Тяжелые металлы в окружающей среде
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| ulescens and S. alfredii than in non-hyperaccumulating relatives. A lower sequestration into root
vacuoles accounts also for the enhanced As translocation in hyperaccumulator compared with non- hyperaccumulator species of Pteris. A key role in heavy metal hyperaccumulation seems to be played by free amino acids, such as histidine and nicotinamine, which form stable complexes with bivalent cations. Free histidine (His) is regarded as the most important ligand involved in Ni hyperaccumulation. In roots of the Ni hyperaccumulator Alyssum lesbiacum, as compared with the non-hyperaccumulator A. montanum, the constitutive overexpression of the TP-PRT1 gene (encoding the ATP-phosphoribosyl transferase enzyme committed in the first step of the biosynthetic pathway) leads to a larger endogenous pool of His, which favors the Ni xylem loading as a Ni-His complex. The high concentrations of His in roots of different Ni hyperaccumulating Thlaspi species suggests that the amino acid may operate in the same way in other hyperaccumulators. Moreover, in hyperaccumulators, but not in non- hyperaccumulators, the Ni–His complexation, besides the involvement in sustaining the Ni release into the xylem, plays an essential role in preventing the heavy metal entrapment in root cell vacu- oles, thus keeping it in the cytosol, in a detoxified form available for translocation. Genes encoding enzymes of the nicotinamine biosynthetic pathway are overexpressed in roots of the Zn/Ni hyperac- cumulators T. caerulescens and A. halleri which contain 3-fold higher amount of nicotinamine than roots of congener non-hyperaccumulating species. However, in T. caerulescens, enhanced nicotin- amine synthesis and nicotinamine–metal chelation show a positive correlation with Ni hyperaccu- mulation, whereas in A. halleri they are involved in Zn hyperaccumulation, with a possible role for the cytosolic nicotinamine–Zn complexes also in keeping metal ions in detoxified form disposable for loading into xylem vessels. A large body of evidence indicates that fast and efficient root-to-shoot translocation of large amounts of heavy metals in hyperaccumulator plants relies on enhanced xylem loading by a consti- tutive overexpression of genes coding for transport systems common to non-hyperaccumulators. The P1B-type ATPases, a class of proteins, also named HMAs (Heavy Metal transporting ATPases), are of particular importance. They operate in heavy metal transport and play a role in metal homeostasis and tolerance. Genes encoding bivalent cation transporters belonging to HMAs (among which HMA4) are overexpressed in roots and shoots of Zn/Cd hyperaccumulators T. caer- ulescens and A. halleri. Moreover, the HMA4 expression is up-regulated when these plants are ex- posed to high levels of Cd and Zn, whereas it is down-regulated in non-hyperaccumulator relatives. The overexpression of HMA4 supports a role of the HMA4 protein (which belongs to the Zn/Co/Cd/Pb HMA subclass and is localized at xylem parenchyma plasma membranes) in Cd and Zn efflux from the root symplasm into the xylem vessels, necessary for shoot hyperaccumulation. This role is upheld by QTL analysis showing co-localization of a major QTL for Zn and Cd toler- ance with the HMA4 gene in A. halleri. Interestingly, it has been demonstrated that the HMA4 ac- tivity positively affects other candidate genes for hyperaccumulation. In fact, the increased expres- sion of HMA4 enhances the expression of genes belonging to the ZIP family, implicated in heavy metal uptake. This strongly suggests that the root-to-shoot translocation acts as a driving force of the hyperaccumulation, by creating a permanent metal deficiency response in roots. The MATE (Multidrug And Toxin Efflux) family of small organic molecule transporters seems to be another kind of transport proteins that are active in heavy metal translocation in hyper- accumulator plants (). FDR3, a gene encoding a member of this family, is constitutively overex- pressed in roots of T. caerulescens and A. halleri. The FDR3 protein, which is localized at root per- icycle plasma membranes, usually operates in the xylem influx of citrate, which is required as a lig- and for Fe homeostasis and transport, but its overexpression in hyperaccumulators suggests that FDR3 might also play a role in translocation of other metals, such as Zn. Whether the long-distance xylem transport of heavy metals can occur in free ionic forms or through metal complexation with organic acids is still controversial. Most of Zn and Cd, for in- 64 65 stance, is present as free hydrated cations