USE OF ELISA AND GUS-TRANSFORMED STRAINS TO STUDY COMPETITION BETWEENPATHOGENIC AND NONPATHOGENIC FUSARIUM-OXYSPORUM FOR ROOT COLONIZATION

To characterize the ability of different strains of Fusarium oxysporum to colonize roots, and to analyze competition for root colonization b etween pathogenic and non-pathogenic strains of F. oxysporum, it was n ecessary to develop specific labelling techniques for quantification o f root colonization. Two methods were selected: the production of poly clonal antibodies, and the use of GUS-transformed strains of F. oxyspo rum. The polyclonal antibodies recognized infected plants, and gave a minimum reaction with healthy plants, but were not specific for indivi dual strains of F. oxysporum. These antibodies enabled total density o f F. oxysporum to be assessed on roots, by ELISA. Metabolic activity o f the root population of GUS-marked strains was assessed by measuring the glucuronidase activity. Strains showed a diversity in their abilit y to colonize roots: patterns of root colonization were similar, but t he intensity and the speed of colonization differed according to the p lant-fungus combination used Results demonstrated competition between the pathogenic and the non-pathogenic strains for root colonization. I n the presence of the non-pathogenic strain Fo 47, the competition see ms to be reciprocal, affecting both the pathogen and non-pathogenic st rain. Other non-pathogenic strains reduced root colonization by the pa thogenic strain, but some strains did not reduce the metabolic activit y of the pathogen, suggesting that different mechanisms are involved i n the interaction between pathogenic and non-pathogenic F. oxysporum.