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 / Active/induced
secondary response
Plant
Animal (e.g. human)

Statements:

Waxy cuticle, cell wall / Preformed antimicrobial substances; the 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:



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 antibodieswhichcan 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 anyproteins 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 itwill 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 againwith 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.

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.


Task 4 - PCR techniques

Using the link below, work through the animation and answer the following questions

  1. What does PCR stand for?
  1. What is the purpose of PCR?
  1. What are the 4 nucleotides that make up DNA?
  1. What is a primer?

  1. What is the role of DNA polymerase?
  1. Why is the DNA heated to 95oC?
  1. What property must a PCR tube have?

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:

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  • Sample DNA
  • A plastic tube or microtiter plate (multi-welled plate)
  • Primers
  • DNA nucleotides
  • Heat stable DNA polymerase (egTaq DNA polymerase)
  • Thermocycler
  • Nucleotides (A, C, G & T)

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Cut out the stages below and put into the correct order using the template on the next page:

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1.
/ 2.

4. / 3.
5. / 6.
8. / 7.
9.

Task 6

Match the observations to the plant disease.

Crown Gall disease / Barley powdery mildew / Tobacco mosaic virus


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