in the xylem sap of T. caerulescens and A. halleri, and only one-third of Ni is bound to citrate in the xylem of hyperaccumulator Stackhousia tryoni (Celastraceae). Conversely, almost all Ni is complexed with citrate and other organic acids in the latex of the extreme Ni hyperaccumulator S. acuminata. Detoxification/sequestration Heavy metal induced-oxidative stress is a naturally occurring phenomenon which plants have evolved to deal with. Heavy metals' signals are perceived by receptors, and receptors transduce signals via cAMP, pH, etc. causing alterations in electron transport systems of the cell, which re- sults into an excess production of reactive oxygen species (ROS). ROS cause damage to macromol- ecules and thus create oxidative stress inside the cell. Plants are coping with it through both toler- ance and detoxification mechanisms in the plant cell. These involve the use of ascorbic acid (AsA), catalase (CAT), cysteine (Cys), c-glutamylcysteinesynthetase (c-ECS), glutamine (Glu), glycine (Gly), glutathione reductase (GR), glutathione synthetase (GS), reduced glutathione (GSH), oxi- dized glutathione (GSSG), hydrogen peroxide (H 2 O 2 ), monodehydroascorbate (MDHA), oxygen molecule (O 2 ), superoxide radicals (O−2), reactive oxygen species (ROS), superoxide dismutase (SOD), etc. On the other hand, in the presence of plant hormones, signals received by them initiate a cascade of signal transduction involving haem oxygenase, transcription factors induced by brassi- nosteroids (BES1 and BZR1) and a gibberellic acid-mediated GA-GID1-DELLA signaling path- way. These factors, in turn, initiate expression of the nuclear genes encoding defense proteins, tran- scription factors (TFs), heat shock proteins (HSP), and metal transporter proteins (MTs). The main role of MTs is to protect electron transport chains against heavy metals by regulating their uptake. Other defense proteins protect plant against ROS under heavy metal stress. Great efficiency in detoxification and sequestration is a key property of hyperaccumulators which allows them to concentrate huge amounts of heavy metals in above-ground organs without suffering any phytotoxic effect. This exceptionally high heavy metal accumulation becomes even more astonishing bearing in mind that it principally occurs in leaves where photosynthesis, essential for plant survival, is accomplished, and that the photosynthetic apparatus is a major target for most of these contaminants. The preferential heavy metal detoxification/sequestration do occur in loca- tions, such as epidermis, trichomes and even cuticle, where they do least damage to the photosyn- thetic machinery. In many cases heavy metals are also excluded from both subsidiary and guard cells of stomata. The detoxifying/sequestering mechanisms in aerial organs of hyperaccumulators consist mainly in heavy metal complexation with ligands and/or in their removal from metabolically active cytoplasm by moving them into inactive compartments, mainly vacuoles and cell walls. Compara- tive transcriptome analyses between hyperaccumulator and related non-hyperaccumulator species have demonstrated that also the sequestration trait relies, at least in part, on constitutive overexpres- sion of genes that, in this case, encode proteins operating in heavy metal transfer across the tono- plast and/or plasma membrane and involved in excluding them from cytoplasm. CDF (Cation Dif- fusion Facilitator) family members, also named MTPs (Metal Transporter Proteins), which mediate bivalent cation efflux from the cytosol, are important candidates. The Zn transport into the vacuole, in fact, may initiate a systemic Zn deficiency response that includes the enhancement of the heavy metal uptake and translocation via the increased expression of ZIP transporters in hyperaccumulator plants. The overexpression of HMA3, coding for a vacuolar P1B-ATPase, plausibly involved in Zn compartmentation, and that of CAX genes encoding members of a cation exchanger family that seems to mediate Cd sequestration, have been noticed in T. caerulescens and A. halleri and sup- posed to be involved in heavy metal hyperaccumulation. Small ligands, such as organic acids, have a major role as detoxifying factors. Such ligands may be instrumental in preventing the persistence of heavy metals as free ions in the cytoplasm and even more in enabling their entrapment in vacuoles where the metal–organic acid chelates are pri- marily located. Citrate, for instance, is the main ligand of Ni in leaves of T. goesingense, while cit- 64 65 stance, is present as free hydrated cations in the xylem sap of T. caerulescens and A. halleri, and only one-third of