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Due to human activities, soils become more and more polluted with metals, which imposes risks for human health and wildlife welfare. As most of the metals end up in the food chain through accumulation in plants, we need to establish science‐based environmental criteria and risk management policies. To meet these necessities, a thorough understanding is required of how these metals accumulate in and affect plants. Many studies have been conducted towards this aim, but strikingly, only a few entries can be found in ecotoxicological databases, especially on Arabidopsis thaliana, which serves as a model species for plant (cell) physiology and genetic studies. As experimental conditions seem to vary considerably throughout literature, extrapolation or comparison of data is rather difficult or should be approached with caution. Furthermore, metal‐polluted soils often contain more than one metal, yet limited studies investigated the impact of metal mixtures on plants. This review aims to compile all data concerning root system architecture under Cu, Cd and Zn stress, in single or multi‐metal exposure in A. thaliana, and link it to metal‐induced responses at different biological levels. Global incorporation into an adverse outcome pathway framework is presented. This review compiles all data on root system architecture under Cu, Cd and Zn, in single or multi‐metal exposure in Arabidopsis thaliana, and links it to the responses at different biological levels. Global incorporation of the data into an adverse outcome pathway framework is presented. [ABSTRACT FROM AUTHOR]

Abstract: Tomato is an important crop worldwide. Cadmium (Cd) concentrations in fruits depend on tomato genotype. This work aimed to study the relation among Cd accumulation, tolerance mechanisms, and fruit features in two tomato cultivars with contrasting tolerance to Cd stress. Tolerant (Yoshimatsu) and sensitive (Tropic Two Orders) plants were grown in control and contaminated soils (0.04 and 3.77 mg/kg Cd, respectively) from the seedling stage to fruit production. Both cultivars were able to acclimatize to Cd exposure, probably through mechanisms associated with reductions in the magnesium status. Cadmium concentrations varied according to the following descending order: roots = leaf blades > (peduncle + sepals) > stem = fruits. However, the tolerant cultivar accumulated more Cd than did the sensitive one. Although Cd reached the fruits from the first to the fourth bunches, peduncle and sepals may act as a barrier to Cd entrance in tomato pulp and peel. The Cd‐induced changes in the fruit mineral profile varied according to plant cultivar, organ, tomato tissue, and bunch position. Moreover, plant yield was not affected by the Cd stress, which was able to improve fruit size and weight in the tolerant cultivar. In conclusion, new insights about the Cd‐induced effects on tomato development and fruit attributes were provided by growing plants in soil, which is the media generally used to cultivate this crop, rather than hydroponics. It was shown that tomato cultivars with contrasting tolerance to Cd toxicity can reach sexual maturity and produce fruits with no yield losses, despite impacts on development from long‐term Cd exposure. This study also revealed the role of floral receptacle and its related structures in limiting, even partially, Cd translocation to the fruits. Furthermore, Yoshimatsu's capacity to produce bigger and heavier fruits, in plants under Cd exposure, may probably be associated with enhanced Cd accumulation. [ABSTRACT FROM AUTHOR]

The basidiomycete Suillus luteus is an important member of the ectomycorrhizal community that thrives in heavy metal polluted soils covered with pioneer pine forests. This study aimed to identify potential heavy metal chelators in S. luteus. Two metallothionein (MT) coding genes, SlMTa and SlMTb, were identified. When heterologously expressed in yeast, both SlMTa and SlMTb can rescue the Cu sensitive mutant from Cu toxicity. In S. luteus, transcription of both SlMTa and SlMTb is induced by Cu but not Cd or Zn. Several putative Cu-sensing and metal-response elements are present in the promoter sequences. These results indicate that SlMTa and SlMTb function as Cu-thioneins. Homologs of the S. luteus MTs are present in 49 species belonging to 10 different orders of the subphylum Agaricomycotina and are remarkably conserved. The length of the proteins, number and distribution of cysteine residues indicate a novel family of fungal MTs. The ubiquitous and highly conserved features of these MTs suggest that they are important for basic cellular functions in species in the subphylum Agaricomycotina. [ABSTRACT FROM AUTHOR]

