Pierson LabPierson Lab

Microbial gene regulationMicrobial gene regulation

Root-associated free-living bacteriaRoot-associated free-living bacteria

Microbial community Microbial community interactionsinteractions

Plant disease Plant disease controlcontrol

The Pseudomonads in Biological Control

Importance of plant diseases

Estimated annual crop production worldwide

Amount lost to disease, insects, weeds using current control measures

Additional losses without current control measures

$1.2 - 1.3 trillion$1.2 - 1.3 trillion

$500 billion$500 billion

$330 billion$330 billion

Verticillium wiltVerticillium wilt

Citrus cankerCitrus canker

Fireblight of pearFireblight of pear Apple scab

Take-all of wheatTake-all of wheat

Plant

Pathogen EnvironmentHumidityH2O

GenotypeStage of growth

Species

Stress

Genotype: hrp, avr raceDispersal, colonization site

Plant factors

Other Microbes

What determines disease?

1995: United States spent $26 billion$26 billion on chemical pesticides

Of this, < 1%< 1% actually gets to where the pathogen is

What happens to the rest?

Ground water

Taken up by the plant

Development of resistance

Current approaches to disease control

Chemical

Identification of resistance genes

Introgressing into commercial cultivars

Problems with development of resistance, pyramiding genes

Breeding

Biological Control is an attractive alternative/supplement

How does Biological Control by Pseudomonads work?

Nutrient Competition

Cross-communication

Biological ControlSite Competition 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.”

Pseudomonas fluorescens Tx-1 Dollar spot of turf (Sclerotinia homoeocarpa)

Pseudomonas aureofaciens 30-84 Take-all of wheat (Gaeumannomyces graminis var. tritici)

Pseudomonas fluorescens Pf-5 Damping off of bean (Rhizoctonia solani)

Pseudomonas fluorescens F113 Damping off of bean (Pythium ultimum)

Pseudomonas aureofaciens AB254 Damping off of bean (Pythium ultimum)

Drechslera leaf spot (D. poae)

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

Examples of Biological Control Pseudomonads

Mechanisms of Biological Control by Fluorescent Pseudomonads

Nutrient Competition

Biological Control

Produces fluorescent siderophores

Chelates Fe in environment

Fe available at 10-18 M

All organisms require Fe

Control of Rhizoctonia solani on cotton by P. cepacia D1

Rhizoctonia solani

P. cepacia D1

R. solani

Cotton

Control

Take-all Disease of Wheat

Caused by Gaeumannomyces graminis var. tritici (Ggt)

No. 1 disease of cereals worldwide (up to 50% yield loss)

One infected root in 10,000 is sufficient to cause an epidemic

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

Years of wheat monoculture

Tak

e-al

l D

isea

se

SuppressiveConducive

Pseudomonas aureofaciens 30-84

Mechanisms of Biological Control by Fluorescent Pseudomonads

Biological Control Antibiosis

Pseudomonas aureofaciens Produces Phenazine Antibiotics

2-OH-PCA2-OH-PCAPCAPCA 2-OH-PZ2-OH-PZ

OHOH

NN

NN COOHCOOHCOOHCOOH

NN

NNOHOH

NN

NN

Pathogen inhibition

Competitive fitness

“Phenazine Phacts”

Broad spectrum

Block respiration

Phenazines required for pathogen inhibition

30-8430-8430-8430-84

Phz-Phz-

Phz-Phz-RestoredRestored

RestoredRestored

Ggt 30-

84.

44-8

(P

hz

30-

84.4

4-8

(P

hz-- ))

30-

84

30-

84

phzIphzI phzRphzR phzFABCD

AHL

oacyl-ACP +

ADO-Met

PRNAPolRNAPol

o

o

PhzIPhzI PhzRPhzR

AHL-mediated Gene Regulationo

o o

oo

phzXYFABCD

CsaRCsaRCsaICsaI

(+)

(+)

(+)GacAGacARpoSRpoS

csaIcsaI csaRcsaR

GacSGacS

oo

(+)

o

o o

o

o

o

o

phzI phzR

(+)(+)(+)

o

oo

PhzIPhzI PhzRPhzR

o

o

o

ExoproteaseExoprotease

o

rpeArpeA

BiofilmsBiofilmsCell SurfaceCell Surface

RpeARpeA

o

oo

o

o

oo

AHL Regulatory System

phzF phzA phzB phzC phzD

P

30-8430-8430-84R (PhzR-)30-84R (PhzR-) 30-84 (PhzR30-84 (PhzR++++))

phzRP

phzR

phzIP

phzI

Lawn of 30-84I (PhzI-)

30-84R (PhzR-)

30-84Z (Phz-, AHL+)

O

O

H

N

O

O

OH

O

H

N

OH

O

OO

H

N

O

O

OO

H

N

O

O

O

H

N

O

O

O

H

N

O

O

O

H

N

O

O

3-oxohexanoyl HSL

Hydroxybutyryl HSL

3-oxooctanoyl HSL

3-oxododecanoyl HSL

Hexanoyl HSL

Octanoyl HSL

Butyryl HSLP. aeruginosa

V. harveyi

V. fischeri

P. aureofaciens

V. fischeri

A. tumefaciens

P. aeruginosa

OHO

H

N

O

O

H3R-hydroxy-7-cis-tetradecenoyl HSL

R. leguminosarum

What about the Microbial Community?

What’s this guy thinking?

He’s Nuts!!

Let’s get together!

Wanna Rumble!

I hear you!

Potential Roles of Bacterial Communication

1. Coordinating gene expression

CompetitionSurvivalPathogen inhibition

2. Interspecies communication

Recognition and defenseConsistency of biological controlBiofilm formation & structure

Positive Negative

Cross-communication

Mechanisms of Biological Control by Fluorescent Pseudomonads

Cross-communication

Biological Control

Why is communication important?

Determine the composition & structure of the root community...

Alter rhizosphere competition...

Reduce pathogen inhibition...

30-84

30-84 + 530-84 + 15

2.7 ± 1.1c

8.9 ± 1.0a

1.3 ± 2.3c

Mixture Ave. Inhibition (mm)

PU-5

PU-15

0 ± 0b

0 ± 0b

Enhance pathogen inhibition...

PU-186

30-84I PU-186 + 30-84I

Can:

Microbial Communities: Biofilms

Conclusions

Biological control an attractive alternative to chemicals

Many biocontrol bacteria identified are Pseudomonads

Plant diseases cause major loss of food and money

Biological control occurs via several mechanisms

Competition

AntagonismCross-communication