SymRK defines a common genetic basis for plant root endosymbioses with arbuscular mycorrhiza fungi, rhizobia, and Frankia bacteria

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SymRK defines a common genetic basis for plant
root endosymbioses with arbuscular mycorrhiza
fungi, rhizobia, and Frankia bacteria

• PNAS 2008
• Volume 105 No. 12, pgs 4928-4932.
• Authors Gherbi, H., Markmann, K., Svistoonoff,
  S., Estevan, J., Autran, D., Giczey, G. Auguy,
  F., Pe ret, B., Laplaze, L., Franche, C.,
  Parniske, M., and Bogusz, D.
• Presented by Erick Breathwaite

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Endosymbiosis

• Root endosymbioses are associations between
  plants and soil microorganisms in which
  microorganisms are accommodated into the host
  cell

http//remf.dartmouth.edu/images/RootNodulesTEM/so
urce/rootnodule_80936_5.html
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Importance

• Endosymbioses contribute to plant nutrition and
  fitness worldwide therefore knowing the genetic
  basis for plant root symbiosis is of much
  importance.

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Types of root endosymbiosis

• The three major types of root endosymbioses that
  occur in plants involve legumes with rhizobia
  bacteria, Frankia (actinomycetes) and
  actinorhizal plants, and arbuscular mycorrhiza
  (AM) fungi with plants.

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Legume/Rhizobia symbiosis

• Several genetic components of host symbiont
  interaction have been found in legumes

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SymRK

• Endre G, et al. (2002) A receptor kinase gene
  regulating symbiotic nodule development. Nature
  417962966.
• Several downstream components of the Nod Factor
  signaling cascade, including the receptor kinase
  gene SymRK are involved in nodulation symbiosis
  in L. japonicus and other legume plants

http//en.wikipedia.org/wiki/Lotus_japonicus
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Evolutionary similarity

• Part of this signaling cascade is also involved
  in transduction of the symbiotic signal in fungi
  symbioses

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The experiment

• Unlike the legume plants, the genetic components
  of host-symbiont interaction in actinorhiza is
  unknown
• In this experiment, CgSymRK, a predicted SymRK
• gene from the actinorhizal tree C. glauca, was
  isolated to analyze its role in root
  endosymbioses

http//www.hear.org/starr/plants/images/image/?q0
40120-0213
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Isolation of CgSymRK

• The C. glauca SymRK candidate, CgSymRK, was
  isolated by using a degenerate priming approach
  based on similarity with legume SymRK sequences

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Knockdown expression of CgSymRK
Table 1. Reduced nodulation in CgSymRK RNAi
composite plants Nodulation was scored 12 weeks
after inoculation with Frankia.
Root systems (genotype) Nodulated/total root systems analyzed nodulated root systems
Nontransgenic 59/63 94
Transgenic (GFP) 24/48 50
CgSymRK RNAi (GFP) 21/78 27
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ARA assays

• To test the ability of CgSymRK RNAi nodules to
  fix nitrogen via acetylene reduction activity
  (ARA) assays.
• N2 6e- 6H   --gt  2NH3
• C2H2 2e-  2H  --gt C2H4

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Is CgSymRK is also involved in AM formation in C.
glauca?
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Can CgSymRK restore nodulation and AM symbioses
in a legume symrk mutant?
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Can CgSymRK restore nodulation and AM symbioses
in a legume (L. japonicus) symrk mutant?
Table 2. Complementation of Nodulation and AM
formation in Lotus symrk mutants carrying
CgSYMRK Nodulation was scored 4, 8, or 15 weeks
after inoculation with M. loti, and AM after 3
weeks of cocultivation with G. intraradices.
Results are compiled from two independent
experiments.
Root systems (genotype) Nodulated/total root systems analyzed AM/total root systems analyzed
symrk-10 control vector (GFP) 0/34 0/34
wild type CgSymRK (GFP) 21/23 31/31
symrk-10 CgSymRK (GFP) 11/63 27/48
wild type LjSymRK (GFP) 11/11 16/17
symrk-10 LjSymRK (GFP) 8/20 7/22
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Conclusion

• These results demonstrate that, in C. glauca as
  in legumes, SymRK is involved in the
  establishment of both nitrogen-fixing nodule and
  AM symbioses, thus supporting the hypothesis that
  signaling genes have been recruited from the more
  ancient AM symbiosis during the evolution of
  nitrogen fixing symbioses