Download Major gene resistance

Heath Ann Bot 80, 713
Differences between animals/plants
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Plants have no RAG (recombinant activating gene)-dependent
immune system
No circulating immune cells – local recognition and response
infection
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Cellular communication via plasmodesmata
sometimes co-opted by bacteria and viruses to move systemically
Whole plant response – Systemic acquired resistance
Plants must differentiate between pathogens and beneficial
symbionts (Rhizobium and mycorrhizal fungi)
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important in nutrient poor soil and/or as biocontrol against pathogens
Triggers of SAR?
Fungal pathogenicity on plants
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Fungal pathogens of plants include
opportunists, necrotrophs and biotrophs
Resistance is seen at several levels
Non-host resistance – durable, broad spectrum,
effective
Passive – attachment/germination and preformed
chemical defenses
 Active – initial colonization, e. g. wall apposition
 “Hyperactive” HR response and apoptosis
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Papillae and wall appositions
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Callose is a -1,3-glucan polymer, different
than cellulose in the connections of the sugars
Papillae contain callose, phenolyics,
hydroxyproline rich (HPR) proteins
Enhance cell wall mediated defense
Part of the basal defense response ?
In susceptible interactions may block / delay
haustorium development
Fungal papillae
Celio, Mims and Richardson Can. J. Bot. 82: 421–429 (2004)
Cell wall modifications
Hypersensitive death
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Triggered before or at
first cell penetration
Multigenic
Durable
www.moreheadplanetarium.org
www.plant.wageningen-ur.nl
Apoptosis
Death program
initiation uses
signalling via MAP
kinase cascades
www.aber.ac.uk
DNA ladders and TUNEL staining
Ryerson and Heath
Plant Cell 8,393
‘Host’ resistance
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Major gene
Systemic acquired/induced
‘horizontal’
Major gene resistance
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After basic compatibility has been established
Plant resistance / host virulence
Speed?
Effectiveness?
Durability?
Major gene resistance
Gene for gene interactions
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Flor 1956  explain inheritance of pathogenicity in the flax rust fungus
Melampsora lini.
Establishment of basic compatibility overcomes nonhost defense for one
pathogen/host combination
Thereafter
Host
Pathogen
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r
susc
susc
Pressure on host to detect pathogen leads to major gene resistance
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A
a
R
resist
susc
Seldom durable
Often used for resistant crop varieties
Pressure on pathogen to overcome/evade resistance
Development of multiple resistance and avirulence genes
Guard hypothesis model of gene for
gene interactions
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R proteins physically interact with cellular targets of
effectors
Recognition of effector-target complex or the products
of this interaction triggers defense signaling
Arabidopsis RPM1 gene recognizes and triggers HR
when either of two Pseudomonas syringae effectors (AvrB
and AvrRpm1) are delivered to the plant cell
Complex of proteins involved in defense signaling
Plant defenses – post infection
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PR proteins
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Defensins
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Structurally diverse group of proteins induced under
pathogen attack or stress by many resistance pathways
often antimicrobial or antifungal
maybe downstream of SAR/SIRgnalling
regulated by plant hormones ethylene and JA (not SA)
structurally similar to insect defensins, such as drosomycin,
and antimicrobials from vertebrates
Conserved strategy in response to microbial attack?
Chemical post infection plant defenses
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Phytoalexins
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Produced by healthy plant cells
adjacent to damage by
wounding or pathogens
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not made in biotrophic
interactions
Usually low molecular weight,
hydrophobic
Roles mostly unclear
Pressure on pathogen to
deotoxify
Gene for gene interaction
can evolve
Phoma
virulent
Pedras and Okanga 2000
Metabolism of analogs of the
phytoalexin brassinin by plant
pathogenic fungi CanJChem
78:338
Phoma
avirulent
Systemic acquired/induced resistance
SIR/SAR/ISR
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usually broad spectrum
often associated with an
enhanced capacity to
mobilize infection-induced,
cellular defense responses,
via ‘priming’
Inducers
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necrotizing attackers,
nonpathogenic, rootcolonizing Pseudomonads,
salicylate, jasmonate
ß-aminobutyric acid (BABA)
Protection of soybean leaves against
Pseudomonas syringae pv. glycinea
• Lower leaves treated with lactofen (not shown)
• 8d later upper leaves (image) were inoculated,
then incubated
www.oardc.ohio-state.edu/soydefense
MGR vs HR
Integrated pest management
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sanitation
crop rotation
cultivation practices
sowing date
plant spacing
resistant cultivars
disease forecasting
biological control
chemical control
IPM projected benefits
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Requirements
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Sprays may be fewer but more complex,
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with components aimed at variety of organisms, e.g. fungi
and insects.
Overall
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preliminary analysis
detailed but flexible planning
reduced cost
reduced chemical pesticide use and dependence
Major targets are fungi and insects
Entomophthorales
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Used as biocontrol
agents
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Entomophaga aulicae
Metarhizium anisopliae
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Beauvaria bassiana
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Cordyceps sinclairii
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Entomophthora muscae
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Conidia attach
Penetrate by enzymatic
digestion
Growth in insect as yeast
or plasmodium or hypha
(sp dependent)
Conidia form at
exoskeleton junctions
High host specificity
Metarhizium anisopliae
Spruce budworm in
North America
Grasshopper control
in Australia “Greenguard”
4 x 1010 spores/g
95% control
Effect on insect behaviour
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Infection can induce
positive phototropism
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Attack nervous
system?
dying insects climb
grass stems and cling
there
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Improved spore
dispersal
www.bioimages.org.uk/
Fungus Saves HoneyBees By Killing Parasitic Mites
WESLACO, Texas, October 21, 2004
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Roles for honeybees
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Varroa mites
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pollinate crops
honey, beeswax
pollen, royal jelly
Bee parasites
Not yet found in Saskatchewan
Chemical control possible but not preferable
Metarhizium anisopliae
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Established biocontrol fungus
Affects mites but not bees