Agriculture and Agrifood Canada, Soils and Crops Development Centre,
2560 Blvd Hochelaga, Ste-Foy, PQ, Canada 2Pest Management Centre, 1391 Sandford Street, London, ON, Canada
Medicago laciniata (cut-leaf medic) is an annual that is nodulated by a restricted range of S. meliloti (Brockwell, Hely 1966), e.g. strain 102L4 (=USDA1170) but not by many strains which nodulate alfalfa, e.g. RCR2011 (=1021) and 41. Our aim was to characterize the S. meliloti 102L4 gene that is responsible for nodulation of M. laciniata.
An 11 kb DNA fragment was isolated from S. meliloti 102L4 on the basis of its ability to complement strain RCR2011 for nodulation of M. laciniata. This DNA fragment contained the common nodulation genes, «oJABCIJ, in a single operon and the overall arrangement of genes was similar to that of strains RCR2011 and 41 (Baev et al. 1991; Torok et al. 1984; S. Long et al. http://www.ncbi.nlm.nih). Mutation of the nod IJ genes of strain 102L4 resulted in a nodulation delay on M. laciniata, M. sativa and on M. alba. A combination of Tn5 mutagenesis and complementation studies with sub-clones of the 11 kb fragment indicated that the nodC gene of strain 102L4 was responsible for the specific nodulation of M. laciniata. The amino acid sequence of the NodC protein from strain 102L4 differed significantly from that of strains RCR 2011 and 41. The rhizobial nodC gene is known to influence the length of the oligosaccharide backbone of lipo-chito-oligosaaccharides (LCOs) thereby influencing the host range for nodulation (Kamst et al. 2000; Schultze et al. 1992; Roche et al. 1991). Our data therefore suggest that S. meliloti 102L4 is able to nodulate M. laciniata because it synthesizes LCOs in which the ratio of C4 to C5 oligosaccharides differ significantly from those produced by strains RCR2011 and 41.
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