Cadmium (Cd) exposure can disturb the homeostasis of essential elements. In Arabidopsis thaliana, Cd induces a squamosa promoter binding protein-like 7 (SPL7)-dependent Cu deficiency response. We investigated how Cd induces a Cu deficiency response. The Cu deficiency response consists of the active SPL7 transcription factor binding to GTAC motifs in promoters of among others several Cu transporters, a Cu chaperone, and cupro-miRNAs to regulate Cu homeostasis. We demonstrated that the addition of supplemental Cu to Cd-exposed A. thaliana plants diminished the Cu deficiency response in roots, while it even disappeared in leaves. Exposure of plants to Cd in combination with extra Cu reduced Cd levels in both roots and leaves resulting in an improved cellular oxidative state. Furthermore, we demonstrated a role for phytochelatins (PCs) in the Cd-induced Cu deficiency response, because it was reduced in roots of cad1-3 mutant plants exposed to Cd. In conclusion, a working mechanism is provided in which it is suggested that Cd increases PC levels that can complex both Cd and Cu. This results in cellular Cu deficiency and subsequently the activation of SPL7 and hence the induction of the Cu deficiency response. [ABSTRACT FROM AUTHOR]

ABSTRACT Stem cells or undifferentiated cells can cope more easily with external stresses. To evaluate the impact of toxic compounds on stem cell dynamics in vivo, in relation to other biological responses, we use the carcinogenic element cadmium and the regenerating model organism Macrostomum lignano. Through both BrdU and anti-histone H3 immunostainings, cadmium-induced effects were investigated at different stages of the stem cell cycle. A 24-h exposure to 100 and 250 μM CdCl2 significantly decreased the number of stem cells (neoblasts) in mitosis, whereas the number of cells in the S phase remained unchanged. After this short-term exposure, the ultrastructure of the neoblasts was minimally affected in contrast to the epidermal tissues. These results were supported by gene expression data: transcripts of cdc2 and pig3 were significantly upregulated during all treatments. Both genes are involved in the cell cycle progression and are transcribed in the gonadal region, where stem cells are highly represented. Based on a substantial increase in gene expression of heat shock proteins (HSP) and their high activity in the gonadal region, we hypothesize that these proteins are key players in the protection of stem cells against external stresses. Apart from the strong HSP induction, other protective processes including cell division, apoptosis and anti-oxidative defence, were also activated. We, therefore, conclude that the protection of stem cells against external stressors may be based on the interplay between stem cell maintenance, i.e. repair and recovery through division, on one hand and apoptosis on the other hand. © 2015 Wiley Periodicals, Inc. Environ Toxicol 31: 1217-1228, 2016. [ABSTRACT FROM AUTHOR]

Although the importance of plant-associated microorganisms for plant growth and health was getting more recognition recently, the role of seed-associated microorganisms, and especially seed endophytic bacteria, still is underestimated. Nevertheless, these associations could be beneficial for germination and seedling establishment as seed endophytic bacteria are already present in these very early plant growth stages. Moreover, bacteria with beneficial characteristics can be selected by the plants and could be transferred via the seed to benefit the next generation. In this paper, the current literature concerning bacterial endophytes that have been isolated from seeds of different plant species is reviewed. Their colonization routes, localization inside seeds and mode of transmission as well as their role and fate during germination and seedling development are discussed. At the end, some examples of bacterial seed endophytes with applications as a plant growth-promoting or biocontrol agent are given. [ABSTRACT FROM AUTHOR]

