Plant gene transfer method
Abstract The birth and development of genetic engineering is inseparable from the emergence and development of gene transfer methods. In order to achieve different goals, a variety of gene transfer methods are needed. In general, the transfer of genes to plants is more difficult than that of microorganisms and animals. However, due to the importance of plant genetic engineering for crop improvement, rapid development has been achieved in recent years, and various gene transfer methods have emerged in an endless stream. This article systematically introduced the existing methods of plant gene transfer. Key words Gene transfer method Foreign gene Ti plasmid co-cultivation vector Transgenic plant Plant genetic engineering was developed on the basis of the in-depth development of molecular biology and the increasingly perfection of recombinant DNA technology. In the plant resistance to diseases, insect resistance, resistance to insecticides and changes in certain components of plants have been transferred to a number of genetic plants, and some have formed strains, in order to improve crop yield, resistance and improve quality, rapid, high-quality, stable production Breeding offers a whole new approach. In the research of genetic engineering of higher plants, the transfer of foreign genes is an extremely important link. Because of the unique nature of plants and their cells, there are also special requirements for gene transfer methods. A number of exogenous gene transfer methods can be divided into two categories according to the form of the vector: that is, the A. tumefaciens containing the exogenous gene Ti plasmid infects the plant and the plant directly transformed with the DNA containing the exogenous gene.
1 Agrobacterium tumefaciens transformed with Agrobacterium tumefaciens Ti plasmid is a gram-negative bacterium, which can infect wounded dicotyledonous plants through wounds, causing the wounded tissues of the plants to proliferate. Crown tumor. Agrobacterium contains a circular DNA that is independent of chromosomes and stably inherited in Agrobacterium cells, namely the Ti plasmid. When Agrobacterium tumefaciens infects plants, a piece of DNA (T-DNA) of the Ti plasmid can be transferred into the plant cells and stably retained in the chromosome of the plant cells, becoming a newly added group of genes in the plant cells and eventually passing through sexual Generations are passed on to offspring. In most cases, tumors or transformed cells induced by wild-type Agrobacterium Ti plasmids are unable to regenerate normal plants due to the imbalance of plant hormones caused by the T-DNA gene product, which makes the transformation characteristics difficult to pass through the seed generation. According to legend. Therefore, the Ti plasmid must be modified and remodeled, even if some of the genes are missing, without affecting their transformation and expression functions.
In 1973, Stanley Cohen of Stanford University and Herbert Boyer of the University of California, et al., carried the plasmid Psc101 with the tetracycline-resistant gene and the plasmid RSF1010 from Salmonella typhimurium (with anti-streptomycin and sulfonamide). Genes) are cut and spliced ​​with endonucleases and ligases to create a new DNA molecule, a hybrid plasmid. After transforming E. coli with it, the transformants were both resistant to tetracycline and streptomycin. Subsequently, Vanlare et al. (1974), Watson et al. (1975), and Zaenen et al. (1971), through continuous efforts, made it possible to use Ti plasmids as vectors for the transfer and expression of plant foreign genes. The key is to transform the Ti plasmid. What they do is to remove the tumor-promoting gene on the Ti plasmid and maintain its T-DNA transfer ability and the ability of foreign genes to express in regenerated plants. In this way, the plant is differentiated into normal plants instead of tumors after being infected with the modified Ti plasmid. The artificially modified Ti plasmid (ie, the Ti plasmid-derived vector) becomes a universal vector capable of introducing genes into plants. There are several methods for gene transfer using Ti plasmid-derived vectors:
(1) The protoplast co-cultivation method is used because the protoplast has genetic consistency and stability, and has the name of "second seed," plus it has no cell walls and is easy to accept a variety of foreign genetic material. The earliest and most widely used receptor system. The protoplasts precultured for 2 to 3 days are mixed with Agrobacterium tumefaciens containing the modified Ti plasmid having the desired foreign gene and incubated in the culture medium for a certain period of time. Then the protoplasts are collected by centrifugation and cultured until regeneration. Become a complete plant.
