1. Heath Ann Bot 80, 713

2. Differences between animals/plants
Plants have no RAG (recombinant activating gene)-dependent immune system
No circulating immune cells – local recognition and response infection
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)
important in nutrient poor soil and/or as biocontrol against pathogens
Triggers of SAR?

3. Fungal pathogenicity on plants
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

4. Papillae and wall appositions
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

5. Fungal papillae
Celio, Mims and Richardson Can. J. Bot. 82: 421–429 (2004)

6. Cell wall modifications

7. Hypersensitive death Triggered before or at first cell penetration
Multigenic
Durable

8. Apoptosis
Death program initiation uses signalling via MAP kinase cascades

9. DNA ladders and TUNEL staining
Ryerson and Heath
Plant Cell 8,393

10. ‘Host’ resistance
Major gene
Systemic acquired/induced
‘horizontal’

11. Major gene resistance After basic compatibility has been established
Plant resistance / host virulence
Speed?
Effectiveness?
Durability?

12. Major gene resistance

13. Gene for gene interactions
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 R r
Pathogen A resist susc
a susc susc
Pressure on host to detect pathogen leads to major gene resistance
Seldom durable
Often used for resistant crop varieties
Pressure on pathogen to overcome/evade resistance
Development of multiple resistance and avirulence genes

14. Guard hypothesis model of gene for gene interactions
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

15. Plant defenses – post infection
PR proteins
Structurally diverse group of proteins induced under pathogen attack or stress by many resistance pathways
often antimicrobial or antifungal
maybe downstream of SAR/SIRgnalling
Defensins
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?

16. Chemical post infection plant defenses
Phytoalexins
Produced by healthy plant cells adjacent to damage by wounding or pathogens
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 Phoma
virulent avirulent
Pedras and Okanga 2000 Metabolism of analogs of the phytoalexin brassinin by plant pathogenic fungi CanJChem 78:338

17. Systemic acquired/induced resistance SIR/SAR/ISR
usually broad spectrum
often associated with an enhanced capacity to mobilize infection-induced, cellular defense responses, via ‘priming’
Inducers
necrotizing attackers,
nonpathogenic, root-colonizing 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

18. MGR vs HR

19. Integrated pest management
sanitation
crop rotation
cultivation practices
sowing date
plant spacing
resistant cultivars
disease forecasting
biological control
chemical control

20. IPM projected benefits
Requirements
preliminary analysis
detailed but flexible planning
Sprays may be fewer but more complex,
with components aimed at variety of organisms, e.g. fungi and insects.
Overall
reduced cost
reduced chemical pesticide use and dependence
Major targets are fungi and insects

21. Entomophthorales Insect eaters Used as biocontrol agents
Entomophaga aulicae
Metarhizium anisopliae
Beauvaria bassiana
Cordyceps sinclairii

22. Entomophthora muscae Conidia attach Penetrate by enzymatic digestion
Growth in insect as yeast or plasmodium or hypha (sp dependent)
Conidia form at exoskeleton junctions

23. High host specificity

24. Metarhizium anisopliae
Spruce budworm in
North America
Grasshopper control
in Australia “Greenguard”
4 x 1010 spores/g
95% control

25. Effect on insect behaviour
Infection can induce positive phototropism
Attack nervous system?
dying insects climb grass stems and cling there
Improved spore dispersal

26. Fungus Saves HoneyBees By Killing Parasitic Mites WESLACO, Texas, October 21, 2004
Roles for honeybees
pollinate crops
honey, beeswax
pollen, royal jelly
Varroa mites
Bee parasites
Not yet found in Saskatchewan
Chemical control possible but not preferable
Metarhizium anisopliae
Established biocontrol fungus
Affects mites but not bees