12.4 - Plant Defences Against Pathogens Flashcards
What 2 defence mechanisms are there ?
- passive
- active
What is a passive defence mechanism ?
- they’re always present
- some are physical barriers that prevent pathogens entering
- some are chemicals that reduce or prevent growth of pathogens
What are active defence mechanisms?
- 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
How does cell signalling play important role in active defence mechanisms ?
Helps coordinate the active defence mechanisms
How do plants recognise an attack?
• 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
What is callose?
A polysaccharide
What does callose contain?
- B-1,3 linkages
- B-1,6 linkages
- between the glucose monomers
What are 3 things which act as a physical barrier to pathogens in plants ?
- waxy cuticle of leaves
- bark on trees
- cellulose cell wall
What happens when plants are attacked by pathogens ?
- They rapidly set up extra mechanical defences
- they produce callose
What are 3 cellular responses to infection in plants ?
- producing defensive chemicals
- physically strengthening cell walls
- sending alarm signals to unaffected cells to trigger their defences
What 2 things can receptors in plant cells respond to upon infection ?
- molecules from the pathogen
- chemicals produced when the plant cell wall is attacked by pathogenic enzymes
What two polysaccharides are made to strengthen plant cell walls upon infection ?
Callose and lignin
What happens within minutes of an 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 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
What do plants do to diseased tissue ?
- seal it off and sacrifice it
- continually growing meristems to replace the damaged parts
What are the 3 parts of the plants cells response to pathogen ?
- Detect
- Communicate
- React
How do plant cells respond to detection of the pathogenic attack ?
- signalling molecules
- alert the nucleus to attack
What are 3 actions that plant cell can take to defend/ attack ?
- Physically strengthening cell walls with callose and lignin
- Defensive molecules directly attack pathogen
- These defensive molecules alert other cells to trigger their defences before they are attacked
What are 3 places that callose is deposited ?
- between cell walls and cell membranes in cells next to infected cells
- blocks sieve plates In phloem
- in plasmodesmata between infected cell and neighbours
What is the role of callose?
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
What is the role of lignin ?
Makes callose deposits even thicker and stronger
What are the 2 actions of powerful chemicals plants produce ?
- Repel insect vectors of disease
- Kill invading pathogens
How do we use plant chemicals ?
- Extract or synthesise to control insects , fungi and bacteria
- Herbs and species because of strong flavour
What are 6 examples of plant defensive chemicals ?
- insect repellent
- insecticides
- antibacterial compounds
- anti-fungal compounds
- anti- oomycetes
- general toxins
What are some examples of insect repellents ?
- Pine resin
- citronella from lemon grass
Example of insecticides
- pyrethrins - made by chrysanthemums and act as insect neurotoxins
- caffeine - toxic to insects and fungi
Example of antibacterial compounds
- 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
Examples of anti fungal compounds
- phenols
- antifungal gossypol produced by cotton
- caffeine- toxic to fungi and insects
Examples of anti - oomycetes
- glucanases , enzymes made by some plants that break down gkucans
- polymers found un the cell walls of oomycetes
Examples of 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
What do chemical defences do ?
Prevent pathogens from growing on the surface of the plant by creating acidic conditions
Name a toxic compound example
Catechol
What does sticky resin found in bark do ?
Traps the pathogens so they can’t spread
What is the importance of cell signalling in plant defence ?
- 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
What are the 4 different phytoalexins mode of transportation?
- 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 )
What are 2 other important signalling molecules involved in plant defence ?
- salicylic acid
- ethylene
Why is salicylic acid an important molecule in plant defence ?
- 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
Why is ethylene an important signalling compound ?
- 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