Uranium (U) causes oxidative stress in Arabidopsis thaliana plants grown at pH 5.5. However, U speciation and its toxicity strongly depend on environmental parameters, for example pH. It is unknown how different U species determine U uptake and translocation within plants and how they might affect the oxidative defense mechanisms of these plants. The present study analyzed U uptake and oxidative stress-related responses in A. thaliana (Columbia ecotype) under contrasted U chemical speciation conditions. The 18-d-old seedlings were exposed for 3 d to 25 µM U in a nutrient solution of which the pH was adjusted to 4.5, 5.5, 6.5, or 7.5. Results indicate that there is a different rate of U uptake and translocation at the different pHs, with high uptake and low translocation at low pH and lower uptake but higher translocation at high pH. After U exposure, an increased glutathione reductase activity and total glutathione concentration were observed in U-exposed roots, pointing toward an important role for glutathione in the root defense system against U either by chelation or by antioxidative defense mechanisms. In leaves, antioxidative defense mechanisms were activated on U exposure, indicated by increased superoxide dismutase and catalase activity. As it seems that U toxicity is influenced by pH, it is important to consider site-specific characteristics when making U risk assessments. Environ Toxicol Chem 2013;32:2125-2133. © 2013 SETAC [ABSTRACT FROM AUTHOR]

Plants suffering from abiotic stress are commonly facing an enhanced accumulation of reactive oxygen species ( ROS) with damaging as well as signalling effects at organellar and cellular levels. The outcome of an environmental challenge highly depends on the delicate balance between ROS production and scavenging by both enzymatic and metabolic antioxidants. However, this traditional classification is in need of renewal and reform, as it is becoming increasingly clear that soluble sugars such as disaccharides, raffinose family oligosaccharides and fructans - next to their associated metabolic enzymes - are strongly related to stress-induced ROS accumulation in plants. Therefore, this review aims at extending the current concept of antioxidants functioning during abiotic stress, with special focus on the emanate role of sugars as true ROS scavengers. Examples are given based on their cellular location, as different organelles seem to exploit distinct mechanisms. Moreover, the vacuole comes into the picture as important player in the ROS signalling network of plants. Elucidating the interplay between the mechanisms controlling ROS signalling during abiotic stress will facilitate the development of strategies to enhance crop tolerance to stressful environmental conditions. [ABSTRACT FROM AUTHOR]

The hypothesis that mitogen-activated protein kinase ( MAPK) signalling is important in plant defences against metal stress has become accepted in recent years. To test the role of oxidative signal-inducible kinase ( OXI1) in metal-induced oxidative signalling, the responses of oxi1 knockout lines to environmentally realistic cadmium ( Cd) and copper ( Cu) concentrations were compared with those of wild-type plants. A relationship between OXI1 and the activation of lipoxygenases and other initiators of oxylipin production was observed under these stress conditions, suggesting that lipoxygenase-1 may be a downstream component of OXI1 signalling. Metal-specific differences in OXI1 action were observed. For example, OXI1 was required for the up-regulation of antioxidative defences such as catalase in leaves and Fe-superoxide dismutase in roots, following exposure to Cu, processes that may involve the MEKK1- MKK2- WRKY25 cascade. Moreover, the induction of Cu/Zn superoxide dismutases in Cu-exposed leaves was regulated by OXI1 in a manner that involves fluctuations in the expression of miRNA398. These observations contrast markedly with the responses to Cd exposure, which also involves OXI1-independent pathways but rather involves changes in components mediating intracellular communication. [ABSTRACT FROM AUTHOR]

We investigated the effect of cadmium (Cd) on leaves of 3-week-old Arabidopsis thaliana with a particular interest on glutathione (GSH) production and consumption, and antioxidative defence. Plants were exposed to either 1 or 10 μM Cd for 1 week. Several genes and correspondent proteins known to participate in the regulation of redox states in plants were analysed in Cd-stressed and unstressed plants. Cd induced a significant increase in the messenger RNA level of genes involved in GSH synthesis (gsh1 and gsh2) and phytochelatin synthase (pcs1). We observed a significant decrease of reduced GSH in Cd-treated plants. In parallel, an increase of phytochelatin (PC), predominantly PC2 was observed. Cd treatment increased the accumulation of glutathione disulphide (GSSG), keeping the GSH/GSSG ratio lower than in control plants. The accumulation of GSSG was accompanied by a decrease of the glutathione reductase (gr) transcript level, while the activities of GR and nicotinamide nucleotide phosphate-reducing enzymes were significantly enhanced. The antioxidative defence mechanism related to the ascorbate (AsA)–GSH cycle was studied in parallel. A general increase of reactive oxygen species scavenging enzymes such as ascorbate peroxidase, catalase or superoxide dismutase was observed. Our data suggest that the plants respond to Cd stress by the AsA–GSH defence network at both transcriptional and enzymatic level. Taken together, it appears that Arabidopsis plants exposed to 1 μM Cd were able to adopt a new metabolic equilibrium, allowing them to cope with this metal. However, when exposed to 10 μM Cd, loss of cellular redox homeostasis resulted in oxidative stress and toxicity. [ABSTRACT FROM AUTHOR]

