4.1.3(Plant defences)

State the 2 different types of plant defences

• Active

• Passive

What is meant by a passive plant defence

• A defence which is present in the plant before infection occurs

• Their role is to prevent entry and spread of pathogens

What is meant by an active plant defence

• A Defence which is specific to when the pathogen enters the plant and is detected by the plant cells

name the 2 types of plant defences

• Physical defences

• Chemical defences

state the 7 types physical defences

• Cellulose cell wall

• Lignin thickening of cell walls

• Waxy cuticles

• Bark

• Stomatal closure

• Callose

• Tylose formation

Describe how the cellulose cell wall defences against pathogens

• It acts as a physical barrier and makes it more difficult for pathogens to enter a cell and causing disease

Describe how lignin thickening of cell walls defends against pathogens

• Lignin is a waterproof and almost completely indigestible and so therefore it makes it harder for pathogens to enter the xylem

Describe how waxy cuticles act as physical barriers against pathogens

• They are physical barrier

• Stops water from collecting on the leaf, preventing infection by pathogen transferred between plants in water

Describe how bark acts as a physical barrier against pathogen

• Physical barrier

• contains tannins which inhibit digestion in insects

Describe how stomatal closure acts as a physical barrier against pathogen

• Pathogens can enter plants through the stomata

• The guard cells control the stomata and they close the stomata when pathogens are detected in that part of the plant

Describe how callose acts as a physical barrier against pathogens

• Callose (a polysaccharide) is deposited in sieve tubes, plasmodesmata and cell wall

• it is deposited around the sieve plates and blocks the flow of the phloem in the sieve tube

• This prevents a pathogen from spreading around a plant

Describe how tyloses act as a physical barrier against pathogens

• Tyloses are balloon like structures in the xylem

• After a period of inactivity xylem vessels are blocked by tyloses and become non-functional

• Once a tylose is fully formed, it plugs the vessel and the vessel can no longer carry water

• this prevents the spread of pathogens through heart wood

• They contain a high concentration of chemicals such as terpenes that are toxic to pathogens

Describe one other role of callose against pathogens

• When a plant is stressed due a pathoegn invasion, callose is deposited between plant cell walls and plasma membranes near the invading pathogen

• Callose deposition make it harder for pathogens to enter the cells

• Callose strengthens the cell wall and blocks plasmodesmata

Describe how passive chemical defences act to prevent a pthogen from entering

• Plants produce anitimicrobial chemicals including antibiotics naturally without being invaded

• They kill and attaack any pathogens that enter the body

Describe how the passive physical defences change once the pathogen has entered the plant

• Once the pathogen has entered the plant, active defences begin to take place

• This includes:

  • Cell walls become thickened and strengthened with additional cellulose

  • Callose is deposited between the plant cell wall and cell membrane near the invading pathogen to strengthen the cell wall and close plasmodesmata

  • Oxidative bursts that produce highly reactive oxygen molecules capable of damaging the cells of invading pathogens

State and describe 5 chemicals used in active chemical plant defences

• Terpenoids:

  • They are a range of volatile oils that have antibacterial and antifungal properties. They also create scent

• Phenols:

  • They have antibacterial and antifungal properties as well and prevent function of enzymes

• Alkaloids:

  • Nitrogen-containing compounds which give a bitter taste to inhibit herbivores feeding

  • They inhibit enzymes

  • Some inhibit protein synthesis

• Defensive proteins(defensins):

  • Small cysteine rich proteins that have a broad anti-microbial activity.

  • They act upon chemicals in the plasma membrane of pathogens, which can inhibit the action of ion transport channels

• Hydrolytic enzymes:

  • These are found in the spaces between cells and they generally break down the pathogen plasma membrane or cell wall

State some examples of terpenoids

• Limonene which is found in citrous fruits

• Digitalis which is found in foxglove plants

• Latex which is found in rubber tree

State and describe the role of some phenols

• Tannins:

  • found in bark inhibit attack by insects. they bind to salivary proteins and digestive enzymes such as trypsin, deactivating the enzyme

  • Insects with a lot of tannins will stop growing and will eventually die which helps to prevent the transmission of pathogens

State and describe the tole of some hydrolytic enzymes

• Chitinases:

  • Break down chitin found in fungal cell walls

• Glucanases:

  • Hydrolyse the glycosidic bond in glucans

• Lysozymes:

  • Capable of degrading bacterial cell walls

Describe and explain the role of necrosis

• Necrosis is deliberate suicide of healthy cells

• This happens to cells surrounding the infection, this limits the access of the pathogen to water and nutrients

• This means that the pathogen can no longer spread in the plant

Describe how necrosis happens

• Necrosis happens due to the intracellular enzymes that are activated by injury.

• These enzymes destroy damaged cells and produce brown spots on leaves or die back

Describe what is meant by a canker

• Canker is a sunken necrotic lesion in the woody tissue siuh as the main stem or branch which causes death of the cambium tissue in the bark