loid)s without showing any apparent heavy damage. In this review, we will deal with different mechanisms that charophytes and bryophytes have evolved to respond to Cd at a cellular level. Particular attention will be addressed to strategies involving Cd vacuolar sequestration and cell wall immobilization, focusing on specific mechanisms that help achieve detoxification. Understanding the effects of metal(loid) pollution and accumulation on the morpho-physiological traits of charophytes and bryophytes can be in fact fundamental for optimizing their use as phytomonitors and/or phytoremediators.

Response of sulfhydryl compounds in subcells of Cladophora rupestris under Pb stress

This study aimed to determine the role of sulfhydryl compounds in the subcells of C. rupestris under Pb stress. Different concentrations (0, 0.5, 1.0, 2.5, 5.0, 7.5, and 10 mg/L) and different exposure days (1, 3, 5, and 7 days) were designed to analyze the subcellular distribution of non-protein thiols (NPT), glutathione (GSH), and phytochelatins (PCs) in C. rupestris. NPT, GSH, and PCs increased significantly with increasing Pb stress in the cell wall and soluble fraction, especially NPT. NPT and GSH slowly increased, and PCs showed no significant difference in the organelle of C. rupestris at low concentrations (< 5.0 mg/L). PCs slightly increased under 5.0 mg/L of Pb stress. PCs/NPT gradually increased with Pb stress at a low Pb concentration. GSH detoxification response lagged behind those of NPT and PCs in response to time. PCs/NPT initially increased and then decreased with Pb stress duration. This study suggested that NPT, GSH, and PCs played an important role in the detoxification of the cell wall and the soluble fraction of C. rupestris under Pb stress. PCs were important in the organelle.

Functional Characterization of Class I Trehalose Biosynthesis Genes in Physcomitrella patens

The function of trehalose metabolism in plants during growth and development has been extensively studied, mostly in the eudicot Arabidopsis thaliana. So far, however, not much is known about trehalose metabolism in the moss Physcomitrella patens. Here, we show that in P. patens, two active trehalose-6-phosphate synthase enzymes exist, PpTPS1 and PpTPS2. Expression of both enzymes in Saccharomyces cerevisiae can complement the glucose-growth defect of the yeast tps1∆ mutant. Truncation of N-terminal extension in PpTPS1 and PpTPS2 resulted in higher TPS activity and high trehalose levels, upon expression in yeast. Physcomitrella knockout plants were generated and analyzed in various conditions to functionally characterize these proteins. tps1∆ and tps2∆ knockouts displayed a lower amount of caulonema filaments and were significantly reduced in size of gametophores as compared to the wild type. These phenotypes were more pronounced in the tps1∆ tps2∆ mutant. Caulonema formation is induced by factors such as high energy and auxins. Only high amounts of supplied energy were able to induce caulonema filaments in the tps1∆ tps2∆ mutant. Furthermore, this mutant was less sensitive to auxins as NAA-induced caulonema development was arrested in the tps1∆ tps2∆ mutant. In contrast, formation of caulonema filaments is repressed by cytokinins. This effect was more severe in the tps1∆ and tps1∆ tps2∆ mutants. Our results demonstrate that PpTPS1 and PpTPS2 are essential for sensing and signaling sugars and plant hormones to monitor the balance between caulonema and chloronema development.

Inhibitory effect of metals on animal and plant glutathione transferases

Glutathione transferases (GSTs) represent a widespread enzyme superfamily in eukaryotes and prokaryotes catalyzing different reactions with endogenous and xenobiotic substrates such as organic pollutants. The latter are often found together with metal contamination in the environment. Besides performing of essential functions, GSTs protect cells by conjugation of glutathione with various reactive electrophiles. The interference of toxic metals with this functionality of GSTs may have unpredictable toxicological consequences for the organisms. In this review results from the recent literature are summarized and discussed describing the ability of metals to inhibit intracellular detoxification processes in animals and plants.

Responses of soil microbial communities and their network interactions to saline-alkaline stress in Cd-contaminated soils

