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Tutor BM6 block 1, week task 2; Crown galls.
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Answers Tutor BM6 block 1, week task 2; Crown galls.
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Tutor weektaak 2 Crown GallsAgrobacterium and crown gall disease
Bacteria – plant symbiosis
Parasitic symbiosis
Bacteria: Agrobacterium tumefaciens
Biotechnology
Tumorous growth (uncontrolled growth) Ti-plasmid = Tumor inducing plasmid
The road to an infection
1. Agrobacterium tumefaciens cells attach to a wound site from the plant.
2. Bacteria produces cellulose microfibrils.
3. Formation of bacterial aggregates on plant cell surface.
4. Plasmid transfer from bacterium to plant.
Ti-plasmid
The entire Ti plasmid is about 200 kilobase pairs (kb) of DNA.
The T-DNA is about 20 kilobase pairs (kb) of DNA.
Ti plasmid transfer: only T-DNA (red part).
Vir-genes (virulent genes) essential for T-DNA transfer (light green part).
T-DNA becomes inserted in plant genome.
T-DNA: genes for tumor induction
onco-genes:
– plant hormone production
– induce tumorigenesis
– opine biosynthesis: carbon, nitrogen and phosphate source
T-DNA transfer in plant
VirA activates VirG by phosphorylation.
VirG activates transcription of other vir genes.
VirD has endonuclease activity that nicks the Ti plasmid, exposing the T-DNA.
VirE is a single-strand binding protein that assist in T-DNA transfer. Plant DNA polymerase
produces the complementary strand to the transferred single strand of T-DNA.
VirB functions as a conjugation bridge between the A. tumefaciens cell and the plant cell.
VirB transfer T-DNA and protein.
Agrobacterium transformation system
Plant biotechnology
Genetically modified transgenic plants
Genes in Ti plasmid (T-DNA fragment):
– resistance to herbicides
– resistance to insects
– resistance to drought
, Questions
1. Which organisms are involved in this interaction?
Agrobacterium tumefaciens bacteria and a plant. A. tumefaciens infects a broad range of
over 1000 different plant species. The disease is a particularly serious problem for grape
vines, walnuts, apple, raspberry and other fruit and nut trees.
2. How do these organisms come together?
When plants are injured, sugars leak out from the wound site into the surrounding soil. A.
tumefaciens bacteria living in the soil ‘smell’ these sugars and swim towards the plant wound
using flagella.
A. tumefaciens then enters the plant through the wound and releases a small loop of DNA (T-
DNA), which gets incorporated into the plant’s own DNA. This DNA forces the plant to
produce excess growth hormones, causing plant cells to grow uncontrollably and tumors
called galls to form. These galls are full of bacteria and act as ‘food factories’. The plant is
also engineered by bacterial DNA to produce sugars, which only the bacteria can make use
of.
3. Is this interaction good or bad?
The A. tumefaciens bacteria profits of the plant, because it gets sugars from the plant. For
the plant the bacteria is a disadvantage. The bacteria extract sugars from the plant, but the
plant doesn’t profit from the bacteria. In fact, the plant will die at a moment, so it is a
parasitic symbiosis.
The plant dies, because galls clog up the plant’s water transport system and can block up to
80% of water uptake. Gall production also uses a lot of the plant’s energy. These stunt plant
growth, production of fruit and nuts and cause leaves to turn yellow. Galls can eventually
result in plant death.
When the plant dies, insects, bacteria and fungi often break down the galls, releasing more
A. tumefaciens back into the soil. The A. tumefaciens bacteria are washed onto neighboring
fields through rainwater, allowing the disease to spread locally. Moving infected plants to
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