Some other variants of these peptides have been also detected in maize and other plants Chassaigne et al. Biological roles of these variant peptides are not well understood at present but should be elucidated in the future in both biochemical and evolutionary aspects. The overall survey for the occurrence of PCs in various plants described above give us a strong enough background for the use of the term "phytochelatins" to describe the heavy-metal binding peptides in plants, as pointed out by previous reviewers Rauser, ; Zenk, ; Cobbett, However, it has been shown that the PC synthase gene is not only restricted to plants, as was once thought, but also has an essential role in the model nematode C.
These studies will develop a new field in PC research and consequently affect the terminology. As shown in table 3 various metal-binding substances have been isolated from yeast and fungi. No di-rect evidence has been proposed for the presence of a native Cd-binding MT in eu-caryotic fungi or plants, with the exception of a strain N of S. No direct evidence has been presented that supports the presence of PCS genes in S. These data provide important information relevant to studies of the evolutionary diversity of MTs and heavy-metal resistance among fungi and plants.
It is not clear why the Cd-binding peptides produced by S. This may be related to the catalytic properties of the PC synthase of the yeast member genus or the strains. Inouhe et al. Only one of these capable of producing PCs was the same genus, Schizosaccharomyces octosporus. Little information on the occurrence of PCs in fungi other than the various yeast species mentioned above is available except for a few reports. More extensive studies for the heavy metal-binding peptides and proteins in other fungi should give us to a greater chance to discover novel types of metal-binding complexes containing various minor metallic elements.
Bacteria and some other prokaryotic cells can produce MT similar to mammalian MTs, as reported in Synechococcus sp. Higham et al. However there are few reports of the production of PCs by prokaryotes. GSH is the most abundant low molecular weight thiol in most organisms Meister and Anderson, ; Rennenberg and Lamoureux, , while some anaerobic bacteria contain very low amounts of GSH Fahey et al.
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This idea is supported by the fact that transformed E. More recently Tsuji et al. PCC The recombinant alr protein strongly catalyzed the first step of PC synthesis from GSH but only weakly the second step, suggesting that the prokaryote retains a more primitive form of functional PCs than eukaryotes. Therefore, we need more information on prokaryotes before reaching a conclusion as to the general lack of PC synthesis or its presence in prokaryotes.
Here, it can be expected that unknown prokaryotes or their DNA remain in deep seas and other harsh environments in the biosphere of the earth's surface, or perhaps in some microhabitats as in a fossil. Ivanova et al. Cyanobacteria can be considered as an ancestor of the chloroplasts in green plants, mainly because the organisms and organelle perform a similar photosynthetic pattern evolving oxygen.
GSH is an effective antioxidant against oxygen toxicity and is abundantly and commonly produced in the prokaryotic and eukaryotic algae as well as in higher plants Rennenberg and Lamoureux, Here, Archaea as a hyperthermophile and sulfur-metabolizing archaebacteria would be an interesting biological tool to assess the possible existence of PCS genes and PC-like peptides, especially when one considers that the cells might provide the origin of the cytoplasm of eukaryotic cells according to an endosymbiosis theory.
PCs were isolated in from several "eukaryote" green algae exposed to heavy metal ions Gekeler et al. Since then many studies have been carried out on PC synthesis in phytoplanktonic algae, including the marine diatom Phaeodactylum tricornutum Morelli and Scarano, ; Scarano and Morelli, , Dunaliella tertiolecta Hirata et al.
Substantial levels of PCs are also found in Euglena gracilis , Fragilaria crotonesis , Sargassum mutium and Porphyridium cruentum Gekeler et al. The synthesis of PCs was compared for several marine algae species as affected by various metals Ahner et al. PCs are induced in the aquatic plants, such as the water fern Salvinia minima Hoffmann et al. The most effective metal to induce PC synthesis in all these plants was Cd, while other metal species were effective to varying extents. It is interesting to note that Zn caused the strongest induction of PC synthesis in Dunalliella tertiolecta Hirata et al.
Zn was ineffective for PC formation in some higher plants, as described later. Physiological roles of PCs and PC-related peptides widely distributed in the plant and other kingdoms have not been fully elucidated, as mentioned earlier. However, their potential roles in heavy metal detoxification in cells have been elucidated Rauser, ; Mehra and Winge, It may be useful here to review their roles in heavy metal tolerance according to the path of evolution from primitive singular cell systems to the more complicated and integrated cell systems in higher plants.
