4.1.3 Plant Defences against Pathogens

Question 1

What are the two different mechanisms that plants have?

Answer 1

Just like animals, plants have defence mechanisms to protect themselves against infection and disease

The different mechanisms are classified into two categories: passive and active

Passive defence mechanisms are always present

Some of these mechanisms are physical barriers that prevent pathogens from entering

Some are chemicals that reduce or prevent the growth of pathogens

Active defence mechanisms in plants are activated when pathogens invade

Hypersensitivity deprives pathogens of resources

The formation of physical barriers by callose plays a major role in limiting the spread of pathogens

Cell signalling plays an important role in coordinating the active defence mechanisms

Question 2

What are the passive defence mechanisms?

Answer 2

Physical barriers make it harder for pathogens to gain entry into plants
Examples of physical barriers:

Waxy cuticle
The only way that viruses and bacteria can penetrate the waxy cuticle of a leaf is if there is a wound on the leaf surface or stem.
Wounds are commonly caused by grazing herbivores

Cellulose cell wall

Closed stomata

Bark

Casparian strip
Some fungi species can invade a plant all the way to the endodermis but they are unable to push past the Casparian strip

Chemical defences prevent pathogens from growing on the surface of the plant by creating acidic conditions

Examples of chemical defences:
Toxic compounds
E.g. Catechol
Sticky resin found in the bark
This traps the pathogens so they can’t spread
Compounds that encourage the growth of competing microorganisms
Microorganisms such as yeast found on the leaf surface are completely harmless to plants. They are strong competitors against harmful pathogens
Enzyme inhibitors
E.g. Tannins
Receptor molecules
They detect the presence of pathogens and trigger other defence mechanisms

Question 3

What are the active defence mechanisms?

Answer 3

Unlike animal cells, plant cells have cell walls. This means that substances can not freely move around the entire plant as the immune cells do in some animals, making cell signalling vital for plant defence

The active defence mechanisms of a plant are activated once a pathogen has invaded

Hypersensitivity is the rapid death of tissue surrounding the infection site
Although quite an extreme response, it is very effective as it deprives the pathogens of host tissue, nutrients and energy

Plants also create physical barriers to reduce the spread of a pathogen
Reinforced cell walls are formed when fungi and bacteria invade
The invasion of pathogens stimulates the release of compounds callose and lignin
These molecules are deposited between the cell surface membrane and the cell wall
Callose is a polysaccharide that forms a matrix shape. Antimicrobial compounds that kill pathogens (hydrogen peroxide and phenols) can be deposited in this shape
Narrowing of the plasmodesmata
Callose helps to reduce the size of the channels that connect neighbouring plant cells
Ingrowths into the xylem vessels (tyloses)
The cytoplasm of nearby cells grows into the xylem to create a wall made of callose
Blockage of the phloem
The sieve pores are filled with callose which prevents phloem sap from being transported

Question 4

What is the importance of cell signalling in plant defence.

Answer 4

Pathogens possess cellulase enzymes that digest the cellulose in plant cell walls

The molecules produced from this breakdown of cellulose act as signals to cell surface receptors

By stimulating these receptors they cause the release of defence chemicals called phytoalexins

Phytoalexins have several modes of action
Disrupting pathogen metabolism
Delaying pathogen reproduction
Disrupting bacterial cell surface membranes
Stimulating the release of chitinases (enzymes that break down the chitin cell walls in fungi)

Salicylic acid is another important signalling molecule involved in plant defence
It migrates through the plant to uninfected areas. Once there it activates defence mechanisms that protect the plant against pathogens for a period of time
This long-term protection is called systemic acquired resistance

Ethylene is a signalling compound that allows plants to communicate
Plants under attack from pathogens secrete ethylene onto their leaves. The ethylene vaporises, stimulating other leaves on the same plant to react (as well as other plants)