Transcriptional up-regulation of genes involved in photosynthesis of the Zn/Cd hyperaccumulator Sedum alfredii in response to zinc and cadmium

Natural colonizers effectively restore heavy metal polluted wasteland

In India, ∼30% of total land is degraded due to pollution, salinization, and nutrient loss. Change in soil-quality at urban waste-dumping site prior and after cow-dung amendment was compared with control agriculture soil. The soil at waste-dumping site had elevated pH, EC, temperature and lowered OC and NPK concentrations when compared to control. Polymetallic pollution of Cr, Cd, Pb, and Ni beyond permissible limits was obtained. Cow-dung amendment restored soil physicochemical properties at the waste-dumping site, with increasing soil moisture, CEC and OC; however, a slight change in soil bulk-density and heavy-metal concentration post-amendment was noted. The seven natural colonizers present at the waste-dumping site accumulated more metals in roots than shoots. Datura innoxia had maximum bioaccumulation of Cr, Calotropis procera of Cd and Ni and Parthenium hysterophorus of Pb in roots. All these plants had Bioacccumulation factor (BAfroot )>1 and translocation factor (Tf) <1 for Cd and serve as its phytostabilizer except Calotropis procera which had BAfroot >1 and Tf >1 and is identified as a phytoextractor for Cd. Cow-dung amendment alone appeared to be insufficient and additionally the revegetation of natural colonizers is recommended for effective reduction in heavy metal load and improving overall soil health at wasteland. Such eco-restoration may also minimize risks to biodiversity in India.

Appropriate sulfur fertilization in contaminated soil enhanced the cadmium uptake by hyperaccumulator Sedum alfredii Hance

The biogeochemical processes of sulfur and heavy metals in the environment are closely related to each other. We investigated the influence of sulfur addition on hyperaccumulator Sedum alfredii Hance growth, cadmium (Cd) accumulation, soil Cd bioavailability, soil bacterial communities and plant transcriptome responses. The results showed that an appropriate rate of sulfur addition (1.0 or 2.5 g/kg) enhanced the growth of Sedum alfredii Hance plants as well as their accumulation of Cd. A high rate of sulfur addition (5.0 or 10.0 g/kg) causes toxicity to Sedum alfredii Hance plants. The application of an appropriate amount of sulfur to the soil increased the abundance of sulfur-oxidizing bacteria such as Sulfuriferula and Thiobacillus; acid-fast bacillus such as Alicyclobacillus; and cadmium-tolerant bacteria such as Bacillus and Rhodanobacter. This led to a decrease in pH and an increase in bioavailable Cd in the soil. RNA sequencing revealed that the addition of sulfur to soils led to the up regulation of most of the differentially expressed genes (DEGs) involved in "photosynthesis" and "photosynthesis, light reaction" in Sedum alfredii Hance leaves. Moreover, the "plant hormone signal transduction" pathway was significantly enriched with sulfur addition. Sulfur assimilation in Sedum alfredii Hance plants may promote photosynthesis and hormone synthesis, leading to Cd tolerance in these plants. Our study revealed that sulfur fertilization enhanced the efficiency of Cd phytoremediation in Sedum alfredii Hance plants.

Physiology and transcriptomic analysis revealed the mechanism of silicon promoting cadmium accumulation in Sedum alfredii Hance

Silicon (Si) effectively promote the yield of many crops, mainly due to its ability to enhance plants resistance to stress. However, how Si helps hyperaccumulators to extract Cadmium (Cd) from soil has remained unclear. In this study, Sedum alfredii Hance (S. alfredii) was used as material to study how exogenous Si affected biomass, Cd accumulation, antioxidation, cell ultrastructure, subcellular distribution and changes in gene expression after Cd exposure. The study has shown that as Si concentration increases (1, 2 mM), the shoot biomass of plants increased by 33.1%-63.6%, the Cd accumulation increased by 31.9%-96.6%, and the chlorophyll, carotenoid content, photosynthetic gas exchange parameters significantly increased. Si reduced Pro and MDA, promoted the concentrations of SOD, CAT and POD to reduce antioxidant stress damage. In addition, Si promoted GSH and PC to chelate Cd in vacuoles, repaired damaged cell ultrastructure, improved the fixation of Cd and cell wall (especially in pectin), and reduced the toxic effects of Cd. Transcriptome analysis found that genes encoding Cd detoxification, Cd absorption and transport were up-regulated by Si supplying, including photosynthetic pathways (PSB, LHCB, PSA), antioxidant defense systems (CAT, APX, CSD, RBOH), cell wall biosynthesis such as pectinesterase (PME), chelation (GST, MT, NAS, GR), Cd absorption (Nramp3, Nramp5, ZNT) and Cd transport (HMA, PCR). Our result revealed the tentative mechanism of Si promotes Cd accumulation and enhances Cd tolerance in S. alfredii, and thereby provides a solid theoretical support for the practical use of Si fertilizer in phytoextraction.

Integrated morphological, physiological and transcriptomic analyses reveal response mechanisms of rice under different cadmium exposure routes

Rice (Oryza sativa) is one of the major cereal crops and takes up cadmium (Cd) more readily than other crops. Understanding the mechanism of Cd uptake and defense in rice can help us avoid Cd in the food chain. However, studies comparing Cd uptake, toxicity, and detoxification mechanisms of leaf and root Cd exposure at the morphological, physiological, and transcriptional levels are still lacking. Therefore, experiments were conducted in this study and found that root Cd exposure resulted in more severe oxidative and photosynthetic damage, lower plant biomass, higher Cd accumulation, and transcriptional changes in rice than leaf Cd exposure. The activation of phenylpropanoids biosynthesis in both root and leaf tissues under different Cd exposure routes suggests that increased lignin is the response mechanism of rice under Cd stress. Moreover, the roots of rice are more sensitive to Cd stress and their adaptation responses are more pronounced than those of leaves. Quantitative PCR revealed that OsPOX, OsCAD, OsPAL and OsCCR play important roles in the response to Cd stress, which further emphasize the importance of lignin. Therefore, this study provides theoretical evidence for future chemical and genetic regulation of lignin biosynthesis in crop plants to reduce Cd accumulation.

Ectopic expression of SaCTP3 from the hyperaccumulator Sedum alfredii in sorghum increases Cd accumulation for phytoextraction

The Cd tolerance protein SaCTP3, which responds to Cd stress, was identified in Sedum alfredii; however, how to improve the efficiency of phytoremediation of Cd-contaminated soil using the CTP gene remains unknown. In this study, the phytoremediation potential of SaCTP3 of Sedum alfredii was identified. In the yeast Cd-sensitive strain Δycf1 overexpressing SaCTP3, the accumulation of Cd was higher than that in the Δycf1 strain overexpressing an empty vector. Transgenic sorghum plants overexpression SaCTP3 were further constructed to verify the function of SaCTP3. Compared to wild-type plants, the SaCTP3-overexpressing lines exhibited higher Cd accumulation under 500 μM Cd conditions. The average Cd content inSaCTP3-overexpressing plants is more than four times higher than that of WT plants. This was accompanied by an enhanced ability to scavenge ROS, as evidenced by the significantly increased activities of peroxidase, catalase, and superoxide dismutase in response to Cd stress. Pot experiments further demonstrated that SaCTP3 overexpression resulted in improved soil Cd scavenging and photosynthetic abilities. After 20 days of growth, the average Cd content in the soil planted with SaCTP3-overexpressing sorghum decreased by 19.4%, while the residual Cd content in the soil planted with wild-type plants was only reduced by 5.4%. This study elucidated the role of SaCTP3 from S.alfredii, highlighting its potential utility in genetically modifying sorghum for the effective phytoremediation of Cd.

