The Arabidopsis/Piriformospora indica interaction system is a nice model system to study beneficial and non-benefical traits in a symbiosis. Establishing symbiotic interaction with P. indica, a root endophytic fun-gus, requires mild and constitutive activation of defense responses. Lesions in different and unrelated defense response pathways resulted in the loss of beneficial traits. Here we describe mutants which do not respond to the fungus and are smaller rather than bigger in its presence. These mutants are defective in defense-signaling components, defense proteins and enzymes, and defense metabolites. They are usually overcolonized by P. indica which leads to the activation of unspecific defense responses therefore shifting the interaction from beneficial to nonbeneficial.