The iandole acetic acid is the principal hormone for plant growing. The Trichoderma and Azospirillum brasilense can produce but this organisms no are estimulated by yuor presence in culture medium.
auxin (and incidentally most other plant growth substances) produced by micro-organisms interferes with the plants own production/location and gives the micro-organisms advantages (e.g. in pathogenesis) .
It is estimated that it intervenes in the associations and in the case of some fungi it is a support factor for the infection of the fungus in the plant.
It is also a factor of interaction between agents. The germinating seed naturally releases tryptophan which is an essential factor for indole acetic acid synthesis