(2) Plant tissue and organ co-culture methods use plant tissues and organs as gene transfer receptors because plant wounds are particularly sensitive to infection by Agrobacterium tumefaciens, and the wound site is where the cells rapidly divide and induce long shoots. Transformed cells into a higher record. In addition, based on the following considerations: 1, not all plants are easy to regenerate plants from the protoplasm. 2. The period of regenerating plants from plant tissues and organs is short; 3. The regenerated plant tissues and mutations or chromosomes are recorded at a low rate. This method is based on Horsch's "Blade Method" based on artificially wounded leaves, upper and lower hypocotyls, stem segments, root segments, etc., soaked in Agrobacterium tumefaciens solution for several seconds to several minutes, and then co-cultured. - 3 days, regenerating plants can be obtained through tissue culture. This method is simple and effective, and in principle, it can transform all dicotyledons that can regenerate plants from the leaves. To date, the pest-resistant transgenic plants obtained on tobacco and other crops have basically been produced as such. In addition, the whole plant horizontal transformation method can also be used: in vivo seeding at the seedling plant stem incision, and then the callus excised from the wound surface for ex vivo cultivation. The above methods of infecting plant cells with Agrobacterium tumefaciens may be limited by the host range, because although there have been few successful reports, it is generally believed that A. tumefaciens is difficult to overcome.
(3) Fusion transformation of plant protoplasts and Agrobacterium spheroplasts Agrobacterium was treated with lysozyme to remove its cell wall and transform rod-shaped bacteria into spherical protoplasm. It is then fused with plant receptor protoplasts with the help of an inducer to recombine the plasmid DNA of Agrobacterium tumefaciens with the plant cell DNA. This conversion method has many advantages. On the one hand, it does not take time to extract and purify the Ti plasmid, and the protoplasm of Agrobacterium also protects the Ti plasmid inside it against nuclease attack. On the other hand, since the process is only a fusion process rather than an infection, it is possible to become a monocotyledonous plant conversion method without being limited by the host range. There are also other natural vectors for gene transfer in nature, such as Agrobacterium rhizogenes and its Ri plasmid. Although the structure and gene expression of the Ri proton T-DNA gene differs from that of the Ti plasmid, it is very similar to the function of the Ti plasmid in transforming plant cells, and the transformation methods used by people are very similar. Although the toxicity to the Ri plasmid is small, and the transformed hairy roots easily form wound tissues and regenerate morphologically normal plants, they have progressed rapidly in recent years.
2 Direct transformation of foreign DNA-containing DNA Direct transformation of DNA involves the introduction of naked foreign DNA into plant cells using physical, chemical and biological means, and the expression of the genes contained in the foreign DNA in plant cells. This method fundamentally overcomes the defect that the Li plasmid can only infect dicotyledonous plants and greatly expands the range of receptor plants.
2.1 Chemical Protoplasts Plant protoplasts in the absence of the carrier, with the help of some chemical reagents can absorb foreign DNA, plasmids and other genetic material, and may be integrated into the plant chromosomes. Commonly used chemical reagents are PEG (polyethylene glycol), PLD (poly ornithine), PVA (polyethylene glycol), among which the most commonly used is PEG. Using this method, Davey and Krens et al. The chemical method breaks the limitation of the Agrobacterium tumefaciens infection scope, making it widely applicable to many kinds of monocotyledonous cereal plants. Now it has realized the cultivation of one grain wheat, perennial rye, rice, high grain and wheat, etc. Transformation of protoplasts and dicotyledon rape, tobacco and petunia. However, the use of chemical methods alone for transformation is more difficult to succeed. If combined with other methods (such as the electrokinetic method and gene gun method described below), the conversion efficiency can be greatly improved. Shillito et al. (1985) combined PEG and electrokinetics to achieve a 2% conversion of tobacco protoplasts, a 1,000-fold improvement over PEG alone.
2.2 Electrokinetics This is a new method that is widely used. It was originally reported by FROMM and was first applied to plant cells by Li Baojian in 1985. The protoplasts are mixed with DNA in solution and then subjected to a short pulse current, and the plastids have been regenerated into fertile transgenic rice and corn plants. Electrokinetics has great potential for gene transformation of plant cells. Not only protoplasts but also intact single cells can use this method, which may be of greater significance to plants that are difficult to regenerate from protoplasts. In addition, the principle and effect of laser microbeam technology and electrokinetic method are very similar.