Land degradation by salinization and sodification changes soil function, destroys soil health, and promotes bioaccumulation of heavy metals in plants, but little is known about their fundamental mechanisms in shaping microbial communities and regulating microbial interactions. In this study, we explored the impact of saline-alkaline (SA) stress on soil bacterial and fungal community structures in different Cd-contaminated soils of Dezhou, Baoding, Xinxiang, Beijing and Shenyang cities from the North China Plain, China. Increased soil salinity and alkalinity enhanced Cd availability, indicated by significant increases in available Cd²⁺ in soil solution of 34.1%-49.7%, soil extractable Cd of 32.0-51.6% and wheat root Cd concentration of 24.5%-40.2%, as well as decreased activities of antioxidative enzymes of wheat root when compared with CK (no extra neutral or alkaline salts added). Soil bacteria were more active in response to the SA stress than fungi, as the significant structural reorganization of soil bacterial microbiota rather than fungal microbiota between SA and CK treatments was illustrated by principal component analysis. Adding neutral and alkaline salts enriched oligotrophic and haloalkaliphilic taxa in the Sphingobacteriaceae, Cellvibrionaceae, and Caulobacteraceae bacterial families, but decreased some Acidobacteria such as subgroup 6_norank, which was a sensitive biomarker that responded only to Cd contamination in CK-treated soils. Conversely, fungi were more sensitive to soil differences than bacteria: the composition of the fungal community was significantly different among different soil types. Phylogenetic molecular ecological network (pMEN) analysis further indicated that the microbial community structure and network interactions were altered to strengthen the adaptability of microorganisms to SA stress; the changes in structure and network interactions were proposed to contribute to competitive interactions. Most of the keystone genera identified in SA-treated soils, such as Blastococcus, Gemmatimonas, RB41, or Candida, had relatively low abundances (<1%), indicating their disproportionate ecological roles in triggering resistance or tolerance to SA stress and Cd toxicity.

Functional characterization of metallothionein-like genes from Physcomitrella patens: expression profiling, yeast heterologous expression, and disruption of PpMT1.2a gene

Physcomitrella patens contains four metallothionein-like genes. Three were shown to confer metal tolerance in yeast. Transcript profiling suggests their roles in senescence and reproductive development or cadmium and oxidative stress. Metallothioneins (MTs) have been suggested to play various roles including metal detoxification, nutrient remobilization, ROS scavenging, stress tolerance, and plant development. However, little is known about the forms and functions of MTs in bryophytes. The moss Physcomitrella patens genome was found to contain four MT-like genes. Amino acid sequence composition showed that the P. patens MTs (PpMTs) were clustered with Type 1 plant MTs, and could be further classified into two sub-types, herein referred to as sub-type 1: PpMT1.1a and PpMT1.1b and sub-type 2: PpMT1.2a and PpMT1.2b. Transcript abundance of PpMT1.1b and PpMT1.2b was upregulated in the gametophore compared to protonema, and all, except PpMT1.2a, were highly induced in senescing gametophytes. PpMT1.1a and PpMT1.1b transcripts were upregulated in protonema treated with cadmium and hydrogen peroxide. Unlike many higher plant MTs, the PpMT transcript abundance was not strongly induced in response to copper and zinc. These results suggest that PpMTs may play a role in protecting P. patens from cadmium and oxidative stress and may be involved in tissues senescence and reproductive development. The PpMTs, except PpMT1.2b, were also able to confer metal tolerance and accumulation when heterologously expressed in the ∆cup1 yeast. A P. patens mutant lacking PpMT1.2a through targeted gene disruption was generated. However, it did not show any alteration in growth phenotypes under senescence-induced conditions or hypersensitivity to cadmium, copper, zinc, H₂O₂, and NaCl stresses. Further characterization of additional P. patens mutants lacking single or multiple PpMTs may provide insight into the physiological roles of bryophytic MTs.

Arsenic detoxification in Eucalyptus: subcellular distribution, chemical forms, and sulfhydryl substances

The Eucalyptus cultivation acreage was large in Guangxi provinces. Guanglin 9 (Eucalyptus grandis × Eucalyptus urophylla) is a widely cultivated Eucalyptus species and has been found to grow normally in soils contaminated by heavy metals such as arsenic (As), but the detoxification mechanisms are not clear yet. Through hydroponic experiment, the adsorption and detoxification of As in Eucalyptus were studied from three aspects, namely subcellular distribution of As, chemical forms of As, and the response of sulfhydryl substances. The subcellular distribution data showed that in the Eucalyptus roots, As was mainly present in the soluble fraction (66.3-79.9%), in leaves in the soluble fraction (50.6-53.8%), and the cell wall fraction (35.6-40.0%) under As stress. The chemical form data showed that in roots, As was mainly present in ethanol extraction state (29.5-40.0%), deionized water extraction state (28.3-31.7%), and sodium chloride extraction state (24.1-33.8%). As stress can induce the formation of non-protein thiols (NPT), glutathione (GSH), and phytochelatins (PCs). With the increasing As concentration, the NPT (maximum increase 55.9%) and GSH (maximum increase 79.9%) contents in roots significantly increased, while the PC content significantly increased and then significantly decreased. It is concluded that the As detoxification mechanisms of Eucalyptus are mainly vacuolar compartmentalization and the chelation of sulfhydryl substances, while cell wall adsorption and As chemical forms have limited effects on As detoxification.