• We examined the subcellular cadmium (Cd) localization in roots and leaves of wild-type Arabidopsis thaliana (ecotype Columbia) exposed to environmentally relevant Cd concentrations. • Energy-dispersive X-ray microanalysis (EDXMA) was performed on high-pressure frozen and freeze-substituted tissues. • In the root cortex, Cd was associated with phosphorus (Cd/P) in the apoplast and sulfur (Cd/S) in the symplast, suggesting phosphate and phytochelatin sequestration, respectively. In the endodermis, sequestration of Cd/S was present as fine granular deposits in the vacuole and as large granular deposits in the cytoplasm. In the central cylinder, symplastic accumulation followed a distinct pattern illustrating the importance of passage cells for the uptake of Cd. In the apoplast, a shift of Cd/S granular deposits from the middle lamella towards the plasmalemma was observed. Large amounts of precipitated Cd in the phloem suggest retranslocation from the shoot. In leaves, Cd was detected in tracheids but not in the mesophyll tissue. • Extensive symplastic and apoplastic sequestration in the root parenchyma combined with retranslocation via the phloem confirms the excluder strategy of Arabidopsis thaliana. [ABSTRACT FROM AUTHOR]

A functional cloning approach using the oxidant-sensitive yeast mutant, Δ yap1, was employed to identify plant genes involved in tolerance of oxidative stress. In this screen, we identified an Arabidopsis late embryogenesis-abundant (LEA)-like protein, AtLEA5, which increased the tolerance of Δ yap1 cells to the oxidants H2O2, diamide, menadione and tert-butyl hydroperoxide. Unlike canonical LEAs, AtLEA5 is constitutively expressed in roots and reproductive organs but not in seeds. In leaves of short-day grown plants, AtLEA5 transcripts exhibited a diurnal pattern of regulation, where transcripts were repressed in the light and abundant in the dark. Expression of AtLEA5 in leaves was induced by oxidants, ABA and dehydration. Use of abi1-1 (ABA-insensitive) and aba1-1 (ABA-deficient) Arabidopsis mutants indicated that drought induction of AtLEA5 required ABA synthesis but was independent of the ABI1 gene product. Abscisic acid and H2O2 induction of AtLEA5 was also independent of the OXI1 protein kinase. Constitutive overexpression of AtLEA5 resulted in increased root growth and shoot biomass, both in optimal conditions and under H2O2 stress. However, in comparison with wild type, photosynthesis in overexpressing plants was more susceptible to drought. These features suggest that AtLEA5 has a unique function among LEA proteins in that it plays a specific role in protection against oxidative stress involving decreased photosynthesis. This protein functions as part of a complex network of defences that contribute to robustness of plants under stress by minimizing the negative effects of oxidation. [ABSTRACT FROM AUTHOR]

