Use of ELISA and GUS‐transformed strains to study competition between pathogenic and non‐pathogenicFusarium oxysporumfor root colonization

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Abstract
To characterize the ability of different strains of Fusarium oxysporum to colonize roots, and to analyze competition for root colonization between pathogenic and non‐pathogenic strains of F. oxysporum, it was necessary to develop specific labelling techniques for quantification of root colonization. Two methods were selected: the production of polyclonal antibodies, and the use of GUS‐transformed strains of F. oxysporum. The polyclonal antibodies recognized infected plants, and gave a minimum reaction with healthy plants, but were not specific for individual strains of F. oxysporum. These antibodies enabled total density of F. oxysporum to be assessed on roots, by ELISA. Metabolic activity of the root population of GUS‐marked strains was assessed by measuring the glucuronidase activity. Strains showed a diversity in their ability to colonize roots: patterns of root colonization were similar, but the intensity and the speed of colonization differed according to the plant—fungus combination used. Results demonstrated competition between the pathogenic and the non‐pathogenic strains for root colonization. In the presence of the non‐pathogenic strain Fo 47, the competition seems to be reciprocal, affecting both the pathogen and non‐pathogenic strain. Other non‐pathogenic strains reduced root colonization by the pathogenic strain, but some strains did not reduce the metabolic activity of the pathogen, suggesting that different mechanisms are involved in the interaction between pathogenic and non‐pathogenic F. oxysporum.