Transcript Wk15-PlantPath.

What’s a plant? They have pathogens? And immunity?
Tobacco mosaic virus (TMV)
 RNA virus discovered by Beijerinck over 100
years ago, as a contagium vivum fluidum that
was filterable and could move through agar
media. (Bacteria were not filterable and
remained fixed in agar).
 CTMV causes mosaic disease in tobacco and
other plants.
 Widely used in host-pathogen research.
Why TMV?
 Mosaic disease of tobacco was detrimental to
the tobacco industry in the early 1900’s.
 Mayer in 1886, in the Netherlands, was the first
to artificially transmit a plant disease (TMV),
the causal agent of which could not be seen or
cultured.
 Around the same time, Pasteur was having a
similar problem with rabies!
Model Systems
 Models become the driving force for
independent steps of discovery,
intervention, and development.
 TMV rapidly accumulates to high titres in
infected plants.
 TMV is not transmitted by insects, fungi,
or nematodes, but is by rub-inoculation.
 TMV symptoms are easy to identify and
can infect a wide range of plants.
TMV in the laboratory
 Easy, cheap, and reliable to produce.
 Omega leader sequence has been
widely used to enhance translation of
foreign genes in transgenic plants.
 cDNA copies of TMV promoters have
been used in gene vectors for expression
of foreign genes in rub-inoculated plants.
Pathogenic Plant Bacteria
 Pseudomonas syringae pv. tomato DC3000
 Bacterial speck disease caused on tomato
and mustard plants.
 >30 Type III Secretion
System effectors.
Arabidopsis thaliana
 Just as animal biologists and medical researchers
have used model organisms such as Drosophila
and the nematode, so plant biologists have relied
upon the mustard Arabidopsis thaliana to help them
understand the genetics, physiology, development
and structure of plants in general.
 Although the efforts of Arabidopsis sequencers
have perhaps been less heralded than those of their
colleagues working on animal genomes, the results
they have yielded are no less scientifically
interesting. Arabidopsis may be a simple plant (its
detractors call it a weed!) with only 50 identified
cells types and no complex nervous system or
behavioural response, but its genome is surprisingly
complex. Chromosomes 2 and 4 of Arabidopsis
were published in Nature at the end of 1999, now
followed by chromosomes 1, 3 and 5.
http://www.nature.com/genomics/papers/a_thaliana.html
Plant Defenses
 Basal defenses:
 Cell wall reinforcements
 Expression of defense-associated proteins
 Activated when LPS or flagellin detected.
 Intracellular Resistance (R) Proteins:
 Detect Type III effectors in plant cells
 Hypersensitive Response (HR) induces
Programmed Cell Death (PCD).
Pseudomonas DC3000
 Full genome has been sequenced.
 DC3000 elicits HR immunity (via apoptosis) in
plants carrying Pto R proteins (serine/threonine
kinase).
 Plant Pto interacts with bacterial DC3000
AvrPto and AvrPtoB effectors as well as Prf for
HR response and plant immunity.
Plants without Pto protein
 Without Pto, the effectors AvrPto and
AvrPtoB promote bacterial growth!
 AvrPto suppresses cell wall-based
defenses.
 AvrPtoB has Cell Death Suppressor
(CDS) activity to prevent HR-based PCD
(apoptosis).
AvrPtoB
 Belongs to VirPphA gene family.
 VirPpha is a virulence factor located on a large
Pph plasmid.
 VirPpha and AvrPtoB have 51% sequence
identity, so they are most likely related but not
homologous proteins.
 AvrPtoB is bigger than AvrPto but has a similar
interaction with Pto.
Cell Death Suppression
 AvrPtoB CDS was discovered with Nicotiana,
because of its robust cell death response.
 AvrPto with Pto results in PCD, but AvrPtoB
with Pto in Nicotiana did not result in
Programmed Cell Death!
 Mutant and pro-apoptotic experiments with
AvrPtoB supported CDS activity.
 Hidden R gene discovered:
 Rsb = Resistance Suppressed by the avrPtoB Cterminus involved in CDS activities.
DC3000 Pathogenesis
 Wild type tomato plants with AvrPtoB and
without functional pto gene = CDS.
 Mutant AvrPtoB led to PCD and immunity.
 Introducing AvrPtoB in trans restored
virulence of DC3000 and CDS!
Rsb gene
 AvrPtoB must be recognized by at least
two resistance genes. Rsb is Pto related:
 Like Pto, Rsb is Prf dependent for CDS.
 Gene silencing experiments of Pto family
members eliminates Rsb resistance in
tomatoes and PCD in Nicotiana plants.
Other DC3000 Effectors
 HoPtoD2, HopPtoE, HopPtoF, HopPtoN,
AvrPphE, AvrPpiB1…
 Cell Death Suppression activity is not
specific and can inhibit various R proteins.
 Most effectors were discovered based on
eliciting the HR immune response, but they
also have the potential for CDS activity.
Trump Model
 Not the Donald, thank goodness!
 The host-pathogen interaction may
involve an R protein as well as a Trump
factor that can suppress the CDS activity.
 This model accounts for the ‘hidden
resistance genes’ and complex plantbacteria interactions.
Trump Model Version 1
 The CDS effector is dominantly expressed if
R protein but no T factor is present.
Trump Model Version 2
 CDS effector is suppressed if both R
protein and T factor are present.
Trump Model Version 3
 CDS effector is dominantly expressed if T
factor but no R protein present.
Other Virulence Activities
 AvrPtoB
 DC3000 mutants without CDS activity cause
a 10-fold growth decrease in tomato plants.
 Plants with prf mutations cannot mount Ptoor Rsb-mediated immunity.
 AvrPtoB can activate ethlyene production in
tomato plants, increasing cell death.
Why would bacteria evolve
to both enhance and
suppress cell death?
 Perhaps early in infection it is
advantageous to suppress PCD to
escape the immune system and establish
the infection.
 Later in the infection it may be
advantageous to enhance PCD to gain
access to new cells and nutrients.
Phytophthora ramorum
 First observed causing bleeding stem
cankers and mortality of tanoak and coast
live oak in coastal CA in the 1990’s, it has
now been identified in North America and
Europe.
 Mortality associated with Fagus genus but
foliar and twig infections of other species
are important for pathogen spread.
Sudden Oak Death
Research
 Individual tree level
 Forest level
 Regional level
Forest Succession
 Mosaic landscapes provide a variety of
niches and microclimates.
 Successional stages follow disturbances
such as fire or logging.
 Tanoak sprouts prolifically following fire, and
is later replaced by Douglas-fir or coast
redwood trees.
The End!