2.3 Microinjection It is a method of injecting foreign DNA directly into plant cells. Its development is largely due to the adhesion of animal cells to the surface of slides. However, because plant cells do not have this property, people At the same time try to fix the plant cells first, the commonly used methods are: (1) agarose embedding method. (2) Polylysine adhesion method. (3) straw support method. The DNA was then injected directly into a single, fixed, living cell using a very fine glass tube (inner diameter 0.1 to 0.5 um). This operation requires a delicate device consisting of a microscope and a slim micromanipulator. One disadvantage of microinjection is that the number of cells injected is small, but the success rate of DNA insertion in each injected cell is high. According to the microinjection of tobacco protoplasts by Corssway et al., the average conversion rate was as high as 6% (cytoplasmic injection) and 14% (intranuclear injection): Indole and corn protoplasts were also successfully transformed by this method. In addition, the microinjection method is applied to pollen, egg cells, or other embryonic tissue, and also overcomes the difficulty in the culture brought about by using the protoplast as a receptor. Chinese scholars have used the pollinated ovary as a receptor and used microinjection to inject the cloned zein gene into the embryo sac of rice. The regenerated plant has a zein gene.
The pollen tube passage method was first established by Zhou Guangyu in 1983. After a certain period of time after plant pollination, exogenous DNA is injected into the axial part of the ovary of the cotton, and the DNA enters the embryo sac along the formed pollen tube channel, thereby transforming fertilized eggs or blast cells that do not have normal cell walls and transform them into The rate is as high as 10%. The establishment of this method has created a precedent for the transformation of whole plants in vivo. In addition, De La Pena injected exogenous DNA into the rye section of the rye in 1987 and also obtained transgenic rye plants. Currently, pollen tube pathways have been used to obtain transgenic plants in rice, cotton, corn and soybeans.
2.5 Gene gun method Klein et al. (1987) first bombarded onion epithelial cells with a gene gun, successfully injected a tungsten bomb containing foreign DNA, and realized the expression of foreign genes in intact tissues. This method uses an anti-gun structure device - a gene gun. The front end of the barrel is sealed. Only a small hole with a diameter of about 1 mm can be used and the warhead cannot pass through. The specific operation is to form a suspension of tungsten powder or other heavy metal powder with a diameter of about 4um in the exogenous DNA, and then the exogenous DNA will be adsorbed on the surface of the tungsten powder particles, and then the metal particles adsorbed with the exogenous genetic material are loaded. To the front end of the cylindrical warhead, after the detonation, the warhead accelerates into the barrel and is blocked near the barrel mouth. The tungsten powder particles on the front end of the warhead disengage from the warhead under inertia and shoot directly through the 1mm hole at high speed. Into the receptor, the foreign DNA adsorbed on the surface also enters the cell. Metallic particles can also be accelerated with high pressure discharge or high pressure gas. Compared with the microinjection method, this method has the advantage that once treatment can transform many cells, the receptor can be plant tissue or cells. Using this method, transformed cells of maize, wheat, rice, and fertile transformed plants such as tobacco and soybean have been obtained. In addition, there are also reports of bombardment and transformation of immature embryos.
2.6 Liposomes The phospholipids are suspended in water. Under appropriate conditions, when treated with high-energy sound waves, the phospholipid molecules cluster together to form dense vesicle-like structures called liposomes. Encapsulating some of the DNA and RNA molecules in it becomes an artificially simulated protoplast, and its outer membrane is equivalent to an artificial cytoplasmic membrane. If liposomes and plant protoplasts are mixed in a suitable medium, endocytosis, fusion or exchange occurs between them, so that the liposomes or their contents are inhaled into plant protoplasts. It is then fused with lysosomes and gradually degraded to release the contents. In this way, the encapsulated nucleic acid can be delivered to various types of cells, which is a liposome-mediated nucleic acid delivery pathway.
All of the above are the more mature and successful gene transfer methods in the current plant genetic engineering work. Each method has its own advantages, but there are some deficiencies or there are some limitations in the scope of application. In the future, it seems that the approach to explore gene transfer can be considered from the following three aspects: First, tap the methodological potential, expand its scope of application and make it more perfect. Second, several methods are used together to achieve a single method that is difficult to achieve. Third, exploring methods and finding vectors.