This relationship can be partially mimicked by that between undifferentiated and differentiated cells in many organisms, and again in dedifferentiated singular cells such as suspension-cultures of higher plants. In the following section, possible roles of PCs in terms of heavy metal-tolerance are described first for suspension cultures and then for various intact plants.
Heavy metal ions or elements discussed here are centered on Cd, Cu, Zn, Ni and As, because these have been most extensively investigated or debated over the years regarding their influence on PC production and other tolerance mechanisms. Studies on heavy metal tolerance in suspension-cultured cells have been performed using common plants such as tomato Bennetzen and Adams, ; Huang et al.
Some other metal-tolerant species have been used as suspension cells for similar studies, including Rauvolfia serpentina , Agrostis tenius , Silene cucubalus Grill et al. In the most culture systems, both the PCs and Cd contents in the cells increase as a function of Cd concentrations in the media. Such characteristics are largely reduced by interference of PC synthesis with the specific inhibitor BSO Steffens et al. Thus the evidence on hand for a role for PCs in Cd-tolerance is very strong.
These results for suspension cells are in good agreement with those of the fission yeast S. The common features regarding these cells are their unicellular nature and their natural habitats are certain liquid media or seawaters. The cells interact with the environmental ions directly through their membrane and cell walls. If toxic ions penetrate into the cells across the barriers then the cells must detoxify the cytoplasm.
Another appreciable homology is the growth pattern of the cells.
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They increase their biomass mainly by cell proliferation. In other words, PC formation might be required for cell division under the influence of Cd and other toxic ions. In fact, tomato cells contain their highest levels of PC activity at an exponential growth stage Chen et al. Similar roles for PC peptides in Cd-detoxification can be partially applied to the cases of macro algae and other aquatic plants growing under water.
However their functions in intact land plants might be restricted to some symplastic locations in the root or shoot tissues, as discussed later. Inorganic arsenic As has been shown to induce the synthesis of PCs in cell cultures of R. Therefore PCs appear to be involved in As-tolerance and detoxification of suspension-cultured cells in many plants Meharg and Hartley-Whitaker, As compared with the abundant evidence for As and Cd tolerance, the roles of PCs in Cu or Zn tolerance in suspension cells are still controversial.
Heavy metal ions affect various processes in plants, including membrane function De Vos et al. The apparent discrepancy for the metal-specific aspects of the activation of PC synthesis cannot be resolved at present, but it may contribute to disclose diversity of the regulation mechanisms in plants. The following is an exceptional case for the effectiveness of Cd on PC formation Inouhe et al. These results suggest that azuki bean cells are lacking in the PC synthase activity per se , hence being Cd-hypersensitive.
While the role of these thiols in the Cd-hypersensitive plant have not been fully examined, it can be generally concluded that PC synthase has a more important role in Cd-tolerance of suspension-cultured cells in general. A possible relationship between Cd-tolerance and Cd-accumulation has been reported at the different family level of plants Kuboi et al.
Furthermore, positive relationships between levels of PCs and Cd-tolerance in intact plants have been reported in maize Nussbaum et. The results clearly showed that mono- and dicotyledonous roots exhibited a substantially different Cd-tolerance, depending on their abilities to produce the Cd-binding PC complexes in the cytoplasmic fractions. More direct evidence for the role of PC in Cd-tolerance in intact plants was obtained using PC-deficient cad1 mutants of Arabidopsis Howden et al.
The lowest Cd concentration to which the cad1 mutants cad were sensitive was 0. Some investigators have shown that PCs have an important role in As-tolerance in intact plants Grill et al. Here we shall briefly survey the mechanisms for the transport and tolerance for As. Plants and microorganisms take up As in the form of the anion AsO 4 3- via a transporter for phosphate PO 4 3- in the cell membranes Willsky and Malamy, ; Fujiwara et al.
One is the restriction of As uptake due to low phosphate uptake activity Rosenberg et al. The inorganic As anions have already been shown to be capable of inducing or binding to PCs in vivo in roots of many plants as well as suspension cells Grill et al. Next are two interesting historical turning points in recent research on the subject. First, the As-tolerance plant Holcus lanatus isolated from an As-Cu contaminated site was considered to be due to the suppression of As uptake Meharg and Macnair, However, this idea was recently revised since the As-tolerant plants were found to contain much higher levels of PCs and As than the wild-type plants when grown in the presence of high levels of As Hartley-Whitaker et al.
However, R. Roles of specific PCs, hPCs and other novel thiols in the As-tolerance mechanism can be expected to be clarified in the near future. There is, however, some confirmed evidence against the role of PCs in heavy metal tolerance in higher plants, since some exceptions have been found as in the case of the suspension cells mentioned earlier.
