The calcium sensor CBL7 is required for Serendipita indica‐induced growth stimulation in Arabidopsis thaliana, controlling defense against the endophyte and K(+) homoeostasis in the symbiosis

Calcium is an important second messenger in plants. The activation of Ca(2+) signalling cascades is critical in the activation of adaptive processes in response to environmental stimuli. Root colonization by the growth promoting endophyte Serendipita indica involves the increase of cytosolic Ca(2+) levels in Arabidopsis thaliana. Here, we investigated transcriptional changes in Arabidopsis roots during symbiosis with S. indica. RNA‐seq profiling disclosed the induction of Calcineurin B‐like 7 (CBL7) during early and later phases of the interaction. Consistently, reverse genetic evidence highlighted the functional relevance of CBL7 and tested the involvement of a CBL7‐CBL‐interacting protein kinase 13 signalling pathway. The loss‐of‐function of CBL7 abolished the growth promoting effect and affected root colonization. The 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. We analysed K(+) contents in wild‐type and cbl7 plants and observed a significant increase of K(+) in roots 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 CBL7 to K(+) transport.