Note: 1. Co-cultivation method: 2. Chemical method: 3. Electrokinetic method: 4. Microinjection method: 5. Pollen tube channel method: 6. Gene gun method: 7. Liposome method: 8. Plant protoplasts Agrobacterium spheroplast fusion transformation 3 References Deng Wanyin et al. Agrobacterium tumefaciens can transform barley and wheat. Chinese Science, (B). 1989.2: 171---175 2 Charles SG Robert TF Genetically engineering plants for crop improvement Science 1989. Vol. 244: 1293~1299 3 Horsch RB et al. A simple and general method for transferring genes into plants. Science. 1985, 217:1229~1231 Horst L. et al. Gene transfer to cereal cells. By protoplast Transformation. MGG. 1985. 199:178. Ingo P. et al. Direct gene transfer to cell of a graminaceous Monocot. Ibid. 1985. 199:183~185.
1 Agrobacterium tumefaciens transformed with Agrobacterium tumefaciens Ti plasmid is a gram-negative bacterium, which can infect wounded dicotyledonous plants through wounds, causing the wounded tissues of the plants to proliferate. Crown tumor. Agrobacterium contains a circular DNA that is independent of chromosomes and stably inherited in Agrobacterium cells, namely the Ti plasmid. When Agrobacterium tumefaciens infects plants, a piece of DNA (T-DNA) of the Ti plasmid can be transferred into the plant cells and stably retained in the chromosome of the plant cells, becoming a newly added group of genes in the plant cells and eventually passing through sexual Generations are passed on to offspring. In most cases, tumors or transformed cells induced by wild-type Agrobacterium Ti plasmids are unable to regenerate normal plants due to the imbalance of plant hormones caused by the T-DNA gene product, which makes the transformation characteristics difficult to pass through the seed generation. According to legend. Therefore, the Ti plasmid must be modified and remodeled, even if some of the genes are missing, without affecting their transformation and expression functions.
In 1973, Stanley Cohen of Stanford University and Herbert Boyer of the University of California, et al., carried the plasmid Psc101 with the tetracycline-resistant gene and the plasmid RSF1010 from Salmonella typhimurium (with anti-streptomycin and sulfonamide). Genes) are cut and spliced ​​with endonucleases and ligases to create a new DNA molecule, a hybrid plasmid. After transforming E. coli with it, the transformants were both resistant to tetracycline and streptomycin. Subsequently, Vanlare et al. (1974), Watson et al. (1975), and Zaenen et al. (1971), through continuous efforts, made it possible to use Ti plasmids as vectors for the transfer and expression of plant foreign genes. The key is to transform the Ti plasmid. What they do is to remove the tumor-promoting gene on the Ti plasmid and maintain its T-DNA transfer ability and the ability of foreign genes to express in regenerated plants. In this way, the plant is differentiated into normal plants instead of tumors after being infected with the modified Ti plasmid. The artificially modified Ti plasmid (ie, the Ti plasmid-derived vector) becomes a universal vector capable of introducing genes into plants. There are several methods for gene transfer using Ti plasmid-derived vectors:
(1) The protoplast co-cultivation method is used because the protoplast has genetic consistency and stability, and has the name of "second seed," plus it has no cell walls and is easy to accept a variety of foreign genetic material. The earliest and most widely used receptor system. The protoplasts precultured for 2 to 3 days are mixed with Agrobacterium tumefaciens containing the modified Ti plasmid having the desired foreign gene and incubated in the culture medium for a certain period of time. Then the protoplasts are collected by centrifugation and cultured until regeneration. Become a complete plant.
(2) Plant tissue and organ co-culture methods use plant tissues and organs as gene transfer receptors because plant wounds are particularly sensitive to infection by Agrobacterium tumefaciens, and the wound site is where the cells rapidly divide and induce long shoots. Transformed cells into a higher record. In addition, based on the following considerations: 1, not all plants are easy to regenerate plants from the protoplasm. 2. The period of regenerating plants from plant tissues and organs is short; 3. The regenerated plant tissues and mutations or chromosomes are recorded at a low rate. This method is based on Horsch's "Blade Method" based on artificially wounded leaves, upper and lower hypocotyls, stem segments, root segments, etc., soaked in Agrobacterium tumefaciens solution for several seconds to several minutes, and then co-cultured. - 3 days, regenerating plants can be obtained through tissue culture. This method is simple and effective, and in principle, it can transform all dicotyledons that can regenerate plants from the leaves. To date, the pest-resistant transgenic plants obtained on tobacco and other crops have basically been produced as such. In addition, the whole plant horizontal transformation method can also be used: in vivo seeding at the seedling plant stem incision, and then the callus excised from the wound surface for ex vivo cultivation. The above methods of infecting plant cells with Agrobacterium tumefaciens may be limited by the host range, because although there have been few successful reports, it is generally believed that A. tumefaciens is difficult to overcome.
