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农杆菌的背景资料 农杆菌的侵染植物
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Agrobacterium tumefaciens
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Jump to: navigation, search
Agrobacterium tumefaciens
A. tumefaciens attaching itself to a carrot cell
Conservation status
Secure
Scientific classification
Kingdom: Bacteria
Phylum: Proteobacteria
Class: Alpha Proteobacteria
Order: Rhizobiales
Family: Rhizobiaceae
Genus: Agrobacterium
Species: A. tumefaciens
Binomial name
Agrobacterium tumefaciens
Smith & Townsend, 1907
Synonyms
Bacterium tumefaciens Smith and Townsend 1907
Pseudomonas tumefaciens (Smith and Townsend 1907) Duggar 1909
Phytomonas tumefaciens (Smith and Townsend 1907) Bergey et al. 1923
Polymonas tumefaciens (Smith and Townsend 1907) Lieske 1928
Agrobacterium tumefaciens is the causal agent of crown gall disease (the formation of tumours) in over 140 species of dicot. It is a rod shaped, Gram negative soil bacterium (Smith et al., 1907). Symptoms are caused by the insertion of a small segment of DNA (known as the T-DNA, for 'transfer DNA') into the plant cell,[1] which is incorporated at a semi-random location into the plant genome.
Agrobacterium tumefaciens (or A. tumefaciens) is an alphaproteobacterium of the family Rhizobiaceae, which includes the nitrogen fixing legume symbionts. Unlike the nitrogen fixing symbionts, tumor producing Agrobacterium are pathogenic and do not benefit the plant. The wide variety of plants affected by Agrobacterium makes it of great concern to the agriculture industry[2].
Economically, A. tumefaciens is a serious pathogen of walnuts, grape vines, stone fruits, nut trees, sugar beets, horse radish and rhubarb.
Contents [hide]
1 Conjugation
2 Method of infection
2.1 Formation of the T-pilus
2.2 Transfer of T-DNA into plant cell
3 Genes in the T-DNA
3.1 Hormones
3.2 Opines
4 Beneficial uses
5 References
6 External links
[edit] Conjugation
In order to be virulent, the bacterium must contain a tumour-inducing plasmid (Ti plasmid or pTi), of 180 kb, which contains the T-DNA and all the genes necessary to transfer it to the plant cell. Many strains of A. tumefaciens do not contain a pTi.
Since the Ti plasmid is essential to cause disease, pre-penetration events in the rhizosphere occur to promote bacterial conjugation - exchange of plasmids amongst bacteria. In the presence of opines, A. tumefaciens produces a diffusible conjugation signal called 30C8HSL or the Agrobacterium autoinducer. This activates the transcription factor TraR, positively regulating the transcription of genes required for conjugation.
[edit] Method of infection
The Agrobacterium tumefaciens infects the plant through its Ti plasmid. The Ti plasmid integrates a segment of its DNA, known as T-DNA, into the chromosomal DNA of its host plant cells.
A. tumefaciens have flagella that allow them to swim through the soil towards photoassimilates that accumulate in the rhizosphere around roots. Chemotaxis: reaction of orientation and locomotion to chemical attractants. Without chemotaxis there will be no cell-cell contact. Some strains may chemotactically move towards chemical exudates coming out from wounded plant such as acetosyringone and sugars. Acetosyringone is recognised by the VirA protein, a transmembrane protein encoded in the virA gene on the Ti plasmid. Sugars are recognised by the chvE protein, a chromosomal gene-encoded protein located in the periplasmic space.[3].
Induction of vir genes: At least 25 vir genes on Ti plasmid are necessary for tumor induction.In addition to their perception role, virA and chvE induce other vir genes. The VirA protein has a kinase activity, it phosphorylates it self on a histidine residu. Then the VirA protein phosphorylates the VirG protein on its aspartate residu.The VirG protein is a cytoplasmic protein traduced from the virG Ti plasmid gene, it's a transcription factor. It induces the transcription of the vir operons. ChvE protein regulates the second mecanism of vir genes activation. It increases VirA protein sensibility to phenolic compounds.[4]
Attachment is a two step process. Following an initial weak and reversible attachment, the bacteria synthesize cellulose fibrils that anchor them to the wounded plant cell. Four main genes are involved in this process: chvA, chvB, pscA and att. It appears that the products of the first three genes are involved in the actual synthesis of the cellulose fibrils. These fibrils also anchor the bacteria to each other, helping to form a microcolony.