Biochemical responses of freshwater microalgae Chlorella sorokiniana to combined exposure of Zn(Ⅱ) and estrone with simultaneous pollutants removal

With the development of livestock industry, contaminants such as divalent zinc ions (Zn (Ⅱ)) and estrone are often simultaneously detected in livestock wastewater. Nevertheless, the combined toxicity of these two pollutants on microalgae is still unclear. Moreover, microalgae have the potential for biosorption and bioaccumulation of heavy metals and organic compounds. Thus, this study investigated the joint effects of Zn (Ⅱ) and estrone on microalgae Chlorella sorokiniana, in terms of growth, photosynthetic activity and biomolecules, as well as pollutants removal by algae. Interestingly, a low Zn (Ⅱ) concentration promoted C. sorokiniana growth and photosynthetic activity, while the high concentration experienced inhibition. As the increase of estrone concentration, chlorophyll a content increased continuously to resist the environmental stress. Concurrently, the secretion of extracellular polysaccharides and proteins by algae increased with exposure to Zn (Ⅱ) and estrone, reducing toxicity of pollutants to microalgae. Reactive oxygen species and superoxide dismutase activity increased as the increase of pollutant concentration after 96 h cultivation, but high pollutant concentrations resulted in damage of cells, as proved by increased MDA content. Additionally, C. sorokiniana displayed remarkable removal efficiency for Zn (Ⅱ) and estrone, reaching up to 86.14% and 84.96% respectively. The study provides insights into the biochemical responses of microalgae to pollutants and highlights the potential of microalgae in pollutants removal.

Identifying cadmium and lead co-accumulation from living rice blade spectrum

Neither cadmium (Cd) nor lead (Pb) is necessary for crop growth, but they both can accumulate in soil and crop tissues, resulting in land degradation and crop reduction. Few researchers have explored how to detect Cd-Pb co-accumulation in leaves using proximal sensing techniques, especially by low-cost, easy-to-use leaf clips that capture hyperspectral reflections at suitable foliar positions. In this study, a hyperspectral imager was employed to collect images of the rice canopy from a designed greenhouse experiment that included 16 pretreatments of Cd-Pb co-accumulation, followed by spectral extractions from 3 foliar positions: the blade root, the middle of the leaf, and the leaf apex. A support vector machine with leave-one-out cross-validation was performed to diagnose the contaminative levels based on the feature wavelengths selected by an improved successive projection algorithm. Partial least squares regression was used to predict Cd-Pb concentrations in rice blades. The results indicated that diagnostic accuracies were varied using spectra of different foliar positions. The blade root and leaf apex of rice blades were the optimal foliar position for detecting Cd and Pb contamination, respectively. At the optimal foliar positions, diagnostic accuracies exceeded 0.80 for distinguishing whether the rice is subject to Cd-Pb contamination. The Cd prediction performed 'very good' with a residual prediction deviation (RPD) of 2.21, a R2 of 0.79, and a root mean square error (RMSE)of 6.14, while that of Pb was 1.62, 0.61, and 186.54. Important wavelengths were identified at 659-694 nm and 667-694 nm to detect Cd and Pb contamination. In summary, our results verified the feasibility and clarified the optimal foliar positions of rice blades to detect Cd-Pb contamination. The wavelengths selecting have the great potential in the design of future leaf clips, and the optimal foliar position can provide suggestions to improve diagnostic performances in field applications.

Phytotoxic effects of polyethylene microplastics combined with cadmium on the photosynthetic performance of maize (Zea mays L.)

Microplastics (MPs) and cadmium (Cd) has attracted increasing attention due to their combined toxicity to terrestrial vegetation. Photosynthesis which utilizes light energy to synthesize organic substances is crucial for crop production. However, the plant photosynthetic response to the joint toxicity of MPs and Cd is still unknown. Here, we studied the effects of polyethylene (PE) MPs on the photosynthetic performance of two maize cultivars Xianyu 335 (XY) and Zhengdan 958 (ZD) grown in a Cd contaminated soil. Results showed that the leaf Cd concentration in XY and ZD reached 26.1 and 31.9 μg g-1, respectively. PE-MPs did not influence the leaf Cd content, but posed direct and negative effects on photosynthesis by increasing the malondialdehyde content, reducing the chlorophyll content, inhibiting photosynthetic capacity, disrupting the PSII donor side, blocking electron transfer in different photosystems, and suppressing the oxidation and reduction states of PSI. Transcriptomic analysis revealed that the inhibitory effect of combined PE-MPs and Cd on maize photosynthesis was attributed to suppressed expression of the genes encoding PSII, PSI, F-type ATPase, cytochrome b6/f complex, and electron transport between PSII and PSI. Using WGCNA, we identified a MEturquoise module highly correlated with photosynthetic traits. Hub genes bridging carbohydrate metabolism, amino acid metabolism, lipid metabolism, and translation provided the molecular mechanisms of PE-MPs and Cd tolerance in maize plants. The comprehensive information on the phytotoxicity mechanisms of Cd stress in the presence or absence of PE-MPs on the photosynthesis of maize is helpful for cloning Cd and PE-MP resistance genes in the future.

Integrated physio-biochemical and transcriptomic analysis revealed mechanism underlying of Si-mediated alleviation to cadmium toxicity in wheat

Cadmium (Cd) contamination has resulted in serious reduction of crop yields. Silicon (Si), as a beneficial element, regulates plant growth to heavy metal toxicity mainly through reducing metal uptake and protecting plants from oxidative injury. However, the molecular mechanism underlying Si-mediated Cd toxicity in wheat has not been well understood. This study aimed to reveal the beneficial role of Si (1 mM) in alleviating Cd-induced toxicity in wheat (Triticum aestivum) seedlings. The results showed that exogenous supply of Si decreased Cd concentration by 67.45% (root) and 70.34% (shoot), and maintained ionic homeostasis through the function of important transporters, such as Lsi, ZIP, Nramp5 and HIPP. Si ameliorated Cd-induced photosynthetic performance inhibition through up-regulating photosynthesis-related genes and light harvesting-related genes. Si minimized Cd-induced oxidative stress by decreasing MDA contents by 46.62% (leaf) and 75.09% (root), and helped re-establish redox homeostasis by regulating antioxidant enzymes activities, AsA-GSH cycle and expression of relevant genes through signal transduction pathway. The results revealed molecular mechanism of Si-mediated wheat tolerance to Cd toxicity. Si fertilizer is suggested to be applied in Cd contaminated soil for food safety production as a beneficial and eco-friendly element.

