Modulation of carbohydrate-binding capacities and attachment ability of Bradyrhizobium sp (Lupinus) to white lupin roots

The attachment of Bradyrhizobium sp. (Lupinus) strain MSDJ718 to excised roots of white lupin was examined. Maximal attachment occurred at early to middle log phases of bacterial growth. This binding was pH dependent, with an optimal value reached at 6.6. Irrespective of the culture age, the attachment was strongly affected by the calcium concentration of the growth medium: a Ca2+ limitation in the Bergersen medium led to optimal attachment of the bacteria. When L-fucose was added during the attachment assay, a significant inhibition was observed. The binding was stimulated when bacteria were cultivated with lupin root extracts, genistein and genistin (two lupin isoflavonoids), and some monosaccharides. In addition, with a spectrofluorimetric method using fluoresceinylated neoglycoproteins, it was shown that the increase of the attachment of bacteria to host cells was correlated to the increase of the L-fucoside binding capacity of the rhizobial cells. Taken together, the results obtained in the present study evidenced a possible role of the L-fucose specific bacterial lectin previously described in the Rhizobium - lupin host cell recognition.