Biology 4218A Study Guide - Final Guide: Verticillium Wilt, Systemic Acquired Resistance, Quantitative Trait Locus

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Host Plant Defenses
Crop Domestication
Forces of crop domestication often oppose those of natural selection
Crop domestication: predictable, uniform, yield; community performance favoured
Nat sel: diversity, persistence, fertility; individual is favoured
Since we are picking out the specific plant with specific traits we want, there will be low
variability (i.e. narrow genetic diversity). This makes crops more susceptible to disease (e.g. late
blight of potato).
Crops and Pathogens
Plants are frequently in contact with pathogens, and there are 3 outcomes:
1. No trace of occurrence because of lack of appropriate pathogen factors, or highly
effective host defenses.
2. Intense interaction and arrest of pathogen development, where host is affected by
pathogen but host defenses stop it
3. Disease (rare)
Steps for pathogen: find host, contact, penetrate, proliferate
Plant defenses:
Structural
Pre-existing (constitutive): plant can have these defenses before it encounters a
pathogen
- Trichomes, hairs, stomata, cell wall, cuticle
Induced: plant can recognize the pathogen presence
- Cork, abscission layers
o Abscission layer grows around infected part so it falls off
- Tyloses, papillae, lignitubers
o Tylose is a defense induced against vascular wilt pathogens
like verticillium wilt, that grows through the vasculature of the
plant, blocking pathogen entry
- Gums and resins
Chemical
Pre-existing (constitutive):
o Antimicrobials (phenolics, terpenoids, alkaloids, glucosinolates)
Glucosinolate is in the vacuole
When an insect chews/pathogen breaks down barriers between
cells, myrosinase enzyme can convert glucosinolate to
aglycone, which defends
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o Defensive proteins
o Proinhibitins, phytoanticipins
o Waxy surfaces, suberin
Induced: (post-infectional)
o Phytoalexins
o Defensive proteins
Nonhost vs Apparent Resistance
NONHOST:
- No compatibility between pathogen and host
- No recognition
- Effective constitutive/structural host defense
- Lack of essential factor or toxin target
- Active/induced defenses
APPARENT:
- Disease escape
- Environmental conditions limiting disease
Quantitative vs Qualitative Resistance
QUANTITATIVE: (Continuous; depends on multiple genes)
- Tolerance, horizontal, durable, general
- Non-specific, field, partial, polygenic
- Quantitative trait loci
- Slows disease, decreases severity
- More subject to env conditions
QUALITATIVE: (Yes/no)
- Vertical, monogenic, race-specific
- R-gene associated
- Host recognition (immune) or detox pathway initiated
- HR, necrosis
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Partial: so it can get attacked but it is not life-threatening
Transient: If you put an R-gene onto a crop plant, you're putting a heavy selective pressure for
path to defeat that R-gene
After they're used for a # of years they get defeated by the pathogen and the pathogen overcomes
that R-gene, and new strains of the pathogen emerge to escape recognition
Durable: because there are many genetic loci so t's not a single chain in a path that enables it to
escape plant resistance; it needs many changes
Complex: because it involves many diff loci
Induced Defenses
Recognition of pathogen by host:
- Plants detect elicitors (molecs produced during interaction, that acts as signaling
molecules; e.g. PAMPs, MAMPs, DAMPs) using immune receptors (PRRs), so plant
induces its defense against the pathogen
- Immune receptors can be on PM or intracellular
- Induced biochemical defenses:
- Phytoalexins released (antimicrobial compounds)
- Defensive proteins/peptides released
- Protease inhibitors: stop proteolysis
- Chitinase: degrade fungal cell wall
- B-glucanase: degrade oomycete cell wall
Hypersensitive response
- Localized induced cell defense at infection site, initiated by recognition of elicitors,
which leads to rapid mobilization of defense cascade, resulting in death of adjacent host
cells in order to stop biotrophic pathogen’s path
- Rate-dependent effectiveness (HR may be trailing behind pathogenicity)
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Document Summary

Forces of crop domestication often oppose those of natural selection. Crop domestication: predictable, uniform, yield; community performance favoured. Nat sel: diversity, persistence, fertility; individual is favoured. Since we are picking out the specific plant with specific traits we want, there will be low variability (i. e. narrow genetic diversity). This makes crops more susceptible to disease (e. g. late blight of potato). Steps for pathogen: find host, contact, penetrate, proliferate. Pre-existing (constitutive): plant can have these defenses before it encounters a pathogen. Cork, abscission layers: abscission layer grows around infected part so it falls off. Tyloses, papillae, lignitubers: tylose is a defense induced against vascular wilt pathogens like verticillium wilt, that grows through the vasculature of the plant, blocking pathogen entry. Pre-existing (constitutive): antimicrobials (phenolics, terpenoids, alkaloids, glucosinolates, glucosinolate is in the vacuole, when an insect chews/pathogen breaks down barriers between cells, myrosinase enzyme can convert glucosinolate to aglycone, which defends, defensive proteins, proinhibitins, phytoanticipins, waxy surfaces, suberin.

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