(3) Fusion transformation of plant protoplasts and Agrobacterium spheroplasts Agrobacterium was treated with lysozyme to remove its cell wall and transform rod-shaped bacteria into spherical protoplasm. It is then fused with plant receptor protoplasts with the help of an inducer to recombine the plasmid DNA of Agrobacterium tumefaciens with the plant cell DNA. This conversion method has many advantages. On the one hand, it does not take time to extract and purify the Ti plasmid, and the protoplasm of Agrobacterium also protects the Ti plasmid inside it against nuclease attack. On the other hand, since the process is only a fusion process rather than an infection, it is possible to become a monocotyledonous plant conversion method without being limited by the host range. There are also other natural vectors for gene transfer in nature, such as Agrobacterium rhizogenes and its Ri plasmid. Although the structure and gene expression of the Ri proton T-DNA gene differs from that of the Ti plasmid, it is very similar to the function of the Ti plasmid in transforming plant cells, and the transformation methods used by people are very similar. Although the toxicity to the Ri plasmid is small, and the transformed hairy roots easily form wound tissues and regenerate morphologically normal plants, they have progressed rapidly in recent years.
2 Direct transformation of foreign DNA-containing DNA Direct transformation of DNA involves the introduction of naked foreign DNA into plant cells using physical, chemical and biological means, and the expression of the genes contained in the foreign DNA in plant cells. This method fundamentally overcomes the defect that the Li plasmid can only infect dicotyledonous plants and greatly expands the range of receptor plants.
2.1 Chemical Protoplasts Plant protoplasts in the absence of the carrier, with the help of some chemical reagents can absorb foreign DNA, plasmids and other genetic material, and may be integrated into the plant chromosomes. Commonly used chemical reagents are PEG (polyethylene glycol), PLD (poly ornithine), PVA (polyethylene glycol), among which the most commonly used is PEG. Using this method, Davey and Krens et al. The chemical method breaks the limitation of the Agrobacterium tumefaciens infection scope, making it widely applicable to many kinds of monocotyledonous cereal plants. Now it has realized the cultivation of one grain wheat, perennial rye, rice, high grain and wheat, etc. Transformation of protoplasts and dicotyledon rape, tobacco and petunia. However, the use of chemical methods alone for transformation is more difficult to succeed. If combined with other methods (such as the electrokinetic method and gene gun method described below), the conversion efficiency can be greatly improved. Shillito et al. (1985) combined PEG and electrokinetics to achieve a 2% conversion of tobacco protoplasts, a 1,000-fold improvement over PEG alone.
2.2 Electrokinetics This is a new method that is widely used. It was originally reported by FROMM and was first applied to plant cells by Li Baojian in 1985. The protoplasts are mixed with DNA in solution and then subjected to a short pulse current, and the plastids have been regenerated into fertile transgenic rice and corn plants. Electrokinetics has great potential for gene transformation of plant cells. Not only protoplasts but also intact single cells can use this method, which may be of greater significance to plants that are difficult to regenerate from protoplasts. In addition, the principle and effect of laser microbeam technology and electrokinetic method are very similar.
2.3 Microinjection It is a method of injecting foreign DNA directly into plant cells. Its development is largely due to the adhesion of animal cells to the surface of slides. However, because plant cells do not have this property, people At the same time try to fix the plant cells first, the commonly used methods are: (1) agarose embedding method. (2) Polylysine adhesion method. (3) straw support method. The DNA was then injected directly into a single, fixed, living cell using a very fine glass tube (inner diameter 0.1 to 0.5 um). This operation requires a delicate device consisting of a microscope and a slim micromanipulator. One disadvantage of microinjection is that the number of cells injected is small, but the success rate of DNA insertion in each injected cell is high. According to the microinjection of tobacco protoplasts by Corssway et al., the average conversion rate was as high as 6% (cytoplasmic injection) and 14% (intranuclear injection): Indole and corn protoplasts were also successfully transformed by this method. In addition, the microinjection method is applied to pollen, egg cells, or other embryonic tissue, and also overcomes the difficulty in the culture brought about by using the protoplast as a receptor. Chinese scholars have used the pollinated ovary as a receptor and used microinjection to inject the cloned zein gene into the embryo sac of rice. The regenerated plant has a zein gene.
