Transcript Fungal relationships with plants

Fungal relationships
with plants
Obligate and facultative
parasitism/pathogenicity
Why is plant pathology important?
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Food quantity
Food quality
Agrinomic practices – tillage, pesticide use
Diversity and stability of ecosystems
Beauty
Better understanding of disease
processes – examples of use
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Disease control through life cycle management
Puccinia graminis tritici on wheat and barberry
 Orchard management practice in Venturia inaequalis
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Optimizing pesticide application
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Disease forecasting
Pesticide design
Breeding for resistance
Fungal pathogenicity on plants
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Plant pathogens can be biotrophic (rusts and
smuts) or hemibiotrophic/necrotrophic
(opportunistic)
Necrotrophs can display high levels of host
specificity, e. g. Magnaporthe grisea
Increasing specialization
Biotroph
Obligate parasite
Increasing host range
Necrotroph
Saprotroph
“Strong”
“Weak”
Facultative parasite Facultative parasite
Symptom types -- necrosis
Blumeriella on plum
Alternaria solani – early blight of potato
Symptom types – wilt
Ophiostoma novo-ulmi
Panama disease: Fusarium oxysporum f.sp. cubense
Symptom types – hormone induced
Hypertrophy auxin
Taphrina deformans
Synchytrium endobioticum
http://www.maff.gov.uk/planth/pestnote/pwd.htm
Symptom types – hormone induced
Etiolation
Bakanae of rice
Fusarium moniliforme = Gibberella fujikuroi
gibberellins
Symptom types – abscission
Hemileia vastatrix
rust.lbl.gov
Symptom types – sterilization
Ustilago maydis
Claviceps purpurea – ergot
Obligate parasites – e. g. Uredinales
Rust fungi may have as many as five different
spore-producing stages in their life cycles
Heteroecism – e.g. wheat stem rust
- two taxonomically different host plants in order to
complete life cycle
- ‘alternate’ host: stages (haploid)
- primary host: stages (diploid)
Autoecism – e.g. bean rust
- entire life cycle completed on a single host species
Microcyclic rusts ≤ 3 spore types
Life Cycle of Puccinia graminis
I
spermatia (n)
insect transported
to receptive hyphae (n)
heterothallic
F
A
L
L
uredinia on grass
from infection by
aeciospores or
urediniospores
spermagonia on barberry from
infection by basidiospores
IV
urediniospores
(n+n)
airborne
II
aeciospores
(n+n)
airborne
O
basidiospore
(n)
airborne
SUMMER
aecia on barberry
(n+n)
meiosis
III
telia on grass
SPRING
teliospore (2n)
germinating on
straw with
promycelium and
basidiospores (n)
karyogamy
(2n)
teliospore on straw
(n+n)
WINTER
Puccinia graminis f.sp. tritici life
cycle
host
ploidy
Overwintering and transport of
urediospores
Stage IV Basidia bearing basidiospores (n)
• in the spring teliospore germinates a promycelium
• diploid nucleus migrates into the promycelium and
undergoes meiosis
• four haploid nuclei migrate into developing sterigmata
& are incorporated into basidiospores
• basidiospores reinfect alternate host
teliospore germinates, gives rise to a short germ tube
of determinate growth known as the promycelium.
Promycelium: site of meiosis & formation of sterigmata and
basidiospores
Stage 0 and I produced on “alternate” host
Stage 0: Spermogonia bearing spermatia (n) and
receptive hyphae (n)
helios.bto.ed.ac.uk/bto/microbes/biotroph.htm
• fertilization of the receptive hyphae by spermatia initiates
the dikaryon and the formation of aecia
Puccinia plasmogamy
www.apsnet.org/education/LessonsPlantPath/StemRustWheat
Stage II: Uredinia bearing urediniospores (n+n)
helios.bto.ed.ac.uk/bto/microbes/biotroph.htm
• reinfect primary host
• amplifies disease within primary host
• uredinia can eventually develop into telia
Teliospore: site of karyogamy
technically part of the basidium
Stage III: Telia bearing teliospores (n+n2n)
• final stage on primary host
• overwinters as dikaryon
Facultative parasitism: Magnaporthe
grisea infection
Magnaporthe grisea / Oryza sativa
Some non pathogenic M. grisea strains can
grow in host plants if wound inoculated
Host resistance and basic
compatibility
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Most plants are not attacked by the vast majority
of potential pathogens
Preformed defenses
Potential pathogens secrete chemicals during
growth that can be detected
What is a pathogenicity gene?
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A gene whose product contributes to successful
fungal establishment in the host
Examples
Hydrolytic enzymes (especially for necrotrophs)
 Compatibility determinants (especially for biotrophs)
 Defense avoidance/detoxification
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What is a resistance gene?
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A gene whose product enables the host to detect
a pathogen and/or mount a defense
The fungal product that is detected
does not have to be directly involved in pathogenesis
 is defined as being produced by an avirulence gene
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Heath – host pathogen interactions
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Ann Bot 80, 713
Fungal pathogenicity on plants
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Fungal pathogens of plants include
opportunists, necrotrophs and biotrophs
Resistance is seen at several levels
Non-host resistance –
Widespread, early onset, effective
 Passive – attachment/germination
 Active – initial colonization, e. g. wall apposition
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Hypersensitivity
Durable
Varietal resistance
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Superimposed on basic compatibility
Often based on a single resistance gene
Typically not durable
Gene for gene interactions
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Host
PathogenA
R
r
resist
susc
a
susc
susc
basic compatibility overcomes nonhost defense
pressure on host to detect pathogen leads to
(temporary) resistance
pressure on pathogen to overcome/evade
resistance
Breeding for resistance
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Identify likely targets, disrupt, look for
attenuation of pathogenicity
Cross commercial susceptible strains to wild
relatives, backcross to retain yield and desireable
characters in resistant strain
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8-10 years; resistance may last 3-5 years
Pyramid strategies
Horizontal resistance