Comparative study of Cu uptake and early transcriptome responses in the green microalga Chlamydomonas reinhardtii and the macrophyte Elodea nuttallii

Microalgae are widely used as representative primary producers in ecotoxicology, while macrophytes are much less studied. Here we compared the bioavailability and cellular toxicity pathways of 2 h-exposure to 10^-6 mol L^-1 Cu in the macrophyte Elodea nuttallii and the green microalga Chlamydomonas reinhardtii. Uptake rate was similar but faster in the algae than in the macrophyte, while RNA-Sequencing revealed a similar number of regulated genes. Early-regulated genes were congruent with expected adverse outcome pathways for Cu with Gene Ontology terms including gene regulation, energy metabolism, transport, cell processes, stress, antioxidant metabolism and development. However, the gene regulation level was higher in E. nuttallii than in C. reinhardtii and several categories were more represented in the macrophyte than in the microalga. Moreover, several categories including oxidative pentose phosphate pathway (OPP), nitrate metabolism and metal handling were only found for E. nuttallii, whereas categories such as cell motility, polyamine metabolism, mitochondrial electron transport and tricarboxylic acid cycle (TCA) were unique to C. reinhardtii. These differences were attributed to morphological and metabolic differences and highlighted dissimilarities between a sessile and a mobile species. Our results highlight the efficiency of transcriptomics to assess early molecular responses in biota, and the importance of studying more aquatic plants for a better understanding on the impact and fate of environmental contaminants.

Foliar spraying of melatonin confers cadmium tolerance in Nicotiana tabacum L

Melatonin is a multifunctional signaling molecule that regulates broad aspects of responses to environmental stresses in plants. Cadmium (Cd) is a persistent soil contaminant that is toxic to all living organisms. Recent reports have uncovered the protective role of melatonin in alleviating Cd phytotoxicity, but little is known about its regulatory mechanisms in plants. In this study, we found that foliar application of melatonin (in particular 100 μmol L-1) remarkably enhanced Cd tolerance of tobacco (Nicotiana tabacum L.) leaves, as evidenced by less Cd accumulation and alleviation of growth inhibition and photoinhibition, compared with nontreated Cd-stressed plants. The addition of melatonin also controlled oxidative damage of Cd on tobacco through direct scavenging and by enhancing the activities of antioxidative enzymes. Melatonin application promoted Cd sequestration in the cell wall and vacuoles based on the analysis of subcellular distribution of Cd in tobacco cells. Structural equation modeling (SEM) analysis revealed that melatonin-induced Cd tolerance in tobacco leaves was modulated by the expression of Cd-transport genes. Molecular evidence illustrated that modulation of IRT1, Nramp1, HMA2, HMA4, and HMA3 genes caused by melatonin could be responsible for weakening Cd uptake, Cd transportation to xylem, and intensifying Cd sequestration into the root vacuoles.

Abscisic acid, cold and salt stimulate conserved metabolic regulation in the moss Physcomitrella patens

Salt and cold are major abiotic stresses that have adverse effects on plant growth and development. To cope with these stresses and their detrimental effects plants have evolved several metabolic, biochemical and physiological processes that are mainly triggered and mediated by the plant hormone abscisic acid (ABA). To elucidate the metabolic responses of the moss Physcomitrella patens, which serves as a model plant for abiotic stress adaptation, we performed GC-MS-based metabolic profiling of plants challenged for 5 and 28 h with either salt, cold or ABA. Our results indicate significant changes in the accumulation of several sugars including maltose, isomaltose and trehalose, amino acids including arginine, histidine, ornithine, tryptophan and tyrosine, and organic acids mainly citric acid and malonic acid. The metabolic responses provoked by ABA, cold and salt show considerable similarities. The accumulation of certain metabolites positively correlates with gene expression data whereas some metabolites do not show correlation with cognate transcript abundance. To place our results into an evolutionary context we compared the ABA- and stress-induced metabolic changes in moss to available metabolic profiles of the seed plant Arabidopsis thaliana. We detected considerable conservation between the species, indicating early evolution of stress-associated metabolic adaptations that probably occurred at the plant water-to-land transition.

Changes in chemical forms, subcellular distribution, and thiol compounds involved in Pb accumulation and detoxification in Athyrium wardii (Hook.)

