Balancing defense and growth—Analyses of the beneficial symbiosis between Piriformospora indica and Arabidopsis thaliana

Abstract

The mutualistic interaction between the endophytic and root-colonizing fungus Piriformospora indica and Arabidopsis thaliana is a nice model system to study beneficial and non-benefical traits in a symbiosis. Colonized Arabidopsis plants are taller, produce more seeds and are more resistant against biotic and abiotic stress. Based on genetic, molecular and cellular analyses, Arabidopsis mutants were identified which are impaired in their beneficial response to the fungus. Several mutants are smaller rather than bigger in the presence of the fungus and are defective in defense responses. This includes mutants with defects in defense-signaling components, defense proteins and enzymes, and defense metabolites. The mutants cannot control root colonization and are often over-colonized by P. indica. As a consequence, the benefits for the plants are lost and they try to restrict root colonization by activating unspecific defense responses against P. indica. These observations raise the question as to how the plants balance defense gene activation or development and what signaling molecules are involved. P. indica promotes the synthesis of phosphatidic acid (PA), which binds to the 3-PHOSPHOINOSITIDE-DEPENDENT-KINASE1 (PDK1). This activates a kinase pathway which might be crucial for balancing defense and growth responses. The review describes plant defense compounds which are necessary for the mutualistic interaction between the two symbionts. Furthermore, it is proposed that the PA/PDK1 pathway may be crucial for balancing defense responses and growth stimulation during the interaction with P. indica.