Plant Disease
Task 1
Both animals and plants have primary and secondary defence mechanisms. Primary defences are
always present and provide barriers to prevent pathogen entry. If this primary barrier is broken and
pathogens enter, then secondary defences are initiated. These can be passive (already formed) or active
(have to be made). Look at the statements on the next page and write the statements into the correct
sections of this table.
Primary
defences
Passive
secondary defences
Plant
Animal
(e.g.
human)
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Active/induced
secondary response
Statements:
Preformed antimicrobial substances; the
Waxy cuticle, cell wall
phytoanticipins e.g. glucosides, saponins
Phagocytes white blood cells
Hypersensitive response - burst of oxygen to
trigger antimicrobial response
Histamine release to attract blood cells
Antimicrobial enzymes such as chitinases
Cell wall reinforcement (callose, lignin, suberin,
cell wall proteins)
Lymphocytes - white blood cells
Antimicrobial substances-phytoalexins (genistein
or camalexin)
Skin, hairs in nose, hydrochloric acid in stomach
etc.
Task 2
To understand the primary defences of plants against invasion, it is important you understand the
structure of a leaf section. On the next page is a net diagram that can be folded into a cube. Using ‘help
sheet 1’ (below), draw onto the net diagram the leaf section and then fold into a cube, using the tabs.
Extension
Using ‘help sheet 2’, repeat this exercise for the skin cell.
What do you notice is similar between the two sections?
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A leaf
Make a 3D model leaf section. Draw in detail at least one palisade cell.
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Help sheet 1
Leaf cell cross section:
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Help sheet 2
Skin cell cross section:
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Task 3 - Determining if a plant is infected or not
Antigens are typically proteins that are present on the surface of all cells. These can be detected in the
laboratory using antibodies which can bind specifically to the antigen. If a plant is infected with a
pathogen the antigens from that pathogen will be present in the plant. To identify if the plant is infected
scientists use the enzyme-linked immunosorbent assay ELISA test to see if the plant contains the
pathogen antigen
A simple summary of this test is shown below:
Method:
1. Obtain your plant sample.
2. Liquidise the plant sample.
3. Add the plant sample to a plastic tube or ‘microtiter’ plate (96 well plate).
4. Leave for 5 minutes for all the proteins in the plant sample to bind to the plastic (including the
disease antigen if present).
5. Wash the wells with a buffered salt solution to wash off any proteins that are not bound to the
plastic.
6. Add a blocking agent (e.g. reconstituted milk powder). This is to block all the plastic that has not
been covered with protein (as the antibody is a protein it will therefore stick to any free plastic and
give a false positive result).
7. Wash again with buffered salt solution to remove the unbound blocking agent.
8. Add an antibody-enzyme complex (an antibody that is chemically bound to an enzyme) that is
specific for the pathogen’s antigen.
9. Wash again with buffered salt solution to remove any unbound antibody-enzyme complex.
10. Add a colourless chemical (substrate) that the enzyme can change into a coloured product.
11. If the plant pathogen (antigen) is in the liquid in the tube will change colour.
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Determining a particular antigen present in a plant sample
Method: Using your knowledge of antigens and antibodies, draw a diagram to summarise the main
stages in carrying out an ELISA test.
1.
2.
3.
4.
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Task 4 - PCR techniques
Using the link below, work through the animation and answer the following questions
http://learn.genetics.utah.edu/content/labs/pcr/
1. What does PCR stand for?
2. What is the purpose of PCR?
3. What are the 4 nucleotides that make up DNA?
4. What is a primer?
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5. What is the role of DNA polymerase?
6. Why is the DNA heated to 95oC?
7. What property must a PCR tube have?
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Task 5 - Polymerase Chain Reaction (PCR)
A single piece of DNA can be amplified in the laboratory using the polymerase chain reaction (PCR).
This process is done in vitro opposed to in vivo (in vitro in Latin means ‘in glass’ although now glass is
typically replaced with plastic, in vivo means ‘within the living’ and typically means ‘in cells’).
The ingredients for the process are:

Sample DNA

A plastic tube or microtiter plate (multi-

Heat stable DNA polymerase (eg Taq
DNA polymerase)
welled plate)

Thermocycler

Primers

Nucleotides (A, C, G & T)

DNA nucleotides
Cut out the stages below and put into the correct order using the template on the next page:
Add the tube to the thermo cycler
Extract DNA using a commercially available kit
Denature DNA at 95C
You have now turned one piece of DNA into two
ATGCGTAGGGCTTAGCTTTCGGATTCTTCTTGCTATTC
TACGCATCCCGAATCGAAAGCCTAAGAAGAACGATAAG

ATGCGTAGGGCTTAGCTTTCGGATTCTTCTTGCTATTC
+
TACGCATCCCGAATCGAAAGCCTAAGAAGAACGATAAG
ATGCGTAGGGCTTAGCTTTCGGATTCTTCTTGCTATTC
TACGCATCCCGAATCGAAAGCCTAAGAAGAACGATAAG
+
ATGCGTAGGGCTTAGCTTTCGGATTCTTCTTGCTATTC
TACGCATCCCGAATCGAAAGCCTAAGAAGAACGATAAG
Add primers (TWO), nucleotides and DNA
Heat to 72C the optimum temperature for the
enzyme
polymerase to the DNA sample in an Eppendorf
tube
ATGCGTAGGGCTTAGCTTTCGGATTCTTCTTGCTATTC
TACGCATCCCGAATCGAAA
+
CGGATTCTTCTTGCTATTC
TACGCATCCCGAATCGAAAGCCTAAGAAGAACGATAAG
Cool to 55C this allows the primers to anneal
ATGCGTAGGGCTTAGCTTTCGGATTCTTCTTGCTATTC
TACGCA
+
CTATTC
TACGCATCCCGAATCGAAAGCCTAAGAAGAACGATAAG
Now repeat steps 5-7 34 times and you will have
made 9 billion copies
Obtain sample (e.g. blood at scene of crime)
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1.
2.
4.
3.
5.
6.
8.
7.
9.
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Task 6
Match the observations to the plant disease.
Crown Gall disease
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Barley powdery mildew
12
Tobacco mosaic virus
2.
1. Circular, powdery white
spots on upper surface of
the leaf
3. As galls grow, plants
often become stunted, weak
and may eventually die
4. Presence of fungus
Erysiphe graminis
5. ‘Mosaic’- like mottling
and discolouration on the
6.
leaves
7. Presence of bacterium
Agrobacterium
tumefaciens
8. Tumour-like growths
above soil level
10.
9. Fungal hyphae are
produced on both upper
and lower leaf surfaces
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