Download Genetics of Plant Disease - Missouri State University

Genetics of Plant
Disease
• Pathogen Has 1 or more Genes for
Pathogenicity, for Specificity &
for Virulence against particular
Host
• The Gene Combination that Makes a Plant
Susceptible Is Present only in that 1 Kind of
Plant or a few Related Kinds
• Nonhost Resistance
– Inability of a Pathogen to Infect a Plant
because the Plant Lacks something the
Pathogen Needs or Contains Substances
Incompatible with the Pathogen
Variability in Resistance & Susceptibility
in Both Pathogens & Plants
– Causes of Variability
• Mutations
– Abrupt Change in the Genetic Material
– Changes in Sequence of Bases in DNA
• Recombination of Genes
Genes & Disease
• Pathogens Are often Host-Specific
– Fusarium oxysporum f. sp.
lycopersici causes tomato wilt but
attacks only tomato
– F. oxysporum f. dianthii
– Puccinia graminis f. sp. tritici
attacks only wheat
http://en.wikipedia.org/wiki/Stem
_rust
• A Few Pathogens Attack many Kinds of
Plants
– Many Diverse Genes for Virulence
– The Genes less Plant Specific than more
Specialized Pathogens
• Each Plant Species Susceptible to fairly
Small Number of different Pathogens
• Plants Have Existing & Induced Defenses to
Resist most Pathogens
– Resistant Plants probably Have Numerous
Genes for Resistance
Mutation Mechanism Often
Favors Pathogens
• Probably not more Frequently
• Pathogens Produce a Great Number of
Progeny
• Monoculture Characteristic of Agriculture
– Segregation & Recombination during Meiotic
Division of the Zygote in Sexual Reproduction
1
Pathogen Designations
• Genus & Species—
– Similar Morphology & Phenotype
• Race or Strain—
– Same Variety but Attack different Varieties of
Plants within a Species
• Variant—
– Offspring of a Race of Pathogens that can
Suddenly Attack New Variety
• Varieties—
– Same Species but Attack different Plant Species
– Special Designations for Varieties
• Biotype—
– The ‗Clones‘ or Offspring of Variants that
Reproduce Asexually
• ―Special Form‖ (Fungi)
– formae specialist = f. sp.
• ―Pathovar‖ (Bacteria)
• ―Type‖ (Viruses)
• ―Race, Biotype, Pathotype, Strain‖ (Nematodes)
Example of Designations
1.
Genus
• Genus & Species = Puccinia
– Daylily Rust Is the Species Puccinia hemerocallidis
– The Pathogen that can Cause Stem Rust in Small Cereal
Grains Is Grouped into the Species Puccinia graminis
• Variety
– Stem Rust in Wheat: the Variety Puccinia graminis tritici
– Stem Rust in Oats: the Variety Puccinia graminis avenae
• Specific Races/Strains of Pathogen can Attack only
Specific Varieties of a Plant Species
Puccinia
Puccinia graminis
2. Species Puccinia hemerocallidis
Puccinia
graminis tritici
3. Variety
4. Races/Strains
P. graminis
tritici-compacti
Puccinia
graminis avenae
P. graminis
tritici-inficiens
– (There Are over 200 Races of P. graminis tritici)
Another Example
• Fusarium oxysporum
– Fusarium Wilt Disease
• Fusarium oxysporum f. sp. niveum
– Fusarium Wilt on Watermelon
• F. oxysporum f. sp. niveum race 0, race 1,
& race 2
– Wilt Disease of Specific Varieties of
Watermelon
Types of Plant Resistance to
Pathogens
• Non-Host Resistance
– Plants Completely Resistant to Certain Pathogens
• True Resistance
– Host, Pathogen Incompatible
• Horizontal Resistance
– Partial Resistance against all Races of a Pathogen
– Controlled by many Genes
• Vertical Resistance
– Complete Resistance to some Races of a Pathogen but not to
other Races
– Controlled by a few Genes
2
Resistance 
Plant Variety B
Resistance 
Plant Variety A
1
2
3
4
5
6
7
1
Races of a Pathogen
• Escape
– All 3 Disease Factors not Optimum
• Tolerance
3
4
5
6
7
Races of a Pathogen
• Apparent Resistance
– Plants usually Susceptible to a Disease Remain
Free of Infection or Symptoms
– They Escape or Tolerate Disease
2
Gene-for-Gene Concept
• Plants & Pathogens Evolved Together
• Genetic Changes in One Is Balanced by
Changes in other
– Ability to Produce Crop even when Infected
Breeding Resistant Varieties
Hypersensitive Response
• Natural Variability Exists
• Plant apparently Recognizes Pathogen
Elicitors
• Activates Cascade of Genes
• Recognition Response also Leads to
Activation of other Defense-Related Genes
that Result in other Types of Resistance,
including Horizontal Resistance
– Breeders Combine most Useful Genes for Yield,
Quality, Uniformity, Hardiness, Resistance
• Sources of Resistance Genes
–
–
–
–
Native or Foreign Varieties
Older, Heirloom Varieties
Wild Plant Relatives
Induced Mutations
• Treating Plant Material with Mutagenic Agents
3
• Recent Work includes Genetic Engineering
Techniques
– Breeders can Introduce Genetic Material (DNA)
into Plant Cells Directly
• Ballistic Devices
• Vectors
• Protoplast Fusion
– Tissue Culture to Obtain Plants with different
Characteristics
• A Few ‗Improved‘ Varieties
Replace Most or all Others
– The most Successful Improved
Varieties Adopted Abroad &
Eventually Replace Local Varieties
– Genetic Base of the Varieties often Narrow
– Vulnerable to Sudden Outbreaks of Catastrophic
Plant Disease Epidemics
• Initial Steps in Breeding
generally Increase Genetic
Variability
– Combines Genes from Plants
Widely Separated by Distance
• One Most Useful Genes Are
Identified, further Breeding
Tends to Eliminate
Variability
• Classical Breeding Techniques for
Disease Resistance
– Depends on Plant‘s Mating System
– Historically Complicated
– Resistance Could Be Determined Only by Making
Plants Diseased
– Recent Molecular Techniques Allow Breeders to
Select for Resistance instead of Inoculating
Plants
• Molecular Markers
• Breeding for Vertical Resistance
–
–
Aimed against Specific Pathogens or Races
Most Effective
•
•
–
Annual Crops that Are Easy to Breed
Directed against Pathogens that Do not Reproduce
& Spread Rapidly or Mutate Frequently
Easier to Manipulate in Breeding Program than
Horizontal Resistance
• Horizontal Resistance Requires Insertion of many
Minor-Effect Genes
–
Can Be Complete at First but may Be ShortLived
• Strategies to Improve Vertical Resistance
–
Multilines
•
–
Plant Mix of Varieties that Differ in Resistance
Genes
Pyramiding
•
Cross Several Varieties that Have different
Resistance Genes & Select Those that Contain
Mixtures
• Breeding for Horizontal Resistance
–
Confers Incomplete but more Durable
Protection
4