PlantMicrobe Interactions

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Plant-Microbe Interactions
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Outline of topics for today and Wednesday

• Terminology
• Microbial plant pathogens
• Coevolution between plants and pathogens
• Rhizosphere interactions
• General biology of mycorrhizae

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Terms describing the location of microbial
habitats related to plants

• Epiphytic organisms growing on the surface of
  photosynthetic organisms
• Phylloplane leaf surface
• Phyllosphere area surrounding the leaf and
  impacted by it
• Rhizoplane root surface
• Rhizosphere area surrounding the root and
  impacted by it

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Profiles of two plant pathogens

• 1. Agrobacterium tumefaciens

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Profiles of two plant pathogens

• 1. Agrobacterium

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• Agrobacterium tumefaciens causes crown gall
  disease of a wide range of dicotyledonous
  (broad-leaved) plants, especially members of the
  rose family such as apple, pear, peach, cherry,
  almond, raspberry and roses.

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• Agrobacterium is significant as a tool to insert
  foreign DNA into a plant. Basically, the
  bacterium transfers part of its DNA to the plant,
  and this DNA integrates into the plants genome,
  causing the production of tumors and associated
  changes in plant metabolism.

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Agrios, 1978
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• tumefaciens is a Gram-negative, non-sporing,
  motile, rod-shaped bacterium, closely related to
  Rhizobium which forms nitrogen-fixing nodules on
  clover and other leguminous plants.
• Most of the genes involved in crown gall disease
  are not borne on the chromosome of A. tumefaciens
  but on a large plasmid, termed the Ti
  (tumour-inducing) plasmid. In the same way, most
  of the genes that enable Rhizobium strains to
  produce nitrogen-fixing nodules are contained on
  a large plasmid termed the Sym (symbiotic)
  plasmid.
• Thus, the characteristic biology of these two
  bacteria is a function mainly of their plasmids,
  not of the bacterial chromosome.

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The central role of plasmids in these bacteria
can be shown easily by "curing" of strains. If
the bacterium is grown near its maximum
temperature (about 30oC in the case of
Agrobacterium or Rhizobium) then the plasmid is
lost and pathogenicity (of Agrobacterium) or
nodule-forming ability (of Rhizobium) also is
lost. However, loss of the plasmid does not
affect bacterial growth in culture - the
plasmid-free strains are entirely functional
bacteria.
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In laboratory conditions it is also possible to
cure Agrobacterium or Rhizobium and then
introduce the plasmid of the other organism.
Introduction of the Ti plasmid into Rhizobium
causes this to form galls introduction of the
Sym plasmid into Agrobacterium causes it to form
nodule-like structures, although they are not
fully functional.
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Studies such as these raise many interesting and
challenging questions about the nature of
bacteria. For example, what does the name of a
bacterial species or genus really mean, if the
organism can change so drastically by loss or
gain of a non-essential plasmid? And how much
gene exchange occurs by means of plasmids and
other mobile genetic elements within natural
populations?
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• It is important to note that only a small part of
  the plasmid (the T-DNA) enters the plant the
  rest of the plasmid remains in the bacterium to
  serve further roles. When integrated into the
  plant genome, the genes on the T-DNA code for
• production of cytokinins
• production of indoleacetic acid
• synthesis and release of novel plant metabolites
  - the opines and agrocinopines.

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The plant hormones upset the normal balance of
cell growth, leading to the production of galls
and thus to a nutrient-rich environment for the
bacteria. The opines are unique amino acid
derivatives, different from normal plant
products, and the agrocinopines similarly are
unique phosphorylated sugar derivatives.
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All these compounds can be used by the bacterium
as the sole carbon and energy source, and because
they are absent from normal plants they provide
Agrobacterium with a unique food source that
other bacteria cannot use.
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• tumefaciens has been used extensively for genetic
  engineering of plants.
• This is achieved by engineering selected genes
  into the T-DNA of the bacterial plasmid in
  laboratory conditions so that they become
  integrated into the plant chromosomes when the
  T-DNA is transferred.

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A few of the commercial applications of T-DNA
technologies
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However, the complexity of the patent landscape
has created by the real and perceived obstacles
to the effective use of this technology.

• Here we show that several species of bacteria
  outside the Agrobacterium genus can be modified
  to mediate gene transfer to a number of diverse
  plants

Broothaerts et al., 2005
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• 2. Phytophthora (plant destroyer)
• The genus Phytophthora contains about 35 spp.,
  many are notorious plant pathogens.

For example, Phytophthora cinnamomi has destroyed
millions of avocado trees in CA and eucalyptus in
Australia. Its motile zoospores are attracted to
root exudates. It produces resistant spores that
can survive up to 6 years in moist soil. Big
concern with this pathogen for Port Orford Cedar
in the Pacific NW.
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From http//www.parks.tas.gov.au/veg/phytop/whatis
.html
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• Phytophthora infestans is famous because it was
  responsible for the great potato famines in
  Ireland in which over a million people died due
  to starvation.

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sporangia germinate either by releasing zoospores
or by producing a hyphal outgrowth.
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• Virtually the entire potato crop was wiped out in
  a single warm, wet week in the summer of 1846.
• This event initiated large-scale emigration.
• Within the decade that followed the population of
  Ireland dropped from 8 million to 4 million
  people.

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Coevolution between plants and pathogens

• Consider pathogens as part of the biotic
  environment that exert a strong selective force
  on populations of plants and animals.