After production of cellulose fibrils a Ca2+ dependent outer membrane protein called rhicadhesin is produced, which also aids in sticking the bacteria to the cell wall. Homologues of this protein can be found in other Rhizobia species.
Possible plant compounds, that initiate Agrobacterium to infect plant cells:[5]
Acetosyringone: Phenolic compound
alpha-Hydroxyacetosyringone
Catechol
Ferulic acid
Gallic acid
p-Hydroxybenzoic acid
Protocatechuic acid
Pyrogallic acid
Resorcylic acid
Sinapinic acid
Syringic acid
Vanillin
[edit] Formation of the T-pilus
In order to transfer the T-DNA into the plant cell A. tumefaciens uses a Type IV secretion mechanism, involving the production of a T-pilus.
The VirA/VirG two component sensor system is able to detect phenolic signals released by wounded plant cells, in particular acetosyringone. This leads to a signal transduction event activating the expression of 11 genes within the VirB operon which are responsible for the formation of the T-pilus.
First, the VirB" pro-pilin is formed. This is a polypeptide of 121 amino acids which requires processing by the removal of 47 residues to form a T-pilus subunit. The subunit is circularized by the formation of a peptide bond between the two ends of the polypeptide.
Products of the other VirB genes are used to transfer the subunits across the plasma membrane. Yeast two-hybrid studies provide evidence that VirB6, VirB7, VirB8, VirB9 and VirB10 may all encode components of the transporter. An ATPase for the active transport of the subunits would also be required.
[edit] Transfer of T-DNA into plant cell
A: Agrobacterium tumefaciens
B: Agrobacterium genome
C: Ti Plasmid : a: T-DNA , b: Vir genes , c: Replication origin , d: Opines catabolism genes
D: Plant cell
E: Mitochondria
F: Chloroplast
G: NucleusThe T-DNA must be cut out of the circular plasmid. A VirD1/D2 complex nicks the DNA at the left and right border sequences. The VirD2 protein is covalently attached to the 5' end. VirD2 contains a motif that leads to the nucleoprotein complex being targeted to the type IV secretion system (T4SS).
In the cytoplasm of the recipient cell, the T-DNA complex becomes coated with VirE2 proteins, which are exported through the T4SS independently from the T-DNA complex. Nuclear localization signals, or NLS, located on the VirE2 and VirD2 are recognised by the importin alpha protein, which then associates with importin beta and the nuclear pore complex to transfer the T-DNA into the nucleus. VIP1 also appears to be an important protein in the process, possibly acting as an adapter to bring the VirE2 to the importin. Once inside the nucleus, VIP2 may target the T-DNA to areas of chromatin that are being actively transcribed, so that the T-DNA can integrate into the host genome.
[edit] Genes in the T-DNA
[edit] Hormones
In order to cause gall formation, the T-DNA encodes genes for the production of auxin or indole-3-acetic acid via the IAM pathway. This biosynthetic pathway is not used in many plants for the production of auxin, so it means the plant has no molecular means of regulating it and auxin will be produced constitutively. Genes for the production of cytokinins are also expressed. This stimulates cell proliferation and gall formation.
[edit] Opines
The T-DNA contains genes for encoding enzymes that cause the plant to create specialized amino acids which the bacteria can metabolize, called opines.[6] Opines are a class of chemicals that serve as a source of nitrogen for A. tumefaciens, but not for most other organisms. The specific type of opine produced by A. tumefaciens C58 infected plants is nopaline (Escobar et al., 2003).