Athyrium wardii is one of the dominant plant species flourishing on the Pb-Zn mine tailings in Sichuan Province, China. A greenhouse pot experiment was conducted to evaluate the chemical forms, subcellular distribution, and thiol compounds in A. wardii under different Pb treatments. The results showed that plants of the mining ecotype (ME) of A. wardii were more tolerant to Pb than those of the non-mining ecotype (NME) in spite of accumulation of higher Pb concentrations. The Pb concentrations in shoots and roots of the ME were 3.2∼8.6 times and 3.0∼24.6 times higher than those of the NME, respectively. The ME was more efficient in Pb uptake than the NME. Moreover, 27.8∼39.0% of the total Pb in ME was sodium chloride (NaCl) extractable and 38.0∼48.5% was acetic acid (HAc) extractable, whereas only a minority of total Pb was in ethanol and H2O extractable. In subcellular level, 77.4∼88.8% of total Pb was stored in the cell walls of ME and 9.0∼18.9% in soluble fractions. Increasing Pb concentrations enhanced sequestration of Pb into the cell walls and soluble fractions of ME tissues to protect organelles against Pb. Synthesis of non-protein thiols (NP-SH) and phytochelatins (PCs) in roots of ME significantly enhanced in response to Pb stress, and significant increases in glutathione (GSH) were observed in shoots of ME. Higher levels of NP-SH, GSH, and PCs were observed in roots of the ME comparing with NME, especially under high Pb treatments. The results indicated that Pb was localized mainly in cell wall and soluble fraction of ME plants with low biological activity by cell wall deposition and vacuolar compartmentalization, which might be the important adapted Pb detoxification mechanisms of ME.

Determination of 3-mercaptopropionic acid by HPLC: A sensitive method for environmental applications

The organic sulfur compound 3-mercaptopropionic acid (3-MPA) is an important thiol intermediate in organic sulfur metabolism in natural environments. It is generated during degradation of sulfur-containing amino acids (e.g. methionine) and from demethylation of dimethylsulfoniopropionate (DMSP). This pathway is an alternative enzymatic process in the DMSP catabolism that routes sulfur away from the climatically-active dimethyl sulfide (DMS). 3-MPA detection and subsequent quantification in different matrices is difficult due to its extreme reactivity. We therefore developed a sensitive method for determination of 3-MPA based on pre-column derivatization with monobromobimane and analysis by high-performance liquid chromatography (HPLC) with fluorescence detection. This methodology was first tested with 3-MPA standards under low (0.005-0.2μmolL(-1)) and high (1-25μmolL(-1)) concentrations. For the optimization of the reaction, CHES and, alternatively, Tris-HCl buffers were evaluated in the derivatization step, with Tris-HCl showing more effective separation of thiol derivatives and a better 3-MPA peak shape. The detection limit was 4.3nmolL(-1) with a 10μL sample injection, and mean recoveries of 3-MPA ranged from 97 to 105% in estuarine waters with different salinities (0.17 and 35.9ppt). The linearity (r>0.99) and repeatability of detector response, with intra- and inter-day precision (% CV) of 2.68-7.01% and 4.86-12.5%, respectively, confirmed the reliability of the method. Previous 3-MPA analytical methods required immediate analysis due to unstable derivatives, but in this method we achieved high stability of the derivatized samples when stored at 4°C, with only a 3-5% loss after more than one year of storage. This method was successfully applied to measure 3-MPA concentrations and rates of 3-MPA production in a variety of intertidal estuarine sediment slurries. Dissolved 3-MPA concentrations in these sediment slurries varied between 2 and 237μmolL(-1) and, 3-MPA net fluxes ranged in wet sediments between -3.6±1.7 and 30±5μmolL(-1)g(-1)h(-1). Thus, the application of this optimized methodology showed an efficient performance for measuring 3-MPA in environmental samples, with a straightforward sample derivatization and a simple analysis of stable 3-MPA derivatives.

Antioxidant response of Phragmites australis to Cu and Cd contamination

Metals are known to induce oxidative stress in plant cells. Antioxidant thiolic compounds are known to play an important role in plants׳ defence mechanisms against metal toxicity but, regarding salt marsh plants, their role is still very poorly understood. In this work, the involvement of non-protein thiols (NPT), such as cysteine (Cys), reduced glutathione (GSH), oxidised glutathione (GSSG) and total acid-soluble SH compounds (total thiols), in the tolerance mechanisms of the marsh plant Phragmites australis against Cu and Cd toxicity was assessed. Specimens of this plant, freshly harvested in an estuarine salt marsh, were exposed, for 7 days, to rhizosediment soaked with the respective elutriate contaminated with Cu (0, 10 and 100 mg/L) or Cd (0, 1, 10 mg/L). In terms of NPT production, Cu and Cd contamination induced different responses in P. australis. The content of Cys increased in plant tissue after plant exposure to Cu, whereas Cd contamination led to a decrease in GSSG levels. In general, metal contamination did not cause a significant variation on GSH levels. Both metals influenced, to some extent, the production of other thiolic compounds. Despite the accumulation of considerable amounts of Cu and Cd in belowground tissues, no visible toxicity signs were observed. So, antioxidant thiolic compounds were probably involved in the mechanisms used by P. australis to alleviate metal toxicity. As P. australis is considered suitable for phytostabilising metal-contaminated sediments, understanding its tolerance mechanisms to toxic metals is important to optimise the conditions for applying this plant in phytoremediation procedures.

