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Rhizobial Adaptation to Hosts, a New Facet in the Legume Root-Nodule Symbiosis

June 2010 , Volume 23 , Number  6
Pages  784 - 790

Marion Koch,1 Nathanaël Delmotte,1 Hubert Rehrauer,2 Julia A. Vorholt,1 Gabriella Pessi,1 and Hauke Hennecke1

1Institute of Microbiology, ETH, Zurich, Switzerland; 2Functional Genomics Center Zurich, University of Zurich and ETH, Zurich, Switzerland

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Accepted 6 February 2010.

Rhizobia are able to infect legume roots, elicit root nodules, and live therein as endosymbiotic, nitrogen-fixing bacteroids. Host recognition and specificity are the results of early programming events in bacteria and plants, in which important signal molecules play key roles. Here, we introduce a new aspect of this symbiosis: the adaptive response to hosts. This refers to late events in bacteroids in which specific genes are transcribed and translated that help the endosymbionts to meet the disparate environmental requirements imposed by the hosts in which they live. The host-adaptation concept was elaborated with Bradyrhizobium japonicum and three different legumes (soybean, cowpea, and siratro). Transcriptomes and proteomes in root-nodule bacteroids were analyzed and compared, and genes and proteins were identified which are specifically induced in only one of the three hosts. We focused on those determinants that were congruent in the two data sets of host-specific transcripts and proteins: seven for soybean, five for siratro, and two for cowpea. One gene cluster for a predicted ABC-type transporter, differentially expressed in siratro, was deleted in B. japonicum. The respective mutant had a symbiotic defect on siratro rather than on soybean or cowpea. This result demonstrates the value of the applied approach and corroborates the host-specific adaptation concept.

© 2010 The American Phytopathological Society