Spatial-Temporal Colonization Patterns of Azospirillum brasilense on the Wheat Root Surface and Expression of the Bacterial nifH Gene during Association. Ann Vande Broek. F.A. Janssens Laboratory of Genetics. Catholic University of Leuven, Willem de Croylaan 42, B-3001 Heverlee, Belgium. Jan Michiels, August Van Gool, and Jos Vanderleyden. F.A. Janssens Laboratory of Genetics. Catholic University of Leuven, Willem de Croylaan 42, B-3001 Heverlee, Belgium.. MPMI 6:592-600. Accepted 28 May 1993. Copyright 1993 The American Phytopathological Society.

Colonization of wheat roots by Azospirillum was studied using the GUS reporter system. The surface distribution of A. brasilense on developing wheat roots was investigated using A. brasilense strains expressing the gusA gene. During the first days of association, bacteria were mainly found in the root hair zones and at the sites of lateral root emergence. Further proliferation of Azospirillum on the wheat root surface was dependent on the nitrogen status of the nutrient solution. Similarly, the expression of a nifH–gusA fusion was monitored during the association. Our results indicate that both oxygen and availability of carbon sources are limiting factors for associative nitrogen fixation.