The capability to synthesize phytochelatins and the presence of constitutive and functional phytochelatin synthases are ancestral (plesiomorphic) characters for basal land plants

Bryophytes, a paraphyletic group which includes liverworts, mosses, and hornworts, have been stated as land plants that under metal stress (particularly cadmium) do not synthesize metal-binding peptides such as phytochelatins. Moreover, very little information is available to date regarding phytochelatin synthesis in charophytes, postulated to be the direct ancestors of land plants, or in lycophytes, namely very basal tracheophytes. In this study, it was hypothesized that basal land plants and charophytes have the capability to produce phytochelatins and possess constitutive and functional phytochelatin synthases. To verify this hypothesis, twelve bryophyte species (six liverworts, four mosses, and two hornworts), three charophytes, and two lycophyte species were exposed to 0-36 μM cadmium for 72 h, and then assayed for: (i) glutathione and phytochelatin quali-quantitative content by HPLC and mass spectrometry; (ii) the presence of putative phytochelatin synthases by western blotting; and (iii) in vitro activity of phytochelatin synthases. Of all the species tested, ten produced phytochelatins in vivo, while the other seven did not. The presence of a constitutively expressed and functional phytochelatin synthase was demonstrated in all the bryophyte lineages and in the lycophyte Selaginella denticulata, but not in the charophytes. Hence, current knowledge according to phytochelatins have been stated as being absent in bryophytes was therefore confuted by this work. It is argued that the capability to synthesize phytochelatins, as well as the presence of active phytochelatin synthases, are ancestral (plesiomorphic) characters for basal land plants.

Evaluation of reference genes for RT qPCR analyses of structure-specific and hormone regulated gene expression in Physcomitrella patens gametophytes

The use of the moss Physcomitrella patens as a model system to study plant development and physiology is rapidly expanding. The strategic position of P. patens within the green lineage between algae and vascular plants, the high efficiency with which transgenes are incorporated by homologous recombination, advantages associated with the haploid gametophyte representing the dominant phase of the P. patens life cycle, the simple structure of protonemata, leafy shoots and rhizoids that constitute the haploid gametophyte, as well as a readily accessible high-quality genome sequence make this moss a very attractive experimental system. The investigation of the genetic and hormonal control of P. patens development heavily depends on the analysis of gene expression patterns by real time quantitative PCR (RT qPCR). This technique requires well characterized sets of reference genes, which display minimal expression level variations under all analyzed conditions, for data normalization. Sets of suitable reference genes have been described for most widely used model systems including e.g. Arabidopsis thaliana, but not for P. patens. Here, we present a RT qPCR based comparison of transcript levels of 12 selected candidate reference genes in a range of gametophytic P. patens structures at different developmental stages, and in P. patens protonemata treated with hormones or hormone transport inhibitors. Analysis of these RT qPCR data using GeNorm and NormFinder software resulted in the identification of sets of P. patens reference genes suitable for gene expression analysis under all tested conditions, and suggested that the two best reference genes are sufficient for effective data normalization under each of these conditions.

Arsenite tolerance in rice (Oryza sativa L.) involves coordinated role of metabolic pathways of thiols and amino acids

Thiolic ligands and several amino acids (AAs) are known to build up in plants against heavy metal stress. In the present study, alteration of various AAs in rice and its synchronized role with thiolic ligand was explored for arsenic (As) tolerance and detoxification. To understand the mechanism of As tolerance and stress response, rice seedlings of one tolerant (Triguna) and one sensitive (IET-4786) cultivar were exposed to arsenite (0-25 μM) for 7 days for various biochemical analyses using spectrophotometer, HPLC and ICPMS. Tolerant and sensitive cultivars respond differentially in terms of thiol metabolism, essential amino acids (EEAs) and nonessential amino acids (NEEAs) vis-á-vis As accumulation. Thiol biosynthesis-related enzymes were positively correlated to As accumulation in Triguna. Conversely, these enzymes, cysteine content and GSH/GSSG ratio declined significantly in IET-4786 upon As exposure. The level of identified phytochelatin (PC) species (PC(2), PC(3) and PC(4)) and phytochelatin synthase activity were also more pronounced in Triguna than IET-4786. Nearly all EAAs were negatively affected by As-induced oxidative stress (except phenylalanine in Triguna), but more significantly in IET-4786 than Triguna. However, most of the stress-responsive NEAAs like glutamic acid, histidine, alanine, glycine, tyrosine, cysteine and proline were enhanced more prominently in Triguna than IET-4786 upon As exposure. The study suggests that IET-4786 appears sensitive to As due to reduction of AAs and thiol metabolic pathway. However, a coordinated response of thiolic ligands and stress-responsive AAs seems to play role for As tolerance in Triguna to achieve the effective complexation of As by PCs.