Morphology, physiology, and biochemistry of zinc-stressed caraway plants

A greenhouse pot experiment was conducted to evaluate the impact of zinc supply (0, 1, and 2 mM Zn as ZnSO₄) on morpho-physiological and biochemical parameters of caraway (Carum carvi L.). Exposure to different Zn concentrations for 12 weeks compromised severely all growth parameters (plant height, number of secondary branches, diameter of primary and secondary branches, fresh and dry weight of aerial parts and roots) yield and its components (number of umbels per primary branches and secondary branches; number of umbel per plant; number of seeds per plant; and the weight of 1000 seeds). These manifestations were intimately linked with excessive accumulation of Zn in roots and leaves, alteration of the content of photosynthetic pigments, and extended lipid peroxidation. A manifest increment of proline and soluble sugar content was also observed in response to Zn application. Lipid content in seeds was dropped in Zn-treated plants and the fatty acid profiles were profoundly affected as they were enriched with saturated fatty acids at the expense of unsaturated ones. While improving their oxidative stability as revealed by the reduced values calculated oxidizability and oxidative susceptibility, Zn treatment reduced the lipid nutritional quality of caraway seeds. Moreover, Zn treatment reduced the essential oil yield and its main component carvone while it enhanced the content of its precursor limonene. It also induced alteration of terpene metabolism as revealed in the redirection of the carbon flux to the shikimate/phenylpropanoid pathway resulting in the stimulation of the production of phenolic compounds and their subsequent antioxidant activities.

Transcriptome alterations of radish shoots exposed to cadmium can be interpreted in the context of leaf senescence

Till now few transcriptome studies have described shoot responses of heavy metal (HM)-sensitive plants to excess Cd and still a unifying model of Cd action is lacking. Using RNA-seq technique, the transcriptome responses of radish (Raphanus sativus L.) leaves to Cd stress were investigated in plants raised hydroponically under control and 5.0 mg L^-1 Cd. The element was mainly accumulated in roots and led to declined biomass and photosynthetic pigments, increased H₂O₂ and lipid peroxidation, and the accumulation of sugars, protein thiols, and phytochelatins. Out of 524 differentially expressed genes (DEGs), 244 and 280 upregulated and downregulated ones were assigned to 82 and 115 GO terms, respectively. The upregulated DEGs were involved in osmotic regulation, protein metabolism, chelators, and carbohydrate metabolisms, whereas downregulated DEGs were related to photosynthesis, response to oxidative stress, glucosinolate, and secondary metabolite biosynthesis. Our transcriptome data suggest that Cd triggers ROS production and photosynthesis decline associated with increased proteolysis through ubiquitin-proteasome system (UPS)- and chloroplast-proteases and in this way brings about re-mobilization of N and C stores into amino acids and sugars. Meanwhile, declined glucosinolate metabolism in favor of chelator synthesis and upregulation of dehydrins as inferred from transcriptome analysis confers shoots some tolerance to the HM-derived ionic/osmotic imbalances. Thus, the induction of leaf senescence might be a major long-term response of HM-sensitive plants to Cd toxicity.

Different responses to joint exposure to cadmium and zinc depends on the sex in Populus cathayana

The alarming increase in soil contamination by heavy metals, such as cadmium and zinc demands immediate attention. The dioecious tree Populus cathayana, a phytoremediation plant, plays an important role in rehabilitating heavy metal contaminated areas. In this study, male and female P. cathayana plants were treated with Cd (20 mg kg^-1) and different levels of Zn (25, 50, or 100 mg kg^-1) to study their physiological responses. The results showed that Cd exposure alone caused stress by inhibiting the growth of both male and female plants. In both males and females, photosynthesis and antioxidant enzymes activities decreased substantially under Cd stress alone. Cd was largely located in the roots, but Zn was present in the shoots of both sexes. Zn supplementation considerably increased the photosynthetic rate from 14.62 % to 60.45 % and also enhanced the antioxidant enzymes activities from 24.11 % to 86.21 %. Zn treatment decreased the translocation ability of Cd compared to the Cd-only treatment, alleviating Cd toxicity. In addition, when sufficient Zn was made available, males showed a high degree of Cd accumulation, low root-to-shoot translocation, elevated antioxidant defense abilities, and an increased photosynthetic rate, while females were less responsive to Cd stress than males. Thus, combined exposure to Cd and Zn caused differential responses in plant growth and physiological processes between males and females P. cathayana. Male plants exhibit better Cd tolerance and accumulation capacity under optimum Zn supplementation. This study increases the fundamental knowledge regarding P. cathayana plants, which can be applied to enhance their remediation capacity in Cd-contaminated soils.

Responses of microbial communities and metabolic profiles to the rhizosphere of Tamarix ramosissima in soils contaminated by multiple heavy metals

Heavy metals (HMs) contamination around smelters poses serious stress to soil microbiome. However, the co-effect of multiple HMs and native vegetation rhizosphere on the soil ecosystem remains unclear. Herein, effects of high HMs level and the rhizosphere (Tamarix ramosissima) on soil bacterial community structure and metabolic profiles in sierozem were analyzed by coupling high-throughput sequencing and soil metabolomics. Plant roots alleviated the threat of HMs by absorbing and stabilizing them in soil. High HMs level decreased the richness and diversity of soil bacterial community and increased numbers of special bacteria. Plant roots changed the contribution of HMs species shaping the bacterial community. Cd and Zn were the main contributors to bacterial distribution in non-rhizosphere soil, however, Pb and Cu became the most important HMs in rhizosphere soil. HMs induced more dominant metal-tolerant bacteria in non-rhizosphere than rhizosphere soil. Meanwhile, critical metabolites varied by rhizosphere in co-occurrence networks. Moreover, the same HMs-tolerant bacteria were regulated by different metabolites, e.g. unclassified family AKYG1722 was promoted by Dodecanoic acid in non-rhizosphere soil, while promoted by Octadecane, 2-methyl- in rhizosphere soil. The study illustrated that high HMs level and rhizosphere affected soil properties and metabolites, by which soil microbial community structure was reshaped.

Transcriptome analysis reveals candidate genes involved in multiple heavy metal tolerance in hyperaccumulator Sedum alfredii

Sedum alfredii Hance is a perennial herb native to China that can particularly be found in regions with abandoned Pb/Zn mines. It is a Cd/Zn hyperaccumulator that is highly tolerant to Pb, Cu, Ni, and Mn, showing potential for phytoremediation of soils contaminated with multiple heavy metals. A better understanding of how this species responds to different heavy metals would advance the phytoremediation efficiency. In this study, transcriptomic regulation of S. alfredii roots after Cd, Zn, Pb, and Cu exposure was analyzed to explore the candidate genes involved in multi-heavy metal tolerance. Although Zn and Cd, Pb and Cu had similar distribution patterns in S. alfredii, distinct expression patterns were exhibited among these four metal treatments, especially about half of the differentially expressed genes were upregulated under Cu treatment, suggesting that it utilizes distinctive and flexible strategies to cope with specific metal stress. Most unigenes regulated by Cu were enriched in catalytic activity, whereas the majority of unigenes regulated by Pb had unknown functions, implying that S. alfredii may have a unique strategy coping with Pb stress different from previous cognition. The unigenes that were co-regulated by multiple heavy metals exhibited functions of antioxidant substances, antioxidant enzymes, transporters, transcription factors, and cell wall components. These metal-induced responses at the transcriptional level in S. alfredii were highly consistent with those at the physiological level. Some of these genes have been confirmed to be related to heavy metal absorption and detoxification, and some were found to be related to heavy metal tolerance for the first time in this study, like Metacaspase-1 and EDR6. These results provide a theoretical basis for the use of genetic engineering technology to modify plants by enhancing multi-metal tolerance to promote phytoremediation efficiency.

