Heath Ann Bot 80, 713

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 www.moreheadplanetarium.org www.plant.wageningen-ur.nl

8. Apoptosis Death program initiation uses signalling via MAP kinase cascades www.aber.ac.uk

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 PhomaPhoma 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 www.oardc.ohio-state.edu/soydefense

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. Insect eaters Entomophthorales • 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. Metarhiziumanisopliae 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 www.bioimages.org.uk/

26. Fungus Saves HoneyBees By Killing Parasitic MitesWESLACO, 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