Week 5 - Plant disease and immunity

Question 1

What is the definition of a plant disease?

Answer 1

Abnormal growth of a plant caused by a pathogen

Question 2

What types of pathogen cause disease?

Answer 2

Viruses
Fungi
Fungi-like microbes
Bacteria
Nematodes

Question 3

What is the difference between plant disease symptoms and signs?

Answer 3

Symptoms are detectable abnormalities caused by the disease, signs are the visible pathogen itself

Question 4

Give 2 signs and 2 symptoms a plant might experience in response to disease caused by fungi

Answer 4

Signs:
Soft rot/ Dry rot
Leaf spots
Leaf curl

Symptoms:
Rust (orangey brown)
Smut (black mass)
Mycelium
Fruiting bodies

Question 5

Give an example of a fungi-like microorganism and symptoms it create in a plant

Answer 5

Oomycota:
Late blight
Necrosis
Downy mildews
leaf infections

Question 6

What signs and symptoms does bacteria cause ?

Answer 6

Symptoms :
Leaf spots
Wilts
Cankers
Diebacks
Rots
Signs:
Ooze

Question 7

What symptoms to viruses produce in plants?

Answer 7

Necrosis
Mosaics
Chlorosis
Stunting
Crinkling

Question 8

What are some signs and symptoms of nematodes ?

Answer 8

Symptoms:
Stunting
Discolouration, decline
Dieback
Root lesions, rots
Necrosis
Chlorosis
Galls
Signs:
Cysts

Question 9

What are the 4 steps of Koch’s postulates in diagnosing a plant disease?

Answer 9

1. Identify the pathogen (organism) in diseased plants
2. Isolate and grow the suspected pathogen (organism) in a lab.
3. Inoculate a healthy plant to see if it gets the same symptoms
4. Reisolate the pathogen (organism) and see if its the same as the original

Question 10

What are some examples of molecular techniques to diagnose plant diseases?

Answer 10

DNA microarrays
RT-PCR
DNA extraction

Question 11

Which 3 factors must be present for a pathogen to cause disease?

Answer 11

Pathogen, environment, host

Question 12

What are the 4 lifestyles of a pathogen?

Answer 12

• Commensal = microbe benefits, plant
  doesn’t
• Mutualistic = both parties benefit
• Pathogenic = causes disease/kills plant
• Saprotrophic = pathogen feeds on dead
  plant matter

Question 13

What are the 3 ways pathogens interact with their host?

Answer 13

Biotrophic = keeps host alive whilst using it
Hemibiotrophic = Pretends to be mutualistic but switches to necrotrophic
Necrotrophic = kills host

Question 14

What are the stages of a disease cycle?

Answer 14

1. Dispersal of primary inoculum
2. Deposition
3. Penetration
4. Infection
5. Colonization
6. Reproduction
7. Survival

Question 15

What are some methods to disperse inocula?

Answer 15

Using host plant
Plant debris
Soil
Alternate hosts

Question 16

How does pathogen survive after infection before it infects a new host ?

Answer 16

Some create structures like oospores which can survive in soil/plants for a long time.

Question 17

Which different ways could we manage plant diseases?

Answer 17

• Cultural practices
• Chemical control
• Biological control (Using an organism to
  control another organism)
• Resistant cultivars
• Biotechnology

Question 18

What is inoculum?

Answer 18

Part of a pathogen capable of causing infection in a host plant.

Question 19

What is meant by the term Non-host resistance?

Answer 19

Ultimate defence (pathogen can never cause disease in that plant)

Question 20

What are the 2 types of defence in plants?

Answer 20

Chemical and physical

Question 21

What is the first line of physical defence?

Answer 21

Dermal tissue:
- Waxy cuticle
- Bark
- Suberin

Question 22

How could plants respond to damaged physical defences?

Answer 22

Strengthen cell wall locally
Close stomata
Formation of tyloses (blockages in xylem vessel- prevents movement)

Question 23

What are some examples of chemical defences?

Answer 23

• Defensins
• Antimicrobial metabolites
• Hypersensitive response

Question 24

What are the 2 components of the plant immune system?

Answer 24

• Basal resistance
• Specific resistance

Question 25

What is Basal resistance ?

Answer 25

Triggers a generalised defence response : Pathogen recognition receptors (PRR) recognise MAMPs/PAMPSs and DAMPs - called Pattern-triggered immunity (PTI) This triggers production of defensins AGAINST MICROBES

Question 26

What is specific resistance?

Answer 26

Triggers a localised then generalised response Can be called gene-for-gene hypothesis (has specific receptors).

Question 27

How does the R gene create an immune response in the specific resistance component?

Answer 27

If plant has the corresponding R protein to the pathogen's Avr protein it triggers defences against the disease. If the plant doesn't have a corresponding R protein to the pathogens Avr gene - plant succumbs to disease.

Question 28

What is the hypersensitive response?

Answer 28

This response will be triggered after the specific response (once R protein is recognised by pathogen). Plant cells produce large quantities of Reactive Oxygen Species, which induce rapid cell death around the site of attack. - Leads to longer term resistance

Question 29

What is Systematic acquired resistance (SAR)?

Answer 29

The hypersensitive response produces Salicylic acid, which is the main signalling molecule in SAR. SAR triggers resistance across the whole of the plant. Allowing it to respond more quickly to a second attack

Question 30

What is induced systematic resistance?

Answer 30

Triggered by non-pathogenic microbes Main signalling molecules = Jasmonic acid (JA) and ethylene (ET) A response to non harmful microbes that helps improve the response of the plant in the future to necrotrophs and insect herbivores

Question 31

Which are SAR and ISR most effective against, Biotrophs or necrotrophs?

Answer 31

SAR = Biotrophs ISR = Necrotrophs

Question 32

Which response involves localised cell death??

Answer 32

Hypersensitive

Question 33

What is involved in defence against viruses?

Answer 33

SAR and gene silencing

Question 34

What is the order of responses based on severity?

Answer 34

First (lowest): Basal resistance Specific resistance Hypersensitive response Systematic acquired resistance (SAR) Induced systematic resistance (ISR) Last (strongest)