Plant Defences 12.4

Question 1

What do plants do with diseases tissue?

Answer 1

They cannot heal it unlike humans so they sacrifice it and seal it off and then grow more tissue from the meristem.

Question 2

How do plants recognise an attack?

Answer 2

Plants are not passive, they will recognise an attack and will produce defenses very quickly. Their receptors will detect the pathogen or they will detect the chemicals produced when a pathogenic enzyme attacks the cell wall. When this is detected, signalling cells will be released which will switch genes on or off in the nucleus. This will in turn trigger the production of defensive chemicals, the alarm signalling to unaffected cells so they can trigger defences or the strengthening of the cell wall.

Question 3

Physical Defenses

Answer 3

When a plant cell wall is being attacked by a pathogen, it will produce high levels of callose which is a polysaccharide with 1,3 and 1,6 beta glucose bonds.
The plant cells directly near the infected cells will synthesise and store the callose in between their cell walls and membranes. This will prevent the pathogens from entering the cell by creating a barrier. This will carry on and lignin will also be added to reinforce it and make it very strong. Callose will also block off the sieve plates of the phloem so the pathogen cannot be spread around the plant and it will also be deposited between the plasmodesmata of neighbouring cells to prevent healthy cells getting infected.

Question 4

Chemical Defences

Answer 4

Plants can produce toxins that repel vectors or kill pathogens when they enter the plant.

• Insect Repellents like citronella from lemon grass
• Insecticides - caffeine is toxic to insects and fungi
• Antibacterials - phenols which are antispectics, defensins which disrupt bacterial membranes, lysosomes which break down bacterial cell walls
• Antifungal - chitinases that break down the cell wall of fungi
• General - Cyanide can be made form certain plants.