40S ribosomal protein S18 is a novel maternal peptidoglycan-binding protein that protects embryos of zebrafish from bacterial infections

Previous studies have shown that ribosomal proteins play important roles in ribosome assembly and protein translation, but other biological functions remain ill-defined. Here it is clearly demonstrated that RPS18 is a newly identified PGN-binding protein which is present abundantly in the eggs/embryos of zebrafish. Recombinant RPS18 not only identifies the bacterial signature molecule PGN, LPS, and LTA, and binds the bacteria as a pattern recognition receptor, but also kills the Gram-positive and Gram-negative bacteria as an antibacterial effector molecule. What is important is that, we reveal that microinjection of rRPS18 into early embryos significantly improved the resistance of the embryos against pathogenic Aeromonas hydrophila challenge, and co-injection of anti-RPS18 antibody could markedly reduced this improved bacterial resistance. In summary, these results indicate that RPS18 is a maternal immune factor that can protect the early embryos of zebrafish against pathogenic attacks. This work also provides another angle for understanding the biological functions of ribosomal proteins.

Exposure of cerium oxide nanoparticles to the hyperaccumulator Sedum alfredii decreases the uptake of cadmium via the apoplastic pathway

Cadmium (Cd) is harmful to the environment and threatens human health. With the increasing use of cerium oxide nanoparticles (CeO₂NPs) in extensive industries, investigating the combination of CeO₂NPs and plants has attracted research interests for phytoremediation. Here, we explored the effects of CeO₂NPs on Cd uptake, transport and the consequent Cd accumulation in Sedum alfredii. Exposure of 50 or 500 mg L^-1 CeO₂NPs alone had no apparent damaging effects on plant growth. However, upon Cd condition, the consistent CeO₂NPs decreased Cd concentrations in the roots and shoots by up to 37%. Furthermore, the application of a metabolic inhibitor revealed that CeO₂NPs mainly decreased the Cd uptake in roots by the apoplastic pathway. Simultaneously, CeO₂NPs accelerated the development of Casparian strips (CSs) and suberin, which was further proven by the elevated expression levels of genes associated with their formation, SaCASP, SaGPAT5, SaKCS20 and SaCYP86A1. Compared to CeO₂NPs added alone, the concurrent Cd decreased the Ce contents in the roots and altered its translocation from root to shoot. Taken together, both CeO₂NPs and Cd influence the interactional uptake of both chemicals in roots of S. alfredii mainly via the apoplastic pathway which is primarily regulated by the development of CSs and suberin.

Meta-omic evaluation of bacterial microbial community structure and activity for the environmental assessment of soils: overcoming protein extraction pitfalls

Microorganisms play unique, essential and integral roles in the biosphere. This work aims to assess the utility of soil's metaomics for environmental diagnosis. Doñana National Park (DNP) was selected as a natural lab since it contains a strictly protected core that is surrounded by numerous threats of pollution. Culture-independent high-throughput molecular tools were used to evaluate the alterations of the global structure and metabolic activities of the microbiome. 16S rRNA sequencing shows lower bacterial abundance and diversity in areas historically exposed to contamination that surround DNP. For metaproteomics, an innovative post-alkaline protein extraction protocol was developed. After NaOH treatment, successive washing with Tris-HCl buffer supplemented with glycerol was essential to eliminate interferences. Starting from soils with different physicochemical characteristics, the method renders proteins with a remarkable resolution on SDS-PAGE gels. The proteins extracted were analysed by using an in-house database constructed from the rRNA data. LC-MS/MS analysis identified 2182 non-redundant proteins with 135 showing significant differences in relative abundance in the soils around DNP. Relevant global biological processes were altered in response to the environmental changes, such as protective and antioxidant mechanisms, translation, folding and homeostasis of proteins, membrane transport and aerobic respiratory metabolism.

Downregulation of Zn-transporters along with Fe and redox imbalance causes growth and photosynthetic disturbance in Zn-deficient tomato

Zinc (Zn) deficiency hinders growth and development in tomato. This study unveils the responses of how Zn starvation affects physiological and molecular processes in tomato. Zn deficiency negatively affected the biomass, cellular integrity, and chlorophyll synthesis in tomato. Also, Zn deficiency decreased the maximum yield of PSII, photosynthesis performance index and dissipation energy per active reaction center, although the antenna size, trapping energy efficiency and electron transport flux were stable in Zn-starved leaves. Further, Zn shortage caused a substantial reduction in Zn and Fe concentrations in both roots and shoots along with decreased root Fe-reductase activity accompanied by the downregulation of Fe-regulated transporter 1, Zn transporter-like (LOC100037509), and Zn transporter (LOC101255999) genes predicted to be localized in the root plasma membrane. The interactome partners of these Zn transporters are predominantly associated with root-specific metal transporter, ferric-chelate reductase, BHLH transcriptional regulator, and Zn metal ion transporters, suggesting that Zn homeostasis may be tightly linked to the Fe status along with BHLH transcription factor in Zn-deficient tomato. We also noticed elevated O₂^.− and H₂O₂ due to Zn deficiency which was consistent with the inefficient antioxidant properties. These findings will be useful in the downstream approach to improve vegetable crops sensitive to Zn-deficiency.

Rapid Hormetic Responses of Photosystem II Photochemistry of Clary Sage to Cadmium Exposure

Five-day exposure of clary sage (Salvia sclarea L.) to 100 μM cadmium (Cd) in hydroponics was sufficient to increase Cd concentrations significantly in roots and aboveground parts and affect negatively whole plant levels of calcium (Ca) and magnesium (Mg), since Cd competes for Ca channels, while reduced Mg concentrations are associated with increased Cd tolerance. Total zinc (Zn), copper (Cu), and iron (Fe) uptake increased but their translocation to the aboveground parts decreased. Despite the substantial levels of Cd in leaves, without any observed defects on chloroplast ultrastructure, an enhanced photosystem II (PSII) efficiency was observed, with a higher fraction of absorbed light energy to be directed to photochemistry (ΦPSΙΙ). The concomitant increase in the photoprotective mechanism of non-photochemical quenching of photosynthesis (NPQ) resulted in an important decrease in the dissipated non-regulated energy (ΦNO), modifying the homeostasis of reactive oxygen species (ROS), through a decreased singlet oxygen (1O2) formation. A basal ROS level was detected in control plant leaves for optimal growth, while a low increased level of ROS under 5 days Cd exposure seemed to be beneficial for triggering defense responses, and a high level of ROS out of the boundaries (8 days Cd exposure), was harmful to plants. Thus, when clary sage was exposed to Cd for a short period, tolerance mechanisms were triggered. However, exposure to a combination of Cd and high light or to Cd alone (8 days) resulted in an inhibition of PSII functionality, indicating Cd toxicity. Thus, the rapid activation of PSII functionality at short time exposure and the inhibition at longer duration suggests a hormetic response and describes these effects in terms of "adaptive response" and "toxicity", respectively.

