Communicable diseases: plant defences

Question 1

How do plants get rid of disease?

Answer 1

Do not heal diseased tissue, but seal it off and sacrifice it. Continually growing at meristems, so replace the damaged part.

Question 2

Describe the process of disease recognition in plants

Answer 2

No passive. Receptors in the plasma membrane can directly recognise the pathogen, whilst some of the by-products of pathogenic enzyme breakdown are recognised. Stimulates the release of signalling molecules in the nucleus, which switch on genes in the nucleus. Trigger in cell responses, including defensive chemicals, polysaccharides (callose and lignin) to strengthen cell wall. Causes direct attack to pathogen and signal to surrounding cells.

Question 3

Describe the physical defences of plants

Answer 3

Produce polysaccharide callose, containing beta- 1,3, and 1,6, linkages between the glucose monomers. Callose is synthesised and deposited between cell wall and membrane. Acts as a barrier, preventing access of pathogen.
Lignin is added increasing mechanical strength.
Callose blocks sieve plates in the phloem, sealing off infected parts - no spread.
Callose in plasmodesmata between infected cells. No access to healthy cells & no spread.

Question 4

Describe the chemical defences of plants

Answer 4

Insect repellants: pine resin and citronella from lemon grass.
Insecticides: pyrethrins from chrysanthemums are neurotoxins and caffeine is toxic.
Antibacterial compounds like antibiotics: phenols (antiseptics) gossypol from cotton, defensins (plant protein disrupts membranes), lysosomes (enzymes that break down cell wall).
Anti-fungal compounds: phenols, gossypol, caffeine, saponins ( chemicals in membranes which disrupt) and chitinases (enzymes break down chitin in fungal cell walls).
Anti-oomycetes: glucanases (enzyme made by plants that break down glucans).
General toxins: chemicals broken down into cyanide.