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Abstract Detail

Symbioses: Plant, Animal, and Microbe Interactions

Agapakis, Christina M [1], Lum, Michelle R [2], Angus, Annette A [1], HIRSCH , ANN M [3].

Legume Nodule Development and Host Range of the β-rhizobial Genus Burkholderia.

For over 100 years, the only bacteria known to establish nitrogen-fixing nodules on legume roots were rhizobia (Rhizobiaceae), a family of α-proteobacteria.  About a decade ago, members of a distinct clade in the β-proteobacterial family Burkholderiaceae, were found to elicit nitrogen-fixing root nodules on legumes.  Although the α- and β-rhizobia diverged well before the evolution of legumes, the genes controlling symbiotic behavior are homologous due to horizontal gene transfer between the clades. Additionally, the development and morphology of the host plant nodule elicited by either rhizobial group are similar because plant genes control these traits.  Thus, siratro nodules elicited by either Burkholderia tuberum STM678 or Rhizobium tropici CIAT899 appear identical (1).  Nonetheless, the nodulating Burkholderia species do differ significantly in their arrangement of nodulation (nod) and nitrogen fixing (nif) genes and lack the typical host-specificity nod genes found in many of the α-rhizobia.  These genes chemically modify the Nod factor signal that begins the process of nodulation, and have long been considered to be crucial for host specificity in the α-rhizobia.  Within the plant-associated Burkholderia, two groups of legume-nodulating species separated by geography and host specificity exist.  The South African species have chromosomally-localized nod and nif genes and nodulate Papilionoid legumes whereas the South American species, which have plasmid-localized symbiotic genes, mainly nodulate Mimosoid legumes.  To understand host specificity in the β-rhizobia better, we studied whether any host overlap existed between B. tuberum STM678 (South African species) and Burkholderia sp. CCGE1002 (isolated in Mexico).  B. tuberum STM678 and B. sp. CCGE1002 share 99% 16S sequence identity as well as significant similarity throughout the core genome.  The symbiotic islands, however, were horizontally transferred separately, reflecting the different rhizosphere environment they inhabit.  Through analysis of these and other South African and South American Burkholderia genomes, we have shown that horizontal gene transfer was a major driver in the evolution of nodulation and the host specificity of this relationship.  
(1) Angus, A.A., Lee, A.S., Lum, M.R., Shehayeb, M., Hessabi, R., et al.  2013. Plant Soil.   362: 543-562.

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1 - UCLA, Molecular, Cell and Developmental Biology, 621 Charles Young Drive South, Los Angeles, CA, 90095-1606, USA
2 - Loyola Marymount University, Biology, Los Angeles, CA, 90045, USA
3 -

symbiotic genes
nodule development
host range.

Presentation Type: Oral Paper:Papers for Topics
Session: 9
Location: Cottonwoods North/Boise Centre
Date: Monday, July 28th, 2014
Time: 10:30 AM
Number: 9002
Abstract ID:198
Candidate for Awards:None

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