Ni is bound to citrate in the xylem of hyperaccumulator Stackhousia tryoni (Celastraceae). Conversely, almost all Ni is complexed with citrate and other organic acids in the latex of the extreme Ni hyperaccumulator S. acuminata. Detoxification/sequestration Heavy metal induced-oxidative stress is a naturally occurring phenomenon which plants have evolved to deal with. Heavy metals' signals are perceived by receptors, and receptors transduce signals via cAMP, pH, etc. causing alterations in electron transport systems of the cell, which re- sults into an excess production of reactive oxygen species (ROS). ROS cause damage to macromol- ecules and thus create oxidative stress inside the cell. Plants are coping with it through both toler- ance and detoxification mechanisms in the plant cell. These involve the use of ascorbic acid (AsA), catalase (CAT), cysteine (Cys), c-glutamylcysteinesynthetase (c-ECS), glutamine (Glu), glycine (Gly), glutathione reductase (GR), glutathione synthetase (GS), reduced glutathione (GSH), oxi- dized glutathione (GSSG), hydrogen peroxide (H 2 O 2 ), monodehydroascorbate (MDHA), oxygen molecule (O 2 ), superoxide radicals (O−2), reactive oxygen species (ROS), superoxide dismutase (SOD), etc. On the other hand, in the presence of plant hormones, signals received by them initiate a cascade of signal transduction involving haem oxygenase, transcription factors induced by brassi- nosteroids (BES1 and BZR1) and a gibberellic acid-mediated GA-GID1-DELLA signaling path- way. These factors, in turn, initiate expression of the nuclear genes encoding defense proteins, tran- scription factors (TFs), heat shock proteins (HSP), and metal transporter proteins (MTs). The main role of MTs is to protect electron transport chains against heavy metals by regulating their uptake. Other defense proteins protect plant against ROS under heavy metal stress. Great efficiency in detoxification and sequestration is a key property of hyperaccumulators which allows them to concentrate huge amounts of heavy metals in above-ground organs without suffering any phytotoxic effect. This exceptionally high heavy metal accumulation becomes even more astonishing bearing in mind that it principally occurs in leaves where photosynthesis, essential for plant survival, is accomplished, and that the photosynthetic apparatus is a major target for most of these contaminants. The preferential heavy metal detoxification/sequestration do occur in loca- tions, such as epidermis, trichomes and even cuticle, where they do least damage to the photosyn- thetic machinery. In many cases heavy metals are also excluded from both subsidiary and guard cells of stomata. The detoxifying/sequestering mechanisms in aerial organs of hyperaccumulators consist mainly in heavy metal complexation with ligands and/or in their removal from metabolically active cytoplasm by moving them into inactive compartments, mainly vacuoles and cell walls. Compara- tive transcriptome analyses between hyperaccumulator and related non-hyperaccumulator species have demonstrated that also the sequestration trait relies, at least in part, on constitutive overexpres- sion of genes that, in this case, encode proteins operating in heavy metal transfer across the tono- plast and/or plasma membrane and involved in excluding them from cytoplasm. CDF (Cation Dif- fusion Facilitator) family members, also named MTPs (Metal Transporter Proteins), which mediate bivalent cation efflux from the cytosol, are important candidates. The Zn transport into the vacuole, in fact, may initiate a systemic Zn deficiency response that includes the enhancement of the heavy metal uptake and translocation via the increased expression of ZIP transporters in hyperaccumulator plants. The overexpression of HMA3, coding for a vacuolar P1B-ATPase, plausibly involved in Zn compartmentation, and that of CAX genes encoding members of a cation exchanger family that seems to mediate Cd sequestration, have been noticed in T. caerulescens and A. halleri and sup- posed to be involved in heavy metal hyperaccumulation. Small ligands, such as organic acids, have a major role as detoxifying factors. Such ligands may be instrumental in preventing the persistence of heavy metals as free ions in the cytoplasm and even more in enabling their entrapment in vacuoles where the metal–organic acid chelates are pri- marily located. Citrate, for instance, is the main ligand of Ni in leaves of T. goesingense, while cit- rate and acetate bind Cd in leaves of S. nigrum. Moreover, most Zn in A. halleri and Cd in T. caer- жүктеу/скачать 6.49 Mb. Достарыңызбен бөлісу:
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