L6 - Plant Disease Flashcards
Why is plant disease a problem?
- Infectious disease always a problem due to the niche of plants made of sugar
- Threat to global food security, 20 - 30% yield loss in major crops
- Sporadic epidemics, pathogen variants overcome resistance mechanism
- Can spoil post harvest product without damaging yield
Describe the pathogens that can exploit plants
- Bacteria, nematodes, viruses, plants, insects, fungi and oomycetes
Same principles underpinning host interaction:
- arms race of defense by plant and counterattack by pathogen
Outline some of the main consequences for humans of pathogens.
Give specific examples
Famine
- E.g. Irish potato famine caused by oomycete pathogen Phytophthora infestans
Post harvest effects:
- E.g. mycotoxins produced by fungi, serious health problems if eaten
- Example mycotoxin = ergot fungus, causes ergotism through cereals
Give the three categories of plant pathogen based on function
What do they all have in common?
1) Necrotrophs
- kills cells and consumes contents
- secretes cell wall degrading enzymes
- wide host range
2) Biotrophs
- plant cells remain alive
- minimal damage
- specific hosts (close association)
3) Hemibiotroph
- EITHER biotrophic stage before necrotrophic
- OR biotrophic on some hosts, necrotrophic on others
- All need to overcome defense and extract nutrients + other resources
Describe the general mechanism of filamentous biotrophs
Filamentous biotrophs - fungi and oomycetes:
- Fungal spore grows on outside of plant cell
- Haustorium grows, requiring proliferation of plant membrane
- No penetration of PM
- Increases SA for exchange of nutrients + water to biotroph
See relevant diagram on pg 5
Describe an example of Nematode biotrophs
Nematode biotrophs:
- Attach to plant roots
- Then induce physiological changes e.g. stimulate cell divisions and cell size to create efficient feeding structure
- Feed from cytoplasm
- Undetected
Which parts of the plant do bacterial biotrophs inhabit?
- Extracellular space
Why are viruses extreme biotrophs? Outline their features
- Intracellular pathogens
- Exploit nutrients, protein structures and cellular structures for replication
- Secretion + exchange systems not required as introduced by mechanical damage or invertebrate vector
Define the effector concept.
Why is it important?
- (generally) proteins produced by pathogen to alter host physiology, development and defense responses to benefit the pathogen
- Required in all pathogens to overcome all plants’ innate resistance
Name and describe the general functioning of a family of effectors from bacteria
Draw the relevant diagram
Transcription Activator-Like Effectors (TALEs)
- Enter the nucleus of the plant cell
- Wrap around major groove of DNA helix
- Bind DNA
- Regulates plant gene expression
Plant TF made by bacteria
See relevant diagram on pg 8
How do TALEs bind to target DNA in the plant?
- TALEs contain a repetition of highly conserved amino acid sequences
- Each repeat has a non-conserved motif of a few amino acids in the “repeat variable domain” (RVD)
- Each RVD binds a specific base
- Sequence of RVDs corresponds to the target DNA base sequence
What is the idea of susceptibility genes?
Give an example as well as the evidence for this
- Plant genomes encode genes required by pathogens in order to infect
- Due to that gene being necessary for plant survival
- E.g. TALEs can target SWEET sucrose transporters
- Sucrose transported into extracellular matrix to feed bacterial growth
- BUT plants need sucrose transporters for feed cells for respiration
- Evidence: Loss of function TALE mutation prevents SWEET activation, reducing bacterial colony
How can plants with susceptibility genes become disease resistant?
Generally: Disease resistant if plant lacks something the pathogen needs - a susceptibility gene
- Disease resistance e.g. due to loss of interaction w/ effector
- E.g. through mutation in DNA binding site for TALEs
- Mutation must not effect plant
- Genetically recessive
