1. Agrobacterium tumefaciens
Unusual disease agent brought into use as plant genetic engineer

2. A range of interactions exist between bacteria and plants
Bacteria associate with plants in ways that benefit both partners - symbiosis.
Other bacteria have no beneficial effects - disease.
Add picture of nodulated root
Rhizobium-legume symbiosis

3. Cell division sites in the plant are restricted
Cells in small regions at the tips of shoots and roots contain cells that expand and divide to increase the size of the plant.
These are called apical meristems

4. Apical meristems
Dividing cells lie at the tip of the root below the region where root hairs emerge. They expand and then divide controlled by 2 hormones -auxin and cytokinin
Apical meristem producing auxin and cytokinin
Cell expansion and division

5. The view inside a root Root hairs Expanding cells Dividing cells
Keiko Fig 2.6
Expanding cells
Dividing cells

6. Some bacteria release the controls on cell division causing a cancerous growth
This happens in crown gall disease, producing large masses of disorganised cells.
Infection usually follows wounding.

7. Broad host range
The disease affects many different plants including economically important species such as these peach trees
Crown gall on Euonymus

8. There are few limits on the final size!
Crown gall on ash.jpg
Crown gall on the trunk of an ash tree

9. All this from one bacterium
The production of these millions of extra cells is caused by a bacterium
Agrobacterium
tumefaciens
Agro on plant surface.jpg

10. Agrobacterium is very common in the soil around roots
Agrobacterium is a close relative of Rhizobium species that form nodules on legumes
Like Rhizobium, it is common in the rhizosphere, the region around roots
rhizosphere.jpg

11. Another molecular conversation but a different outcome
Just like Rhizobium exchanging signals with its legume host, Agrobacterium and its future host exchange signals
These activate a mechanism in the bacterium that transfers some bacterial DNA to take control of the plant

12. DNA - information carrier
DNA carries the genetic instructions all organisms (including us) receive from our parents
Those instructions determine all inherited features - that make us different ( hair colour, eye colour, blood group etc) and all the features we share
DNA directs activities in all cells
One enormously long DNA molecule forms each chromosome
The information on each chromosome is broken down into many genes
Each gene provides the information to make one protein

13. Agrobacterium controls its plant host by putting a small piece of T-DNA into one of the plant chromosomes
The transferred genes form the T-DNA (transferred DNA) region of the Ti plasmid (tumour inducing).
T-DNA is less than 10% of the whole plasmid, encodes only 3 or so genes and enters a plant that has at least genes

14. But these few genes are enough to take control of the plant
Changes the plant’s metabolism to produce food materials (opines) that only the bacterium can use
Produces the plant hormones auxin and cytokinin that remove the controls that normally limit cell division and cell expansion.
Result - cells showing altered metabolism multiply uncontrollably

15. Hence the massive disorganised growths

16. T-DNA, a small region of the Ti plasmid is transferred
Functions for DNA transfer and using opines

17. SUMMARY
Gene trans in crowngall.jpg
Even if the T-DNA originally change only one cell in the plant, because that cell divides uncontrollably and passes on those bacterial genes to all the new cells, there are soon millions of cells feeding the bacterium at the expense of the plant.

18. Plant scientists can use Agrobacterium to put other genes into plant chromosomes and so understand what they do

19. Disarming the Ti plasmid
Remove and replace these genes

20. New genes in the Ti plasmid
New genes inserted between borders

21. “Floral dip” to transfer genes

22. How my research uses Agrobacterium
Mutants have a fault in one gene which stops it directing the cell’s activities in the normal way.
Mutants tell us what function a gene normally serves.
Make diagram of genetic changes leading to trait

23. Albino mutants have a faulty gene needed to make leaves green

24. Plants with a downturned branches have a defective gene needed to make them grow upright
cipoupright.gif and cipoweep.gif

25. Our mutants have defective genes needed to make cellulose
We selected mutants unable to make the plant’s major structural material - cellulose.
Cellulose makes vegetables crisp, wood strong, paper thin but strong and cotton fibres tough enough to make jeans

26. Cellulose forms the walls that surround all plant cells
Keiko Fig 2.6
When cellulose production stops, the restraint imposed by the wall is removed and the root swells instead of increasing in length

27. Cellulose structure and appearance in em

28. The cotton boll
Cotton fibres are very long hair cells that form on the cotton seed.
They develop thick walls that are almost pure (>95%) cellulose.
The fibres are spun into threads to make cotton garments
“cotton boll.jpg” Copyright blanked out not for publication

29. Our mutants have defective genes needed to make cellulose
We selected mutants unable to make the plant’s major structural material - cellulose.
Cellulose makes vegetables crisp, wood strong, paper thin but strong and cotton fibres tough enough to make jeans

30. “Which of the 25,000 genes has the fault?”
Preliminary work narrows the choice of genes
Agrobacterium puts a new copy of each of the suspected genes into the mutant.
If the mutant now looks normal, the introduced gene has repaired the fault and the mutant had a faulty copy of the gene delivered by Agrobacterium.

31. With a new copy of the faulty gene, the mutant looks normal

32. WITH THE RIGHT GENE, THE CHANGE IS DRAMATIC
Make diagram of plasmid with test gene inserted
Mutant - swelling not elongating
Normal plant
Mutant + 1 gene introduced by Agrobacterium

33. GFP making organelles visible
Golgi bodies
Endoplasmic reticulum

34. Some T-DNA insertions inactivate genes in the host plant
No effect
Inactivates
Gene 2
This makes a new insertional mutant

35. From disease agent to research tool
With some changes to the Ti-plasmid, Agrobacterium has been brought from the field to make possible new experiments to understand the function of plant genes

36. “Floral dip” and seed selection