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Plants have two general responses to pathogen
attack

• Passive Constitutive defenses
• Active Induced defenses

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Two general plant responses to pathogen attack,
from Dickinson Lucas 1982
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• 1. Passive, constitutive defenses ( defenses
  that are constantly available
• Structural physical e.g. waxy or thickened
  cuticle, hairy stomates, structures to nurture
  associations with ants, etc.
• Chemical e.g. tannins, terpines, resins,
  alkaloids, ... many drugs vices)

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• 2. Active, induced defenses acquired after the
  plant is attacked.
• a. Structural localize responses at the site of
  entry e.g. necrosis, callose deposition,
  lignification, abscission layers, tyloses etc.
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• b. Chemical i.e. systemic acquired resistance
  (SAR) e.g. phytoalexins including polyphenolic,
  flavonoid or proteinaceous antimicrobial
  compounds. Salicylic acid plays a role in
  activating the genes coding for these compounds.

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• What is the relationship between pathogens and
  genetic diversity in plant populations and
  species diversity in plant communities?
•  

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• What is the relationship between pathogens and
  genetic diversity in plant populations and
  species diversity in plant communities?
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• Microbial parasites, pathogen, and herbivores may
  be responsible for maintaining a high degree of
  genetic polymorphism in plant populations and a
  high degree of species diversity within plant
  communities.

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• Dan Jansen suggested that pathogen pressures are
  responsible for maintaining the incredibly
  diverse tropical forests.

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• Keith Clay and Jim Bever suggest that infection
  of plants by mutualistic fungi may be a
  prerequisite for survival and persistence of
  plant species (i.e. a stabilizing force), but
  parasitic fungi may prevent plant communities
  from becoming dominated by one or several species
  (i.e. destabilizing force).

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• Back to the Red Queen analogy of a coevolutionary
  arms race between plants and microbial pathogens.
  

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Red Queen hypothesis

• Genetic systems determining virulence in the
  pathogen will be paralleled by genes conferring
  resistance in the host.
• This is because any mutation to virulence in a
  pathogen population will be countered by the
  selection of hosts able to resist this more
  aggressive pathogen.
• Thus, in a ideal world, we might expect a
  perpetual stalemate, with host and pathogen
  populations being closely matched in resistance
  and virulence.
• Hence, over time disease would be neither
  completely absent nor epidemic.

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• So what happened in Ireland in 1846?
• Why did P. infestans wipe out virtually all of
  the potatoes?
•  

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• Disease epidemics often occur when genetic
  diversity of plant populations is eliminated by
  human intervention.

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IV. Rhizosphere interactions
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• Roots exude a tremendous quantity of carbon into
  the rhizosphere.
• Why?

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Microbial biomass C (mg Cg1 soil dry wt)
measured by using substrate-induced respiration
method in rhizosphere continuums of three
experiments shows that root exudates increase
microbial biomass. From Bonkowski et al., 2000
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• Rhizosphere bacteria and fungi generally
  immobilize nitrogen and phosphorus

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• 3. Grazing by microfauna can influence the
  whether bacteria and fungi in the rhizosphere
  mineralize or immobilize the nutrients.
• Grazing of microflora by microbivores seems to
  be a crucial mechanism to maintain the balance in
  the competition between micro-organisms and
  plants. (Bonkowski et al. p. 137)

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These microshredders, immature oribatid mites,
skeletonize plant leaves. This starts the
nutrient cycling of carbon, nitrogen, and other
elements. Collohmannia sp. Credit Roy A.
Norton, College of Environmental Science
Forestry, State University of New York
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Interactions between soil bacteria and microfauna

• Generally, protozoa increase mineralization in
  soil, whereas the effects of bacterial feeding
  nematodes appear to depend on the status of the
  populations with nutrients released only if
  nematode populations decline or are in the
  presence of nematode predators. (Bonkowski et
  al. p.136-137)

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Respiration by protozoa account for 2/3 of all
respiration by soil-animals
Flagellates, photo by Sarah Spaulding
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Unlike bacteria and the substrates that they
consume, protozoa and their bacterial prey differ
little in respect to their CNP ratios. Why
is this important?
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Protozoa use only 10 to 40 of the prey carbon
for biomass production and excrete the excess N
and P. Also, when protozoa die (e.g. in the
winter) they release a flush of highly
decomposable protozoan tissue.
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Protozoa are picky eaters and they feed
selectively on certain species of bacteria. The
species composition of soil communities can
impact their ecosystem function (e.g.
flagellates, amoebae and ciliates stimulate
nitrifying bacteria), which can feedback on plant
community composition.
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Interactions between soil fungi and microfauna

• Collembola and oribatid mites feed selectively on
  certain soil fungi. Not all fungi are grazed
  equally.
• EARLY DOGMA Fungivorous microarthropods
  influence plant growth through grazing on
  mycorrhizal fungi.
• However, studies have turned this idea
  upside-down!

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A mushroom of Laccaria bicolor fruiting with a
white pine seedling. The size of the mushroom
indicates that abundant photosynthate must be
transported from the needles of the seedling to
its root/mycorrhiza system. Photo by Christian
Godbout and Andre Fortin
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Close up of springtails, Folsomia candida
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• Klironomos Hart discovered a surprising
  relationship between one fungus and a putative
  collembolan grazer
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• Klironomos, J. and M.M. Hart. 2001. Animal
  nitrogen swap for plant carbon. Nature
  410651-652.
• This study showed that mycorrhizal fungi can
  parasitize soil insects and transfer
  insect-derived nitrogen to their plant partners.
  Up to 25 of total plant nitrogen may be of
  insect origin.

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• Our results reveal a nitrogen cycle of far
  greater flexibility and efficiency than was
  previously assumed, where the fungal partner uses
  animal-origin-nitrogen to barter for carbon
  from the host tree. The host, in turn, supplies
  its fungal associate with carbon to synthesize
  proteolytic enzymes. Should this phenomenon
  prove to be widespread, forest-nutrient cycling
  may turn out to be more complicated and tightly
  linked than was previously believed.
• Klironomos Hart, 2001