Strong accumulation capacity of hyperaccumulator Solanum nigrum L. for low or insoluble Cd compounds in soil and its implication for phytoremediation

This experiment is to explore whether one hyperaccumulator shows the strongly accumulative capacities for low or insoluble Cd compounds in soil. Soil potting experiment was conducted to analyze the accumulation capacity of Solanum nigrum L. for 10 different Cd compounds under two levels. The results clearly indicated: The Cd concentrations of shoots and roots were very high for different Cd compounds in soils even with low or insoluble Cd compounds compared with easily soluble Cd in the treatments of soil contaminated with Cd at different concentrations. Furthermore, the EFs and TFs were all larger than 1 either. Based on the results, although the bioavailabilities of some Cd compounds in soil were lower, S. nigrum's ability to accumulate them was still very strong. Phytoremediation may be widely used to treat with soil contaminated by different cadmium compounds. In addition, the total Cd content is also very important in evaluating the risk of Cd contamination in soil. Thus, phytoextraction is promising.

Overexpression of Trx CDSP32 gene promotes chlorophyll synthesis and photosynthetic electron transfer and alleviates cadmium-induced photoinhibition of PSII and PSI in tobacco leaves

Cadmium stress causes a decrease in chlorophyll content and inhibits photosynthesis in tobacco leaves. The role of thioredoxin-like protein CDSP32 expressed in plant chloroplasts is to alleviates the reduced enzymes expression involved in chlorophyll synthesis of tobacco leaves due to Cd exposure, effectively preventing chlorophyll degradation and promoting increased tobacco biomass. Overexpression of Trx CDSP32 can protect the oxygen-evolving complex on the PSII donor side and promote electron transfer on the PSII acceptor side of tobacco leaves under Cd stress. Trx CDSP32 not only significantly increase the PSI activity of tobacco leaves, but also alleviate cadmium-induced PSI photoinhibition. Although Trx CDSP32 has no significant effect on the expression of PC and FNR proteins in tobacco leaves under Cd stress, it can alleviate the decreased expression of protein subunits involved in photosynthetic electron transfer such as Cyt b6/f complex subunits, Fd, and ATP synthase subunits. Trx CDSP32 can promote the synthesis of chlorophyll, stabilize the electron transfer chain, and promote ATP synthase activity to alleviate cadmium-induced photoinhibition of PSII and PSI in tobacco leaves.

Toxic effects of heavy metal Cd and Zn on chlorophyll, carotenoid metabolism and photosynthetic function in tobacco leaves revealed by physiological and proteomics analysis

To explore the mechanisms underlying the action of the heavy metals Cd and Zn on the photosynthetic function of plant leaves, the effects of 100 μmol L-1 Cd and 200 μmol L-1 Zn stress (the exposure concentrations of Cd and Zn in the culture medium were 2.24 mg kg-1 and 5.36 mg kg-1) on the chlorophyll and carotenoid contents as well as the photosynthetic function of tobacco leaves (Long Jiang 911) were studied. The key proteins in these physiological processes were quantitatively analyzed using a TMT-based proteomics approach. Cd stress was found to inhibit the expression of key enzymes during chlorophyll synthesis in leaves, resulting in a decrease of the Chl content. However, Zn stress did not significantly influence the chlorophyll content. Leaves adapted to Zn stress by upregulating CAO expression and increase the Chl b content. Although the Car content in leaves did not significantly change under either Cd or Zn stress, the expressions of ZE and VDE during Car metabolism decreased significantly under Cd stress. This was accompanied by damages to the xanthophyll cycle and the NPQ-dependent energy dissipation mechanism. In contrast, under Zn stress, leaves adapted to Zn stress by increasing the expression of VDE, thus improving NPQ. Under Cd stress, the expressions of three sets of proteins were significantly down-regulated, including PSII donor-side proteins (PPD3, PPD6, OEE1, OEE2-1, OEE2-2, OEE2-3, and OEE3-2), receptor-side proteins (D1, D2, CP43, CP47, Cyt b559α, Cyt b559β, PsbL, PsbQ, PsbR, Psb27-H1, and Psb28), and core proteins of the PSI reaction center (psaA, psaB, psaC, psaD, psaE-A, PsaE-B, psaF, psaG, psaH-1, psaK, psaL, psaN, and psaOL). In comparison, only eight of the above proteins (PPD6, OEE3-2, PsbL, PsbQ, Psb27-H1, psaL, and psaOL) were significantly down-regulated by Zn stress. Under Cd stress, both the donor side and the receptor side of PSII were damaged, and PSII and PSI experienced severe photoinhibition. However, Zn stress did not decrease either PSII or PSI activities in tobacco leaves. In addition, the expression of electron transport-related proteins (cytb6/f complex, PC, Fd, and FNR), ATPase subunits, Rubisco subunits, and RCA decreased significantly in leaves under Cd stress. However, no significant changes were observed in any of these proteins under Zn stress. Although Cd stress was found to up-regulate the expressions of PGRL1A and PGRL1B and induce an increase of PGR5/PGRL1-CEF in tobacco leaves, NDH-CEF was significantly inhibited. Under Zn stress, the expressions of ndhH and PGRL1A in leaves were significantly up-regulated, but there were no significant changes in either NDH-CEF or PGR5/PGRL-CEF. Under Cd stress, the expressions of proteins related to Fd-dependent nitrogen metabolism and reactive oxygen species (ROS) scavenging processes (e.g., FTR, Fd-NiR, and Fd-GOGAT) were significantly down-regulated in leaves. However, no significant changes of any of the above proteins were identified under Zn stress. In summary, Cd stress could inhibit the synthesis of chlorophyll in tobacco leaves, significantly down-regulate the expressions of photosynthesis-related proteins or subunits, and suppress both the xanthophyll cycle and NDH-CEF process. The expressions of proteins related to the Fd-dependent nitrogen metabolism and ROS scavenging were also significantly down-regulated, which blocked the photosynthetic electron transport, thus resulting in severe photoinhibition of both PSII and PSI. However, Zn stress had little effect on the photosynthetic function of tobacco leaves.

Interaction of zinc and IAA alleviate aluminum-induced damage on photosystems via promoting proton motive force and reducing proton gradient in alfalfa

BACKGROUND

In acidic soils, aluminum (Al) competing with Zn results in Zn deficiency in plants. Zn is essential for auxin biosynthesis. Zn-mediated alleviation of Al toxicity has been rarely studied, the mechanism of Zn alleviation on Al-induced photoinhibition in photosystems remains unclear. The objective of this study was to investigate the effects of Zn and IAA on photosystems of Al-stressed alfalfa. Alfalfa seedlings with or without apical buds were exposed to 0 or100 μM AlCl3 combined with 0 or 50 μM ZnCl2, and then foliar spray with water or 6 mg L- 1 IAA.

