Chapter 12-12.4- Plant defences against pathogens

Question 1

What are some of the ways plants have evolved to defend themselves against communicable diseases?

Answer 1

Waxy cuticle

Bark

Cellulose cell walls- barrier

Plants do not heal diseased tissue as animals do- they seal it off and sacrifice it. Because they are continually growing at the meristems, the can then replace the damaged parts.

Question 2

How do plants recognise an attack?

Answer 2

Plants are not passive- they respond rapidly to pathogen attacks.

Receptors in the cells respond to molecules from the pathogens, or to chemicals produced when the plant cell wall is attacked.

Tis stimulates the release of signalling molecules that appear to switch on genes in the nucleus. This in turn triggers cellular responses, which include producing defensive chemicals, sending alarm signals to unaffected cells to trigger their defences, and physically strengthening the cell walls.

Revise diagram in book on p314

Question 3

What are the physical defences of a plant?

Answer 3

When plants are attacked by pathogens they rapidly set up extra mechanical defences. They produce high levels of a polysaccharide called callose, which contains (beta)-1,3 linkages and (beta)-1,6 linkages between the glucose monomers. Scientists still do not fully understand the roles played by callose in the defence mechanisms of the plant but current research suggests that:

• Within minutes of an initial attack, callose is synthesised and deposited between cell walls and the cell membranes in cells next to the infected cells.

These callose papillae act as barriers, preventing the pathogens entering the plant cells around the site of infection.

• Large amounts of callose continue to be deposited in cell walls, after the initial infection. Lignin is added, making the mechanical barrier to invasion even thicker and stronger.
• Callose blocks sieve plates in the phloem, sealing off the infected parts and preventing the spread of pathogens.
• Callose is deposited in the plasmodesmata between the infected cells and their neighbours, sealing them off from the healthy cells and helping to prevent the pathogen spreading.

Question 4

What are the chemical defences of a plant?

Answer 4

Many plants produce powerful chemicals that either repel the insect vectors of disease or kill invading pathogens. Some of these chemicals are so powerful that we extract and use them or synthesise them to help us control insects, fungi and bacteria.

Some examples have strong flavours and are used as herbs and spices. Examples of plant defensive chemicals include:

• Insect repellents- for example, pine resin and citronella from lemon grass.
• Insecticides- for example pyrethrins- these are made by chrysanthemums and act as insect neurotoxins; and caffeine- toxic to insects and fungi
• Antibacterial compounds including antibiotics- for example, phenols- antiseptics made in many different plants; antibacterial gossypol produced by cotton; defensins- plant proteins that disrupt bacterial and fungal cell membranes; lysosomes- organelles containing enzymes that break down bacterial cell walls.
• Antifungal compounds- for example, phenols- antifungals made in many different plants; antifungal gossypol produced by cotton; caffeine- toxic to fungi and insects; saponins- chemicals in many plant cell membranes that interfere with fungal cell membranes; chitinases- enzymes that break down the chitin in fungal cell walls.
• Anti-oomycetes- for example, glucanases- enzymes made by some plants that break down glucans; polymers found in cell walls of oomycetes (e.g. P.infestans).
• General toxins- some plants make chemicals that can be broken down to form cyanide compounds when the plant cell is attacked. Cyanide is toxic to most living things.