Agrobacterium tumefaciens Crown Gall Disease

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Agrobacterium tumefaciens Crown Gall Disease
or, some very weird sex
Crown gall bacteria injecting DNA into host plant
cells
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Agrobacterium tumefaciens Crown Gall Disease
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Agrobacterium tumefaciens Crown Gall Disease
An example of Selfish Genes - A. tumefaciens
genetically engineers plants to make specialized
food for it
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Bio-engineering plants with Agrobacterium
engineered gene
selectable marker gene
Left T-DNA border
Right T-DNA border
Plant Transformation Vector
Note opine genes and plant hormone genes are
deleted from the plant transformation vector
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• Applications of Agrobacterium-mediated plant
  transformation
• Bt crops that resist insects because they express
  an insecticidal toxin gene from soil bacterium
  Bacillus thuringiensis
• Roundup-Ready crops that tolerate herbicide
  because they contain an alternate
  herbicide-insensitive enzyme from a bacterium
• Virus-resistant crops that block virus
  replication by expressing a bit of the virus
  genome, triggering degradation of viral RNAs by
  plant RNAses called Slicer and Dicer
• And many others

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Papaya trees exposed to papaya ringspot virus
(PRSV)
parent
transgenic
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Agrobacterium tumefaciens Crown Gall Disease

• A successful example of biocontrol
• Agrobacterium radiobacter strain K84 makes the
  antibiotic Agrocin 84
• Crop rootstocks dipped in cultures of K84 are
  colonized by the A. radiobacter strain
• K84 produces Agrocin 84, active against
• A. tumefaciens
• A. tumefaciens doesnt infect these plants

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Other bacterial pathogens cause galls, including
some species of Pseudomonas and Erwinia
However, these galls are due to hormones
produced by the bacteria and not caused by DNA
transfer into plants.
Olive knot (Pseudomonas savastanoi)
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Agrobacterium tumefaciens Crown Gall Disease -
REVIEW
Pathogen Gram-negative rod Symptoms galls /
tumors Virulence mechanism transformation of
plants with a piece of bacterial DNA, causes
plant to form a tumor and produce food for
Agrobacterium cells. Control biocontrol with
Agrobacterium radiobacter, which produces a toxin
that kills tumor forming Agrobacterium strains,
and through sanitation of equipment Industrial
use plant transformation, construction of
genetically-modified plants
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Important Concepts in Phytobacteriology
Plant pathogenic bacteria act in groups, power in
numbers Bacterial pathogens can acquire new
virulence genes via horizontal transfer Bacterial
pathogens can manipulate plant cells by
injecting DNA into the plant cells Some
bacterial pathogens can be controlled with
biocontrol microbes
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Pseudomonas syringae spots, specks, and blights
Grows on leaves, fruit, stems, and germinating
seeds Pseudomonas syringae can infect most plant
species, but individual strains are limited to a
few species or even a few cultivars within a
species (pathovars)
Bacterial blight of lilac
Bacterial speck of tomato
Blister spot of Mutsu apple
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Pseudomonas syringae spots, specks, and blights
Symptoms patches of dead cells (lesions
necrotic patches) sometimes surrounded by a
yellow halo (chlorosis) Signs can see
bacterial streaming from the lesions
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Pseudomonas syringae spots, specks, and blights
Plant death is rare (with exception of seed
decay) P. syringae affects quality of harvested
produce
Bacterial brown spot of bean
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Growth of bacteria in leaves
Log of bacteria
days
The Hypersensitive Response (HR) - rapid collapse
of plant cells in response to an incompatible
pathogen
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Pseudomonas syringae spots, specks, and blights
Same genes are required for Pseudomonas syringae
to cause the HR on tobacco and disease symptoms
on bean. What do these genes do? -encode the
Type 3 Secretion System and its effectors
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Pseudomonas syringae requires the Type 3
Secretion System (T3SS) to grow in plant leaves.
T3SS a syringe-like apparatus that injects
proteins (effectors) directly into host plant
cells
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Injected T3SS effectors ( ) suppress
plant disease resistance
defenses off
bacterium
nucleus
plant cell
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Injected T3SS effectors ( ) suppress
plant disease resistance
plant defense responses
nucleus
Result Bacteria multiply cause Disease
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BUTplants can evolve to recognize injected T3SS
effectors and launch plant disease resistance
(often HR)
plant defense responses
nucleus
Result Rapid plant cell death, bacteria cant
multiply Resistance
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Plant recognition of pathogen effectors
(avirulence factors) is the basis of
gene-for-gene disease resistance
plant defense responses
nucleus
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Pseudomonas syringae spots, specks, and blights
Toxins also play an important role in bacterial
disease Example Coronatine
Wild-type coronatine- mutant
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Coronatine - toxin produced by many plant
pathogenic Pseudomonads -Causes chlorosis
(halo)
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Plant stomates are surrounded by guard cells.
scanning electron micrograph of stomates
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Plant stomates are surrounded by guard
cells. These close when they sense bacteria
!!
scanning electron micrograph of stomates
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(No Transcript)
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P. syrinage strain DC3000 can re-open stomates
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But coronatine-deficient mutant DC3118 cant
How does coronatine re-open stomates?
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Coronatine toxin mimics the plant defense
signaling molecule methyl jasmonate and the
senescence hormone ethylene. These block basal
defenses that close stomates.
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Pseudomonas syringae spots, specks, and blights
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wild-type
coronatine-
Green fluorescent protein (GFP)-labelled P.
syringae moving through open stomates
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Pseudomonas syringae spots, specks, and blights
Coronatine toxin keeps leaf stomata open, may
help bacteria gain entrance into leaves.
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Pseudomonas syringae spots, specks, and blights
Pathogen gram-negative rod bacterium Symptoms
brown spots and/or yellow chlorosis on leaves,
sometimes wilting diseases. T3SS protein
secretion system that injects bacterial proteins
into host cells. Proteins suppress resistance,
but can also trigger plant immune system. Toxins
play important role in disease. They can suppress
immune system, cause cell death, and help
bacteria enter leaves
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Bacterial Wilt Disease

