What

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Whats a plant? They have pathogens? And immunity?
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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.

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Why TMV?

• Mosaic disease of tobacco was detrimental to the
  tobacco industry in the early 1900s.
• 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!

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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.

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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.

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Pathogenic Plant Bacteria

• Pseudomonas syringae pv. tomato DC3000
• Bacterial speck disease caused on tomato and
  mustard plants.
• gt30 Type III Secretion
• System effectors.

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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.h
tml
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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).

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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.

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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).

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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.

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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
  C-terminus involved in CDS activities.

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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!

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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.

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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.

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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 plant-bacteria interactions.

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Trump Model Version 1

• The CDS effector is dominantly expressed if R
  protein but no T factor is present.

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Trump Model Version 2

• CDS effector is suppressed if both R protein and
  T factor are present.

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Trump Model Version 3

• CDS effector is dominantly expressed if T factor
  but no R protein present.

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Other Virulence Activities

• AvrPtoB
• DC3000 mutants without CDS activity cause a
  10-fold growth decrease in tomato plants.
• Plants with prf mutations cannot mount Pto- or
  Rsb-mediated immunity.
• AvrPtoB can activate ethlyene production in
  tomato plants, increasing cell death.

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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.

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Phytophthora ramorum

• First observed causing bleeding stem cankers and
  mortality of tanoak and coast live oak in coastal
  CA in the 1990s, 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.

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Sudden Oak Death Research

• Individual tree level
• Forest level
• Regional level

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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.

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The End!