The Pseudomonads in Biological Control
Pierson Lab
Microbial gene regulation
Root-associated free-living bacteria
Microbial community
interactions
Plant disease
control
Importance of plant diseases
Estimated annual crop production worldwide
$1.2 - 1.3 trillion
Amount lost to disease, insects, weeds using
current control measures
$500 billion
Additional losses without current control measures
$330 billion
Apple scab
Fireblight of pear
Take-all of wheat
Citrus canker
Verticillium wilt
What determines disease?
Plant
Species
Genotype
Stage of growth
Stress
Pathogen
Genotype: hrp, avr race
Dispersal, colonization site
Environment
Humidity
H2O
Plant factors
Other Microbes
Current approaches to disease control
Chemical
1995: United States spent $26 billion
on chemical pesticides
Of this, < 1% actually gets to
where the pathogen is
Breeding
Identification of resistance genes
Introgressing into commercial
cultivars
What happens to the rest?
Ground water
Problems with development of
resistance, pyramiding genes
Taken up by the plant
Development of resistance
Biological Control is an attractive alternative/supplement
How does Biological Control by
Pseudomonads work?
Nutrient Competition
Site Competition
Biological
Control
Crosscommunication
Antibiosis
The Rhizosphere
The zone of soil influenced by the
plant root
Plants can exude ca. 70% of fixed
carbon through their roots
Rhizosphere is a dynamic environment
The rhizosphere comprises  50% of the biomass of the plant
(From Kutschera, L & Lichtenegger, E. 1992 Wurzelatlas Mitteleuropaischer Grundpflanzen Gustav Fischer Verlag Stuttgart)
“There is more biomass below the earth’s surface than above it.”
Examples of Biological Control Pseudomonads
Pseudomonas fluorescens Tx-1
Take-all of wheat (Gaeumannomyces graminis
var. tritici)
Dollar spot of turf (Sclerotinia homoeocarpa)
Pseudomonas fluorescens Pf-5
Damping off of bean (Rhizoctonia solani)
Pseudomonas aureofaciens 30-84
Drechslera leaf spot (D. poae)
Pseudomonas fluorescens F113
Damping off of bean (Pythium ultimum)
Pseudomonas aureofaciens AB254
Damping off of bean (Pythium ultimum)
Pseudomonas fluorescens WCS365
Rhizoctonia solani
Pseudomonas fluorescens A506
Fireblight of pear (Erwinia amylovora)
Pseudomonas putida
Phytophthora root rot of citrus
Pseudomonas syringae pv. tagetis
Canadian thistle
Mechanisms of Biological Control by
Fluorescent Pseudomonads
Nutrient Competition
Biological
Control
Control of Rhizoctonia solani on cotton by P. cepacia D1
Rhizoctonia solani
Produces fluorescent siderophores
Chelates Fe in environment
All organisms require Fe
Fe available at 10-18 M
P. cepacia D1
Control
Cotton
R. solani
Take-all Disease of Wheat
No. 1 disease of cereals worldwide (up to 50% yield loss)
One infected root in 10,000 is sufficient to cause an epidemic
Caused by Gaeumannomyces graminis var. tritici (Ggt)
No varieties of wheat or barley exist with specific resistance to take-all.
No direct method of chemical control is presently available.
Take-all disease of wheat
Pathogen: Gaeumannomyces
graminis var. tritici
Invades root vascular tissues
Physically blocks water &
nutrient transport
Take-all Decline- An Example of Natural Suppression
Conducive
Suppressive
Pseudomonas
aureofaciens 30-84
Years of wheat monoculture
Mechanisms of Biological Control by
Fluorescent Pseudomonads
Biological
Control
Antibiosis
Pseudomonas aureofaciens Produces
Phenazine Antibiotics
N
COOH
N
COOH
N
OH
OH
N
PCA
N
N
2-OH-PCA
2-OH-PZ
“Phenazine Phacts”
Broad spectrum
Block respiration
Pathogen inhibition
Competitive fitness
Phenazines required for pathogen inhibition
Restored
Phz-
30-84
30-84
Phz-
Restored
Ggt
AHL-mediated Gene Regulation
o
o
o
o
o
AHL
+
acyl-ACP
ADO-Met
PhzI
o
o
PhzR
o
phzI
phzR
P
RNAPol
phzFABCD
Cell Surface
Biofilms
o
o
o
o
RpeA
(+)
o
rpeA
o
o
Exoprotease
o
o
o
o
o
o
o
o
o
PhzI
o
PhzR
o
o
o
CsaR
o
CsaI
o
o
(+)
(+)
(+)
phzI phzR phzXYFABCD
csaI csaR
(+)
(+)
RpoS
(+)
GacA
GacS
AHL Regulatory System
PphzI
P
phzI phzR
phzF
phzA
phzB
phzC phzD
PphzR
30-84R (PhzR-)
30-84 (PhzR++)
30-84
30-84R (PhzR-)
30-84Z (Phz-, AHL+)
Lawn of 30-84I (PhzI-)
O
P. aeruginosa
O
N
Butyryl HSL
H
O
H OH O
V. harveyi
Hydroxybutyryl HSL
H
O
V. fischeri
O
N
O
O
3-oxohexanoyl HSL
O
N
H
O
O
P. aureofaciens
Hexanoyl HSL
O
N
H
O
O
V. fischeri
Octanoyl HSL
H
O
A. tumefaciens
O
R. leguminosarum
3R-hydroxy-7-cistetradecenoyl HSL
O
N
H
3-oxododecanoyl HSL
O
O
3-oxooctanoyl HSL
P. aeruginosa
O
N
O
O
O
N
H
O
H OH O
O
N
H
O
What about the Microbial Community?
Let’s get together!
What’s this guy
thinking?
I hear you!
He’s Nuts!!
Wanna Rumble!
Potential Roles of Bacterial Communication
1.
Coordinating gene expression
Competition
Survival
Pathogen inhibition
2.
Interspecies communication
Recognition and defense
Consistency of biological control
Biofilm formation & structure
Cross-communication
Positive
Negative
Mechanisms of Biological Control by
Fluorescent Pseudomonads
Biological
Control
Crosscommunication
Why is communication important?
Can:
Alter rhizosphere competition...
Determine the composition & structure of the root community...
PU-186
Enhance pathogen inhibition...
30-84I
Reduce pathogen inhibition...
Mixture
30-84
PU-5
PU-15
30-84 + 5
30-84 + 15
Ave. Inhibition (mm)
8.9 ± 1.0a
0 ± 0b
0 ± 0b
2.7 ± 1.1c
1.3 ± 2.3c
PU-186 + 30-84I
Microbial Communities: Biofilms
Conclusions
Plant diseases cause major loss of food and money
Biological control an attractive alternative to chemicals
Many biocontrol bacteria identified are Pseudomonads
Biological control occurs via several mechanisms
Competition
Antagonism
Cross-communication