L9 - Crop protection against infectious disease 1 Flashcards
Outline the genetics solutions to plant disease and the general downsides
- Speeding up conventional breeding
- GM solutions based on ETI, PTI or manipulation of post recognition pathway
- Gene editing
Disadvantages:
- Takes several years
- Many not give durable resistance to new strains
Describe a recent discovery that has been very important in understanding and developing plant immunity
- PTI and ETI can be considered a single entity
Evidence:
- Distinct ligands (PAMP and effectors) but similar outcomes (not conclusive)
- BAK1 required for ETI-associated pathogen resistance against Hpa (but doesn’t isolate effects of PTI and ETI) (not conclusive)
- Separation of PAMPS and ETI show dependency (some NLRs require PRR for HR) (conclusive)
Describe the process of conventional plant breeding for disease resistance.
- Introgression used to introduce desired R gene from donor to elite variety
- Knowledge of R gene not needed but selection marker accelerates process
Disadvantages:
- Linkage drag can introduce undesirable genes adjacent to R gene
- Hard w/ recessive genes e.g. susceptibility genes
How can GM be used in developing disease resistant plants? What are the general cons
Describe the two sub categories of GM
Can introduce R gene
PROS:
-Eliminates linkage drag
CONS:
- R gene must be molecularly characterised
- Don’t know where to place genes - may replace important gene or be silenced
Cisgenesis:
- Gene transfer between hybridisable species
- Not well accepted publicly
Transgenesis:
- Gene transfer between non-hybridisable species
- Even less public (and regulatory) acceptance
Give an example of where cisgenesis is used in the field
Protection against Phytophthora infestans
- Causes major disease, especially in potatoes (e.g. Irish Potato Famine)
- Produces extra and intercellular effectors
- Can be controlled via GM introduction of NLR R genes from potato relatives to non-resistant potatoes e.g. Solanum tuberosum
Name some problems with transgenic plants in the field and how these can be overcome
Transfer of only sensor OR helper but not both
- Develop more knowledge
Pathogen evolution overcomes resistance
- Stack resistant mechanisms
- Identify more R genes
How can the rapid identification of R genes be done to improve cisgenics and breeding approaches
- Library of oligonucleotides designed to correspond to all NLR genes in genome
- NLRs functionally identified by linking variation in NLR-ome to variation in resistance
- NLRs can be transferred
When are transgenic approaches to disease resistance effective?
- To transfer phytoalexin pathways between species
- To transfer PRRs between species
- To overexpress components of signal transduction pathways
Can PTI be transferred between species?
- Yes, some forms of PTI can be transferred
- E.g. The RLK EFR transferred from Arabidopsis to Solanaceae enhances bacterial resistance
- EFR binds the PAMP ELF18
- Bacteria in Solanaceae not used to this specific type of EFR
Under what conditions in the transgenic overexpression of genes in a post recognition pathway beneficial?
Give an example of where it is useful
- Effect of overexpression must not be constitutive
- Constitutive expression = cost of resistance, e.g. HR in absence of pathogen
- Useful if not constitutive, e.g. overexpression of NPR1
- NPR1 protein accumulates in multimeric form in cytoplasm
- Only released to active form upon SA perception from pathogen-induced change
- Increased copies = faster + stronger response
How is gene editing usually used to produce disease resistance?
Give an example
- To introduce mutations into defined target genes, usually susceptibility genes
- Two types of susceptibility genes targeted: effector targets and negative regulators of immunity
Negative regulators:
- Optimal levels of negative regulation different in crops to wild species
- “Negativity” can be reduced if crop can tolerate higher immunity levels
- Supported by natural mutation of mlo in barley, conferring resistance to powdery mildew
Give an example of how gene editing can be used to engineer resistance to TAL effectors
- E.g. Engineering of a regulating gene that turns off a susceptibility gene only upon binding of a TAL
Why is the durability of resistance a problem?
What factors affect the durability of genetically engineered resistance? GIve examples do demonstrate this point
- Pathogens have tendency to overcome resistance via evolution (especially in monocultures)
- Dominant R genes far easier to overcome than recessive R genes
- Due to “easy to break things, hard to make things”
- E.g. mutations in pathogen makes binding surface of dominant gene obsolete
- E.g. Altering binding site of a TAL target gene requires mutations in TAL to match
- Low likilihood that effector mutation will leave function intact
What has the conventional use of chemistry for crop protection consisted of?
Why may this not be the best idea and what alternatives may do better?
- Crop protection chemicals are typically microbicides
- Imposes strong selection pressure on resistant pathogens
- Using chemicals to target host may be better
- E.g. by priming or sensitise plant immune system
- Several potential compounds that don’t have an effect until activation of PTI or ETI pathways
- Potentially long lived priming effect = infrequency application/in seed
