L7 - Disease resistance Flashcards
Split disease resistance into two broad classes
Passive resistance:
- Plant lacks a susceptibility gene
- Genetically recessive
Active resistance:
- Plant actively does something to block the pathogen
- Resistance factor or resistance gene (R-gene) present
- Genetically dominant
What is the most simple things plants do to protect themselves? Give examples
- Build physical or chemical walls
Preformed physical barrier examples:
- Cuticle on epidermal cells
- Primary and secondary cell walls incl. lignin, suberin, waxes etc…
Preformed chemical barrier examples:
- Secondary metabolites with antimicrobial or pesticidal properties e.g. caffiene and coffee!
How do fungi and oomycetes penetrate physical barriers?
Sketch the relevant diagram
- Spores of these filamentous pathogens germinate
- Turgor in appressorium causes penetration peg to penetrate cuticle
- Invasive hypha grow between cells
- Occasionally invade cell wall, producing intracellular haustoria w/ large SA for two way exchange
- PM crucially never penetrated
See relevant diagram on pg 3
How do pathogenic bacteria penetrate physical barriers?
- Form different secretion systems
- Type III secretion system most common
- Inject macromolecules into intercellular space or into cell
Macromolecule properties:
- EITHER hydrolases - break down barriers and release nutrients
- OR effectors e.g. proteins/DNA that adapt plant cells
How do nematodes penetrate physical barriers?
Give two points about the evolution of the mechanisms used
- Use needle like stylets to puncture plant tissue
- Allows either migration inside host tissue or delivers effectors e.g. cell wall degrading enzymes
- Stylets have evolved 4 times in nematode phylum, convergent evolution (prerequisite for parasitism)
- Cell wall degrading enzymes horizontally acquired from bacteria + fungi
How do viruses penetrate physical barriers?
- Dependent on damage to plant cell OR transported by invertebrate vectors (e.g. nematodes, insects) or fungi
- Once inside transported via plasmodesmata
Give an example of a chemical barrier used by plants against pathogens.
Describe what it protects against as well as whether it’s constitutive or not and why.
Avenacin
- Preformed antimicrobial compound in oats
- Protects against Gaeumannomyces graminis fungus that causes Take All disease in wheat
- Wheat susceptible as doesn’t produce avenacin
- Constitutive (on all the time)
- Possible as it’s a saponin - inserts into fungal but not plant membrane (no/little effect on host)
- Causes aggregation of membrane and pore formation = loss of fungal cell content
- Gaeumannomyces graminis variants can infect oats by producing avenicinase
Active resistance can be further categorised into two categories. Name and describe these categories
Constitutive response:
- Always present in the cell
- Must cause little to no damage to plant
- Cannot be very severe as a result
Inducible response:
- Has a cost to the host
- Must be localised as a result
- More localised = more severe, up to cell death
- Requires detection of pathogen (directly) or action of pathogen (indirectly)
Generally, what features of pathogens do plants use for detection?
Describe an example
- Highly conserved region that a whole pathogenic group contains
- E.g. Fungi chitin
- E.g. Pseudomonas species + other gram - plant pathogenic bacteria have flagellae
- flg22 = short, highly conserved peptide forming part of flagella
- flg22 elicits plant defense, e.g. cell death
- flg22 = PAMP or MAMP
- Conserved regions essential for bacteria
How is flg22 detected by plants?
- Mutant screen identified FLS2 gene in plant responsible (explain)
FLS2:
- receptor like protein kinase
- has extracellular leucine rich repeat domain (LRR) that recognises flg22
- has intracellular protein kinase domain (PK) to signal detection
Describe the evidence for FLS2’s action
- Binding assay shows direct contact between FLS2 and flg22
- WT FLS2 gene added to loss of disease FLS2 mutant exhibits disease resistance
- FLS2 = PAMP recognition receptor and starts PAMP triggered immunity
How does FLS2 initiate PAMP triggered immunity
- Recognition of flg22 stimulates FLS2 to associate BAK1
- FLS2/BAK1 complex initiates signalling cascade
- Causes PAMP triggered immunity (PTI)
Elaborate on the following statement: “All members of a plant species are resistant against all members of a pathogen species”
- PAMP generally conserved in pathogens
- PTI fundamental and conserved in plants
- Hence generally a plant will always be resistant to a pathogen
How do plants avoid recognition and PTI?
Two general strategies:
- Avoid recognition
- Block downstream PTI pathway
- E.g. Ps syringae and other biotrophic gram - bacteria suppress PTI using type 3 secretion system (TTSS)
- Effector prevents interaction of BAK1 with PAMP recognition receptor
- E.g. Fungi block plant pathway which recognises conserved Chitin oligomers and produces Chitinases to degrade fungal cell wall :
- Avr4 effector blocks chitinase activity
- LysM effectors sequester chitin oligomers, reducing detection
