Journal Article

Evolution of <i>Bradyrhizobium–Aeschynomene</i> Mutualism: Living Testimony of the Ancient World or Highly Evolved State?

Takashi Okubo, Shohei Fukushima and Kiwamu Minamisawa

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 53, issue 12, pages 2000-2007
Published in print December 2012 | ISSN: 0032-0781
Published online November 2012 | e-ISSN: 1471-9053 | DOI:
Evolution of Bradyrhizobium–Aeschynomene Mutualism: Living Testimony of the Ancient World or Highly Evolved State?

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  • Biochemistry
  • Molecular and Cell Biology
  • Plant Sciences and Forestry


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Until recently it had been well established that the initial step in legume–rhizobia symbioses was flavonoid and Nod factor (NF) signaling. However, NF-independent symbiosis is now known to occur between Bradyrhizobium and some species of Aeschynomene. Since its discovery, this unusual symbiotic system has attracted attention, and efforts have been devoted to revealing the NF-independent symbiotic mechanism, although the molecular mechanisms of nodule initiation still remain to be elucidated. NF-independent symbiosis is also interesting from the perspective of the evolution of legume–rhizobia symbiosis. In this mini-review, we discuss the current literature on the NF-independent symbiotic system in terms of phylogeny of the partners, infection, bacteroid differentiation, nodule structure, photosynthesis, endophytic features and model host plant. We also discuss NF-independent symbiosis, which is generally regarded to be more primitive than NF-dependent symbiosis, because the bacteria invade host plants via ‘crack entry’. We propose three possible scenarios concerning the evolution of NF-independent symbiosis, which do not exclude the possibility that the NF-independent system evolved from NF-dependent interactions. Finally, we examine an interesting question on Bradyrhizobium–Aeschynomene mutualism, which is how do they initiate symbiosis without NF. Phylogenetic and genomic analyses of symbiotic and non-symbiotic bradyrhizobia with A. indica may be crucial to address the question, because of the very narrow phylogeny of natural endosymbionts without nod genes compared with other legume–rhizobia symbioses.

Keywords: Aeschynomene; Bradyrhizobium; Co-evolution; Endophyte; Nod factors; Nodulation

Journal Article.  4276 words.  Illustrated.

Subjects: Biochemistry ; Molecular and Cell Biology ; Plant Sciences and Forestry

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