12.4 - Plant Defences Against Pathogens

Question 1

How do plants react and recognise an attack?

Answer 1

• Receptors in cells respond to molecules from pathogens, or chemicals produced when plant cell wall is attacked
• Stimulates release of signalling molecules that switch genes on in nucleus
• This triggers cellular responses – e.g.: Producing defensive chemicals; sending alarm signals to unaffected cell walls to trigger defence; physical strengthening of cell wall (callose and lignin strengthen cell walls)

Question 2

What is an example of physical defences by a plant against disease?

Answer 2

• Plant is attacked, high levels of the polysaccharide callose (with β-1,3 and β-1,6 bonds between glucose monomers). (Not fully understood the role callose plays in defence).

1. Callose is synthesised and deposited between cell walls and cell membranes in cells surrounding infected ones. Callose papillae are physical barriers preventing pathogens entering around infection site.
2. Large quantities of callose are deposited in cell wall after initial infection. Lignin is also added to increase the mechanical strength of the barrier
3. Callose blocks sieve plates of phloem, sealing pathogenic parts of plants to stop the spreading
4. Callose is then deposited in plasmodesmata between infected and neighbouring cells. Sealing these corridors from healthy cells – prevents pathogens spreading

Question 3

What uses do chemicals in plants have?

Answer 3

• Chemicals are produced that either repel insect disease vectors or kill invading pathogens.
• Humans also extract these chemicals due to their strength – helps control pathogen types and insects

Question 4

What are examples of chemical defences in plants?

Answer 4

1. Terpenoids - oils with antibacterial & antifungal properties
2. Phenols (e.g. tannin) - has antibacterial & antifungal properties. Tannins inhibit insect attack by intefering with insect digestion
3. Alkaloids (e.g. caffeine, nicotine, cocaine, morphine) - bitter taste to inhibit herbivores. Intefere with insect metabolism by inhibiting or activating enzymes
4. Hydrolytic enzymes - found in cell sapces - e.g.
   
   1. chitinases, break down chitin in fungal cell walls
   2. glucanases - hydrolyse glycosidic bonds in glucans of bacterial cell walls
   3. lysosymes - degrade bacterial cell walls
5. Defensins - cytesine-rich proteins inhibiting ion transport in pathogenic plasma membranes