Тяжелые металлы в окружающей среде
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- Improving heavy metal tolerance in plants – biotechnological potential
| ulescens are complexed with malate.
The heavy metal detoxification in hyperaccumulators, in contrast with tolerant non- hyperaccumulator plants, does not rely on high molecular mass ligands, such as phytochelatins, likely because of the excessive sulphur amounts and the prohibitive metabolic cost that a massive synthesis of this kind of chelators would require. Overexpression of antioxidation-related genes, as well as enhanced synthesis of glutathione (GSH) as pivotal antioxidant molecule, do occur, instead, in hyperaccumulators, as a strategy to reinforce the cell antioxidant system and cope with the risk of ROS rise due to heavy metal stress. The major detoxification strategy in Se hyperaccumulators is to get rid of selenoaminoacids, mainly selenocysteine (SeCys), derived from selenate assimilation in leaf chloroplasts. This detoxification occurs through methylation of Se-Cys to the harmless non- protein amino acid methylselenocysteine in a reaction catalyzed by a selenocysteine methyltransfer- ase, which is constitutively expressed and activated only in leaves of hyperaccumulator species. Improving heavy metal tolerance in plants – biotechnological potential The broad picture described above opens a multitude of options for improving HM tolerance in crop plants. Various proposed pathways for dealing with HM are targeted at the main processes that influence plants' susceptibility to heavy metals – the sensing, uptake and transport of heavy metal ions, the formation of phytochelatin-heavy metal complexes, and their sequestration in vacu- ole. Secondary processes that are also important include defense reactions based on the antioxidant system and the preservation of membrane lipid integrity. All of these processes are subject to both transcriptional and posttranslational controls of a wide array of genes. It has been demonstrated that the heavy metal tolerance of plants can be changed significantly using a wide range of genetic, mo- lecular, and biotechnological tools. The capacity of the cell wall for rapid dynamic reorganization in response to heavy metal stress has been demonstrated by proteomic studies, which have revealed dramatic changes in the abundance of proteins involved in cell wall metabolism following exposure to metal ions. As a re- sult, the properties of cell walls and methods for manipulating them have attracted great interest be- cause of their potential biotechnological applications in improving plant stress tolerance and productivity. In fact, the plasma membranes of the root cells are the first physiological barriers to the entrance of heavy metals into the symplast. Another primary target for biotech applications are metal transporters as they are crucially involved in tolerance, generally by increasing deposition of the toxic elements in the vacuole. Some, such as HMAs (heavy metal associated transporters) and NRAMPs (natural resistance- associated macrophage proteins), are good targets for increasing heavy metal tolerance through ge- netic engineering. Importantly, their overexpression increases heavy metal tolerance and does not have detrimental effects on plant physiological homeostasis. The beneficial effects of metal trans- porters in the higher heavy metal tolerance primarily stem from their ability to increase heavy metal sequestration to vacuoles, where the formation of heavy metal-chelate complexes greatly reduces their potential toxicity. Another group of proteins that seem good targets are the cation diffusion facilitator (CDF) family of proteins which function as antiporters of Zn, Cd, Co and/or Ni with protons, and are local- ized to vacuolar membranes. As specific amino acid sequences in CDFs are responsible for their selectivity and for the sensing of cytoplasmic HM levels point mutations in these sequences confer higher Co and Cd tolerance, suggesting that engineering these proteins may provide a general way of broadening their ion specificity. Transcription factors represent another very promising tool for improving plant heavy metal tolerance. The Fe-homeostasis transcription factor FIT (FER-like Deficiency Induced Transcripition Factor) and basic-region leucine zipper (bZIP) are two better studied representatives from this group that were found to be able to modify HM tolerance. Yet another broad group of potential targets includes metallothioneins (MTs) and phyto- chelatins (PCs) – ubiquitous low-molecular-weight, cysteine-rich proteins that bind to metals and 66 67 are important in metal detoxification. Their mode of action is often connected to their ability to scavenge reactive oxygen species (ROS). Identification of the above effects suggests that bolstering plants' antioxidant defense re- sponses could have beneficial effects on heavy metal tolerance. The simultaneous overexpression of Cu/Zn superoxide dismutase and ascorbate peroxidase under an oxidative stress-induced promoter was found to increase plants resistance to Cd, Cu and As. Similarly, the overexpression of aldehyde dehydrogenase gene conferred increased tolerance of Cu and Cd. At present, there are two main biotechnological approaches to reducing the impact of heavy metal toxicity in nature. One involves enhancing the heavy metal uptake capacity of certain plant species. The second involves improving plant resistance to heavy metal toxicity, which would pre- vent the inhibition of plant growth and productivity. Genetic manipulation of the expression, activity and localization of heavy metal ion trans- porters is promising approach to this goal, since these proteins can directly control the uptake, dis- tribution and accumulation of various elements in plants. Bioengineered plants tolerant to the presence of toxic levels of metals like Cd, Zn, Cr, Cu, Pb, As and Se have been reported. A combination of transporter genes has also been used in rapidly growing plant species leading to promising results. Transgenic B. juncea, grown either in hydropon- ic or in soils, shows higher uptake of Se and enhanced Se tolerance than the wild species. To engi- neer Se tolerance the selenocysteine methyltransferase (SMT) gene has been transferred from the Se hyperaccumulator A. bisulcatus to Se-non-tolerant B. juncea. SMT transgenic plants of B. juncea grown in a contaminated soil accumulate 60% more Se than the wild-type. A promising biotechnological approach for enhancing the potential for metal phytoextrac- tion, may be to improve the hyperaccumulator growth rate through selective breeding, or by the transfer of metal hyperaccumulation genes to high biomass species. In an effort to correct for small sizes of hyperaccumulator plants, somatic hybrids have been generated between T. caerulescens and Brassica napus. High biomass hybrid selected for Zn toler- ance indicates that the transfer of the metal hyperaccumulating phenotype is feasible. Furthermore, somatic hybrids from T. caerulescens and B. juncea were able to remove significant amounts of Pb. The transgenic plant approach has shown to be promising, but only very few studies have been performed till now under field conditions. Moreover, it has to be considered that tolerance and accumulation of heavy metals and thus phytoextraction potential of a given plant are controlled by many genes, so that genetic manipulations to improve these traits in fast-growing plants will require to change the expression levels in a number of genes, and to cross them to determine the number of genes involved and their characteristics. In laboratory as well as filed studies, two strategies have been used for plant hormone- mediated increase in stress tolerance as well as crop yield. These strategies include exogenous ap- plication of plant hormones and genetic manipulation of their endogenous contents. Both approach- es have given promising results for increasing crop yield and enhancing stress tolerance in a variety of crop species. In this context, modulation of endogenous hormone and hormone-like levels by ge- netic engineering has emerged as an efficient strategy for heavy metal tolerance as well as achiev- ing high yield with desired agronomic traits by using salicylic acid (SA), brassinosteroids (BRs), and gibberellins (GA). Stress tolerance is a genetically complex process that involves many components of signal- ing pathways, multigenic in nature, and thus, comparatively more difficult to control and engineer. Therefore, plant-engineering strategies for heavy metal tolerance depend on the expression of gene(s) whose product(s) are involved either in signaling and regulatory pathways or in the synthe- sis of functional and structural proteins and metabolites that confer heavy metal stress tolerance. Functions and regulations of genes involved in metal exclusion, uptake, root-to-shoot translocation, detoxification / sequestration mechanisms need to be fully understood to render transgenic approach applicable to solving the problem. 