Heath Ann Bot 80, 713Heath Ann Bot 80, 713

Differences between Differences between animals/plantsanimals/plants

Plants have no RAG (recombinant activating gene)-Plants have no RAG (recombinant activating gene)-dependent immune systemdependent immune system

No circulating immune cells – local recognition and No circulating immune cells – local recognition and response infectionresponse infection Cellular communication via plasmodesmata Cellular communication via plasmodesmata sometimes co-opted by bacteria and viruses to move sometimes co-opted by bacteria and viruses to move

systemicallysystemically Whole plant response – Systemic acquired resistanceWhole plant response – Systemic acquired resistance Plants must differentiate between pathogens and Plants must differentiate between pathogens and

beneficial symbionts (beneficial symbionts (RhizobiumRhizobium and mycorrhizal and mycorrhizal fungi)fungi) important in nutrient poor soil and/or as biocontrol against important in nutrient poor soil and/or as biocontrol against

pathogenspathogens Triggers of SAR?Triggers of SAR?

Fungal pathogenicity on Fungal pathogenicity on plantsplants

Fungal pathogens of plants include Fungal pathogens of plants include opportunists, necrotrophs and biotrophsopportunists, necrotrophs and biotrophs

Resistance is seen at several levelsResistance is seen at several levels Non-host resistance – durable, broad Non-host resistance – durable, broad

spectrum, effective spectrum, effective Passive – attachment/germination and Passive – attachment/germination and

preformed chemical defensespreformed chemical defenses Active – initial colonization, e. g. wall Active – initial colonization, e. g. wall

appositionapposition ““Hyperactive” HR response and apoptosisHyperactive” HR response and apoptosis

Papillae and wall Papillae and wall appositionsappositions

Callose is a Callose is a -1,3-glucan polymer, -1,3-glucan polymer, different than cellulose in the different than cellulose in the connections of the sugarsconnections of the sugars

Papillae contain callose, Papillae contain callose, phenolyics, hydroxyproline rich phenolyics, hydroxyproline rich (HPR) proteins (HPR) proteins

Enhance cell wall mediated defenseEnhance cell wall mediated defense

Part of the basal defense Part of the basal defense response ?response ?

In susceptible interactions may In susceptible interactions may block / delay haustorium block / delay haustorium developmentdevelopment

Fungal papillaeFungal papillae

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

Cell wall modificationsCell wall modifications

Hypersensitive deathHypersensitive death

Triggered before Triggered before or at first cell or at first cell penetrationpenetration

MultigenicMultigenic DurableDurable

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

ApoptosisApoptosis

www.aber.ac.uk

Death program initiation uses signalling via MAP kinase cascades

DNA ladders and TUNEL DNA ladders and TUNEL stainingstaining

Ryerson and HeathPlant Cell 8,393

‘‘Host’ resistanceHost’ resistance

Major geneMajor gene Systemic acquired/inducedSystemic acquired/induced ‘‘horizontal’horizontal’

Major gene resistanceMajor gene resistance

After basic compatibility has been After basic compatibility has been establishedestablished

Plant resistance / host virulencePlant resistance / host virulence

Speed?Speed? Effectiveness? Effectiveness? Durability?Durability?

Major gene resistanceMajor gene resistance

Gene for gene Gene for gene interactionsinteractions

Flor 1956 Flor 1956 explain inheritance of pathogenicity in the flax explain inheritance of pathogenicity in the flax rust fungus rust fungus Melampsora liniMelampsora lini..

Establishment of basic compatibility overcomes nonhost Establishment of basic compatibility overcomes nonhost defense for one pathogen/host combinationdefense for one pathogen/host combination

Thereafter Thereafter

HostHost RR rrPathogenPathogen AA resistresist suscsusc

aa suscsusc suscsusc

Pressure on host to detect pathogen leads to major gene Pressure on host to detect pathogen leads to major gene resistanceresistance Seldom durableSeldom durable Often used for resistant crop varietiesOften used for resistant crop varieties

Pressure on pathogen to overcome/evade resistancePressure on pathogen to overcome/evade resistance Development of multiple resistance and avirulence genesDevelopment of multiple resistance and avirulence genes

Guard hypothesis model of Guard hypothesis model of gene for gene interactionsgene for gene interactions

R proteins physically interact with cellular R proteins physically interact with cellular targets of effectorstargets of effectors

Recognition of effector-target complex or the Recognition of effector-target complex or the products of this interaction triggers defense products of this interaction triggers defense signalingsignaling

Arabidopsis Arabidopsis RPM1 gene recognizes and RPM1 gene recognizes and triggers HR when either of two triggers HR when either of two Pseudomonas Pseudomonas syringaesyringae effectors (AvrB and AvrRpm1) are effectors (AvrB and AvrRpm1) are delivered to the plant celldelivered to the plant cell