RESULTS

Our results showed that Al stress significantly decreased plant growth rate, net photosynthetic rate (Pn), quantum yields and electron transfer rates of PSI and PSII. Exogenous application of Zn and IAA significantly alleviated the Al-induced negative effects on photosynthetic machinery, and an interaction of Zn and IAA played an important role in the alleviative effects. After removing apical buds of Al-stressed alfalfa seedlings, the values of pmf, gH+ and Y(II) under exogenous spraying IAA were significantly higher, and ΔpHpmf was significantly lower in Zn addition than Al treatment alone, but the changes did not occur under none spraying IAA. The interaction of Zn and IAA directly increased Y(I), Y(II), ETRI and ETRII, and decreased O2- content of Al-stressed seedlings. In addition, the transcriptome analysis showed that fourteen functionally noted genes classified into functional category of energy production and conversion were differentially expressed in leaves of alfalfa seedlings with and without apical buds.

CONCLUSION

Our results suggest that the interaction of zinc and IAA alleviate aluminum-induced damage on photosystems via increasing pmf and decreasing ΔpHpmf between lumen and stroma.

Phytoremediation of cadmium-polluted soil assisted by D-gluconate-enhanced Enterobacter cloacae colonization in the Solanum nigrum L. rhizosphere

Microbe-assisted phytoremediation for Cd-polluted soil is being regarded increasingly. However, the availability of microbes that can collaborate with Cd-hyperaccumulators effectively has become one of bottlenecks restricting the remediation efficiency. A siderophore-producing bacterium (Y16; Enterobacter cloacae) isolated from the rhizospheric soil of Cd-hyperaccumulator Solanum nigrum L. was identified by 16S rRNA gene sequencing and biochemical analysis, and then used for analyzing microbial chemotaxis, carbon source utilization, and insoluble P/Cd mobilization capacities. Besides, a soil-pot trial was performed to underlie the phytoremediation mechanism of Cd-polluted soil assisted by D-gluconate-enhanced Enterobacter cloacae colonization (DEYC) in the Solanum nigrum L. rhizosphere. Results displayed that D-gluconate was an effective chemoattractant and carbon source strengthening Y16 colonization, and Y16 exhibited strong abilities to mobilize insoluble P/Cd in shake flask by extracellular acidification (p < 0.05). In the soil-pot trial, DEYC observably enhanced soil Cd phytoextraction by Solanum nigrum L., and increased microbial diversity according to alpha- and beta-diversity analysis (p < 0.05). Taxonomic distribution and co-occurrence network analysis suggested that DEYC increased relative abundances of dominant microbial taxa associated with soil acidification (Acidobacteria-6), indoleacetic acid secretion (Ensifer adhaerens), soil fertility improvement (Flavisolibacter, Bdellovibrio bacteriovorus, and Candidatus nitrososphaera), and insoluble Cd mobilization (Massilia timonae) at different classification levels. Importantly, COGs analysis further shown that DEYC aroused the up-regulation of key genes related to chemotactic motility, carbon fixation, TCA cycle, and propanoate metabolism. These results indicated that DEYC drove the rhizospheric enrichment of pivotal microbial taxa directly or indirectly involved in soil Cd mobilization, meanwhile distinctly promoted plant growth for accumulating more mobilizable Cd. Therefore, Y16 could be used as bio-inoculants for assisting phytoremediation of Cd-polluted soil.

Pseudomonas fluorescens promote photosynthesis, carbon fixation and cadmium phytoremediation of hyperaccumulator Sedum alfredii

Plant growth-promoting bacteria (PGPB) can promote photosynthesis and biomass production of hyperaccumulators, achieving enhanced phytoremediation efficiency of cadmium (Cd). A better understanding of the mechanisms controlling photosynthesis of hyperaccumulating plants by PGPB is necessary for developing strategies that promote the practical phytoextraction of Cd-polluted soils. In this study, chlorophyll fluorescence, gas exchange, and transcriptome sequencing were conducted to evaluate the physiological and transcriptional changes on photosynthesis and carbon fixation in hyperaccumulator Sedum alfredii after inoculation with PGPB Pseudomonas fluorescens. The results showed that bacterial inoculation significantly enhanced maximum quantum yield of PS II (Fv/Fm), effective quantum yield of PS II (ΦPSII), photochemical quenching (qP) and chlorophyll concentration, while reduced non-photochemical quenching (NPQ) of S. alfredii. Further, inoculation resulted in an increased net photosynthetic rates (Pn), intercellular CO₂ concentration (Ci), transpiration rate (Tr) and stomatal conductance (Gs) of the studied plant. At the transcriptional level, 70 photosynthetic genes and 42 C4-pathway carbon fixation related genes were significantly up-regulated in response to inoculation, which could be the reason for enhanced photosynthesis and dry biomass. To sum up, this P. fluorescens strain can simultaneously promote growth and Cd uptake of S. alfredii, which can be a promising bacterial agent applied to Cd phytoremediation practices.

Toxic effects of heavy metals Pb and Cd on mulberry (Morus alba L.) seedling leaves: Photosynthetic function and reactive oxygen species (ROS) metabolism responses

To explore the mechanism of how lead (Pb) and cadmium (Cd) stress affects photosynthesis of mulberry (Morus alba L.), we looked at the effects of different concentrations of Pb and Cd stress (at 100 and 200 μmol L^-1), which are two heavy metal elements, on leaf chlorophyll (Chl), photosynthesis gas exchange, Chl fluorescence, and reactive oxygen species (ROS) metabolism in mulberry leaves. The results showed that higher concentrations of Pb and Cd reduced leaf Chl content, especially in Chl a where content was more sensitive than in Chl b. Under Pb and Cd stress, the photosynthetic carbon assimilation capacity of mulberry leaves was reduced, which was a consequence of combined limitations of stomatal and non-stomatal factors. The main non-stomatal factors were decreased photosystem II (PSII) and photosystem I (PSI) activity and carboxylation efficiency (CE). Damage to the donor side of the PSII reaction center was greater than the acceptor side. After being treated with 100 μmol L^-1 of Pb and Cd, mulberry leaves continued to be able to dissipate excess excitation energy by starting non-photochemical quenching (NPQ), but when Pb and Cd concentrations were increased to 200 μmol L^-1, the protection mechanism that depends on NPQ was impaired. Excessive excitation energy from chloroplasts promoted a great increase of ROS, such as superoxide anion (O₂^•-) and H₂O₂. Moreover, under high Pb and Cd stress, superoxide dismutase (SOD) and ascorbate peroxidase (APX) were also inhibited to some extent, and excessive ROS also resulted in a significantly higher degree of oxidative damage. Compared with Cd, the effect of Pb stress at the same concentration level displayed a significantly lower impact on Chl content, photosynthetic carbon assimilation, and stomatal conductance. Meanwhile, Pb stress mainly damaged activity of the oxygen-evolving complex (OEC) located on PSII donor side, but it reduced the electronic pressure on the PSII acceptor side and PSI. Furthermore, under Pb stress, the NPQ, SOD, and APX activity were all significantly higher than those under Cd stress. Thus under Pb stress, the degree of photoinhibition and oxidative damage of PSII and PSI in mulberry leaves were significantly lower than under Cd stress.