Copper-imposed oxidative stress and antioxidative defence responses were investigated in the primary leaves of Phaseolus vulgaris L. plants grown on hydroponics containing 50 μM CuSO[sub 4]. Copper mainly accumulates in roots; therefore, an increase of the copper content in the leaves was only observed 48 h after the start of the copper supply. Nevertheless, an increase of the thiobarbituric acid reactive metabolites (TBArm) content, an indication of stress, occurred immediately following copper application. Because the ascorbate-glutathione pathway is considered as a major antioxidative defence mechanism, the evolution of the enzymes and the related metabolites involved in this pathway were studied in the primary leaves as a function of plant copper assimilation. The capacities of monodehydroascorbate reductase (EC 1.6.5.4), dehydroascorbate reductase (EC 1.8.5.1), and glutathione reductase (EC 1.6.4.2) were increased before elevated amounts of copper could be detected in the leaves. The early enhancement of glutathione reductase was only temporary. After copper accumulation in the leaves, a second increase of the glutathione reductase capacity and also an increase of the ascorbate peroxidase capacity (EC 1.11.1.11) were observed. These changes in enzymatic capacity modified the level of the metabolites involved. Increase of the ascorbate pool and maintenance in its reduced form was observed immediately after the start of the treatment. In the beginning of the experiment, the glutathione disulphide/reduced glutathione ratio was higher in the treated plants as compared to the controls. However, towards the end of the experiment, the total glutathione pool, as well as the reduced glutathione content, increased, resulting in a lower ratio value for the treated plants. In conlusion, copper-imposed oxidative stress, as well as the antioxidative defence response in the leaves, appears to be biphasic. An indirect preventive effect on the antioxidative defence system was observed during the first phase before the leaf copper content increased. A root‐to‐shoot signalling system appears to be involved. Direct oxidation by copper of reduced cell metabolites occurred during the second phase when the leaf copper content was enhanced. [ABSTRACT FROM AUTHOR]

The involvement of the ascorbate-glutathione cycle in the defence against Cu-induced oxidative stress was studied in the roots of Phaseolus vulgaris L. cv. Limburgse vroege. All the enzymes of this cycle [ascorbate peroxidase (APOD), EC 1.11.1.11; monodehydroascorbate reductase (MDHAR), EC 1.6.5.4; dehydroascorbate reductase (DHAR), EC 1.8.5.1; glutathione reductase (GR), EC 1.6.4.2] were increased, and the total ascorbate and glutathione pools rose after a 15 μM root Cu treatment. In the first hours after the start of the experiment, the accumulation of dehydroascorbate (DHA), formed as a result of a Cu-mediated direct oxidation of ascorbate (AA), was limited by a non-enzymatic reduction using glutathione (GSH) as the reductant. At 24 h, the enzyme capacities of both DHAR and GR were increased to maintain the redox status of the AA and GSH pools. After 72 h of Cu application, the DHAR capacity was inhibited and MDHAR was responsible for maintaining the AA pool in its reduced form. Although the GR capacity was enhanced after 72 h in the treated plants, the GSSG/GSH ratio was increased. This could be due to direct participation of GSH in the detoxification of Cu through reduction and complexation. [ABSTRACT FROM AUTHOR]

In previous studies exposing animals to Cd caused oxidative stress and kidney damage. Mostly high doses were applied, often by injection. In the present study mice were exposed up to 23 weeks to low Cd concentrations (10 and 100 mg CdCl2/l) in the drinking water. Antioxidant gene expression levels as well as glutathione, ascorbate and lipid peroxidation levels were measured. Metallothionein 1 and 2 were upregulated from 1 week of exposure on. An early induction of the Prdx2 gene suggested that peroxiredoxin might be involved in the early response as well. After 8 weeks Cd reduced antioxidant expression of Bcl2, Prdx2 and Sod1 which might indicate a toxic effect. No significant effect was seen on lipid peroxidation however, and the overall redox status remained in balance throughout the whole experiment. Levels of reduced glutathione and ascorbate and of transcription of Sod2 remained stable. This suggested that the energy maintenance in mitochondria was under control. A second response was observed after 23 weeks. Interestingly, the expression of renal NADPH oxidase 4 (Nox4) increased. Nox4 has not been studied yet in Cd nephrotoxicity. The antioxidants catalase, glutathione peroxidase 4 and heme oxygenase 1 also responded. In conclusion our study reveals a two-step oxidative stress response in the kidney. Clearly the kidney was in control of Cd-induced oxidative stress after exposure to low Cd concentrations. [ABSTRACT FROM AUTHOR]