Role of extracellular polysaccharide in the colonization of wheat (Triticum vulgare L.) roots by N2-fixing cyanobacteria

The characteristics of the mucilaginous sheaths of the cyanobacteria Nostoc 2S9B and Anabaena C5 and their role in the formation of associations with the roots of wheat plants grown in liquid culture have been assessed. Light and scanning electron microscopy revealed that the filaments of Nostoc 2S9B that formed a tight association with the root surface were contained in a firm mucilaginous shell. In contrast, filaments of Anabaena C5 formed a loose association and were easily detached from the mucilage that had a sheet-like appearance and tended to disintegrate as the culture aged. Similarly, there was a tight attachment of the isolated polysaccharide from Nostoc 2S9B to the root surface and a loose attachment of the Anabaena C5 polysaccharide. When the crude polysaccharide from Nostoc 2S9B was freed from proteins by phenol or pronase treatment, its ability to adhere to the root surface was lost or considerably reduced, suggesting that a protein component contributes to the tight attachment of Nostoc 2S9B. The crude polysaccharide preparation from Nostoc 2S9B contained 2.8% (w/w) protein while that from Anabaena C5 was only 0.6% (w/w) protein. The purified exopolysaccharide from Nostoc 2S9B contained three neutral sugars and glucuronic acid, whereas fucose and a uronic acid were the main components of that from Anabaena C5. Washing the roots or treating them with different sugars did not alter the ability of Nostoc 2S9B to colonize the root surface, indicating that cyanobacterial attachment may not be specific.