Heath Ann Bot 80, 713

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?

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

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

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

Cell wall modifications

Hypersensitive death 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 Major gene Systemic acquired/induced ‘horizontal’

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

Major gene resistance

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

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

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?

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

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

MGR vs HR

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

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

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

Entomophthora muscae 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 Infection can induce positive phototropism Attack nervous system? dying insects climb grass stems and cling there Improved spore dispersal www.bioimages.org.uk/

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