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Conditional Requirement for Exopolysaccharide in the Mesorhizobium–Lotus Symbiosis

March 2013 , Volume 26 , Number  3
Pages  319 - 329

Simon J. Kelly,1,2 Artur Muszyński,3 Yasuyuki Kawaharada,2,4 Andree M. Hubber,1 John T. Sullivan,1,2 Niels Sandal,2,4 Russell W. Carlson,3 Jens Stougaard,2,4 and Clive W. Ronson1,2

1Department of Microbiology and Immunology, University of Otago, Dunedin, New Zealand; 2Centre for Carbohydrate Recognition and Signalling, Aarhus University, Aarhus, Denmark; 3Complex Carbohydrate Research Center, University of Georgia, Athens, U.S.A.; 4Department of Molecular Biology and Genetics, Aarhus University


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Accepted 22 October 2012.

Rhizobial surface polysaccharides are required for nodule formation on the roots of at least some legumes but the mechanisms by which they act are yet to be determined. As a first step to investigate the function of exopolysaccharide (EPS) in the formation of determinate nodules, we isolated Mesorhizobium loti mutants affected in various steps of EPS biosynthesis and characterized their symbiotic phenotypes on two Lotus spp. The wild-type M. loti R7A produced both high molecular weight EPS and lower molecular weight (LMW) polysaccharide fractions whereas most mutant strains produced only LMW fractions. Mutants affected in predicted early biosynthetic steps (e.g., exoB) formed nitrogen-fixing nodules on Lotus corniculatus and L. japonicus ‘Gifu’, whereas mutants affected in mid or late biosynthetic steps (e.g., exoU) induced uninfected nodule primordia and, occasionally, a few infected nodules following a lengthy delay. These mutants were disrupted at the stage of infection thread (IT) development. Symbiotically defective EPS and Nod factor mutants functionally complemented each other in co-inoculation experiments. The majority of full-length IT observed harbored only the EPS mutant strain and did not show bacterial release, whereas the nitrogen-fixing nodules contained both mutants. Examination of the symbiotic proficiency of the exoU mutant on various L. japonicus ecotypes revealed that both host and environmental factors were linked to the requirement for EPS. These results reveal a complex function for M. loti EPS in determinate nodule formation and suggest that EPS plays a signaling role at the stages of both IT initiation and bacterial release.



© 2013 The American Phytopathological Society