The pollen tube passage method was first established by Zhou Guangyu in 1983. After a certain period of time after plant pollination, exogenous DNA is injected into the axial part of the ovary of the cotton, and the DNA enters the embryo sac along the formed pollen tube channel, thereby transforming fertilized eggs or blast cells that do not have normal cell walls and transform them into The rate is as high as 10%. The establishment of this method has created a precedent for the transformation of whole plants in vivo. In addition, De La Pena injected exogenous DNA into the rye section of the rye in 1987 and also obtained transgenic rye plants. Currently, pollen tube pathways have been used to obtain transgenic plants in rice, cotton, corn and soybeans.
2.5 Gene gun method Klein et al. (1987) first bombarded onion epithelial cells with a gene gun, successfully injected a tungsten bomb containing foreign DNA, and realized the expression of foreign genes in intact tissues. This method uses an anti-gun structure device - a gene gun. The front end of the barrel is sealed. Only a small hole with a diameter of about 1 mm can be used and the warhead cannot pass through. The specific operation is to form a suspension of tungsten powder or other heavy metal powder with a diameter of about 4um in the exogenous DNA, and then the exogenous DNA will be adsorbed on the surface of the tungsten powder particles, and then the metal particles adsorbed with the exogenous genetic material are loaded. To the front end of the cylindrical warhead, after the detonation, the warhead accelerates into the barrel and is blocked near the barrel mouth. The tungsten powder particles on the front end of the warhead disengage from the warhead under inertia and shoot directly through the 1mm hole at high speed. Into the receptor, the foreign DNA adsorbed on the surface also enters the cell. Metallic particles can also be accelerated with high pressure discharge or high pressure gas. Compared with the microinjection method, this method has the advantage that once treatment can transform many cells, the receptor can be plant tissue or cells. Using this method, transformed cells of maize, wheat, rice, and fertile transformed plants such as tobacco and soybean have been obtained. In addition, there are also reports of bombardment and transformation of immature embryos.
2.6 Liposomes The phospholipids are suspended in water. Under appropriate conditions, when treated with high-energy sound waves, the phospholipid molecules cluster together to form dense vesicle-like structures called liposomes. Encapsulating some of the DNA and RNA molecules in it becomes an artificially simulated protoplast, and its outer membrane is equivalent to an artificial cytoplasmic membrane. If liposomes and plant protoplasts are mixed in a suitable medium, endocytosis, fusion or exchange occurs between them, so that the liposomes or their contents are inhaled into plant protoplasts. It is then fused with lysosomes and gradually degraded to release the contents. In this way, the encapsulated nucleic acid can be delivered to various types of cells, which is a liposome-mediated nucleic acid delivery pathway.
All of the above are the more mature and successful gene transfer methods in the current plant genetic engineering work. Each method has its own advantages, but there are some deficiencies or there are some limitations in the scope of application. In the future, it seems that the approach to explore gene transfer can be considered from the following three aspects: First, tap the methodological potential, expand its scope of application and make it more perfect. Second, several methods are used together to achieve a single method that is difficult to achieve. Third, exploring methods and finding vectors.
Note: 1. Co-cultivation method: 2. Chemical method: 3. Electrokinetic method: 4. Microinjection method: 5. Pollen tube channel method: 6. Gene gun method: 7. Liposome method: 8. Plant protoplasts Agrobacterium spheroplast fusion transformation 3 References Deng Wanyin et al. Agrobacterium tumefaciens can transform barley and wheat. Chinese Science, (B). 1989.2: 171---175 2 Charles SG Robert TF Genetically engineering plants for crop improvement Science 1989. Vol. 244: 1293~1299 3 Horsch RB et al. A simple and general method for transferring genes into plants. Science. 1985, 217:1229~1231 Horst L. et al. Gene transfer to cereal cells. By protoplast Transformation. MGG. 1985. 199:178. Ingo P. et al. Direct gene transfer to cell of a graminaceous Monocot. Ibid. 1985. 199:183~185.
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