Calcium (Ca ²⁺) is an important second messenger in plants. The activation of Ca ²⁺ signaling cascades is critical in the activation of adaptive processes in response to perceived environmental stimuli, including biotic stresses. The colonization of roots by the plant growth promoting endophyte Serendipita indica involves the increase of cytosolic Ca ²⁺ levels in Arabidopsis thaliana. In this study, we investigated transcriptional changes in Arabidopsis roots during symbiosis with S. indica. RNA-seq profiling disclosed the significant induction of CALCINEURIN B-LIKE 7 ( CBL7) during early- and later phases of the interaction. Consistent with the transcriptomics analysis, reverse genetic evidence and yeast two-hybrid studies highlighted the functional relevance of CBL7 and tested the involvement of a CBL7-CBL-INTERACTING PROTEIN KINASE 13 (CIPK13) signaling pathway in the establishment of the mutualistic relationship that promotes plant growth. The loss-of-function of CBL7 abolished the growth promoting effect of S. indica and affected the colonization of the root by the fungus. The subsequent transcriptomics analysis of cbl7 revealed the involvement of this Ca ²⁺ sensor in activating plant defense responses. Furthermore, we report on the contribution of CBL7 to potassium transport in Arabidopsis. Triggered by the differential expression of a small number of K ⁺ channels/transporter genes, we analyzed K ⁺ contents in wild-type and cbl7 plants and observed a significant accumulation of K ⁺ in root of cbl7 plants, while shoot tissues demonstrated K ⁺ depletion. Taken together, our work associates CBL7 with an important role in the mutual interaction between Arabidopsis and S. indica and links the CBL7 Ca ²⁺ receptor protein to K ⁺ transport.