Ernst and his group first noticed that the Cd-tolerant and Cd-sensitive plants of Silene vulgaris are both capable of producing equal levels of PC peptides in response to Cd Verkleij et al. They found that greater amounts of Cd and acid-labile sulfides are associated more so with the PCs in the tolerant plants than in the sensitive ones, suggesting a more important role of sulfides rather than PC itself in the Cd-detoxification process in the plants. Actually the general importance of the incorporation of acid-labile sulfides in the formation of the more stable and functional Cd-binding complexes has been demonstrated in the other plants and microorganisms Murasugi et al.
The Cd-treatments are also known to stimulate sulfur assimilation required for sulfide formation in plants Nussbaum et al. Therefore it must be concluded that a variety of factors other than PC synthase are involved in the tolerance phenotypes of intact plants to heavy metals. So far various plants more than a hundred species have been recognized as the heavy-metal hyper-accumulators that accumulate Zn, Cd, Ni, Cu, and As etc preferentially in the shoot organs.
Their suction powers of metals from soils or waters are also believed to contribute to phytoremediation of contaminated sites with various heavy metals. However at present such evidence supporting a role for PC in metal hyperaccumulation and tolerance are not so strong. Ebbs et al. Further roles of PC in constitutive and adaptive heavy metal-tolerances were studied in hyperaccumulator and non-hyperaccumulator metallophytes, S.
BSO also caused an increase in Cd-sensitivity in non-metallicolous plants but did not in hypertolerant plants. From this and other evidence Salt et al. Although it would be interesting to discuss the mechanisms for the PC-independent metal-accumulation and transport to shoots in the hyperaccumulators, this is beyond the scope of the present review.
One of the most well-known properties of the hyperaccumulator plants concerns their long-distance transport systems mainly governed by the transpiration stream. This apoplastic route which carries the stream of heavy metal-containing water transported from roots to shoots leaves can be a collective proof that higher land plants evolved from their ancestor plants or microbes living in water as described earlier.
In the future, relationships between symplast and apoplast for metal binding and movement will be of great importance in research projects in both the pure and applied fields of science. PCs as biochemical indicators for heavy metal contamination. Finally, the possible importance of the formation PCs and PC-related peptides in plant tissues as the biochemical indicators for heavy metal contamination is briefly described. As described earlier, PC formation in plant tissues occurs in function of external metal concentrations in general, irrespective of whether they are specific or non-specific to metals.
Thus quantitative biochemical assays for the metal adjustments are also possible. For example, as shown in figure 3 , chickpea provides very capable indicators for heavy metal contamination Gupta et al. The roots are very sensitive in producing PCs and hPCs in a specific manner in response to Cd and As, and the shoots, as well as the roots of the plant, are sensitive to various metals in terms of the rapid increases in the levels of GSH, hGSH and cysteine.
These changes can be used in the biochemical evaluation of some specific or overall heavy metal contamination in various habitats. Furthermore, the signaling and transport systems between root and shoot in terms of heavy-metal response will be a matter of great importance in the future. However, as for such a possible utilization of PC peptides as biochemical signals for contamination, Rauser has already cautioned that PC formation is not very sensitive at lower concentrations of Cd.
He claimed that it should be recognized that many other mechanisms may prevail in plants growing in Cd-polluted soils where internal Cd concentrations are much lower than those used in most experimental model systems such as suspension cultures Rauser, Nevertheless, there is a recent positive report showing that very low free ionic Cd 10 Parris Island. Here, the plants were grown in a flow-through hydroponic FTH system Maier et al.
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The results bring a new insight to the field and point the direction of further research. Morphological changes in metal hypersensitive plants can be used as a biological indicator for similar assessments for the metal contamination by counting the number of plants affected by the phyto-toxic effects of different metals. However this kind of evaluation may have some limitations for actual application because the justifications are made based upon the survival of the plants and thus need a large number of individuals and will only respond at higher concentrations of the contaminating metals.
On the other hand, the biochemical indices of PC formation can be used for more sensitive and more specific assessments of environmental contamination by different heavy metal ions. The former involves several different sub-processes: the activation of PC synthase, GSH biosynthesis, the accumulation of acid-labile sulfides, sulfur assimilation and transport of the Cd-PC complexes to the vacuole. All these would be required for the formation of stable and nontoxic Cd-complexes in the vacuole or other symplastic sites in the cells of most plants and some fungi and worms, where PC synthase is the key factor for the tolerance phenotypes to Cd and other ions.
