12.4 - Plant Defences Against Pathogens

Question 1

What 2 defence mechanisms are there ?

Answer 1

• passive
• active

Question 2

What is a passive defence mechanism ?

Answer 2

• they’re always present
• some are physical barriers that prevent pathogens entering
• some are chemicals that reduce or prevent growth of pathogens

Question 3

What are active defence mechanisms?

Answer 3

• they’re activated when pathogens invade
• hypersensitivity deprives pathogens of resources
• formation of physical barriers by callose plays a major role in limiting the spread of pathogens

Question 4

How does cell signalling play important role in active defence mechanisms ?

Answer 4

Helps coordinate the active defence mechanisms

Question 5

How do plants recognise an attack?

Answer 5

• receptors in the cells respond to molecules from the pathogens or to chemicals produced when the plant cell wall is attacked

• this 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

Question 6

What is callose?

Answer 6

A polysaccharide

Question 7

What does callose contain?

Answer 7

• B-1,3 linkages
• B-1,6 linkages
• between the glucose monomers

Question 8

What are 3 things which act as a physical barrier to pathogens in plants ?

Answer 8

• waxy cuticle of leaves
• bark on trees
• cellulose cell wall

Question 9

What happens when plants are attacked by pathogens ?

Answer 9

• They rapidly set up extra mechanical defences
• they produce callose

Question 10

What are 3 cellular responses to infection in plants ?

Answer 10

• producing defensive chemicals
• physically strengthening cell walls
• sending alarm signals to unaffected cells to trigger their defences

Question 11

What 2 things can receptors in plant cells respond to upon infection ?

Answer 11

• molecules from the pathogen
• chemicals produced when the plant cell wall is attacked by pathogenic enzymes

Question 12

What two polysaccharides are made to strengthen plant cell walls upon infection ?

Answer 12

Callose and lignin

Question 13

What happens within minutes of an initial attack ?

Answer 13

• 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 infected parts and preventing the spread of pathogens
• callose Is deposited in the plasmodesmata between infected cells and their neighbours, sealing the, off from the healthy cells and prevent the pathogen spreading

Question 14

What do plants do to diseased tissue ?

Answer 14

• seal it off and sacrifice it
• continually growing meristems to replace the damaged parts

Question 15

What are the 3 parts of the plants cells response to pathogen ?

Answer 15

1. Detect
2. Communicate
3. React

Question 16

How do plant cells respond to detection of the pathogenic attack ?

Answer 16

• signalling molecules
• alert the nucleus to attack

Question 17

What are 3 actions that plant cell can take to defend/ attack ?

Answer 17

1. Physically strengthening cell walls with callose and lignin
2. Defensive molecules directly attack pathogen
3. These defensive molecules alert other cells to trigger their defences before they are attacked

Question 18

What are 3 places that callose is deposited ?

Answer 18

• between cell walls and cell membranes in cells next to infected cells
• blocks sieve plates In phloem
• in plasmodesmata between infected cell and neighbours

Question 19

What is the role of callose?

Answer 19

acts as a barrier sealing off infected area and preventing pathogens entering plant cells around the site of infection to prevent the spread of the pathogens

Question 20

What is the role of lignin ?

Answer 20

Makes callose deposits even thicker and stronger

Question 21

What are the 2 actions of powerful chemicals plants produce ?

Answer 21

1. Repel insect vectors of disease
2. Kill invading pathogens

Question 22

How do we use plant chemicals ?

Answer 22

1. Extract or synthesise to control insects , fungi and bacteria
2. Herbs and species because of strong flavour

Question 23

What are 6 examples of plant defensive chemicals ?

Answer 23

• insect repellent
• insecticides
• antibacterial compounds
• anti-fungal compounds
• anti- oomycetes
• general toxins

Question 24

What are some examples of insect repellents ?

Answer 24

• Pine resin
• citronella from lemon grass

Question 25

Example of insecticides

Answer 25

• pyrethrins - made by chrysanthemums and act as insect neurotoxins
• caffeine - toxic to insects and fungi

Question 26

Example of antibacterial compounds

Answer 26

• antibiotics for eg. Phenols
• antiseptics made in 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

Question 27

Examples of anti fungal compounds

Answer 27

• phenols
• antifungal gossypol produced by cotton
• caffeine- toxic to fungi and insects

Question 28

Examples of anti - oomycetes

Answer 28

• glucanases , enzymes made by some plants that break down gkucans
• polymers found un the cell walls of oomycetes

Question 29

Examples of general toxins

Answer 29

• 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

Question 30

What do chemical defences do ?

Answer 30

Prevent pathogens from growing on the surface of the plant by creating acidic conditions

Question 31

Name a toxic compound example

Answer 31

Catechol

Question 32

What does sticky resin found in bark do ?

Answer 32

Traps the pathogens so they can’t spread

Question 33

What is the importance of cell signalling in plant defence ?

Answer 33

• pathogens posses cellulose enzymes that digest the cellulose in plant cell walls
• 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

Question 34

What are the 4 different phytoalexins mode of transportation?

Answer 34

• 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 )

Question 35

What are 2 other important signalling molecules involved in plant defence ?

Answer 35

• salicylic acid
• ethylene

Question 36

Why is salicylic acid an important molecule in plant defence ?

Answer 36

• migrates through the plant to uninflected areas
• once there it activates defence mechanisms that protect the plant against pathogens for a period of time
• long term protection is called systematic acquired resistance

Question 37

Why is ethylene an important signalling compound ?

Answer 37

• allows plants to communicate
• plants under attack from a thorns secrete ethylene onto their leaves
• ethylene Vaporises , stimulating other leaves in the same plant to react