Hyperaccumulation of Cd by Rorippa globosa (Turcz.) Thell. from soil enriched with different Cd compounds, and impact of soil amendment with glutathione (GSH) on the hyperaccumulation efficiency

Rorippa globosa (Turcz.) Thell. is known as Cd hyperaccumulator, however neither hyperaccumulation nature, nor affecting factors like the effect of Cd compounds entering soil from different sources, or of specific soil amendments, are not yet satisfactorily clarified. In the pot culture experiment, Cd accumulation by R. globosa from soils spiked with 3 and 9 mg Cd kg^-1 in the form of Cd(NO₃)₂, CdCl₂, CdBr₂, CdI₂, CdSO₄, CdF₂, Cd(OH)₂, CdCO₃, Cd₃(PO₄)₂, CdS and effect of soil amendment with glutathione (GSH) were investigated. Accumulation capacity of R. globosa for Cd appeared to reflect its extractability in soils and was about two-fold bigger for high soluble compounds than for low-soluble ones. At that, the differences between the accumulation of Cd originating from high soluble compound group did not exceed 20%, while the differences within the low soluble compound group were insignificant (p < 0.05). The analysis of Cd uptake, uptake factor (UF), enrichment factor (EF) and translocation factor (TF) patterns revealed that Cd hyperaccumulating properties of R. globosa are based on the high water/nutrients demand and strong tolerance to Cd, although weak protection against Cd uptake by root system was also observed. Amendment with GSH enhanced Cd availability to plant and its uptake from soil, but exerted no effect on Cd translocation in plants. In the light of the results, the use of R. globosa for phytoremediation of moderately polluted agricultural lands as forecrop or aftercrop, and the GSH-assisted phytoremediation of highly polluted post-industrial sites seem to be viable options.

Molecular dissection of cadmium-responsive transcriptome profile in a low-cadmium-accumulating cultivar of Brassica parachinensis

Brassica parachinensis L., a daily consumed leaf vegetable, is a high-Cd accumulator that substantially threatens human health. Screening and breeding Cd pollution-safe cultivars (Cd-PSCs) of crops is a low-cost strategy to restrict human Cd intake from contaminated soils via the food chain. However, little is known about the molecular mechanisms underlying the low-Cd-accumulating traits of B. parachinensis Cd-PSCs. In the current study, we analyzed the transcriptomes of the Cd-treated (5 μM) roots and shoots of a low-Cd-accumulating cultivar (SJ19) and a high-Cd-accumulating cultivar (CX4) of B. parachinensis to reveal the molecular mechanisms in response to Cd stress. Compared to CX4, many pathways involved in carbohydrate and amino acid metabolisms were exclusively up-regulated in SJ19 roots upon exposure to low Cd concentrations, which may produce more energy and metabolites for Cd detoxification. Antioxidant enzymes in the peroxisome were up-regulated in both SJ19 and CX4 roots in response to Cd, while glutathione biosynthesis was only activated in SJ19 roots. In SJ19 shoots, pathways of photosynthesis and cell growth were activated to mitigate Cd-induced damages. Furthermore, Cd transport genes, such as MTP1, HMA3 and CAX family genes, were highly induced by Cd stress in SJ19 roots in accordance with the high Cd concentration in roots, while genes involved in root-to-shoot Cd translocation such as FRD3 and CESA3 were suppressed, which may contribute to the low Cd concertation in edible part of SJ19. Our study provides a genetic basis for further Cd-PSCs screening and breeding.

Toxic Effects of Cd and Zn on the Photosynthetic Apparatus of the Arabidopsis halleri and Arabidopsis arenosa Pseudo-Metallophytes

Hyperaccumulation and hypertolerance of Trace Metal Elements (TME) like Cd and Zn are highly variable in pseudo-metallophytes species. In this study we compared the impact of high Cd or Zn concentration on the photosynthetic apparatus of the Arabidopsis arenosa and Arabidopsis halleri pseudo-metallophytes growing on the same contaminated site in Piekary Slaskie in southern Poland. Plants were grown in hydroponic culture for 6 weeks, and then treated with 1.0 mM Cd or 5.0 mM Zn for 5 days. Chlorophyll a fluorescence and pigment content were measured after 0, 1, 2, 3, 4, and 5 days in plants grown in control and exposed to Cd or Zn treatments. Moreover, the effect of TME excess on the level of oxidative stress and gas-exchange parameters were investigated. In both plant species, exposure to high Cd or Zn induced a decrease in chlorophyll and an increase in anthocyanin contents in leaves compared to the control condition. After 5 days Cd treatment, energy absorbance, trapped energy flux and the percentage of active reaction centers decreased in both species. However, the dissipated energy flux in the leaves of A. arenosa was smaller than in A. halleri. Zn treatment had more toxic effect than Cd on electron transport in A. halleri compared with A. arenosa. A. arenosa plants treated with Zn excess did not react as strongly as in the Cd treatment and a decrease only in electron transport flux and percentage of active reaction centers compared with control was observed. The two species showed contrasting Cd and Zn accumulation. Cd concentration was almost 3-fold higher in A. arenosa leaves than in A. halleri. On the opposite, A. halleri leaves contained 3-fold higher Zn concentration than A. arenosa. In short, our results showed that the two Arabidopsis metallicolous populations are resistant to high Cd or Zn concentration, however, the photosynthetic apparatus responded differently to the toxic effects.

The metal-binding domain of wheat heavy metal ATPase 2 (TaHMA2) is involved in zinc/cadmium tolerance and translocation in Arabidopsis

Cysteine in the N-terminal metal-binding domain (N-MBD) of TaHMA2 participates in Zn²⁺/Cd²⁺ binding and translocation in Arabidopsis. Wheat heavy metal ATPase 2 (TaHMA2) can transport Zn²⁺ and Cd²⁺ across membranes. A previous study showed that cysteine (Cys) and glutamate residues in the N-terminal metal-binding domain (N-MBD) were necessary for metal-binding and translocation of TaHMA2 in yeast. However, the function of TaHMA2 in plants was not fully revealed. In this study, we investigated the roles of the CCxxE and CPC motifs in the N-MBD and the N/C-terminal regions of TaHMA2 in Zn²⁺/Cd²⁺ translocation in root and shoot of Arabidopsis. Compared with the wild type, overexpression of TaHMA2 and the TaHMA2 derivative (glutamic substituted for alanine from CCxxE) in Arabidopsis increased root length, fresh weight and enhanced Zn²⁺/Cd²⁺ root-to-shoot translocation. The plants with a truncated N/C-terminal of TaHMA2 were impaired in Zn²⁺/Cd²⁺ tolerance and translocation, while mutagenesis of Cys in the N-MBD reduced the tolerance and transport activity of TaHMA2, suggesting the involvement of Cys in Zn²⁺/Cd²⁺ binding and translocation in Arabidopsis. This study therefore provides a theoretical possibility for the application of TaHMA2 in transgenic breeding to regulate metal element balance in crop plants.