Complex of proteins involved in defense Complex of proteins involved in defense signalingsignaling

Plant defenses – post Plant defenses – post infectioninfection

PR proteinsPR proteins Structurally diverse group of proteins induced under Structurally diverse group of proteins induced under

pathogen attack or stress by many resistance pathogen attack or stress by many resistance pathwayspathways

often antimicrobial or antifungaloften antimicrobial or antifungal maybe downstream of SAR/SIRgnallingmaybe downstream of SAR/SIRgnalling

DefensinsDefensins regulated by plant hormones ethylene and JA (not SA)regulated by plant hormones ethylene and JA (not SA) structurally similar to insect defensins, such as structurally similar to insect defensins, such as

drosomycin, and antimicrobials from vertebratesdrosomycin, and antimicrobials from vertebrates Conserved strategy in response to microbial Conserved strategy in response to microbial

attack?attack?

Chemical post infection plant Chemical post infection plant defensesdefenses

PhytoalexinsPhytoalexins Produced by healthy Produced by healthy

plant cells adjacent to plant cells adjacent to damage by wounding damage by wounding or pathogensor pathogens

not made in biotrophic not made in biotrophic interactionsinteractions

Usually low molecular Usually low molecular weight, hydrophobic weight, hydrophobic

Roles mostly unclearRoles mostly unclear Pressure on pathogen Pressure on pathogen

to deotoxifyto deotoxify Gene for gene Gene for gene

interaction can evolveinteraction can evolve

Phoma Phomavirulent avirulent

Pedras and Okanga 2000 Metabolism of analogs of the phytoalexin brassinin by plant pathogenic fungi CanJChem 78:338

Systemic acquired/induced Systemic acquired/induced resistance SIR/SAR/ISRresistance SIR/SAR/ISR

usually broad usually broad spectrumspectrum

often associated with an often associated with an enhanced capacity to enhanced capacity to mobilize infection-mobilize infection-induced, cellular induced, cellular defense responses, via defense responses, via ‘priming’‘priming’

Inducers Inducers necrotizing attackers, necrotizing attackers, nonpathogenic, root-nonpathogenic, root-

colonizing Pseudomonads, colonizing Pseudomonads, salicylate, jasmonatesalicylate, jasmonate ß-aminobutyric acid ß-aminobutyric acid

(BABA) (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 HRMGR vs HR

IntegratedIntegrated pest pest managementmanagement

sanitationsanitation crop rotationcrop rotation cultivation practicescultivation practices sowing datesowing date plant spacingplant spacing resistant cultivarsresistant cultivars disease forecastingdisease forecasting biological controlbiological control chemical control chemical control

IPM projected benefitsIPM projected benefits Requirements Requirements

preliminary analysispreliminary analysis detailed but flexible planningdetailed but flexible planning

Sprays may be fewer but more complex, Sprays may be fewer but more complex, with components aimed at variety of with components aimed at variety of

organisms, e.g. fungi and insects. organisms, e.g. fungi and insects. Overall Overall

reduced costreduced cost reduced chemical pesticide use and reduced chemical pesticide use and

dependencedependence Major targets are fungi and insectsMajor targets are fungi and insects

EntomophthoralesEntomophthorales

Used as biocontrol Used as biocontrol agentsagents Entomophaga Entomophaga

aulicaeaulicae Metarhizium Metarhizium

anisopliae anisopliae

Beauvaria bassianaBeauvaria bassiana

Cordyceps sinclairiiCordyceps sinclairii

Entomophthora muscaeEntomophthora muscae

Conidia attachConidia attach Penetrate by Penetrate by

enzymatic digestionenzymatic digestion Growth in insect as Growth in insect as

yeast or plasmodium yeast or plasmodium or hypha (sp or hypha (sp dependent)dependent)

Conidia form at Conidia form at exoskeleton junctionsexoskeleton junctions

High host High host specificityspecificity

MetarhiziumMetarhizium anisopliaeanisopliae

Spruce budworm inNorth America

Grasshopper controlin Australia “Greenguard”4 x 1010 spores/g95% control

Effect on insect Effect on insect behaviourbehaviour

Infection can Infection can induce induce positive positive phototropismphototropism Attack nervous Attack nervous

system?system? dying insects dying insects

climb grass stems climb grass stems and cling thereand cling there Improved spore Improved spore

dispersaldispersal

www.bioimages.org.uk/

Fungus Saves HoneyBees By Killing Fungus Saves HoneyBees By Killing Parasitic MitesParasitic Mites

WESLACO, Texas, October 21, 2004WESLACO, 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 fungusEstablished biocontrol fungus Affects mites but not beesAffects mites but not bees