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Crown Gall Disease
It is caused by Agrobacterium tumefaciens, which is a common plant disease (bacterial). The disease mostly affects dicotyledon species such as woody & herbaceous plants. Can be identified by the appearance of tumors of various size & shape at lower stem & main roots of the plant. Tumor cells use plant metabolites to form nutrients for bacterial growth. Crown gall disease affects many commercially important crops such as:
- Sugar beet
- Nut trees rose
- Many shrubs
Image Source: Backyard Gardener
- It induces crown gall disease.
- Gram-negative, rods, obligate
- Enters easily through plant wounds.
- It also forms a tumor.
- Also referred to as nature’s smallest genetic engineer.
- Common soil
- Associates with the roots of many plants by
- Contains a Ti (tumor-inducing)
The ability to cause disease is that tumor tissue growth depends on the ability of Agrobacterium species to transfer the bacterial genes into the plant genome. Agrobacterium tumefaciens naturally affect the wound site in the dicotyledonous plant causing the formation of crown gall tumors.
Agrobacterium tumefaciens consist of two main types:
- A6 STRAIN: Ti plasmids pTiA6, pTiAch, pTiB6S3 are referred to as octopine Ti plasmid. Transfer 2 fragments of DNA (T2 and TR) to plant cells.
- C58 STRAIN: Ti plasmid pTiC58 is a nopaline Ti plasmid transfer of one DNA fragment (TDNA) to plant cells.
- Circular chromosome (2.8Mb)
- Linear chromosome (2.0 Mb)
- At plasmid (450 Kb)
- Ti plasmid (210 Kb)
Pathogenicity of bacteria depends on Ti-plasmid that is a tumor-inducing plasmid. The interesting feature of the Ti plasmid is T-DNA (transfer DNA) which is actually transferred from bacterium to the plant genome. The ability of Agrobacterium tumefaciens to caused crown gall disease depends on the Ti-plasmid. This is a megaplasmid double-stranded circular DNA approximately 200Kbp present within the bacterium. Analysis of the nuclear DNA in the plant tumor indicated the presence of the part of the bacterial plasmid DNA which was integrated into the host plant.
Important regions on Ti plasmid
- Ti plasmid consists of the following regions;
- Origin of replication. (which enable plasmid to replicate independently of the bacterial chromosome).
- Plasmid conjugative transfer region (which decide to compatibility with other strain)
- Vir region (consist of Vir genes).
- Opine catabolism region (that breaks down opine to provide nutrition and energy for bacterial proliferation).
- T DNA region (transfer to the host plant cell).
Induction of Vir operon
- Vir reagent determine the pathogenicity of the bacterium in response to phenolic signals from wounded plant cells the vir A protein present in the bacterial membrane is autophosphorylated and then transfer the phosphate signal to Vir G protein,both Vir A & Vir G are expressed in Agrobacterium, the activated Vir G binds to the promotors of the other Vir gene in the Vir operon.
T-DNA enter in the plant cell and integrated into the plant genome thus genetically modify the plant. T-DNA region which is transfer to the host plant is bounded by the left & right border which consists of an imperfect direct repeat sequence of 25bp. Within the border, a gene involved in auxin & cytokinin biosynthesis called oncogenes and genes involved in opine synthesis.
The T-DNA region is cut at the borders and single standard DNA is transferred to the host plant. All the gene in TDNA have eukaryotic promotors since they are express in the plant. The oncogenes lead to change the level of auxin & cytokinin in transformed cell & caused rapid growth/multiplication of plant cell leading to tumor formation. Tumor cells synthesize opines.
Secondary amine derivatives formed by condensation of an amino acid and sugar/keto acid. More than 30 opines have been identified in Agrobacterium strains and the most common opines are octopine, mannopine, nopaline, agropine. Opines cannot be metabolized by plants. Agrobacterium Ti plasmid has an opine catabolism gene that codes for an enzyme that metabolizes opines. Opines provide building blocks and energy for bacterial growth and proliferation.
Cause of Death
Uncontrolled cell proliferation at the infection site leads to tumors or galls. Poor cell differentiation in root galls leads to decreased uptake of water and nutrients for the plant, eventually leading to plant death. Galls may fall off after plant death, exposing surrounding plants to A. tumefaciens. A. tumefaciens may also be transferred by biting insects or harvesting equipment.
What treatments would you suggest?
1- Plant A. Tumefaciens Resistant Crops.
2- Use biochemical agents.
Spray antibiotics such as agrocin on the entire field. Agrocin structurally resembles the opine agrocinopine A, taken up by A. Tumefaciens. Agrocin inhibits nucleic acid and protein synthesis by obstructing the intake of amino acid residues by pathogenic A. tumefaciens.
- Replant the entire field with cereal crops for a long period of time.
- Cereal crops are not in the host range of A. tumefaciens.
- Lack of hosts significantly decreases A. tumefaciens population.
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Crown Gall Disease