Wheat expansin gene TaEXPA2 is involved in conferring plant tolerance to Cd toxicity

Cadmium (Cd) is a severe and toxic heavy metal pollutant that affects plant growth and development. In this study, we found that the expression of an expansin gene, TaEXPA2, was upregulated in wheat leaves under CdCl2 toxicity. We characterized the involvement of TaEXPA2 in conferring Cd tolerance. Tobacco plants overexpressing TaEXPA2 showed higher germination rate, root elongation, and biomass accumulation compared to the wild-type (WT) plants upon CdCl2 treatment. The improved photosynthetic parameters and lesser cellular damage in transgenic plants exposed to Cd compared to that in the WT plants suggest that TaEXPA2 overexpression improves Cd tolerance in plants. Furthermore, we noticed that Cd was efficiently effluxed out of the cytoplasm in the transgenic plants owing to the enhanced activities of H+-ATPase, V-ATPase, and PPase, which helped in conferring Cd tolerance. Moreover, Cd concentration and ROS accumulation were lower in the transgenic plants than in WT plants as a consequence of enhanced antioxidant enzyme activities in the former. In addition, atexpa2, an Arabidopsis mutant, exhibited lower biomass and shorter primary root compared to its WT under Cd toxicity; however, the phenotype was recovered upon expression of TaEXPA2 in these mutants. Our results demonstrate that TaEXPA2 confers tolerance to Cd toxicity. The changed absorption/transportation of Cd and the antioxidative capacity may be involved in the improved tolerance of the transgenic plants with overexpression of TaEXPA2 to CdCl2 toxicity.

Conceptualizing plant systems evolution

Organisms inhabiting extreme environments are emerging models in systems evolution, enabling us to identify the molecular alterations effecting major phenotypic divergence through comparative approaches. Here I discuss possible physiological mechanisms underlying evolutionary adaptations to extreme environments both theoretically and in relation to experimental observations. Reasoning leads me to the 'conserved steady-state' hypothesis of evolutionary adaptation: Between closely related plants adapted to differently composed soils, the homeostatically controlled steady-state set point cytosolic (buffered) concentrations of mineral ions are conserved. Subsequently, I compare molecular alterations expected to contribute to physiological adaptations with our present knowledge. Key roles of enhanced gene product dosage in plant evolutionary adaptations question the widespread stimulus response-centric paradigm. As a broader implication, co-regulation networks can lack decisive functional network elements. With this article, I hope to stimulate a discussion across research fields and provide an initial conceptual framework for future experimental testing and for quantitative modelling.

Effects of Different Metals on Photosynthesis: Cadmium and Zinc Affect Chlorophyll Fluorescence in Durum Wheat

A comparative study of the effects of exposure to high Cd²⁺ (50 µM) and excess Zn²⁺ (600 µM) on photosynthetic performance of hydroponically-grown durum wheat seedlings was performed. At day 8, Cd and Zn were added to the nutrient solution. After 7-days exposure, the chosen concentrations of both metals resulted in similar relative growth rate (RGR) inhibitions of about 50% and comparable retardations of the CO₂ assimilation rates (about 30%) in the second developed leaf of wheat seedlings. Analysis of chlorophyll a fluorescence indicated that both metals disturbed photosynthetic electron transport processes which led to a 4- to 5-fold suppression of the efficiency of energy transformation in Photosystem II. Non-specific toxic effects of Cd and Zn, which prevailed, were an inactivation of part of Photosystem II reaction centres and their transformation into excitation quenching forms as well as disturbed electron transport in the oxygen-evolving complex. The specificity of the Cd and Zn modes of action was mainly expressed in the intensity of the toxicity effects: despite the similar inhibitions of the CO₂ assimilation rates, the wheat photochemistry showed much more sensitivity to Cd than to Zn exposure.

Trace metal metabolism in plants

Many trace metals are essential micronutrients, but also potent toxins. Due to natural and anthropogenic causes, vastly different trace metal concentrations occur in various habitats, ranging from deficient to toxic levels. Therefore, one focus of plant research is on the response to trace metals in terms of uptake, transport, sequestration, speciation, physiological use, deficiency, toxicity, and detoxification. In this review, we cover most of these aspects for the essential micronutrients copper, iron, manganese, molybdenum, nickel, and zinc to provide a broader overview than found in other recent reviews, to cross-link aspects of knowledge in this very active research field that are often seen in a separated way. For example, individual processes of metal usage, deficiency, or toxicity often were not mechanistically interconnected. Therefore, this review also aims to stimulate the communication of researchers following different approaches, such as gene expression analysis, biochemistry, or biophysics of metalloproteins. Furthermore, we highlight recent insights, emphasizing data obtained under physiologically and environmentally relevant conditions.

Adaption of Ulva pertusa to multiple-contamination of heavy metals and nutrients: Biological mechanism of outbreak of Ulva sp. green tide

The multiple-contamination of heavy metals and nutrients worsens increasingly and Ulva sp. green tide occurs almost simultaneously. To reveal the biological mechanism for outbreak of the green tide, Ulva pertusa was exposed to seven-day-multiple-contamination. The relation between pH variation (V_pH), Chl a content, ratio of (Chl a content)/(Chl b content) (R_chla/chlb), SOD activity of U. pertusa (A_SOD) and contamination concentration is [Formula: see text] (p<0.05), C_chla=0.88_±0.09-0.01_±0.00×C_Cd (p<0.05), [Formula: see text] (p<0.05), and [Formula: see text] (p<0.05), respectively. C_ammonia, C_Cd and C_Zn is concentration of ammonia, Cd²⁺ and Zn²⁺, respectively. Comparing the contamination concentrations of seawaters where Ulva sp. green tide occurred and the contamination concentrations set in the present work, U. pertusa can adapt to multiple-contaminations in these waters. Thus, the adaption to multiple-contamination may be one biological mechanism for the outbreak of Ulva sp. green tide.

Hyperaccumulating potential of Bidens pilosa L. for Cd and elucidation of its translocation behavior based on cell membrane permeability

Phytoremediation with the use of hyperaccumulating plant species to remove excess trace metals from contaminated soil and water is considered a cost-effective non-invasive technique. Over 400 plant taxa worldwide have been identified as natural hyperaccumulators, but only very few are reported to hyperaccumulate Cd. Bidens pilosa L. is a newly found, promising Cd hyperaccumulator, although its potential to accumulate Cd and mechanism of this process are not yet well known. This paper was aimed at exploring hyperaccumulation capacity of B. pilosa for Cd, and its translocation behavior related to cell membrane permeability. The highest Cd concentration in shoots of B. pilosa grown in soil was 405.91 mg kg^-1 and of that cultured in nutrient solution 1651.68 mg kg^-1, indicating very high accumulation potential. Cd concentrations in the root, stem, leaf, and shoot of B. pilosa cultured in nutrient solution were all much higher than those in soil, while biomass development was considerably lower. This resulted in lesser differences between Cd maximum accumulation loads in the shoot (462 and 365 μg pot^-1) and in the root (100 and 96 μg pot^-1) of B. pilosa grown in solution and in soil, respectively. Relative electric conductivity (REC), K⁺ relative permeability ratio, and MDA (malondialdehyde) contents, which are major indices expressing cell membrane permeability, appeared to be closely related to Cd translocation and accumulation. The relative molecular mechanism of Cd accumulation/translocation in B. pilosa was found of importance and needs to be elucidated.