66 67 are important in metal detoxification. Their mode of action is often connected to their ability to scavenge reactive oxygen species (ROS). Identification of the above effects suggests that bolstering plants' antioxidant defense re- sponses could have beneficial effects on heavy metal tolerance. The simultaneous overexpression of Cu/Zn superoxide dismutase and ascorbate peroxidase under an oxidative stress-induced promoter was found to increase plants resistance to Cd, Cu and As. Similarly, the overexpression of aldehyde dehydrogenase gene conferred increased tolerance of Cu and Cd. At present, there are two main biotechnological approaches to reducing the impact of heavy metal toxicity in nature. One involves enhancing the heavy metal uptake capacity of certain plant species. The second involves improving plant resistance to heavy metal toxicity, which would pre- vent the inhibition of plant growth and productivity. Genetic manipulation of the expression, activity and localization of heavy metal ion trans- porters is promising approach to this goal, since these proteins can directly control the uptake, dis- tribution and accumulation of various elements in plants. Bioengineered plants tolerant to the presence of toxic levels of metals like Cd, Zn, Cr, Cu, Pb, As and Se have been reported. A combination of transporter genes has also been used in rapidly growing plant species leading to promising results. Transgenic B. juncea, grown either in hydropon- ic or in soils, shows higher uptake of Se and enhanced Se tolerance than the wild species. To engi- neer Se tolerance the selenocysteine methyltransferase (SMT) gene has been transferred from the Se hyperaccumulator A. bisulcatus to Se-non-tolerant B. juncea. SMT transgenic plants of B. juncea grown in a contaminated soil accumulate 60% more Se than the wild-type. A promising biotechnological approach for enhancing the potential for metal phytoextrac- tion, may be to improve the hyperaccumulator growth rate through selective breeding, or by the transfer of metal hyperaccumulation genes to high biomass species. In an effort to correct for small sizes of hyperaccumulator plants, somatic hybrids have been generated between T. caerulescens and Brassica napus. High biomass hybrid selected for Zn toler- ance indicates that the transfer of the metal hyperaccumulating phenotype is feasible. Furthermore, somatic hybrids from T. caerulescens and B. juncea were able to remove significant amounts of Pb. The transgenic plant approach has shown to be promising, but only very few studies have been performed till now under field conditions. Moreover, it has to be considered that tolerance and accumulation of heavy metals and thus phytoextraction potential of a given plant are controlled by many genes, so that genetic manipulations to improve these traits in fast-growing plants will require to change the expression levels in a number of genes, and to cross them to determine the number of genes involved and their characteristics. In laboratory as well as filed studies, two strategies have been used for plant hormone- mediated increase in stress tolerance as well as crop yield. These strategies include exogenous ap- plication of plant hormones and genetic manipulation of their endogenous contents. Both approach- es have given promising results for increasing crop yield and enhancing stress tolerance in a variety of crop species. In this context, modulation of endogenous hormone and hormone-like levels by ge- netic engineering has emerged as an efficient strategy for heavy metal tolerance as well as achiev- ing high yield with desired agronomic traits by using salicylic acid (SA), brassinosteroids (BRs), and gibberellins (GA). Stress tolerance is a genetically complex process that involves many components of signal- ing pathways, multigenic in nature, and thus, comparatively more difficult to control and engineer. Therefore, plant-engineering strategies for heavy metal tolerance depend on the expression of gene(s) whose product(s) are involved either in signaling and regulatory pathways or in the synthe- sis of functional and structural proteins and metabolites that confer heavy metal stress tolerance. Functions and regulations of genes involved in metal exclusion, uptake, root-to-shoot translocation, detoxification / sequestration mechanisms need to be fully understood to render transgenic approach applicable to solving the problem. With the advent of modern biotechnological solutions it appeared possible to apply integra- tive approach to this field. The availability of the large scale data generated by the next generation sequencing (NGS) and new bioinformatics tools allows for implementing metabolomics, tran- scriptomics and genomics as an integrated toolset. While the practical use of the results obtained few years back would have required the use of transgenics, the development of latest gene-editing tools opens the opportunities for much easier and quicker access to the outcomes of the knowledge generated. Furthermore, it allows for producing both more precise and less controversial outcomes, thus avoiding unnecessary hurdles to scientific research. жүктеу/скачать 6.49 Mb. 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