Genotypic variation in sulfur assimilation and metabolism of onion (Allium cepa L.) III. Characterization of sulfite reductase

Genomic and cDNA sequences corresponding to a ferredoxin-sulfite reductase (SiR) have been cloned from bulb onion (Allium cepa L.) and the expression of the gene and activity of the enzyme characterized with respect to sulfur (S) supply. Cloning, mapping and expression studies revealed that onion has a single functional SiR gene and also expresses an unprocessed pseudogene (φ-SiR). Northern and qPCR analysis revealed differences in expression pattern between the SiR gene and the pseudogene. Western analysis using antibodies raised to a recombinant SiR revealed that the enzyme is present in chloroplasts and phylogenetic analysis has shown that the onion protein groups with lower eudicots. In hydroponically-grown plants, levels of SiR transcripts were significantly higher in the roots of S-sufficient when compared with S-deficient plants of the pungent cultivar 'W202A' but not the less pungent cultivar 'Texas Grano'. In these same treatments, a higher level of enzyme activity was observed in the S-sufficient treatment in leaves of both cultivars before and after bulbing. In a factorial field trial with and without sulfur fertilization, a statistically significant increase in SiR activity was observed in the leaves of the pungent cultivar 'Kojak' in response to added S but not in the less pungent cultivar 'Encore'.

Expression of phytochelatin synthase from aquatic macrophyte Ceratophyllum demersum L. enhances cadmium and arsenic accumulation in tobacco

Phytochelatin synthase (PCS), the key enzyme involved in heavy metal detoxification and accumulation has been used from various sources to develop transgenic plants for the purpose of phytoremediation. However, some of the earlier studies provided contradictory results. Most of the PCS genes were isolated from plants that are not potential metal accumulators. In this study, we have isolated PCS gene from Ceratophyllum demersum cv. L. (CdPCS1), a submerged rootless aquatic macrophyte, which is considered as potential accumulator of heavy metals. The CdPCS1 cDNA of 1,757 bp encodes a polypeptide of 501 amino acid residues and differs from other known PCS with respect to the presence of a number of cysteine residues known for their interaction with heavy metals. Complementation of cad1-3 mutant of Arabidopsis deficient in PC (phytochelatin) biosynthesis by CdPCS1 suggests its role in the synthesis of PCs. Transgenic tobacco plants expressing CdPCS1 showed several-fold increased PC content and precursor non-protein thiols with enhanced accumulation of cadmium (Cd) and arsenic (As) without significant decrease in plant growth. We conclude that CdPCS1 encodes functional PCS and may be part of metal detoxification mechanism of the heavy metal accumulating plant C. demersum.

KEY MESSAGE

Heterologous expression of PCS gene from C. demersum complements Arabidopsis cad1-3 mutant and leads to enhanced accumulation of Cd and As in transgenic tobacco.

The relevance of compartmentation for cysteine synthesis in phototrophic organisms

In the vascular plant Arabidopsis thaliana, synthesis of cysteine and its precursors O-acetylserine and sulfide is distributed between the cytosol, chloroplasts, and mitochondria. This compartmentation contributes to regulation of cysteine synthesis. In contrast to Arabidopsis, cysteine synthesis is exclusively restricted to chloroplasts in the unicellular green alga Chlamydomonas reinhardtii. Thus, the question arises, whether specification of compartmentation was driven by multicellularity and specified organs and tissues. The moss Physcomitrella patens colonizes land but is still characterized by a simple morphology compared to vascular plants. It was therefore used as model organism to study evolution of compartmented cysteine synthesis. The presence of O-acetylserine(thiol)lyase (OAS-TL) proteins, which catalyze the final step of cysteine synthesis, in different compartments was applied as criterion. Purification and characterization of native OAS-TL proteins demonstrated the presence of five OAS-TL protein species encoded by two genes in Physcomitrella. At least one of the gene products is dual targeted to plastids and cytosol, as shown by combination of GFP fusion localization studies, purification of chloroplasts, and identification of N termini from native proteins. The bulk of OAS-TL protein is targeted to plastids, whereas there is no evidence for a mitochondrial OAS-TL isoform and only a minor part of OAS-TL protein is localized in the cytosol. This demonstrates that subcellular diversification of cysteine synthesis is already initialized in Physcomitrella but appears to gain relevance later during evolution of vascular plants.

