1. Microbial community interactions
The Pseudomonads in Biological Control
Pierson Lab
Microbial gene regulation
Root-associated free-living bacteria
Microbial community interactions
Plant disease control

2. Importance of plant diseases
Estimated annual crop production worldwide
$ 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

3. Plant Pathogen Environment What determines disease? Species Genotype
Stage of growth
Stress
Pathogen
Environment
Genotype: hrp, avr
race
Humidity
Dispersal, colonization site
H2O
Plant factors
Other Microbes

4. Current approaches to disease control
Chemical
Breeding
1995:
United States spent $26 billion on chemical pesticides
Identification of resistance genes
Of this, < 1% actually gets to where the pathogen is
Introgressing into commercial cultivars
What happens to the rest?
Problems with development of resistance, pyramiding genes
Ground water
Taken up by the plant
Development of resistance
Biological Control is an attractive alternative/supplement

5. How does Biological Control by Pseudomonads work?
Nutrient Competition
Site Competition
Antibiosis
Biological Control
Cross-communication

6. 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

7. The rhizosphere comprises  50% of the biomass of the plant
(From Kutschera, L & Lichtenegger, E Wurzelatlas Mitteleuropaischer Grundpflanzen Gustav Fischer Verlag Stuttgart)
“There is more biomass below the earth’s surface than above it.”

8. Examples of Biological Control Pseudomonads
Pseudomonas aureofaciens 30-84
Take-all of wheat (Gaeumannomyces graminis var. tritici)
Pseudomonas fluorescens Tx-1
Dollar spot of turf (Sclerotinia homoeocarpa)
Pseudomonas fluorescens Pf-5
Damping off of bean (Rhizoctonia solani)
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

9. Mechanisms of Biological Control by Fluorescent Pseudomonads
Nutrient Competition
Biological Control

10. Control of Rhizoctonia solani on cotton by P. cepacia D1
Produces fluorescent siderophores
Chelates Fe in environment
All organisms require Fe
Fe available at M

11. P. cepacia D1
Control
Cotton
R. solani

12. 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.

13. Take-all disease of wheat
Pathogen: Gaeumannomyces graminis var. tritici
Invades root vascular tissues
Physically blocks water & nutrient transport

14. Pseudomonas aureofaciens 30-84
Take-all Decline- An Example of Natural Suppression
Conducive
Suppressive
Pseudomonas aureofaciens 30-84
Take-all Disease
Years of wheat monoculture

15. Mechanisms of Biological Control by Fluorescent Pseudomonads
Antibiosis
Biological Control

16. Pseudomonas aureofaciens Produces Phenazine AntibioticsOH N
COOH
PCA
2-OH-PCA
2-OH-PZ
“Phenazine Phacts”
Broad spectrum
Block respiration
Pathogen inhibition
Competitive fitness

17. Phenazines required for pathogen inhibition
(Phz-)
30-84
Restored
Phz-
30-84
30-84
Restored
Phz-
Ggt

18. AHL-mediated Gene Regulation
acyl-ACP + o
ADO-Met
PhzI
PhzR o P
RNAPol
phzI
phzR
phzFABCD

19. phzI phzR phzXYFABCD csaI csaR (+) PhzI PhzR CsaR CsaI (+) (+) (+) (+)
Cell Surface
Biofilms o o o
RpeA
(+) o o
Exoprotease o
rpeA o o o o o o o o o o o
PhzI
PhzR o o o o
CsaR
CsaI o o
(+)
(+)
(+)
phzI
phzR
phzXYFABCD
csaI
csaR
(+)
(+)
RpoS
GacA
(+)
GacS

20. AHL Regulatory System P P phzF phzA phzB phzC phzD 30-84
phzR P
phzI P
phzF
phzA
phzB
phzC
phzD
30-84
30-84R (PhzR-)
30-84 (PhzR++)
Lawn of 30-84I (PhzI-)
30-84R (PhzR-)
30-84Z (Phz-, AHL+)

21. 3R-hydroxy-7-cis-tetradecenoyl HSL R. leguminosarum
P. aeruginosa
Butyryl HSL OH O H N
V. harveyi
Hydroxybutyryl HSL O H N
V. fischeri
3-oxohexanoyl HSL O
P. aureofaciens
Hexanoyl HSL O N O H O
V. fischeri
Octanoyl HSL O N O H O H N
A. tumefaciens
3-oxooctanoyl HSL O H N
P. aeruginosa
3-oxododecanoyl HSL
3R-hydroxy-7-cis-tetradecenoyl HSL H OH O
R. leguminosarum O N O H

22. What about the Microbial Community?
Let’s get together!
What’s this guy thinking?
I hear you!
He’s Nuts!!
Wanna Rumble!

23. Potential Roles of Bacterial Communication
Coordinating gene expression
Competition
Survival
Pathogen inhibition
2. Interspecies communication
Recognition and defense
Consistency of biological control
Biofilm formation & structure

24. Cross-communication
Positive
Negative

25. Mechanisms of Biological Control by Fluorescent Pseudomonads
Cross-communication

26. Why is communication important?
Can:
Alter rhizosphere competition...
Determine the composition & structure of the root community...
Enhance pathogen inhibition...
PU-186
30-84I
PU I
Reduce pathogen inhibition...
30-84
2.7 ± 1.1c
8.9 ± 1.0a
1.3 ± 2.3c
Mixture
Ave. Inhibition (mm)
PU-5
PU-15
0 ± 0b

27. Microbial Communities: Biofilms

28. 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