Pathogenesis v. symbiosis

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• Pathogenesis v. symbiosis
• how do plants recognize beneficial organisms and
  not respond with defence activation?
• how do symbionts recognize and communicate with
  a host?
• do commonalities exist?

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Major symbionts of plants include rhizobial
bacteria and mycorrhizal fungi Rhizobia -
Nitrogen fixation (specific to legumes) Mycorrhiz
ae - Nutrient uptake, protection from pathogens
(most plants) Both initially induce plant
defenses short lived localized to few cells
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• Plants must first recognize colonization by the
  symbiont
• elicitors may be exogenous (produced by
  symbiont)
• or endogenous (produced by the host)
• synthesis of chalcone synthase and phytoalexins
• in legumes induced by mycelial extracts!
• nod factor
• SA, ROS induced, but short lived
• suppression or degradation?
• 2 possibilities
• symbionts produce weak response
• down regulation accomplished by further
  crosstalk

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http//commtechlab.msu.edu/sites/dlc-me/zoo/zdrr01
25.html
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Gelvin (2000) Agrobacterium and plant genes
involved in T-DNA transfer and integration. Ann
Rev Plant Physiol and Mol Biol. 51223-256
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Bittinger MA, Gross JA, Widom J, Clardy J, and
Handelsman J. (2000) Rizobium etli CE3 carries
vir Gene Homologs on a Self-Transmissilbe
Plasmid. MPMI 131019-1021
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Four major events are important in activating and
suppressing plant defense during the interaction
of plants and benficial symbionts

• Plant recognition of the symbiont
• Signal transduction
• Activation and expression of plant defense genes
• Suppression of plant defense genes/degradation of
• Elicitor molecules

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• Nematodes and nodules
• root-knot nematodes share a common pathway with
  rhizobia,
• yet are pathogenic!

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RKN life cycle
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http//plantpath.caes.uga.edu/personnel/faculty/Hu
ssey.html
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Enod40 and ces52 are induced during nodule
formation and during Giant cell formation
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Phan and knox are induced during nodule formation
and during Giant cell formation
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Weerasinghe RR, Bird DMcK, and Allen NS. (2005)
Root-knot nematodes and bacterial Nod factors
elicit common signal transduction events in Lotus
japonicus. PNAS 1023147-3152
nem factor (nemF) induces similar effects as nod
factor and is also a small secreted molecule
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M. incognita nod-L

• Protein sequence bacteria-like
• 58 amino-acid identity (8.8e-54) to nodL from
  Rhizobium leguminosarum

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RKN nod-L
Scholl, Thorne, McCarter, Bird. 2003.
Horizontally transferred genes in plant-parasitic
nematodes A high-throughput genomic approach.
Genome Biology, 4 R39.
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M. incognita nod-L

• Protein sequence bacteria-like
• 58 amino-acid identity (8.8e-54) to nodL from
  Rhizobium leguminosarum
• Gene structure typical of a nematode
• 2 introns
• mRNA trans-spliced at the 5-end polyadenylated
  at the 3-end
• eukaryotic promoter
• codon usage (codon adaptation index)
  nematode-like, not rhizobia-like

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Detection of nod-L
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Nod-L key enzyme for Nod factor biosynthesis
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nod-L in RKN

• What is the function of this gene in RKN?
• Does RKN make a Nod factor?
• Does the RKN-plant interaction have similarities
  to the rhizobia-plant interaction?

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Weerasinghe RR, Bird DMcK, and Allen NS. (2005)
Root-knot nematodes and bacterial Nod factors
elicit common signal transduction events in Lotus
japonicus. PNAS 1023147-3152
nem factor (nemF) induces similar effects as nod
factor and is also a small secreted molecule
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nod-L in RKN

• What is the function of this gene in RKN?
• Does RKN make a Nod factor?
• Does the RKN-plant interaction have similarities
  to the rhizobia-plant interaction?
• How did RKN acquire nod-L?
• Horizontal gene transfer?
• Does RKN have other rhizobial-like genes?

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Other horizontal candidates?

• Global search of the Meloidogyne EST set
• MI00754 - 2-hyrdoxymuconic semialdehyde hydrolase
• MI00252 - exo-polygalacturonase
• MI00426 - glutamine sythetase
• MI01644 - L-threonine aldolase
• MI00109 - hypothetical conserved protein
• b-endoglucanases (ancient paralogues)