The PC-independent mechanisms are apparently present in more differentiated higher plants that inhabit terrestrial environments. Readjustment of both the symplastic and apoplastic activities of metal-binding in shoots may work more effectively for the required phytoremediation processes of contaminated soil and water environments.
MOFs therefore offer great potential for their effective integration and exploration in various sensing applications. MOFs can be put together arbitrarily like Lego bricks and outperform every previously known class of material in terms of flexibility. The physicochemical properties of materials are governed by the synergistic effects of structures and compositions, and MOFs are fascinating examples of how the unique structure of hollow-structured materials can provide a whole raft of advantageous features.
Among them are enhanced surface-to-volume ratio; low density; microreactor environment; higher loading capacities; and reduced transmission lengths of mass and charge. Consequently, the preparation of hollow structures for technological applications has long been a popular research field for chemists and materials scientists.
However, the synthesis of porous or hollow-structured materials with controllable — and especially complex — structures and certain composition in a controlled manner has always been a challenge for scientists. Enter MOFs — crystalline hybrid materials created from both organic and inorganic molecules via molecular self-assembly.
Pioneered in the late s "Design and synthesis of an exceptionally stable and highly porous metal-organic framework" by Prof. So far, more than 20 different MOFs have been fabricated and characterized. Numerous applications in many fields are being developed that exploit MOFs' cage-like structure, such as gas storage and separation, liquid separation and purification, electrochemical energy storage, catalysis, and sensing. In addition to direct applications, MOFs have been used as unique precursors for the construction of inorganic functional materials with unparalleled design possibilities, such as carbons, metal-based compounds, and their composites.
Currently, carbonaceous materials are attracting much interest for their extensive applications including adsorption, catalysis, batteries, fuel cells, supercapacitors, and drug delivery and imaging. In addition, some sensors are also one of the important applications of carbonaceous materials, because they are closely related to human health. There are varieties of approaches for the preparation of these carbon materials, but among them, directly carbonizing from organic precursors is the most frequently used method to prepare nanoporous carbons due to its flexibility and simplicity.
These materials present certain drawbacks, though, such as low surface areas, disordered structures, and non-uniform sizes, which will greatly limit their applications. However, researchers found that carbon materials derived from metal-organic frameworks MOFs could overcome these limitations read more: " Applications of carbon materials derived from metal-organic frameworks ". Typically, the instruments that can detect traces of a specific gas in the air are large, expensive, energy-intensive machines.
One promising way to make small, inexpensive, and energy-efficient gas sensors involves porous materials like metal-organic frameworks MOFs. MOFs' high surface area also is a beneficial aspect for high-performance gas sensors. One example is a thin-film a tailor-made MOF, coated onto an electrode, that forms an electronic sensor that could detect traces of sulfur dioxide gas.
Sulfur dioxide molecules red and yellow are selectively taken up by pores in the metal-organic framework.
Image: Valeriya Chernikova. Scientists also found that a metal—organic framework, MFM Al , not only effectively filters harmful nitrogen dioxide gas, but it also has outstanding capabilities for ammonia storage. One particular MOF material exhibits an unprecedented cooperative mechanism for carbon dioxide capture-and-release with only small shifts in temperature.
The discovery paves the way for designing more efficient materials that dramatically reduce overall energy cost of carbon capture. Such materials could be used for carbon capture from fossil-fuel-based power plants as well as from the atmosphere, mitigating the greenhouse effect. Atomic structure of the adsorbed carbon dioxide grey sphere bonded to two red spheres inserted between the manganese green sphere and amine blue sphere groups within the novel metal-organic framework, forming a linear chain of ammonium carbamate top.
Some hydrogen atoms white sphere are omitted for clarity. In other research, Mg-MOF, an open metal site MOF , has emerged as one of the most promising strategies for capturing and storing greenhouse gases. By applying concepts developed in micro- and nanorobotics, researchers have demonstrated the controlled motion and delivery of cargo payloads embedded in MOFs.
These helical MOF-based micromachines, termed MOFBOTs , are propelled by artificial bacterial flagella, can swim and follow complex trajectories in three dimensions under the control of weak rotational magnetic fields. Similar to the carbon capture application described above, researchers are exploring ways how MOFs may help lower energy consumption for air conditioning by engineering them to hold onto a large amount of refrigerant gases.
The high attachment of this gas — an environmentally friendly fluorocarbon called R and water — to MOFs hold promise for their use in adsorbent cooling systems that a can be powered by waste heat.