Degradation of glutathione S-conjugates in Physcomitrella patens is initiated by cleavage of glycine

Glutathione-dependent detoxification is a key pathway that allows plants to efficiently remove toxic compounds like heavy metals or electrophilic xenobiotics. Under persistent exposure to toxins plants need to respond to continuous demand with efficient synthesis of glutathione (GSH) and ideally fast and efficient removal of potentially toxic glutathione S-conjugates. With the aim of studying the respective degradation pathway in Physcomitrella patens we initially characterized fluorescence labeling of protonema cultures with GSH-specific xenobiotic monochlorobimane (MCB). Incubation of protonema with 200 μM MCB for 24 h resulted in a steady increase of total bimane label, which was not confined to glutathione S-bimane (GS-B), but predominantly present in γ-glutamylcysteine S-bimane (γ-EC-B) and cysteine S-bimane (Cys-B). Pulse-chase experiments identified γ-EC-B and Cys-B as degradation products of GS-B, suggesting initial cleavage of the C-terminal glycine, followed by cleavage of the γ-glutamyl bond. The amount of GS-B formed, increased linearly at 90 nmol GSH g fw⁻¹ h⁻¹ for 24 h and after ∼1.5 h already surpassed the amount of GSH present in control protonema. This demand-driven biosynthesis of GSH depends on sufficient supply of sulfate in the incubation medium.

Comparing copper resistance in two bryophytes: Mielichhoferia elongata Hornsch. versus Physcomitrella patens Hedw

The bryophyte Mielichhoferia elongata is known to occur on copper-rich substrate, but the exact resistance level remained to be determined by in vitro experiments. Here, we tested its copper tolerance in graded copper solutions and compared the results to the moss Physcomitrella patens that is not known to inhabit heavy metal sites. Our results confirm the survival of M. elongata in classical resistance experiments of up to 10 mM Cu-ethylenediaminetetraacetic acid (EDTA) solution. Interestingly, P. patens is equally resistant. Cultured on copper-enriched agar plates for over 5 weeks, P. patens survived even higher copper levels of up to 100 mM Cu-EDTA and an increment of growth was detected on all concentrations tested. Obviously, P. patens is able to withstand harmfully high levels of copper in both solution and substrate. In this short communication, we give a detailed description of the growth rates and discuss the results in comparison to other moss species and heavy metals.

Regulation of sulfate assimilation in Physcomitrella patens: mosses are different!

Sulfur is an essential nutrient, taken up as sulfate from soil, reduced and incorporated into bioorganic compounds in plant cells. The pathway of sulfate assimilation is highly regulated in a demand-driven manner in seed plants. To test the evolutionary conservation of the regulatory mechanisms, we analyzed regulation of the pathway in the model for basal plants, the moss Physcomitrella patens. While in Arabidopsis the key enzyme of sulfate assimilation, adenosine 5'-phosphosulfate reductase (APR), is feedback repressed by thiols and induced by reduced levels of glutathione, in P. patens such regulation does not occur. The control of the pathway was not moved to other components as these conditions affected neither mRNA accumulation of other genes of sulfate assimilation nor sulfate uptake. Other treatments known to regulate APR, O-acetylserine, cadmium and sulfur deficiency affected APR transcript levels, but not enzyme activity. It appears that the sulfate assimilation pathway in P. patens is much more robust than in seed plants. Thus, the regulatory networks controlling the pathway have probably evolved only later in the evolution of the seed plants after separation of the bryophytes.

Targeted knock-out of a gene encoding sulfite reductase in the moss Physcomitrella patens affects gametophytic and sporophytic development

A key step in sulfate assimilation into cysteine is the reduction of sulfite to sulfide by sulfite reductase (SiR). This enzyme is encoded by three genes in the moss Physcomitrella patens. To obtain a first insight into the roles of the individual isoforms, we deleted the gene encoding the SiR1 isoform in P. patens by homologous recombination and subsequently analysed the DeltaSiR1 mutants. While DeltaSiR1 mutants showed no obvious alteration in sulfur metabolism, their regeneration from protoplasts and their ability to produce mature spores was significantly affected, highlighting an unexpected link between moss sulfate assimilation and development, that is yet to be characterized.

Comparative biochemical and transcriptional profiling of two contrasting varieties of Brassica juncea L. in response to arsenic exposure reveals mechanisms of stress perception and tolerance