Two nopaline type Ti plasmids, pTi-SAKURA and pTiC58, were fully sequenced. A. tumefaciens C58, the first fully sequenced pathovar, was first isolated from a cherry tree crown gall. The genome was simultaneously sequenced by Goodner et al.[7] and Wood et al.[8] in 2001. The genome of A. tumefaciens C58 consists of a circular chromosome, two plasmids, and a linear chromosome. The presence of a covalently bonded circular chromosome is common to Bacteria, with few exceptions. However, the presence of both a single circular chromosome and single linear chromosome is unique to a group in this genus. The two plasmids are pTiC58, responsible for the processes involved in virulence, and pAtC58, coined the “cryptic” plasmid.[7][8]
The pAtC58 plasmid has been shown to be involved in the metabolism of opines and to conjugate with other bacteria in the absence of the pTiC58 plasmid.[9] If the pTi plasmid is removed, the tumor growth that is the means of classifying this species of bacteria does not occur.
[edit] Beneficial uses
Plants that have undergone transformation with Agrobacterium.The DNA transmission capabilities of Agrobacterium have been extensively exploited in biotechnology as a means of inserting foreign genes into plants. Marc Van Montagu and Jeff Schell, (University of Ghent and Plant Genetic Systems, Belgium) discovered the gene transfer mechanism between Agrobacterium and plants, which resulted in the development of methods to alter Agrobacterium into an efficient delivery system for genetic engineering in plants.[10] The plasmid T-DNA that is transferred to the plant is an ideal vehicle for genetic engineering.[11] This is done by cloning a desired gene sequence into the T-DNA that will be inserted into the host DNA. This process has been performed using firefly luciferase gene to produce glowing plants. This luminescence has been a useful device in the study of plant chloroplast function and as a reporter gene.[12] It is also possible to transform Arabidopsis by dipping their flowers into a broth of Agrobacterium, the seed produced will be transgenic. Under laboratory conditions the T-DNA has also been transferred to human cells, demonstrating the diversity of insertion application.[13]
The mechanism by which Agrobacterium inserts materials into the host cell by a type IV secretion system, is very similar to mechanisms used by pathogens to insert materials (usually proteins) into human cells by type III secretion. It also employs a type of signaling conserved in many Gram-negative bacteria called quorum sensing. This makes Agrobacterium an important topic of medical research as well.
农杆菌的侵染植物~
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Agrobacterium tumefaciens
A. tumefaciens attaching itself to a carrot cell
Conservation status
Secure
Scientific classification
Kingdom: Bacteria
Phylum: Proteobacteria
Class: Alpha Proteobacteria
Order: Rhizobiales
Family: Rhizobiaceae
Genus: Agrobacterium
Species: A. tumefaciens
Binomial name
Agrobacterium tumefaciens
Smith & Townsend, 1907
Synonyms
Bacterium tumefaciens Smith and Townsend 1907
Pseudomonas tumefaciens (Smith and Townsend 1907) Duggar 1909
Phytomonas tumefaciens (Smith and Townsend 1907) Bergey et al. 1923
Polymonas tumefaciens (Smith and Townsend 1907) Lieske 1928
Agrobacterium tumefaciens is the causal agent of crown gall disease (the formation of tumours) in over 140 species of dicot. It is a rod shaped, Gram negative soil bacterium (Smith et al., 1907). Symptoms are caused by the insertion of a small segment of DNA (known as the T-DNA, for 'transfer DNA') into the plant cell,[1] which is incorporated at a semi-random location into the plant genome.
Agrobacterium tumefaciens (or A. tumefaciens) is an alphaproteobacterium of the family Rhizobiaceae, which includes the nitrogen fixing legume symbionts. Unlike the nitrogen fixing symbionts, tumor producing Agrobacterium are pathogenic and do not benefit the plant. The wide variety of plants affected by Agrobacterium makes it of great concern to the agriculture industry[2].
Economically, A. tumefaciens is a serious pathogen of walnuts, grape vines, stone fruits, nut trees, sugar beets, horse radish and rhubarb.
Contents [hide]
1 Conjugation
2 Method of infection
2.1 Formation of the T-pilus
2.2 Transfer of T-DNA into plant cell
3 Genes in the T-DNA
3.1 Hormones
3.2 Opines
4 Beneficial uses
5 References
6 External links
[edit] Conjugation
In order to be virulent, the bacterium must contain a tumour-inducing plasmid (Ti plasmid or pTi), of 180 kb, which contains the T-DNA and all the genes necessary to transfer it to the plant cell. Many strains of A. tumefaciens do not contain a pTi.
