Plant defense against pathogens

What are some examples of ways that plants are evolved to defend themselves against pathogens?

• Waxy cuticle of plant leaves

• Bark on trees

• Cellulose cell walls of plant cells

why can plants use barrier adaptions to prevent themselves against pathogens?

• they act as barriers which prevent pathogens getting in

• plants don’t heal diseased tissue

• they seal it off, sacrifice it - able to do this as they are continually growing at the meristems so they can replace the damaged parts

what are some features of plants recognizing an attack?

• they are passive

• they respond rapidly to pathogen attacks

how do plants recognize an attack?

1. receptors in cells respond to molecules from the pathogen, or to chemicals produced when the plant cell wall is attacked

2. stimulates the release of signalling molecules that appear to switch on genes in the nucleus

3. triggers cellular responses, including producing defensive chemicals, sending alarm signals to unaffected cells to trigger their defences and physically strengthen the cell walls

fill in the diagram to help show how plants recognise an attack

what are some physical defences in plants?

• When plants are attacked by pathogens they rapidly set up extra mechanical defences

• They produce high levels of a polysaccharide called callose, which contains β - 1,3 linkages and β - 1,6 linkages between the glucose monomers

what does the current research into callose defence mechanisms in plants?

• Within minutes of initial attack, callose is synthesised and deposited between the cell walls and the cell membrane 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 part and preventing the spread of pathogens

• Callose is deposited in the plasmodesmata between infected cells and their neighbors, sealing them off from the healthy cells and helping to prevent the pathogen spreading 

what are plant chemical defences?

• 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 have strong flavours and are used as herbs and spices.

what are some examples of plant chemical defences?

• Insect repellents - e.g. pine resin

• Insecticides - e.g. pyrethrins

• Antibacterial compounds including antibiotics - e.g. phenols

• Antifungal compounds - e.g. phenols - antifungals made in many different plants

• Anti-oomycetes - e.g. glucanases - enzymes made by some plants that break down glucans

• 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.