The mechanisms of perception of arsenic (As)-induced stress and ensuing tolerance in plants remain unresolved. To obtain an insight into these mechanisms, biochemical and transcriptional profiling of two contrasting genotypes of Brassica juncea was performed. After screening 14 varieties for As tolerance, one tolerant (TPM-1) and one sensitive (TM-4) variety were selected and exposed to arsenate [As(V)] and arsenite [As(III)] for 7 d and 15 d for biochemical analyses. The tolerant variety (TPM-1) demonstrated higher accumulation of As upon exposure to both 500 microM As(V) and 250 microM As(III) [49 microg g(-1) and 37 microg g(-1) dry weight (dw) after 15 d] as well as a better response of thiol metabolism as compared with the responses observed in the sensitive variety (TM-4). Transcriptional profiling of selected genes that are known to be responsive to sulphur depletion and/or metal(loid) stress was conducted in 15-d-old seedlings after 3 h and 6 h exposure to 250 microM As(III). The results showed an up-regulation of sulphate transporters and auxin and jasmonate biosynthesis pathway genes, whereas there was a down-regulation of ethylene biosynthesis and cytokinin-responsive genes in TPM-1 within 6 h of exposure to As(III). This suggested that perception of As-induced stress was presumably mediated through an integrated modulation in hormonal functioning that led to both short- and long-term adaptations to combat the stress. Such a coordinated response of hormones was not seen in the sensitive variety. In conclusion, an early perception of As-induced stress followed by coordinated responses of various pathways was responsible for As tolerance in TPM-1.

Interaction of heavy metals with the sulphur metabolism in angiosperms from an ecological point of view

The metabolism of sulphur in angiosperms is reviewed under the aspect of exposure to ecologically relevant concentrations of sulphur, heavy metals and metalloids. Because of the inconsistent use of the term 'metal tolerance', in this review the degree of tolerance to arsenic and heavy metals is divided into three categories: hypotolerance, basal tolerance and hypertolerance. The composition of nutrient solutions applied to physiological experiments let see that the well-known interactions of calcium, sulphate and zinc supply with uptake of heavy metals, especially cadmium are insufficiently considered. Expression of genes involved in reductive sulphate assimilation pathway and enzyme activities are stimulated by cadmium and partially by copper, but nearly not by other heavy metals. The synthesis of the sulphur-rich compounds glucosinolates, metallothioneins and phytochelatins is affected in a metal-specific way. Phytochelatin levels are low in all metal(loid)-hypertolerant plant species growing in the natural environment on metal(loid)-enriched soils. If laboratory experiments mimic the natural environments, especially high Zn/Cd ratios and good sulphur supply, and chemical analyses are extended to more mineral elements than the single metal(loid) under investigation, a better understanding of the impact of metal(loid)s on the sulphur metabolism can be achieved.

The role of the novel adenosine 5'-phosphosulfate reductase in regulation of sulfate assimilation of Physcomitrella patens

Sulfate assimilation provides reduced sulfur for the synthesis of the amino acids cysteine and methionine and for a range of other metabolites. The key step in control of plant sulfate assimilation is the reduction of adenosine 5'-phosphosulfate to sulfite. The enzyme catalyzing this reaction, adenosine 5'phosphosulfate reductase (APR), is found as an iron sulfur protein in plants, algae, and many bacteria. In the moss Physcomitrella patens, however, a novel isoform of the enzyme, APR-B, has recently been discovered lacking the co-factor. To assess the function of the novel APR-B we used homologous recombination to disrupt the corresponding gene in P. patens. The knock-out plants were able to grow on sulfate as a sole sulfur source and the content of low molecular weight thiols was not different from wild type plants or plants where APR was disrupted. However, when treated with low concentrations of cadmium the APR-B knockout plants were more sensitive than both wild type and APR knockouts. In wild type P. patens, the two APR isoforms were not affected by treatments that strongly regulate this enzyme in flowering plants. The data thus suggest that in P. patens APS reduction is not the major control step of sulfate assimilation.

Sulfate assimilation in basal land plants - what does genomic sequencing tell us?

Sulfate assimilation is a pathway providing reduced sulfur for the synthesis of cysteine, methionine, co-enzymes such as iron-sulfur centres, thiamine, lipoic acid, or Coenzyme A, and many secondary metabolites, e.g., glucosinolates or alliins. The pathway is relatively well understood in flowering plants, but very little information exists on sulfate assimilation in basal land plants. Since the finding of a putative 3'-phosphoadenosine 5'-phosphosulfate reductase in PHYSCOMITRELLA PATENS, an enigmatic enzyme thought to exist in fungi and some bacteria only, it has been evident that sulfur metabolism in lower plants may substantially differ from seed plant models. The genomic sequencing of two basal plant species, the Bryophyte PHYSCOMITRELLA PATENS, and the Lycophyte SELAGINELLA MOELLENDORFFII, opens up the possibility to search for differences between lower and higher plants at the genomic level. Here we describe the similarities and differences in the organisation of the sulfate assimilation pathway between basal and advanced land plants derived from genome comparisons of these two species with ARABIDOPSIS THALIANA and ORYZA SATIVA, two seed plants with sequenced genomes. We found differences in the number of genes encoding sulfate transporters, adenosine 5'-phosphosulfate reductase, and sulfite reductase between the lower and higher plants. The consequences for regulation of the pathway and evolution of sulfate assimilation in plants are discussed.