Since the Ti plasmid is essential to cause disease, pre-penetration events in the rhizosphere occur to promote bacterial conjugation - exchange of plasmids amongst bacteria. In the presence of opines, A. tumefaciens produces a diffusible conjugation signal called 30C8HSL or the Agrobacterium autoinducer. This activates the transcription factor TraR, positively regulating the transcription of genes required for conjugation.
[edit] Method of infection
The Agrobacterium tumefaciens infects the plant through its Ti plasmid. The Ti plasmid integrates a segment of its DNA, known as T-DNA, into the chromosomal DNA of its host plant cells.
A. tumefaciens have flagella that allow them to swim through the soil towards photoassimilates that accumulate in the rhizosphere around roots. Chemotaxis: reaction of orientation and locomotion to chemical attractants. Without chemotaxis there will be no cell-cell contact. Some strains may chemotactically move towards chemical exudates coming out from wounded plant such as acetosyringone and sugars. Acetosyringone is recognised by the VirA protein, a transmembrane protein encoded in the virA gene on the Ti plasmid. Sugars are recognised by the chvE protein, a chromosomal gene-encoded protein located in the periplasmic space.[3].
Induction of vir genes: At least 25 vir genes on Ti plasmid are necessary for tumor induction.In addition to their perception role, virA and chvE induce other vir genes. The VirA protein has a kinase activity, it phosphorylates it self on a histidine residu. Then the VirA protein phosphorylates the VirG protein on its aspartate residu.The VirG protein is a cytoplasmic protein traduced from the virG Ti plasmid gene, it's a transcription factor. It induces the transcription of the vir operons. ChvE protein regulates the second mecanism of vir genes activation. It increases VirA protein sensibility to phenolic compounds.[4]
Attachment is a two step process. Following an initial weak and reversible attachment, the bacteria synthesize cellulose fibrils that anchor them to the wounded plant cell. Four main genes are involved in this process: chvA, chvB, pscA and att. It appears that the products of the first three genes are involved in the actual synthesis of the cellulose fibrils. These fibrils also anchor the bacteria to each other, helping to form a microcolony.
After production of cellulose fibrils a Ca2+ dependent outer membrane protein called rhicadhesin is produced, which also aids in sticking the bacteria to the cell wall. Homologues of this protein can be found in other Rhizobia species.
Possible plant compounds, that initiate Agrobacterium to infect plant cells:[5]
Acetosyringone: Phenolic compound
alpha-Hydroxyacetosyringone
Catechol
Ferulic acid
Gallic acid
p-Hydroxybenzoic acid
Protocatechuic acid
Pyrogallic acid
Resorcylic acid
Sinapinic acid
Syringic acid
Vanillin
[edit] Formation of the T-pilus
In order to transfer the T-DNA into the plant cell A. tumefaciens uses a Type IV secretion mechanism, involving the production of a T-pilus.
The VirA/VirG two component sensor system is able to detect phenolic signals released by wounded plant cells, in particular acetosyringone. This leads to a signal transduction event activating the expression of 11 genes within the VirB operon which are responsible for the formation of the T-pilus.
First, the VirB" pro-pilin is formed. This is a polypeptide of 121 amino acids which requires processing by the removal of 47 residues to form a T-pilus subunit. The subunit is circularized by the formation of a peptide bond between the two ends of the polypeptide.
Products of the other VirB genes are used to transfer the subunits across the plasma membrane. Yeast two-hybrid studies provide evidence that VirB6, VirB7, VirB8, VirB9 and VirB10 may all encode components of the transporter. An ATPase for the active transport of the subunits would also be required.
[edit] Transfer of T-DNA into plant cell
A: Agrobacterium tumefaciens
B: Agrobacterium genome
C: Ti Plasmid : a: T-DNA , b: Vir genes , c: Replication origin , d: Opines catabolism genes
D: Plant cell
E: Mitochondria
F: Chloroplast
G: NucleusThe T-DNA must be cut out of the circular plasmid. A VirD1/D2 complex nicks the DNA at the left and right border sequences. The VirD2 protein is covalently attached to the 5' end. VirD2 contains a motif that leads to the nucleoprotein complex being targeted to the type IV secretion system (T4SS).