• - soil-transmitted vascular pathogen
• - most destructive plant disease n the tropics
• broadly distributed, wide host range tobacco,
  peppers, potato, bananas, ginger, cloves, tomato,
  peanut, etc.
• Symptoms stunting, wilting, sudden death

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Infection and Colonization of Plants by Ralstonia
solanacearum
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Diverse Hosts of R. solanacearum
plantains
tomato
bananas
geraniums
potato
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Visual diagnosis for bacterial wilt After 10-15
minutes in water, fine white threads flow from
cut stems
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Control of Bacterial Wilt

• Plant resistant lines
• Sanitation - clean tools and tubers/ cuttings
• Nematode control (make wounds)
• Control floodwaters
• 5-year rotations, especially with grains or
  non-host dicots (carrots, cabbage)

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Citrus Huanglongbing - citrus greening disease
Huanglongbing yellow shoot disease in
Chinese Extremely destructive emerging citrus
disease Struck Brazil in 2004, Florida in 2005
(after hurricanes) Infected trees develop yellow
shoots and mottled leaves
Trees eventually die but before that, fruit is
distorted and bitter, cant be used for juice.
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Citrus Huanglongbing - citrus greening disease
Transmitted by phloem-feeding citrus psyllid
insects often a latent period before symptoms
appear. Pathogen is probably the Candidatus
Liberibacter asiaticus bacteria. It has not yet
been cultured. There are no good control
methods no resistant varieties, no rapid
sensitive diagnostics
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Citrus Huanglongbing - citrus greening disease
HLB poses a very serious threat to Florida citrus
production Eradication efforts are on-going May
result in end of shipment of fresh citrus from
Florida in order to protect industries in Texas
and California.
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Important Concepts in Phytobacteriology
Bacterial pathogens gain access to plants through
wounds or by exploiting natural openings.
Bacterial pathogens can manipulate plant cells
by injecting DNA or proteins into plant cells or
with toxins Plants can fight back by evolving
ability to recognize these effectors launch
resistance/defense mechanisms Some bacterial
plant pathogens are insect-transmitted. Control
of bacterial plant diseases is very difficult
exclusion and sanitation may be the best options