In the cytoplasm of the recipient cell, the T-DNA complex becomes coated with VirE2 proteins, which are exported through the T4SS independently from the T-DNA complex. Nuclear localization signals, or NLS, located on the VirE2 and VirD2 are recognised by the importin alpha protein, which then associates with importin beta and the nuclear pore complex to transfer the T-DNA into the nucleus. VIP1 also appears to be an important protein in the process, possibly acting as an adapter to bring the VirE2 to the importin. Once inside the nucleus, VIP2 may target the T-DNA to areas of chromatin that are being actively transcribed, so that the T-DNA can integrate into the host genome.
[edit] Genes in the T-DNA
[edit] Hormones
In order to cause gall formation, the T-DNA encodes genes for the production of auxin or indole-3-acetic acid via the IAM pathway. This biosynthetic pathway is not used in many plants for the production of auxin, so it means the plant has no molecular means of regulating it and auxin will be produced constitutively. Genes for the production of cytokinins are also expressed. This stimulates cell proliferation and gall formation.
[edit] Opines
The T-DNA contains genes for encoding enzymes that cause the plant to create specialized amino acids which the bacteria can metabolize, called opines.[6] Opines are a class of chemicals that serve as a source of nitrogen for A. tumefaciens, but not for most other organisms. The specific type of opine produced by A. tumefaciens C58 infected plants is nopaline (Escobar et al., 2003).
Two nopaline type Ti plasmids, pTi-SAKURA and pTiC58, were fully sequenced. A. tumefaciens C58, the first fully sequenced pathovar, was first isolated from a cherry tree crown gall. The genome was simultaneously sequenced by Goodner et al.[7] and Wood et al.[8] in 2001. The genome of A. tumefaciens C58 consists of a circular chromosome, two plasmids, and a linear chromosome. The presence of a covalently bonded circular chromosome is common to Bacteria, with few exceptions. However, the presence of both a single circular chromosome and single linear chromosome is unique to a group in this genus. The two plasmids are pTiC58, responsible for the processes involved in virulence, and pAtC58, coined the “cryptic” plasmid.[7][8]
The pAtC58 plasmid has been shown to be involved in the metabolism of opines and to conjugate with other bacteria in the absence of the pTiC58 plasmid.[9] If the pTi plasmid is removed, the tumor growth that is the means of classifying this species of bacteria does not occur.
[edit] Beneficial uses
Plants that have undergone transformation with Agrobacterium.The DNA transmission capabilities of Agrobacterium have been extensively exploited in biotechnology as a means of inserting foreign genes into plants. Marc Van Montagu and Jeff Schell, (University of Ghent and Plant Genetic Systems, Belgium) discovered the gene transfer mechanism between Agrobacterium and plants, which resulted in the development of methods to alter Agrobacterium into an efficient delivery system for genetic engineering in plants.[10] The plasmid T-DNA that is transferred to the plant is an ideal vehicle for genetic engineering.[11] This is done by cloning a desired gene sequence into the T-DNA that will be inserted into the host DNA. This process has been performed using firefly luciferase gene to produce glowing plants. This luminescence has been a useful device in the study of plant chloroplast function and as a reporter gene.[12] It is also possible to transform Arabidopsis by dipping their flowers into a broth of Agrobacterium, the seed produced will be transgenic. Under laboratory conditions the T-DNA has also been transferred to human cells, demonstrating the diversity of insertion application.[13]
The mechanism by which Agrobacterium inserts materials into the host cell by a type IV secretion system, is very similar to mechanisms used by pathogens to insert materials (usually proteins) into human cells by type III secretion. It also employs a type of signaling conserved in many Gram-negative bacteria called quorum sensing. This makes Agrobacterium an important topic of medical research as well.
农杆菌的侵染植物~
根癌农杆菌侵染植物是一个非常复杂的过程。根癌农杆菌具有趋化性，即植物的受伤组织会产生一些糖类和酚类物质吸引根癌农杆菌向受伤组织集中。研究证明，主要酚类诱导物为乙酰丁香酮和羧基乙酰丁香酮，这些物质主要在双子叶植物细胞壁中合成，通常不存在于单子叶植物中，这也是单子叶植物不易被根癌农杆菌侵染的原因。还发现一些中性糖，如L-阿拉伯糖、D-木糖等也有诱导作用。酚类物质和糖类物质既可以作为根瘤农杆菌的趋化物，又可以作为农杆菌中Ti质粒上 Vir区（毒性区）基因的诱导物，使Vir区基因活化，导致T-DNA的加工和转移，从而侵染植物细胞。需要注意的是农杆菌中不同的菌株，侵染能力有差别，在基因工程中需要加以选择使用。GV3101、AGL1、LBA440、EHA105等都有不同的特性，利用农杆菌侵染单子叶植物进行遗传转化时，是需要加上述酚类物质的，同时单子叶植物种类不同，农杆菌侵染进行遗传转化的效果也有很大差异。
根癌农杆菌的Ti质粒上有一段转移DNA（T-DNA），具有向植物细胞传递外源基因的能力，而细菌本身并不进入受体细胞。农杆菌转化植物细胞涉及一系列复杂的反应，主要包括：①受伤的植物细胞为修复创伤部位，释放一些糖类、酚类等信号分子。②在信号分子的诱导下，农杆菌向受伤组织集中，并吸附在细胞表面。③转移DNA上的毒粒基因被激活并表达，同时形成转移DNA的中间体。④转移DNA进入植物细胞，并整合到植物细胞基因组中。因为单子叶植物不是农杆菌的天然寄主，况且其不能合成起诱导作用的信号分子，所以限制了农杆菌介导法在单子叶植物中的应用。不过近年来大量成功转化的实例表明，植物、真菌、哺乳动物甚至人类细胞都可以作为农杆菌的受体
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15813076549：农杆菌怎么小摇和大摇
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15813076549：发根农杆菌应该如何活化
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15813076549：植物转基因技术有哪些?
姬饶答：农杆菌介导法、DNA直接插入法、花粉管通道法和植物病毒介导法。农杆菌介导法就是指利用农杆菌转化受体细胞，将目标基因插入受体细胞基因组的转化技术。该方法是目前研究最多，机理最清楚，应用最为广泛和成熟的植物转基因技术。DNA直接插入法是指不需要借助质粒载体的转化而实现外源目标基因插入的方法。
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姬饶答：用限制性核酸内切酶切割目的基因与Ti农杆菌质粒 然后用DNA连接酶连接目的基因与Ti农杆菌质粒 把含目的基因的Ti质粒转入农杆菌中 然后繁殖（含有目的基因的Ti质粒的）农杆菌 然后利用（含有目的基因的Ti质粒的）农杆菌侵染植物细胞的DNA 然后就可以在植物细胞内翻译成蛋白质 参考资料：看不懂的看看高二的...
15813076549：生物科技详细资料大全
姬饶答：对于双子叶植物(例如:大豆、西红柿、棉花)来说,基因转殖通常是借由冠缨农杆菌(Agrobacterium tumefaciens)来达成。冠缨农杆菌能够自然地(也就是不经任何人工处理)感染种类繁多的植物,其感染乃是透过将其自身的一段DNA直接的插入受感染植物的DNA中,是以只要将冠缨农杆菌DNA中的致癌基因去除,并将所欲植入的外来基因...
15813076549：农杆菌介导的转基因玉米种子的获得?
姬饶答：这种简单的基因转化方法,结合了花粉通道与农杆菌侵染的优势,如果能够被其它实验室所证实,将在小麦等的基因转化上发挥主导作用。 在农杆菌介导小麦基因转导方面,Cheng等于1997打破了十几年的沉默,首次获得的正常的转基因小麦植株。2年后,我国科学工作者夏光敏等报道获得农杆菌介导的转基因小麦植株,笔者的研究组也同样...
15813076549：大肠杆菌转化脓杆菌的方法步骤
姬饶答：3、将目的基因导入受体细胞。将含目的基因的重组质粒导入农杆菌（农杆菌为受体细胞）。4、目的基因的检测与鉴定。用DNA分子杂交技术/分子杂交技术/抗原-抗体杂交/个体生物学水平鉴定。5、最后将成功表达的细胞导入植物体内，对植物体进行个体生物学水平鉴定。参考资料：http://baike.baidu.com/view/3114841...
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